Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 1

Influence of Temporal Overlap on Time Course of the Simon Effect

Giulia Baroni

Antonello Pellicano

Luisa Lugli

Roberto Nicoletti

Robert W. Proctor

Correspondence to:

Giulia Baroni (giulia.baroni4@unibo.it)

Department of Communication Studies, University of Bologna

Via A. Gardino 23, 40122 Bologna, Italy

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 2

Abstract

Two experiments are reported in which we manipulated relevant and irrelevant stimulus dimensions

to assess whether an increase in temporal overlap would influence the time course of a “standard”

Simon effect (obtained when visual stimuli are presented on the left/right of the screen and left/right

responses are performed with uncrossed hands). This procedure is new in two ways: First, the

manipulations were intended to reduce, instead of increase, the distance between conditional and

unconditional response-activation processes. Second, we manipulated the relevant and irrelevant

stimulus dimensions in a manner that did not vary stimulus onset asynchronies (SOAs), precues or

go/no go trials, or alter the stimulus quality. Results were consistent with the hypothesis that when

the two response processes are shifted closer to each other, the Simon effect would be sustained

across time, instead of decreasing as typically found. These findings are discussed in line with the

temporal overlap hypothesis and with an automatic activation account.

Keywords: Simon effect, temporal overlap, time-course analysis, conditional accuracy function,

automatic activation.

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 3

Introduction

In the Simon task, participants are to respond to the colour or shape of a stimulus (i.e., the

relevant dimension) while ignoring its spatial position (i.e., the irrelevant dimension). The Simon

effect refers to the phenomenon wherein the spatial position of the stimulus, despite being

irrelevant, influences performance: Responses are faster and more accurate when the stimulus and

response locations correspond to each other than when they do not (see Proctor & Vu, 2006, and

Simon, 1990, for reviews). Thus, if participants are instructed to press a right key to a red stimulus

and a left key to a green stimulus, their reaction times (RTs) will be shorter and accuracy higher

when the red stimulus is on the right (corresponding trials) than when it is on the left (non-

corresponding trials).

Dual-route models (e.g., De Jong, Liang, & Lauber, 1994) are widely used to account for the

Simon effect. According to these models, two different response processes are involved: On the

one hand, the irrelevant spatial position automatically activates an ipsilateral response through an

unconditional route; on the other hand, the relevant feature of the stimulus is translated, following

the task instructions, into the correct response through a conditional route. Performance on

corresponding trials is faster and more accurate because the same response is activated through the

unconditional and conditional routes. Conversely, non-corresponding trials are slower and less

accurate because a conflict occurs between the automatically activated and the voluntarily translated

response.

The time-courses of these two response processes have been investigated through analysis of

the RT distributions (bin analysis; De Jong et al., 1994; see Proctor, Miles, & Baroni, 2011, for a

review). This technique has revealed that the Simon effect can have two different time courses: A

decreasing effect function (i.e., the magnitude of the Simon effect diminishes as RT increases) is

found when the unconditional response activation occurs soon after the stimulus onset and then

dissipates over time (e.g., Proctor, Yamaguchi, Zhang, & Vu, 2009; Rubichi & Pellicano, 2004).

An increasing or constant effect function is observed, though, when the irrelevant response needs

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 4

more time to reach complete activation and thus to exert the maximum influence on performance

(e.g., Vallesi, Mapelli, Schiff, Amodio, & Umiltà, 2005; Wühr, 2006).

To explain these different effect functions, Wascher, Schatz, Kuder, and Verleger (2001)

proposed that the Simon effect can be generated by two different and dissociable mechanisms: A

visuomotor facilitation of same-side responses and a cognitive interference of codes. The decaying

effect function, yielded by the visuomotor process, would be generated by a “natural” spatial

anatomical mapping, that is, when stimuli and responses are both on the horizontal axis and the

hands are placed in an uncrossed position (see Buhlmann, Wascher, & Umiltà, 2007, Experiments 1

and 2; Wascher et al., 2001, Experiment 1). This task setting has come to be prototypical and is

often referred to as “standard”. Conversely, the stable or increasing effect function, yielded by the

cognitive process, would be due to the lack of a “natural” relation between a visual stimulus and

anatomical effector, namely when (a) the task is performed with the hands crossed (Wascher et al.,

2001, Experiment 1), (b) the stimuli are along the vertical axis and are matched, through a fixed

mapping, with left/right responses (Proctor, Vu, & Nicoletti, 2003; Wiegand & Wascher, 2005;

Wiegand & Wascher, 2007), or (c) the stimuli are presented in an auditory modality (Wascher et al.,

2001, Experiment 2).

The focus of the current study is on the dynamics underlying the decreasing time course of

the “standard” Simon effect. De Jong et al. (1994) firstly attributed this distribution pattern to an

automatic, but short lived, response activation. This interpretation has gathered consistent evidence

in behavioural and neuropsychological literature (see Proctor et al., 2011), and it also matches with

the same-side response activation subsequently illustrated by Wascher et al. (2001) for their so-

called visuomotor Simon effect.

The idea of an automatic activation pattern is also supported by Hommel’s (1993) temporal

overlap hypothesis, which postulates that the Simon effect magnitude is strongly influenced by the

temporal distance between codes from the relevant and the irrelevant stimulus dimension. More

precisely, Hommel claimed, “Every experimental manipulation that markedly increases the

temporal distance between the formation of the relevant stimulus code and the irrelevant spatial

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 5

code leads to a smaller Simon effect – that is, to a smaller effect of irrelevant spatial correspondence

between stimulus and responses” (p. 289). In several of Hommel’s experiments, the Simon effect

tended to disappear whenever the coding of the relevant dimension was postponed relative to the

coding of the irrelevant one. This happened, for example, when the code of the relevant dimension

was slowed both through decreasing the signal quality with a pattern mask or a reduction in contrast

relative to the background, as well as when the stimulus built up gradually (i.e., its colour appeared

at different time gaps, also called stimulus onset asynchronies, SOAs). A similar result was also

obtained in a further study (Hommel, 1994b) in which higher stimulus complexity made the

discrimination of the relevant stimulus value less immediate (see also Hommel, 1994a, and Rubichi,

Iani, Nicoletti, & Umiltà, 1997). Lastly, a Simon effect smaller in magnitude was found even when

the response to lateralised stimuli had to be withheld until a go signal occurred (Simon, Acosta,

Mewaldt, & Speidel, 1976; Vallesi & Umiltà, 2009, Experiment 1) or when a spatial cue preceded

the target stimuli (Vallesi & Umiltà, 2009, Experiment 2).

Hence, consistent with the automatic activation account, all the above mentioned results

demonstrate that when the temporal gap between the relevant and irrelevant dimension is increased,

by delaying the relevant dimension or precuing the irrelevant one, the Simon effect decreased. This

result pattern implies that the spatial code had time to dissipate before response selection occurred.

Recently, Miles and Proctor (2009) provided further support for this automatic activation

account and demonstrated that temporal overlap manipulations can influence the time course of the

Simon effect, as well as its magnitude, when central spatial indicators (“left”/ “right” words) are

presented as stimuli. In their Experiment 1 participants were to press a left or right key according to

the colour of the central word, which could be intact (high discriminability condition) or masked

with a noise overlay (low discriminability condition). The former condition showed an increasing

Simon effect, as is typically obtained for location words (e.g., Pellicano, Lugli, Baroni, & Nicoletti,

2009, Experiment 3). However, for the latter condition, in which the colour processing was

disrupted, the Simon effect was nonsignificant overall and showed a flat effect function across the

RT distribution. The authors interpreted these results as consistent with the temporal overlap

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 6

hypothesis: The delay of the conditional process allowed the automatic activation to decay prior to

the conflict arising. This interpretation was extended to the results of Experiment 2, in which the

discriminability of both the relevant and the irrelevant dimensions was reduced in order to delay

both the conditional and unconditional response processes. The Simon effect in this low

discriminability condition was significant and increased across bins, as for the non-disrupted

condition of Experiment 1. The only difference between these manipulations was the position of

the Simon effect distribution function along the abscissa (RT): The function for Experiment 2 was

shifted rightward by more than 100 ms because of the overall higher task complexity.

The aim of the current study was to investigate more deeply how temporal overlap

manipulations can shape the time course of the Simon effect. However, we followed an opposite

rationale with respect to most of the above discussed studies. Our experiments aimed at decreasing,

instead of increasing, the temporal distance between stimulus codes relative to a baseline condition,

thus yielding greater temporal overlap between the resulting response processes. As in Miles and

Proctor’s (2009) study, an automatic activation account was used as the theoretical framework. But

unlike that study, all of our tasks shared a spatial-anatomical mapping between stimuli and

responses. Participants were in fact always faced with lateralised, instead of centrally presented,

stimuli. This difference is not trivial since, according to Wascher et al. (2001), such a “natural” task

set should yield a decreasing time course.

We hypothesized, though, that greater temporal overlap would turn this typical decreasing

effect found for the “standard” Simon task into a sustained one. Indeed, reducing the distance

between the relevant and irrelevant stimulus dimensions would make participants respond during

the time window in which the automatic activation had not yet dissipated. As a consequence,

performance would be influenced by the irrelevant information for a longer time span, thus yielding

a significant Simon effect across the whole RT distribution. To assess this hypothesis, we

conducted two experiments in which the temporal overlap was enhanced through discrete

manipulations to reduce the time to process the relevant stimulus dimension (i.e., Experiment 1,

wide coloured stimuli condition) or increase the time to process the irrelevant stimulus dimension

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 7

(i.e., Experiment 2, arrow stimuli condition). Note that, unlike most of the previously discussed

studies, the temporal proximity was increased without dissociating the two stimulus codes by means

of SOAs, precues, or go/no go signals. Moreover, we also avoided altering the stimulus

discriminability through masking patterns or variations of the stimulus contrast.

Experiment 1 was composed of three conditions, baseline, wide coloured stimuli and wide

unfilled stimuli. The baseline condition aimed at replicating the “standard” Simon settings:

Lateralized coloured squares were used as stimuli and matched with left/right keypress responses

performed by the left and the right hand, respectively. In the wide coloured stimuli condition

participants were faced with the same coloured squares, but larger in size. This manipulation aimed

at facilitating and speeding up the colour discrimination (i.e., the relevant distinction), thus shifting

the conditional component back in time, closer to the automatic one. Finally, in the wide unfilled

stimuli condition, participants were also faced with wide squares, but coloured only along their

borders, instead of their whole area. This condition served as a control for the wide coloured stimuli

condition, in order to disambiguate the possible effects of increased colour information from that of

increased size. Indeed, the stimulus width was kept equal to the wide coloured stimuli condition but

the amount of colour information was set similar to the baseline condition.

Two different results were thus expected for these three conditions: In line with the above

mentioned literature, a decreasing time course was expected for the baseline and also for the wide

unfilled stimuli condition, since the temporal proximity between the relevant and irrelevant stimulus

codes is assumed to be similar. Conversely, in the wide coloured stimuli condition, the increased

temporal overlap was expected to yield a constant or increasing Simon effect.

A sustained time course was also hypothesized for the arrow stimuli condition of

Experiment 2, where the temporal overlap was enhanced, though, by a manipulation on the

irrelevant stimulus dimension. Lateralised arrow signals, similar in width to the baseline stimuli,

were used in fact in place of squares. The spatial code given by the direction of such indicators was

expected to sum up to the one provided by their location in space, leading to a stronger and longer

lasting stimulus spatial code with respect to that of the baseline condition of Experiment 1.

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 8

To summarize, the goal of the wide coloured and arrow stimuli conditions of Experiments 1

and 2, respectively, was to increase the temporal overlap between the two stimulus codes. As a

consequence, a sustained Simon effect function was expected to be found for both conditions,

despite their “standard” task set would typically yield a decaying time course. Results that match

our predictions would provide reliable further evidence that temporal overlap influences the time

course of the Simon effect.

Experiment 1

The experiment included baseline, wide coloured stimuli, and wide unfilled stimuli

conditions, all implemented with a “standard” Simon task: A square was presented to the left or to

the right on the computer screen, and the responses to its colour was performed with uncrossed

hands by pressing one of two buttons placed on the left and right of the body midline. The

difference between the three conditions was in the saliency of the colours (see Figure 1).

The aim of the colour saliency manipulation was as follows. First of all, presenting wide

stimuli filled with colour (i.e. wide coloured stimuli condition) was meant to facilitate and speed up

the colour discrimination task, relative to that of the baseline condition, in order to shift the

selection of the correct response (conditional process) closer to the (fast) activation of the

unconditional one. This goal of a higher degree of temporal overlap had a converse rationale with

respect to Hommel’s (1994a, 1994b) studies (see also Rubichi et al., 1997), in which the stimulus

complexity was enhanced to slow down identification of the relevant dimension. Presenting wide

squares, but only coloured along their borders (i.e., wide unfilled stimuli), should not facilitate the

task-instructed colour discrimination, thus yielding a similar degree of temporal overlap between

stimulus codes to that of baseline.

Our predictions were as follows. For the baseline condition, we expected the Simon effect to

be largest in the shortest RT bins and then to decrease across the longer ones. This result would

replicate the findings shown in the current literature (e.g., Proctor et al., 2009; Rubichi & Pellicano,

2004) and would be consistent with the evidence that such a declining effect function is bound to a

spatial-anatomical mapping between stimuli and responses (Wascher et al., 2001). For the wide

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 9

coloured stimuli condition, the easier colour discrimination was expected to yield overall shorter

RT with respect to the baseline condition and, more importantly, a stable or increasing Simon effect

function. The perceptual manipulation on the relevant stimulus dimension was intended to allow

participants to respond during the time window in which the automatic activation had its maximum

strength and it had not yet dissipated. Finally, in the wide unfilled stimuli condition we

hypothesized that reducing the amount of colour information within the stimulus compared to the

wide coloured stimuli would result in the conditional response process not being shifted closer to

the unconditional one. Consequently, we expected that condition to yield an RT range and a Simon

effect function similar to those of the baseline condition.

To summarize, we hypothesized that, within a “standard” Simon task, a higher degree in

temporal overlap would shift the typically found decreasing time course, into a sustained or

increasing one.

Method

Participants. Thirty-six students from the University of Bologna (24 females; mean age =

20.4 years; SD = 1.8 years) volunteered to participate in the experiment. Twelve took part in the

baseline condition, 12 in the wide coloured stimuli condition and the remaining 12 in the wide

unfilled stimuli one. The participants were all right-handed, had normal or corrected-to-normal

vision, and were naïve as to the purpose of the experiment. They received extra credit for their

participation in the study.

Apparatus and stimuli. The experiment was carried out in a dimly lit and noiseless room.

The participant was seated facing a 17 in. cathode-ray tube screen driven by a 700 MHz processor

computer. His/her head was positioned in an adjustable head-and-chin rest. The screen was

positioned so that stimuli appeared at eye level at a distance of 47 cm. Stimulus selection, response

timing and data collection were controlled by the E-Prime software (www.pstnet.com; Psychology

Software Tools, Inc.)

A fixation cross 1-cm wide and 1-cm high (1.2° x 1.2° of visual angle) was presented at the

centre of the screen against a black background. For the baseline and wide coloured stimuli

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 10

conditions, the stimuli were filled green and red squares presented at the left or right of the fixation

cross. For the wide unfilled stimuli condition, though, the stimuli were black squares with green/red

borders also presented at the left/right screen edges. The square width and height was 0.8 cm (1° x

1°), for the baseline condition, and 4.6 cm (5.6° x 5.6°) for the wide coloured and wide unfilled

stimuli conditions (see Figure 1). The width of the border for the wide unfilled stimuli condition

was 0.4 cm. The stimulus total coloured area for the baseline, wide coloured and wide unfilled

stimuli conditions was: 0.6; 21.2 and 13 cm2, respectively. Stimuli of all three conditions were

centred at a distance from the fixation, being separated by 9.4 cm (11.5° centre to centre).

Responses were made by pressing either the ‘‘Z’’ or the ‘‘M’’ key (left and right keys) on the

computer keyboard with the left or the right index finger, respectively.

Insert Figure 1 about here

Procedure. At the beginning of the experiment, instructions were presented on the screen,

and each participant was required to place the tip of his/her left and right index finger on the left

and right key, respectively. When the participant’s fingers were appropriately placed on the keys,

the experimenter started the experiment by pressing the space bar. The central fixation cross was

presented and then remained visible across the whole experiment. After a 1000-ms intertrial

interval, the stimulus appeared on the left or on the right of the screen and remained visible for 250

ms followed by a 1000-ms blank interval for response collection. Stimulus presentations occurred

according to a random sequence. RT was recorded as the time between the stimulus onset and the

pressing of the response key. The instructions stressed the speed and accuracy of performance.

One-third of the participants was randomly assigned to the baseline condition, another third

to the wide coloured stimuli condition and the remaining third to the wide unfilled stimuli one

(between-subjects design). Within each condition, the instructions were balanced since half of the

participants were instructed to respond to the red colour by pressing the right key and to the green

colour by pressing the left key, whereas the other half were instructed with the reverse assignment.

An error message was given as feedback for omitted (> 1250 ms), anticipated (< 150 ms), and

incorrect responses. The feedback for incorrect responses was also enhanced by a low pitch tone of

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 11

600 ms in duration. Each participant completed a block of 40 practice trials, which was followed by

400 experimental trials divided into 4 blocks of 100 trials each (25 for each stimulus type).

Participants were allowed to take a short break between blocks.

Results

We excluded from the analysis trials for which the RT was more than two standard

deviations smaller (0.1 %) or greater (3.9 %) than the participant’s overall mean RT. Incorrect

responses (4.3 %) were also discarded. Mean correct RTs and arcsin-transformed accuracy rates

(ARs), that is, percentage of correct responses, were analyzed separately.

To measure the Simon effect, corresponding (i.e., the position of the response key

corresponded to the position of the square) and non-corresponding (i.e., the response key did not

correspond to the position of the square) responses for each condition were compared. The time

course of the Simon effect for RT was investigated by applying the Vincentizing procedure

(Ratcliff, 1979). The RT distribution for each participant and correspondence condition was divided

into deciles (bins), and the mean of RT for each decile was calculated. Because the variance was

higher for the last bin than for the others, it was eliminated from the analysis (see, e.g., Buhlmann,

et al., 2007; Wascher et al., 2001). We calculated the size of the Simon effect for each remaining

bin, subtracting the mean RT for the corresponding condition from the mean RT for the non-

corresponding condition.

Mean reaction times. RTs were submitted to a three-factor ANOVA with Bin (1–9) and

Correspondence (corresponding vs. non-corresponding S-R pairings) as within-subjects factors and

Condition (Baseline vs. Wide Coloured Stimuli vs. Wide Unfilled Stimuli) as a between-subjects

factor. It should be noted that, considering the way that the data were grouped, the Bin main effect

necessarily turned out to be significant in all analyses. Therefore, it is not reported and discussed

here or in Experiment 2. When sphericity was violated, the Huynh-Feldt correction was applied,

although the original degrees of freedom are reported.

The Correspondence factor was significant, F(1,33) = 113.43, p < .001, indicating that

corresponding responses were faster than non-corresponding ones (381 vs. 404 ms). The Condition

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 12

factor was close to the .05 level, F(2,33) = 3.13, p = .058, showing a trend for responses to be faster

in the wide coloured stimuli condition (363 ms) than in the baseline and the wide unfilled stimuli

conditions (411 vs. 403 ms, respectively).

The Correspondence × Condition interaction was not significant, F(1,22) < 1, indicating that

the Simon effect was of similar magnitude for the baseline (400 vs. 423 ms), wide coloured stimuli

(351 vs. 374 ms), and wide unfilled stimuli (392 ms vs. 415 ms) conditions (see Table 1).

Insert Table 1 about here

Crucially, the Bin × Correspondence x Condition interaction was significant, though,

F(16,264) = 3.71, p = .005 (see Figure 2). Consequently, we performed two post-hoc ANOVAs to

test the predictions that (a) the baseline condition would yield a similar (i.e., decaying) time course

to that of the wide unfilled stimuli condition and (b) the time course of both these conditions would

differ from that of the wide coloured stimuli condition (i.e., sustained).

First, a three-factor ANOVA with Bin × Correspondence × Condition (Baseline vs. Wide

Unfilled Stimuli) was run. As hypothesized, the baseline and wide unfilled stimuli conditions

yielded similar overall RTs (411 and 403, respectively), as shown by the lack of significance of the

Condition main effect, F(1,22) < 1.0. Moreover, they had similar time courses as shown by the

nonsignificant Condition × Correspondence × Bin interaction, F(8,176) = 1.65, p = .196. In the

baseline condition, paired sample t-tests showed that the Simon effect was significant at all bins

except for the last one, ts(11) > 3.70, ps < .003, and t(11) = 1.1, p = .295, for bins 1 to 8 and for bin

9, respectively. Furthermore, Helmert contrasts showed that the size of the Simon effect did not

differ significantly from bin 1 to bin 2 (27 and 35 ms), F(1,11) = .957, p = .349, but then decreased

significantly from bin 2 to bin 9 (35, 31, 29, 25, 22, 18, 13 and 5 ms , respectively), Fs(1,11) > 8.05,

ps < .02. In the wide unfilled stimuli condition, paired sample t-tests showed that the Simon effect

was also significant at all bins except for the last one, ts(11) > 4.00, ps = .002, and t(11) = 1.80, p =

.099, for bins 1 to 8 and for bin 9, respectively. Helmert contrasts showed that the size of the Simon

effect did not differ significantly from bin 1 to bin 5 (21, 27, 28, 28 and 29 ms), Fs(1,11) > .425; ps

> 0.68, but then decreased significantly from bin 5 to bin 9 (29, 28, 23, 17 and 10 ms, respectively),

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 13

Fs(1,11) > 5.85, ps = .034 (see Figure 2). Thus, significant decreasing effect functions were found

for both baseline and wide unfilled stimuli conditions.

A second three-factor ANOVA compared the time course of the wide coloured stimuli

condition to those of the baseline and wide unfilled stimuli conditions averaged together. As

hypothesized, the Condition × Correspondence × Bin interaction was significant, F(8,272)= 5.74, p

= .003, indicating that while the Simon effect in the baseline and wide unfilled stimuli conditions

had a decreasing time course, the wide coloured stimuli condition yielded a sustained one. This

sustained effect function was confirmed by a Bin × Correspondence ANOVA on the single wide

coloured stimuli condition which displayed a nonsignificant interaction F(8,88) = 1.71, p = .192,

indicating that the size of the Simon effect did not change reliably across bins: It was 16, 20, 23, 24,

24, 27, 29, 27 and 19 ms from bin 1 to bin 9, respectively (see Figure 2).

Insert Figure 2 about here

Accuracy rates (ARs). Arcsine-transformed ARs of each condition were submitted to a

repeated-measures ANOVA with Correspondence and Condition as the within- and between-

subjects factors, respectively. For clarity, the original accuracy rates are reported. When sphericity

was violated, the Huynh-Feldt correction was applied, although the original degrees of freedom are

reported.

The main effect of Correspondence was significant, F(1,33) = 11.60, p = .002, indicating

that corresponding responses were more accurate than non-corresponding ones (96.9 % vs. 94.8%).

Paired sample t tests on the single conditions, indicated that the Simon effect for ARs was

significant for both wide coloured and wide unfilled stimuli conditions (2.7 % vs. 3.3 %), ts(12) >

2.52, p < .03, respectively, but failed to reach significance for the baseline condition, t(12) > 1 (see

Table 1). The Condition factor was not significant, F(2,33) < 1, but the Condition ×

Correspondence interaction approached the .05 level, F(2,33) = 3.17, p = .055. Independent sample

t tests indicated that the Simon effect for ARs was larger for the wide unfilled stimuli than for the

baseline condition, t(22) = 2.45, p = .023, and the wide coloured stimuli condition also tended to

have a larger Simon effect with respect to that of the baseline condition, t(22) = 1.90, p = 0.71.

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 14

Discussion

The results of the time-course investigations confirmed our hypothesis: A decreasing time

course was found for both the baseline and the wide unfilled stimuli conditions and, crucially, a

sustained time course was shown by the wide coloured stimuli condition (see Figure 2). The

baseline result was consistent with current literature (e.g., Proctor et al., 2009) and confirmed the

fast and transient nature of the automatically activated response (De Jong et al., 1994). This

transient activation pattern was later attributed by Wascher et al. (2001) to visuomotor facilitation of

same side responses, depending on an S-R spatial-anatomical mapping. It is worth mentioning that,

to make this condition an effective baseline for the following ones, we used stimuli similar in

dimensions to those of previously conducted studies that used lateralised coloured shapes (e.g., De

Jong et al., 1994; Proctor, Pick, Vu & Anderson, 2005; Proctor et al., 2009; Vu & Proctor, 2008).

Findings from the wide coloured stimuli condition provided evidence that temporal overlap

was an important factor in shaping the effect function of a “standard” Simon task. In that condition,

the Simon effect did not differ reliably across the distribution function, showing a tendency for the

largest Simon effect to occur later in the RT distribution, when the temporal proximity between the

two stimulus codes, and thus between the resulting response processes, was increased. This shift in

the effect function was independent of the S-R task settings, which remained spatial-anatomical as

it was for the baseline condition. Moreover, since our stimuli were intact, this change in the time

course could not be attributed to temporal dissociation between the two stimulus dimensions nor to

imposition of masking patterns or overlays (see Hommel, 1993, 1994a, 1994b; Miles & Proctor,

2009; Vallesi & Umiltà, 2009).

Someone might object to these conclusions, though, by saying that wide coloured squares

may not only have facilitated the task-relevant colour discrimination but also altered some aspects

related to processing of the irrelevant stimulus spatial value and, thus, linked to spatial attention

mechanisms. For instance, increasing stimulus size may have, on one hand, decreased the perceived

distance between fixation and the stimulus and, on the other hand, affected location processing by

altering the probability and/or size of attention shifts toward the stimulus. To avoid this possible

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 15

critique, the wide unfilled stimuli condition was run, where stimulus width was the same as that of

the wide coloured stimuli condition but with colour presented only along the squares’ borders

instead of within their whole area. A decreasing time course, similar to that of baseline, would

corroborate the hypothesis that the sustained time course found for the wide coloured stimuli

condition was due to the facilitation of colour discrimination and thus to an increase in temporal

overlap. Results confirmed our expectations. First, the time course of the baseline condition did not

differ from that of the wide unfilled stimuli condition. Namely, the Simon effect tended to dissipate

across time for both these conditions. Second, the time course of the wide stimuli condition differed

from that of the baseline and wide unfilled stimuli conditions, being sustained rather than

decreasing. Third, the difference in RT ranges supported the hypothesis that the colour

discrimination was easier for the wide coloured stimuli condition with respect to the remaining

conditions. Indeed, there was a trend for the wide stimuli condition to have the fastest RTs and,

conversely, for the baseline and wide unfilled stimuli conditions to have similar mean RTs.

Hence, taken together, these findings broaden the evidence in favour of the automatic

activation account, which has previously been investigated mainly by way of manipulations which

decreased, instead of increasing, the temporal overlap. To corroborate our conclusions, we

conducted a second experiment in which an increased temporal overlap was also obtained by

manipulating the irrelevant, instead of the relevant, stimulus dimension.

Experiment 2

In line with the wide coloured stimuli condition of Experiment 1, the current experiment was

conducted to assess the time-course of the “standard” Simon effect when the temporal overlap

between the conditional and unconditional response mechanisms was increased. However, as

opposed to Experiment 1, we achieved this goal by manipulating the task irrelevant spatial

dimension of the stimulus instead of the relevant dimension. Lateralised arrows were employed as

visual stimuli in place of the squares used in Experiment 1. Converging evidence has been provided

about the strength and effectiveness of such spatial indicators: Even when presented centrally,

arrows have been found to quickly activate a spatial code and also to trigger a reflexive shift of

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 16

attention similar to the one caused by the abrupt onset of a peripheral non-predictive visual stimulus

(e.g., Bonato, Priftis, Marenzi, & Zorzi, 2008; Hommel, Pratt, Colzato, & Godijn, 2001; Ristic &

Kingstone, 2006). This finding was also confirmed by a recent study conducted by Pellicano et al.

(2009) that focused on the time course of such indicators. The authors demonstrated that the spatial

value of centrally presented arrows affected performance from the shortest RTs, as typically found

for lateralised stimuli. Furthermore, they influenced performance for a longer time than lateralised

stimuli, yielding a large Simon effect along the whole RT distribution (see also Proctor et al., 2009;

Ricciardelli, Bonfiglioli, Iani, Rubichi, & Nicoletti, 2007).

Considering the above mentioned findings, in the current experiment we decided to use

arrows presented at corresponding lateralised locations, instead of centrally presented ones. These

stimuli should produce two spatial codes, one given by the arrow direction and the other by its

location on the screen. Because the two always corresponded, the resulting spatial value, and also

the following unconditional response activation, would be stronger and longer lasting with respect

to that yielded by the square stimuli in the baseline condition of Experiment 1.

Hence, we hypothesized that a sustained Simon effect function, instead of a decreasing one

(i.e. baseline condition of Experiment 1), would be observed. Moreover, we also hypothesized that

the analysis of the conditional accuracy function (CAF), that is, the time course analysis of ARs,

would provide further evidence of such stronger unconditional activation. Many studies by

Ridderinkhof and colleagues (see Ridderinkhof, 2002a, 2002b; Wylie et al., 2009; Wylie,

Ridderinkhof, Bashore, & van den Wildenberg, 2010) suggest that the greater the strength of the

automatic response activation, the lower the ARs for non-corresponding trials mostly during shorter

RTs, since participants are more affected by the stimulus spatial value. A belated reaching of a near

perfect level of accuracy, thus, was expected for the current experiment compared to the baseline

condition of Experiment 1.

To summarize, the goal and the expected results of the current experiment were in line with

those of the wide coloured stimuli condition of Experiment 1: Increased temporal overlap was

supposed to yield a sustained time course for the Simon effect. However, in place of facilitating the

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 17

relevant colour task, we decided to enhance the irrelevant stimulus code. Hence, lateralised arrow

signals replaced the previously utilised squares stimuli. Their stronger spatial code was supposed to

influence performance for a longer time gap, thus yielding a constant or increasing Simon effect

across the RTs distribution and, also, a delayed increase of the ARs level in the CAF.

Method

Twelve new right-handed students from the same pool (6 females; mean age = 23.1 years,

SD = 2.1 years) were selected as before. The apparatus, stimuli and procedure were the same as in

Experiment 1 except that the stimuli were coloured (green/red) arrows, pointing to the left or to the

right, presented on the left/right of the screen (see Figure 1). Arrows were 1.5 cm wide and 1 cm

high (1.8° x 1.2°). The total coloured area for the arrow stimuli was: 0.65 cm2. As for the previous

Experiment, the distance of each stimulus from fixation was 9.4 cm (11.5° centre to centre). The

arrow direction was always congruent with its position: Arrows pointing to the left were presented

on the left side of the screen and the opposite was true for arrows pointing to the right.

Results

As for the previous experiment, we excluded from the analysis responses that were more

than two standard deviations smaller (0.1 %) or greater (4.2 %) than the participant’s overall mean

RT. Incorrect responses were also discarded (4.6 %). Mean correct RTs and ARs were analyzed

separately.

Mean reaction times. To test whether the shape of the effect function for RTs changed

significantly from the baseline condition of Experiment 1 to the arrow stimuli condition of

Experiment 2, we performed a repeated-measures ANOVA with Condition (Baseline vs. Arrow

Stimuli) as the between-subjects factor and Correspondence and Bin factors as the within-subjects

ones. When sphericity was violated, the Huynh-Feldt correction was applied, although the original

degrees of freedom are reported.

The Correspondence factor was significant, F(1,22)= 70.06, p < 001, indicating that

corresponding responses were faster than non-corresponding ones (377 vs. 401 ms). The Condition

factor was significant, F(1,22) = 5.15, p = .033, indicating that participants had faster performance

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 18

in the arrow stimuli condition compared to baseline (367 vs. 411 ms). The lack of significance of

the Correspondence × Condition interaction, F(1,22) < 1, indicates that the arrow stimuli condition

did not differ from the baseline condition as regard to the size of the Simon effect (354 vs. 380 ms

and 400 vs. 423 ms, for corresponding and non-corresponding trials of the arrow stimuli and

baseline condition, respectively, see Table 1). As hypothesized, the Bin × Correspondence ×

Condition interaction was significant, F(8,176) = 4.40 p = .005, showing that the arrow stimuli and

the baseline conditions yielded different time courses being stable the former and decreasing the

latter (see Figure 2). Indeed, the nonsignificant Bin × Correspondence interaction, F(8,88) = 2.21, p

= .121, of an ANOVA conducted on the single arrow stimuli condition, indicated that the size of the

Simon effect did not change reliably across bins: It was 20, 26, 29, 30, 29, 26, 26, 23 and 19 ms

from bin 1 to bin 9, respectively (see Figure 2).

Accuracy rates. To assess the CAF, that is, the time course of accuracy rates, ARs were

calculated for each of the nine RT bins and both corresponding and non-corresponding trials. Data

were then submitted to a repeated-measures ANOVA with the same factors as those of the RTs

analysis. When sphericity was violated, the Huynh-Feldt correction was applied, although the

original degrees of freedom are reported.

The Condition factor was significant, F(1,22) = 5.06, p = .035, showing that participants had

a less accurate performance for the arrow stimuli condition with respect to the baseline condition.

Taken together with the RT results, these finding indicate that participants adopted a faster but less

accurate criterion in responding in the arrow stimuli condition compared to baseline (367 vs. 411

ms and 94.5% vs. 96.4 %, for RTs and ARs, respectively). Thus, these results denote that a less

conservative speed-accuracy criterion was adopted on average in Experiment 2. The

Correspondence factor and the Correspondence × Condition interaction both failed to reach

significance, Fs(1,22) = 2.57, p =.123 and = 3.10, p = .092, respectively. The latter result indicates

that that two conditions did not differ as regard to the size of the Simon effect, even if the arrow

stimuli condition showed a clear trend to have a larger amplitude compared to the baseline one (0.1

% vs. 3.6 %, see Table 1).

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 19

Crucially, the Bin × Correspondence × Condition interaction was significant, F (8,176) =

3.06, p = . 004, indicating, in line with our hypothesis, that the ARs function for the arrow stimuli

condition differed to that of baseline. To investigate deeper this result, a repeated-measure ANOVA

on non-corresponding trials only was run where Bin and Condition were the within and between

subjects factors, respectively. In line with our prediction, the Bin × Condition interaction was

significant, F(8,176) = 3.02 p = .006, indicating that the arrow stimuli curve took a longer time to

reach an almost perfect level of accuracy compared to that of baseline (see Figure 3). Indeed, in an

ANOVA conducted on the non-corresponding trials of the baseline condition only, the Bin factor

resulted significant, F(8,88) = 9.05, p < .001, and Helmert contrasts showed that the ARs for non-

corresponding trials steeply increased from bin 1 to bin 3 (83 %, 93% and 97%, respectively),

Fs(1,11) > .9.36, ps < .02, for then remaining constant around a very high accuracy rate until bin 9

(97 %, 98%, 99%, 97% 99%, 98% and 99%, respectively), Fs(1,11) > 1. Crucially, when the same

ANOVA was run for the arrow stimuli condition, the Bin factor also turned out to be significant,

F(8,88) = 18.40, p < .001 and Helmert contrasts showed that, in line with our prediction,

participants took longer to reach a close-to-perfection level. Indeed, ARs for non-corresponding

trials increased significantly from bin 1 to bin 4 (78%, 87%, 90% and 93%, respectively), Fs(1,11)

> 17.17, ps < .003, slightly decreased from bin 4 to bin 5 (93 % and 91%), F(1,11) =15.90, p =

.002, increased again from bin 5 to bin 6 (91% and 97%, respectively), F(1,11) =12.00, p = .005,

and then remained constant until the last ninth bin (97%, 99%, 98% and 99%, respectively),

Fs(1,11) > 1 (See Figure 3). An ANOVA on corresponding trials only, with Bin and Condition as

the within- and between-subjects factors, respectively, was also run. As hypothesized, the ARs

functions of the two conditions did not differed between each others, as indicated by the

nonsignificant Bin × Condition interaction F(8,176) < 1. Indeed, for both the baseline and arrow

stimuli conditions participants had an almost perfect level of accuracy for corresponding trials along

all bins.

Discussion

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 20

The results of both the RTs and ARs time course investigations confirmed our predictions:

Lateralised arrow stimuli produced a stronger and longer lasting interference effect on performance.

The time course analysis revealed that the Simon effect was sustained across the whole RT

distribution, a result in line with that obtained for the wide coloured stimuli condition of Experiment

1, but different with respect to the decaying time course yielded by the baseline condition.

Moreover, the time course analysis of accuracy rates (see Ridderinkhof, 2002a, 2002b; Wylie et al.,

2008, Wylie et al., 2010) supported the idea of a stronger interference effect on performance along

bins: A near perfect level of accuracy for non-corresponding trials was reached later in time in the

current experiment compared to the baseline condition of Experiment 1. Hence, this finding

indicated that arrow stimuli had a stronger spatial code, compared to the squares one, which caused

participants to be more error prone for a longer time gap.

To conclude, these results supported the previously reported evidence showing that an

increased temporal overlap can affect the time course of a “standard” Simon effect, turning it from

decaying to stable. Moreover, these findings were further corroborated by the CAF analysis results,

which showed a stronger activation of the automatic response process.

General Discussion

The current study aimed to investigate the role of temporal overlap in shaping the time

course of a “standard” Simon effect. Two main conclusions can be drawn from our results. First,

enhancing the temporal proximity between stimulus codes, and thus between the resulting response

processes, yielded a sustained Simon effect across the whole RT distribution. This finding is new

with respect to literature where a decreasing time course has typically been found when a

“standard” setting is implemented, that is, when lateralised visual stimuli are matched to left/right

responses performed with uncrossed hands (e.g., Proctor et al., 2009). However, an automatic

activation account can encompass and explain both of these outcomes.

De Jong et al. (1994) were the first to attribute the decreasing time course of the “standard”

Simon effect to a fast, but stimulus locked, spatially correspondent response activation. Despite

being questioned (Roswarski & Proctor, 2003; Zhang & Kornblum, 1997), this hypothesis has

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 21

gathered converging evidence from both neuropsychological and behavioral studies. Among the

first ones are studies employing lateralized readiness potentials (LRPs), a technique developed in

the late 1980s (De Jong, Wierda, Mulder, & Mulder, 1988; Gratton, Coles, Sirevaag, Eriksen, &

Donchin, 1988) and which has been widely used within the Simon effect paradigm to investigate

early response preparation (see Leuthold, 2011, for a review). More precisely, the LRP: “exhibits

greater negativity in the motor cortex contralateral then ipsilateral to the responding hands a few

milliseconds before overt response onset” (Leuthold, 2011, p. 205). Several studies by Wascher and

colleagues provided data showing that this cortical asymmetry tended to disappear, or be drastically

reduced, when a non “standard” Simon task was implemented, that is, when stimuli were presented

in a vertical position or in the auditory modality, or when responses were made with crossed hands

(Buhlmann et al., 2007; Wascher et al., 2001; Wiegand & Wascher, 2005; Wiegand & Wascher,

2005). Such findings led Wascher et al. (2001) to claim that automatic activation was only yielded

by a good visuomotor connection, that is, when stimuli and responses share a spatial-anatomical

mapping.

Of particular relevance to our research are the results from behavioural studies which

showed a reduction, or even inversion, of the Simon effect for cases in which the temporal distance

between the conditional and unconditional response processes was increased. In these cases, the

automatically activated response had enough time to dissipate before response selection occurred

(Hommel, 1993, 1994a, 1994b; Simon et al, 1976; Vallesi & Umiltà, 2009). In a recent study, Miles

and Proctor (2009) showed that, besides its magnitude, such a decrease in temporal overlap had an

impact on the Simon effect function, when centrally presented spatial words (“left”/ “rights”) were

used.

The temporal overlap hypothesis was embraced by our experiments as well. However, we

assessed it by mean of a converse rationale with respect to the above mentioned studies: The

relevant and irrelevant stimulus dimensions were manipulated in such a way as to reduce, instead of

increase, the temporal distance between the conditional and unconditional responses. More

precisely, we first facilitated the discrimination of the relevant stimulus dimension (i.e., wide

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 22

coloured stimuli condition, Experiment 1), presenting stimuli with a wider coloured area compared

to those of baseline. In line with our prediction, the magnitude of the Simon effect did not vary with

time, indicating that the facilitation of the color discrimination shifted the conditional process closer

to the unconditional one. To make sure that this result was due to temporal overlap manipulations

and not to spatial attention matters related to coding of the irrelevant stimulus dimension, we ran a

control condition where the same stimuli were presented but with color presented only along their

borders, instead of within their whole area (i.e., wide unfilled stimuli condition). As hypothesized,

this condition yielded a decreasing Simon effect function similar to that of the baseline condition.

In Experiment 2, we then manipulated the irrelevant stimulus dimension using arrow stimuli in

place of squares (i.e., arrow stimuli condition) and found, as hypothesized, a sustained time course

as it was for the wide coloured stimuli condition. Hence, we interpreted our results as being in line

with an automatic activation account: Increasing the temporal overlap made participants respond

during the time span wherein the automatic activation had not decayed yet, thus yielding a stable

Simon effect function even for “standard” Simon tasks.

Although the manipulations intended to increase temporal overlap yielded stable, instead of

decreasing, effect functions, the overall mean Simon effect was no larger in those conditions than in

the baseline condition. The main reason why the mean effects did not differ seems to be that more

responses occurred prior to the build up of peak activation in the conditions with increased temporal

overlap (the wide coloured stimuli and arrow stimuli conditions) than in the baseline condition (see

Fig. 2). Thus, in addition to moving slow responses into the range prior to dissipation of the

corresponding-response activation, the conditions with increased temporal overlap apparently

moved more fast responses into the range during which the automatic activation was increasing.

An alternative possibility is that the mean Simon effect is determined by factors other than those

influencing the distribution functions, but given that mean data often show differences in Simon-

effect size expected on the basis of temporal overlap, this possibility seems less likely (see Proctor

et al., 2011, for a review).

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 23

It is worth noting that we manipulated the two stimulus dimensions, neither dissociating

them by mean of SOAs, pre- or post-cues, nor by altering the quality of the stimulus signal or

contrast. That is, we avoided adopting the most predominant paradigms that have been used in the

literature so far (Hommel, 1993, 1994a, 1994b; Miles & Proctor, 2009; Rubichi et al., 1997; Simon

et al., 1976; Vallesi & Umiltà, 2009).

The second conclusion concerns the CAF analysis. We showed in fact that this tool was

further informative on the effects of the strengthened stimulus spatial code and the consequent

automatic response activation. The spatial code of arrow indicators used for Experiment 2 was

demonstrated to be stronger with respect to the one of the square stimuli presented in the baseline

condition of Experiment 1, because participants tended to be error prone for a longer period in the

former condition than in the latter condition, thus showing greater difficulty in containing the

influence that arrows had on their performance. These results integrate well with those from RTs,

where arrows were found to exert a prolonged influence on performance, as shown by their

sustained effect function.

The CAF analysis was first implemented by Ridderinkhof and colleagues (Ridderinkhof

2002a, 2002b; Wylie et al., 2009, 2010) with the aim of investigating individual and group

differences in the activation and subsequent containment of the automatic response process. We

think that the CAF analysis could be a valid and informative tool to place side by side to the time

course investigation of RTs, even when the subject pool is composed of healthy adults and it is not

focused on individual or group differences.

To summarize, the current study aimed at increasing, instead of reducing, the temporal

overlap between the conditional and unconditional responses processes. As hypothesized, this

manipulation yielded a positive Simon effect with a sustained time course even for a “standard”

Simon task, a result which is new with respect to the literature. However, our interpretation was in

line with the temporal overlap hypothesis and with an automatic activation account.

More research is needed to further investigate the role of temporal overlap in shaping the

time course of the Simon effect. In particular, the current study only focused on the “standard”

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 24

Simon task, without investigating the other non “standard” tasks (e.g., see Wühr ,2006), for which

Wascher et al. (2001) argued that the Simon effect has a different, “cognitive” basis. Moreover, our

results did not disentangle the issue of whether the dissipation of the automatically activated

response is due to a passive decay or to an active inhibition. Evidence on both sides has been

gathered, but, to our knowledge, a definitive answer has yet to be given. What our results do show

is that by increasing temporal overlap, all responses can be made prior to much dissipation being

evident.

Acknowledgements

This work was supported by the EU FP7 project “Emergence of communication in Robots

through Sensorimotor and Social Interaction” (ROSSI), Grant Agreement 216125

(www.rossiproject.eu). The authors would also thank Christian Frings, Peter Wühr and Edmund

Wascher for helpful comments during the revision process.

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 25

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Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 29

Table 1: Corresponding (C) and Non corresponding (NC) values and the resulting Simon effect

(SE; * = significant) for RT (ms) and AR (%) measures.

EXPERIMENT C NC SE

Experiment 1 - Baseline

Mean RTs (ms) 400 423 23*

ARs (%) 96.5 % 96.4 % 0.1 %

Experiment 1 – Wide Coloured Stimuli

Mean RTs (ms) 351 374 23*

ARs (%) 96.9 % 94.2 % 2.7* %

Experiment 1 – Wide Unfilled Stimuli

Mean RTs (ms) 392 415 23*

ARs (%) 97.2 % 93.9 % 3.3* %

Experiment 2 – Arrow Stimuli

Mean RTs (ms) 354 380 26*

ARs (%) 96.3 % 92.7 % 3.6 %

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 30

Figure 1: The stimuli used for Experiment 1 and for Experiment 2. The stimuli used for the baseline

condition (on the left side of the rightmost and leftmost panels) are matched with the stimuli used

for the wide coloured stimuli condition of Experiment 1 (right side of the leftmost panel) and with

those presented for arrow stimuli condition of Experiment 2 (right side of the rightmost panel). The

central panel displays the stimuli used for the wide coloured stimuli condition of Experiment 1

(right side) and those of the wide unfilled stimuli (left side).

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 31

Figure 2: The magnitude of the Simon effect (RTs) across bins for the four conditions in

Experiments 1 and 2.

0

10

20

30

40

250 300 350 400 450 500 550

MEAN RT (MS)

SE

SIZ

E (

MS

)

BASELINE (Exp 1) WIDE COLOURED STIMULI (Exp 1)

WIDE UNFILLED STIMULI (Exp 1) ARROW STIMULI (Exp 2)

Running head: Influence of Temporal Overlap on Time Course of the Simon Effect 32

Figure 3: Accuracy rates for non-corresponding trials across bins for the baseline condition of

Experiments 1 and the arrow stimuli condition of Experiment 2. Accuracy for corresponding

condition was close to 100%.

70%

75%

80%

85%

90%

95%

100%

200 250 300 350 400 450 500 550

BASELINE (Exp 1) ARROW STMULI (Exp 2)