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By definition, preattentive change detection refers to theability to continuously monitor the environment forpotentially important changes, but without consumingattentional resources. When a salient environmental

change occurs, however, attention is brought to bear on thechanged stimulus and it is processed further [1]. The mecha-nism underlying preattentive change detection in the auditorysystem is usually studied by recording the brain electrical activi-ty elicited during a passive oddball paradigm in which partici-pants experience rare deviant stimuli embedded in a train ofstandard stimuli. Participants typically watch a video or read abook as the foreground task during tone presentation and areinstructed to ignore the background auditory stimuli. Althoughthe tones occur in background, a complete representation of therepeating standard stimuli is established and stored as a memorytrace [2]. If there is a change in one or more physical features ofthe incoming stimulus (e.g., frequency, intensity, duration, ortime of occurrence), the comparison of the representation of thestandard stimulus and the deviant stimulus elicits a mismatchresponse [3]. This preattentive mismatch response is reflected inthe event-related potential (ERP) component called the mis-match negativity (MMN). The MMN refers to the increasednegativity in the waveform that is revealed when the electro-physiological brain response to the standard stimuli is subtract-ed from the response to the deviant stimuli. It usually peaksbetween 100 and 200 ms after deviant onset with the maximumdifference found at central frontal electrode sites [2].

The results of lesion studies [4], [5], electrophysiologicalsource analysis studies [6], [7], and brain imaging studies [8]–[11]suggest that the MMN comprises two subcomponents that arisefrom activity in distinct areas of the temporal and frontal cortices.

An ERP source analysis study [6] showed peak temporalcortex activation in the right hemisphere at 160 ms followedby peak frontal cortex activation 8 ms later. However, thefrontal and temporal cortex sources could not be fully separat-ed from one another in the spatial and temporal dimensions inthat study because of spatial smearing of the electrophysiolog-ical signal. More recently, combined fMRI and ERP analysesalso suggested sequential activation of superior temporal gyrus(STG) and the inferior frontal gyrus (IFG) [8], [10].

Temporal cortex activity has been associated with changedetection, whereas there are several nonmutually exclusive

hypotheses about the functional role of the frontal cortex acti-vation (see [11] for a summary). Some authors [3], [12], [13]suggest that the frontal cortex activity is related to attentionswitching or reorientation that occurs after change detection.Several recent fMRI studies [8], [10], [11] showed a correla-tion between the magnitude of the deviant change and themagnitude of the blood-flow change in the STG and the IFG.STG activation increased with increasing deviance, whereasIFG activation decreased with increasing deviance. Based onthis pattern of results, Doeller et al. [8] and Opitz et al. [10]suggested that the IFG activity could reflect contrast enhance-ment. This hypothesis claims that IFG activity is related totuning the sensitivity of the auditory system for change detec-tion, but not attention switching itself. In this explanatoryframework, greater activity in the IFG supports greater audi-tory system sensitivity to small changes. Thus, IFG activity isgreater for less distinct stimuli than highly salient stimuli. Incontrast, Rinne et al. [11] suggested that the IFG is part of aninhibition system that allows one to ignore change when achange-related response is not needed. Because small changesare less salient, and more likely to be ignored, stronger inhibi-tion of response, and IFG activation, would be expected.

The event-related optical signal (EROS) has also been usedto study preattentive auditory change detection [10],[14]–[17]. For example, Rinne et al. [14] used EROS to dis-criminate the optical counterparts of the auditory N1 andMMN; Fabiani et al. [16] showed that suppression of the opti-cal N1, but not the optical MMN, is related to changes in sen-sory processing during normal aging.

The EROS technique relies on the difference in light-scat-tering properties of active versus inactive brain tissue [18].Specifically, the mean photon flight time of light pulsesbetween a light source and a detector [19], [20] is longer foractive neural tissue because active tissue is less light scatteringthan inactive tissue [21], [22]. Differences in photon flighttime between conditions indicate differences in brain activity.The spatial resolution of EROS is determined by the configu-ration of light sources and detectors used in a given experi-ment and is generally on the order of a square centimeter orless. EROS can provide a temporal resolution of severalmilliseconds. For example, the sampling period of a two-lightsource system can be as fast as 3.2 ms. However, in order to

Preattentive ChangeDetection Using the Event-Related Optical SignalOptical Imaging of Cortical Activity Elicited by Unattended Temporal Deviants

BY CHUN-YU TSE AND TREVOR B. PENNEY

© BRAND X PICTURES

improve the signal-to-noise ratio and to increase the area ofbrain being sampled, multiplexed light sources are used andthe typical sampling rate is 20–30 ms. Therefore, EROS iseffective in distinguishing spatially disparate activations withhigh temporal resolution.

Tse et al. [15] used EROS to study the temporal dynamics ofSTG and IFG activation in preattentive detection of infrequentomitted and early stimuli. The standard SOA was 84 ms andthe SOA for the early deviants was 57 ms. That study revealeda significant increase in right STG activity that peaked 140 msafter the onset of the omission event, followed by a peak inright IFG activity 60 ms later. This result provides support forthe presence of a temporal-frontal cortical network in preatten-tive change detection. However, early deviants failed to elicit asignificant increase in STG or IFG activity. Tse et al. suggest-ed that this was due to the small difference in stimulus onsetasynchrony (SOA) between the standard and deviant stimuliand rapid presentation rate of the stimuli.

The goal of the present study was to use EROS to furtherinvestigate preattentive change detection of early and omis-sion deviants with an SOA in the hundreds of millisecondsrange. Although ERP studies [23] have typically failed to findan omission MMN elicited by SOAs larger than 200 ms, thisdoes not rule out an EROS effect because there are intrinsicdifferences between the ERP and EROS signals. Furthermore,comparison of the activation patterns elicited by early andomission deviants may help to determine the functional role ofthe IFG in preattentive change detection.

Methods

ParticipantsNineteen undergraduate students (eight males and 11 females;age range 19–22) from The Chinese University of Hong Kongparticipated in this study. They were naive to the experimentalhypothesis and, according to self-report, had normal hearing,were not taking any psychoactive medications, and had nohistory of neurological disorders or head trauma. Scores onthe Edinburgh Handedness Inventory [24] showed that all ofthe participants were right-handed. Informed consent wasobtained from each participant before the experiment began.

Stimuli and ProcedureDuring the experiment, participants watched a self-selected,subtitled movie, played without sound, while a sequence of440-Hz pure tones, which the participants were instructed toignore, was presented via headphones. Although watching asilent video may not be the best attention control task, giventhe length of the present experiment was approximately three hours per session, we believe that it was the best choiceavailable for the present experiment.

All of the tones were 75 ms in duration and the standardSOA was 1,500 ms. The SOA was 1,000 ms for the earlydeviants (i.e., the tone appeared 500 ms earlier than expected),and for the omission deviants the expected tone was omitted,leaving a silent interval of 2,925 ms between tones (Figure 1).An SOA decrement of 33.3% was selected for the early

Fig. 1. The stimulus sequences in each experimental block are illustrated here. Omission and early deviants were randomly pre-sented during the train of standard stimuli with the constraint that at least two consecutive standard stimuli were presentedbefore each deviant event and the first deviant event was presented after at least five standard stimuli. The standard stimulusonset asynchrony (SOA) was 1,500 ms and it was shortened to 1,000 ms in the early deviant condition. The standard stimuluswas omitted in the omission deviant condition.

Start of Block End of Block

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1,500 ms 1,500 ms 1,500 ms 1,500 ms

Standard Stimulus (440 Hz; 75 ms)

Early Deviant Omission Deviant

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Omission Stimulus

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deviant because this magnitude of deviance elicited the great-est IFG activity in two previous fMRI studies [8], [10]. In thepresent experiment both omission and early deviants were pre-sented in each block. Each block comprised 150 deviantevents of each type (p = 0.12) randomly inserted in a train of950 standard stimuli (i.e., 1250 stimuli in total) with the con-straint that at least two standard stimuli (tones with the stan-dard SOA) preceded each deviant event and five standardstimuli were presented at the beginning of the block. A total of40 blocks of stimuli were presented in two test sessions (i.e.,20 blocks each) separated by less than a month. The opticalsignal from either the temporal or the frontal region wasrecorded in the first session, while the remaining region wasrecorded in the second session.

EROS Recording and AnalysisOptical data were recorded using a frequency-domain oxime-ter (Imagent, ISS Inc., Champaign, Illinois, USA). Light (690nm) emitted from laser diodes and modulated at 110 MHzwas channeled to the participant’s head via 2.5-meter-longplastic clad silica optical fibers (400-µm-diameter core).Light that passed through the participant’s scalp, skull, andbrain to reach the points on the head where detector fiber-optic bundles (3 mm diameter) were located was carried tophoto multiplier tubes (PMTs). The PMTs were modulated at110.01 MHz and generated a 10 kHz heterodyning frequency

(i.e., cross correlation frequency). The analog to digital (A-D)sampling rate was 50 kHz and the output current was fastFourier transformed (FFT). dc (average) intensity, ac (ampli-tude) intensity, and relative phase delay (in degrees) measureswere computed. As phase data is most commonly used inEROS studies, only phase delay data are presented here.

The light source and detector fibers were held in placeusing a custom-built head mount system. Two montages(i.e., two sets of light source/detector configurations) wereused to interrogate the STG and each montage comprised 24source-detector pairs (four detectors × six light sources) perhemisphere (Figure 2, Montages A and B). Another twomontages were used to interrogate the IFG (Figure 2,Montages C and D). For the frontal cortex montages, wereport data from 12 source-detector pairs (two detectors ×six light sources) per hemisphere only because one of thesystem A-D boards malfunctioned during the study and theexperiment was completed using fewer detector channels forthe frontal cortex than were used for the temporal cortex.(The malfunction of the A-D board produced abnormallylarge negative dc values, but it only affected the PMTs con-nected to that A-D board. The PMTs connected to the otherA-D board and all of the light sources were unaffected.) Thesampling frequency was 49.2 Hz (i.e., it took approximately20 ms to cycle through the six light sources). Montage orderwas counterbalanced across participants.

The locations of sources and detec-tors, as well as the nasion and preauricu-lar points of each participant weredigitized with a Polhemus Fastrak3Space 3-D digitizer (Colchester,Vermont, USA). A T-1 weighted brainanatomical MRI was obtained for eachparticipant using a Philips ACS-NT 1.5Tesla scanner. The nasion and preauricu-lar points were marked by vitamin Epills in the MRI scan for coregistrationof the functional optical data with thestructural MRI. The individually coreg-istered data were then Talairach trans-formed [25].

Preprocessing of the optical dataincluded correction for phase wrapping,normalization by subtracting the phasemean, pulse correction [26], filteringwith a 1–12-Hz bandpass filter, andthen averaging for each channel, timepoint, and condition using a 100-msprestimulus baseline. In order toremove noisy channels from the analy-sis, channels with standard deviationsof phase greater than 14 degrees and/ora source–detector distance outside the22.5 mm to 50 mm range were exclud-ed from the analysis [27].

The averaged data were analyzedusing Opt-3D [28]. The optical signal fora given voxel was defined as the meanvalue of the channels that overlapped atthat particular voxel [29]. t-statistics ofthe phase data were calculated at grouplevel for each voxel and were converted

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Fig. 2. Four montages were used to record data from a total of 144 source-detectorpairs. The A and B montages, optimized for detection of STG activity, comprisedeight detectors, four placed over each hemisphere, and six frequency-modulatedlight (690 nm) sources for a total of 48 source-detector pairs in each montage. TheC and D montages were used to interrogate the IFG. They comprised four detec-tors, two over each hemisphere, and six frequency modulated light (690 nm)sources for a total of 24 source-detector pairs in each montage.

Montage A Montage C

Montage B

Detector

Source

Montage D

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to Z-scores. Statistical maps of the optical signal for each datapoint were generated by projecting the Z-score onto the lateralsurface of a template brain in Talairach space, with an 8-mmspatial filter, according to the location information from thecoregistration procedure. The Talairach coordinates reportedhere comprise y (front and back) and z (top and bottom) val-ues only because the statistical map was surface projectedonto the template brain.

The regions of interest (ROIs) for the statistical analysis ofbrain activation (20 ms time windows) in the STG and theIFG, which are identical to those used by Tse et al. [15], werebased on previous PET and fMRI studies [8], [10], [30].Green boxes in the optical data figure (Figure 3) indicate thelocation of the 2-cm cube that defined each of the ROIs, awhite cross within the box indicates the point of peak activity,

and the dark gray shading shows the area of cortex interrogat-ed by the montage design in the present study.

For the early deviant condition, the statistical analysis wasperformed by subtracting the optical response elicited by thestandard stimulus preceding the deviant from the responseelicited by the deviant stimulus. The brain response to theomission deviant was tested against zero in the statisticalanalysis, an approach that is similar to the analysis procedureused by Yabe et al. [23].

ResultsStatistical maps of the EROS results are shown in Figure 3.Statistically significant increases in the EROS signal werefound for both the early deviant and the omission deviant. Asshown in Figure 3(a), infrequent omissions of the standard

Fig. 3. Analysis time windows with statistically significant EROS results are illustrated here. The green boxes indicate the region ofinterest (ROI) for the STG and the IFG, the white cross represents the location showing peak activity, and the dark gray shadingshows the area of cortex interrogated by the montage design in the present study. Yellow to red coloring indicates the Z scoreranged from +2.00 to +3.00 or above. Blue to dark blue coloring indicates the Z score ranged from −2.00 to −3.00 or below.Time point zero refers to the onset of the deviant event. (a) The contrast testing omission deviant activity against zero showedright STG activity followed by right IFG activity. (b) Comparison of the activity elicited by early deviants and that elicited by thestandard stimuli immediately preceding the early deviants showed early right IFG activation, then STG activity, and then rightIFG activity again. Late left IFG activity was also found.

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RIFG 80 to 100 ms RSTG 140 to 160 ms RSTG 160 to 180 ms

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stimulus elicited increased activity in the right STG from180 ms to 200 ms, peak Z score = 2.33, Zcritical = 2.23 (all Zscores reported as significant were p < 0.05 following adjust-ment for the spatial smoothing of the data [31]), Talairachcoordinate: y = −30, z = 0, after the onset of the omission,followed by increased activity in the right IFG from 240 ms to260 ms and 260 ms to 280 ms, peak Z scores = 2.41 and 2.40,Zcritical = 2.37 and 2.39, y = 24, z = −2 and y = 27, z = 0,respectively. Omission deviant events failed to elicit signifi-cant STG or IFG activity in the left hemisphere.

Similar right hemisphere activation was found for theearly deviants [Figure 3(b)]. A significant increase in activitywas shown in the right STG from 140 ms to 160 ms andfrom 160 ms to 180 ms after the onset of the deviants, peakZ scores = 3.07 and 2.97, Zcritical = 2.14 and 2.05,y = −30, z = 12 and y = −30, z = 12, respectively. Thiswas followed by an increase in right IFG activity from 200ms to 220 ms, peak Z score = 2.81, Zcritical = 2.25, y = 14,z = −2. Interestingly, there was an early activation in theright IFG that was not obtained in the omission deviant con-dition here, nor in the earlier study from our lab [15].Specifically, early deviants elicited increased IFG activitybetween 80 ms and 100 ms, peak Z score = 2.6, Zcritical =2.26, y = 29, z = 9. Finally, early deviants also elicited a lateleft IFG activity increase from 240 ms to 260 ms and 260 msto 280 ms, peak Z scores = 2.61 and 2.70, Zcritical = 1.94and 1.92, y = 12, z = 9 and y = 12, z = 9, respectively. Thetime course for the peak activity in the right hemisphere for

the omission and early deviant events is shown in Figure 4(a)and Figure 4(b), respectively.

DiscussionThe primary finding in the present study was that deviantevents in a passive oddball task, where deviance was definedas a stimulus that occurred earlier than the regularly sched-uled 1,500-ms SOA or not at all, elicited activity in the rightSTG and the right IFG. As such, these results partially repli-cated an earlier preattentive deviance detection study fromour lab [15] in which the standard SOA was 87 ms and thedeviant event was a stimulus omission. In that study [15],peak STG activation occurred between 140 and 160 ms andpeak IFG activation between 200 and 220 ms. In the presentstudy, the peak of the STG activation occurred between 180and 200 ms, followed by peak IFG activation between 240and 260 ms, a delay of 20–40 ms relative to the activationpattern observed in an earlier experiment. This 20–40-msdelay was likely due to the much longer standard SOA usedin the present study. For example, if temporal deviance detec-tion occurs when the memory representation of the standardSOA is exceeded by some proportional amount, which isprobable given Weber’s law for time [32], then the amount oftime that the SOA must be exceeded by prior to a stimulusomission being detected should be greater for a standard SOAof 1,500 ms than for a standard SOA of 87 ms [33].

As noted at the beginning of this article, Yabe et al. [23].failed to find an omission MMN when the SOA was longer

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Fig. 4. The activation time courses for each of the activation peaks in the right STG and right IFG are shown here. Time zerorefers to the onset of the deviant event. (a) Activation elicited by omission deviants peaked between 180 and 200 ms (circle)in the STG (Talairach coordinates: y = −30, z = 0) and between 240 and 260 ms in the IFG (triangle) at 240 ms (y = 24, z = −2).(b) Early deviants elicited peak IFG activity (square) between 80 and 100 ms (y = 29, z = 9), peak STG activity (circle) between140 and 160 ms (y = −30, z = 12), and then a second IFG activity peak (triangle) between 200 and 220 ms (y =14, z = −2).

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change detection, whereas there are several

nonmutually exclusive hypotheses about the

functional role of the frontal cortex activation.

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than 200 ms. In contrast, an EROS response was elicited byomission deviants in the present study. This discrepancybetween EROS and ERP responses to omission deviants maybe due to differences in the nature of the signal being mea-sured by ERP and EROS. The EROS is sensitive to changesin the light-scattering properties of neural tissue that occurwith activity. These activity-dependent changes in scatteringproperties are due to movement of ions across the cell mem-brane and changes in ion concentrations around the mem-brane [20]. Moreover, unlike ERPs, EROS is not affected byneuron orientation or volumetric conduction. On the otherhand, EROS can measure brain activity in tissue 3–5 cm fromthe surface of the skull only. Given that the two methodshave different characteristics and constraints, differencesbetween the present study and previous ERP studies are notentirely surprising.

Our previous study [15] also included an early deviant con-dition (SOA = 57 ms), but those early deviants failed to elicitan increase in STG or IFG activity. Interestingly, here theearly deviants elicited activation in the right STG and IFGwith a time course that was more similar to that of the omis-sion deviants in our earlier report than the omission deviantresponse reported here. However, a stimulus that occurs tooearly violates expectation immediately, so a mismatchresponse should be elicited earlier than for an omission.Therefore, the time course differences in STG and IFG activa-tion between the omission and early deviant conditions in thepresent study are not surprising. Rather, the consistentsequence of STG and IFG activation, as well as the generallysimilar time courses, both within the present study andbetween this and the earlier study, indicate a similar mecha-nism for preattentive detection of stimulus omissions andearly deviants.

The early right IFG activation presented between 80 and100 ms for the early deviants, but for the omission deviantshere or in our earlier study, it was unexpected. However,frontal activation preceding temporal activity in preattentivechange detection has been reported in at least one prior ERPstudy [7]. Those authors suggested that the early onset of pre-frontal activity in their experiment could be a consequence ofa prefrontal N1 response to the frequency deviant, or due to achange detection mechanism in the thalamus, similar to thatfound in nonhuman animals [34], [35], that sends projectionsto the frontal cortex. In either case, given that the early frontalactivity in the present experiment was elicited by a reductionof the SOA, it appears that early frontal activity is not specificto stimulus frequency analysis.

Although the current study showed an IFG-STG-IFG acti-vation sequence, this result does not contradict the attentionorientation model [3], [12]. Rather, the consistent pattern ofSTG activity followed by late IFG activity elicited by tempo-

ral deviants found here and in our previous study [15] sug-gests multiple functional roles for the IFG in preattentivechange detection.

Assuming that the STG activity is associated with changedetection, it would be logical to also assume that any IFGactivity occurring after STG activity onset is likely to berelated to attention reorientation or the inhibition of a volun-tary response, whereas IFG activation that precedes the STGactivity is more likely associated with contrast enhancementor preprocessing of the stimulus representation before changedetection occurs. Indeed, the failure of omission deviants toelicit early IFG activation may be because omission deviantsare highly distinctive and do not require activation of contrastenhancement mechanisms to ensure detection of missing stim-uli [15]. However, contrast enhancement may be needed forearly deviants, especially when the degree of deviance issmall. Although the current study does not prove that the STGand IFG play specific functional roles in preattentive changedetection, the results are consistent with the theoretical frame-works described in the beginning of this article. Moreover, thetime course of the early IFG-STG-late IFG EROS patternobtained here is similar to those reported in ERP source analy-sis studies. For example, Yago et al. [7] found early frontalactivity at 94 ms followed by temporal activity at 154 ms andRinne et al. [6] showed temporal activity at about 160 ms fol-lowed by late frontal activity at around 168 ms. Of course, asdescribed above, there are intrinsic differences between EROSand ERP measures (e.g., volume conduction of the ERP sig-nal), so it is not surprising that the EROS and ERP time cours-es are similar, but not identical.

Finally, the early deviants elicited a late left IFG activa-tion whereas omission deviants did not. Bilateral IFG acti-vation has also been found in fMRI studies of pitch [8] andearly deviant [11] detection, although another fMRI studyusing pitch deviants failed to find a left IFG effect [10].Doeller et al. [8] suggested that processing of musical fea-tures in addition to the physical characteristics of the pitchstimuli could be responsible for the bilateral IFG activity intheir experiment. A musical features explanation could beapplied here if one considers an early stimulus presentationas a violation of a musical rhythm. However, more researchis needed to confirm this account.

SummaryThe current study revealed right STG activation followed byIFG activation in response to temporal deviant events. Thesequence and general time course of the activation patternreplicates our earlier findings for omission deviants, but withan SOA that was an order of magnitude larger. It also extendsthe findings to include early deviants, and thereby it strength-ens claims about the spatial localization of the deviance

During the experiment, participants watched a

self-selected, subtitled movie, played without

sound, while a sequence of 440-Hz pure tones,

which the participants were instructed to ignore,

was presented via headphones.

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response and the functional roles the STG and the IFG play inthat response. Finally, the IFG activity elicited by the earlydeviants that preceded the STG activity suggests the IFG mayplay a dual role in preattentive change detection. Future EROSstudies using parametric modulations of deviance magnitudewill help to provide more information about the functional roleof IFG in preattentive change detection.

AcknowledgmentsThanks to Kei-rui Tien, Eric Ng, and Alex Lee for valuableassistance in data collection and computer programming. Thisproject was supported by grants 4322/01H and 4264/03Hawarded to T.B. Penney by the Research Grants Council of theHong Kong SAR.

Chun-Yu Tse received the B.S.Sc. andM.Phil. in psychology from The ChineseUniversity of Hong Kong in 2003 and2005, respectively. He is currently study-ing for his Ph.D. at the University ofIllinois Urbana Champaign under thesupervision of Profs. Monica Fabiani andGabriele Gratton. His research interests

include optical brain imaging of language processes andpreattentive change detection, functional connectivity in thebrain, and time perception.

Trevor B. Penney received the B.Sc. andM.Sc. in psychology from the MemorialUniversity of Newfoundland, Canada, in1990 and 1993, respectively, and a Ph.D. inpsychology from Columbia University, NewYork, in 1997. He was a postdoctoral fellowat the Max Planck Institute of CognitiveNeuroscience in Leipzig, Germany, from

1997 to 2000. Subsequently, he was an assistant professor(2000–2005) and an associate professor (2005–2006) in theDepartment of Psychology at the Chinese University of HongKong. Since July, 2006 he has been an associate professor in theDepartment of Psychology at the National University ofSingapore. His research interests include the cognitive and neur-al substrates underlying interval timing and memory.

Address for Correspondence: Trevor B. Penney,Department of Psychology, Faculty of Arts and SocialSciences, National University of Singapore, Singapore117570. E-mail: penney@hus.edu.sg.

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