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Context superiority in detection 7

the noise elements are f 14O 'and' f 7 6 O , representing an absolute angle difference (AAD) with respect to the horizontal and vertical target elements of 14'. The visibility of the target elements and consequently of the entire pattern is directly dependent on the magnitude of the absolute angle difference (compare figures la, lb, and l c with figure Id) which was used as an independent variable.

Our experimental technique allows an important theoretical and methodological generalization of the context-superiority effect. First, in contrast with previous studies, the detection of our patterns (ie of the context) is not possible without the detection of their individual elements (but the reciprocal is not true). Any observed superiority effect must then be interpreted as showing the simultaneous activation of lower and higher visual processing stages interacting in enigmatic ways. Second, the measurement

Figure 1. Examples of the stimuli used in the experiments. In (a),,(b), and (c) target stimuli are embedded among lines oriented at f 45'. These angle differences between target and back- ground elements induced 100% correct detection performances and were not used in the actual experiments. Stimulus (d), where the 'noise' elements are oriented at f 14' and *76', was used in the experiments. (a) and (d) show the 'face' stimulus (positioned in the middle-right part of the latter). The symmetrical 'nonface' and the asymmetrical stimuli are illustrated in (b) and (c) respectively.

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14 A Gorea, B Julesz

Moreover, the procedure used in the present experiments permitted estimation of the psychometric functions describing this phenomenon and thus its quantification in terms of a sensitivity (threshold) shift.

The detection face-superiority effect represents an average sensitivity gain of about lo (ie 5% of the average sensitivity). In terms of detection percentages, it represents a maximum increase of about 5% (at small absolute angle difference; see figure 2). This effect is comparable with the 7% gain obtained by Purcell and Stewart (1988) under more or less similar conditions, but substantially smaller than most of the previously reported object-superiority (Weisstein and Harris 1974; Williams and Weisstein 1978) and face-superiority (Homa et a1 1976; van Santen and Jonides 1978) effects of up to 20% increase in percentage correct responses. There are two obvious reasons for this difference.

One is that our face stimulus (figures l a and Id) is far more schematic than those used in previous studies. In this respect one may argue that our conclusions cannot be generalized to a larger family of 'faces' and that, in the last resort, our stimulus does not represent a face at all. It should be noted that the principle by which our stimuli are constructed does not allow the design of more- than one generic 'face' stimulus which could have been displayed at different spatial scales, namely with more or fewer noise elements intercalated among the target elements. Addition of such noise elements would have counteracted the cluster effect which, in turn, would have decreased the context-superiority effect. On the other hand, had we obtained no context-superiority effect, we would have also argued that there is nothing in common between our face stimulus and a real face. However, the present results argue in favour of the particular perceptual status of the face stimulus.

The second obvious reason is that, in contrast to previous studies, the presence of a context in our conditions could not be dissociated from the presence of a target. It is clear that under these experimental conditions a context effect will be strongly attenuated. The fact that the context (face) effect was, nevertheless, measurable and statistically significant is of particular interest since it demonstrates the existence of an intricate bottom-uphop-down loop whereby perceptual decisions are reached through a recursive exchange of information between low (detection) and high (context- identification) processing levels.

The face-superiority effect measured in the detection task does not seem to be accompanied by a context-identification face-superiority effect (figures 4 and 5). However, one cannot be affirmative in this respect, given the overfl low identification performances on which our threshold estimates were based. It should be noted that observer AG's percentage correct identification results, which are substantially higher than those of the other observers, do display an identification face-superiority effect of about 7' (figure 5). Previous reports by Purcell and Stewart (1988) confirm this obsemation, Under the assumption that the face pattern is processed by specific higher- order units (see Gross et a1 1972; Perrett et a1 1982, 1985), it should be concluded that the activation of the latter, even at subthreshold levels, is sufficient to reinforce the activation of lower-level orientation-selective units.

Taken altogether, the identification performances are significantly below the detec- tion performances (compare figures 2 and 4 and 3 and 5). There are two reasons why one does not need to postulate a dichotomy between corresponding 'attentive' and 'preattentive' processes (Sagi and Julesz 1985a, 1985b) in order to account for this difference between the identification and detection performances in our experiments.

First, under our experimental conditions, the identification performances for the four-target stimuli are limited by probability multiplication considerations. That these performances are in fact higher than those predicted by probability multiplication (figure 4), indicates the existence of facilitatory interactions among clustered detectors

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