THE RESPONSES OF NEURONS IN THE TEMPORAL CORTEX OF PRIMATES, AND FACEIDENTIFICATION AND DETECTION

The ability of a human observer to detect the presence of a briefly fl ashed picture of a face can depend on the picture's spatial configurat ion, that is on whether its features are rearranged (jumbled) or are i n their normal configuration. The face-detection effect (FDE) is found under conditions of backward masking, when the presence of a face can be detected with shorter masking intervals when it is in the normal. than when in the rearranged configuration. A similar effect is found w hen the subject is asked to classify the face as rearranged or not - t he face-classification effect (FCE). Part of the interest of the FDE a nd the FCE is that they show how the configuration of a stimulus can b e an important factor in the perceptual processing which leads to dete ction and classification of the stimulus. To analyse these effects we recorded from single neurons in the cortex in the superior temporal su lcus of macaques when they were shown (in a visual fixation task) norm al and rearranged faces under backward masking conditions shown in exp eriments 2 and 3 to produce, with the same apparatus, the FCE, and als o to produce comparable effects on the identification of which face wa s present (called hereafter the face-identification effect), and also of the clarity of the face. We found in experiment 1 that there are so me face-selective neurons which respond to faces only, or better, when the features in the faces are in their normal configuration rather th an rearranged. We also showed in this experiment that the difference i n the response to the normal as compared to the rearranged faces becam e greater when the masking stimulus was delayed more. Thus, at interme diate delays, there are more neurons active for the normal than for th e rearranged face. We therefore propose that the FDE, the FCE, and the face-identification effect arise because the total number of neurons activated by faces in their normal configuration is greater than that activated by rearranged faces, because of the sensitivity of some face -selective neurons to the spatial arrangement of the features. The exp eriments also show that backward visual masking does produce abrupt te rmination of the firing of neurons in the temporal cortical visual sys tem, so that the duration of a neuronal response is very short when vi sual stimuli can just be perceived.