TRANSLATION-INVARIANCE IN THE RESPONSES TO FACES OF SINGLE NEURONS INTHE TEMPORAL VISUAL CORTICAL AREAS OF THE ALERT MACAQUE

1. The responses of single neurons in the inferior temporal cortex and the cortex in the banks of the anterior part of the superior temporal sulcus of three awake, behaving macaques were recorded during a visua l fixation task. Stimulus images subtending 17 or 8.5 degrees were pre sented in the center of the display area, and fixation was either at t he center of the display area, or at one of four positions that were o n the stimulus, or several degrees off the edge of the test stimulus. The experiments were performed with face-selective cells, and the resp onses were compared for fixation at each position for both effective a nd noneffective face stimuli for each cell. 2. The firing rates of mos t neurons to an effective image did not significantly alter when visua l fixation was as far eccentric as the edge of the face, and they show ed only a small reduction when the fixation point was up to 4 degrees from the edge of the face. Moreover, stimulus selectivity across faces was maintained throughout this region of the visual field. 3. The cen ters of the receptive fields of the cells, as shown by the calculated ''centers of gravity,'' were close to the fovea, with almost all being within 3 degrees of the fovea. 4. The receptive fields of the cells t ypically crossed the vertical midline for at least 5 degrees. 5. Infor mation theory procedures were used to analyze the spike trains of the visual neurons. Nearly six times more information was carried by these neurons' firing rate about the identity of an image than about its po sition in the visual field. Thus the information theory analysis showe d that the responses of these neurons reflected information about whic h stimulus had been seen in a relatively translation invariant way. 6. Principal component analysis showed that principal component 1 (PC1) is related primarily to firing rate and reflected information primaril y about stimulus identity. (For identity PC2 added only 14% more infor mation to that contained in PC1.) Principal component 2 (PC2) was more closely related to neuronal response latencies, which increased with increasing eccentricity of the image in the visual field. PC2 reflecte d information about the position of the stimulus in the visual field, in that PC2 added 109% more information to that contained in PC1 about the position of the stimulus in the visual field. 7. These findings s how that in the temporal cortical visual areas there exists a populati on of visual cells whose responses are largely translation invariant, and that stimulus selectivity is maintained independently of retinal p osition for at least several degrees of the visual field near the fove a.