Functional mapping of the inferior temporal cortex (IT) of the macaque monkey

$212 FUNCTIONAL MAPPING OF THE INFERIOR TEMPORAL CORTEX (IT) OF THE MACAQUEMONKEY. KEIJI TANAKA, HIDE-AKI SAITO, YOSHIRO FUKADA# and MADOKAFUKUMOTO$*, NHK Sci. & Tech. Res. Labs., #Teikyo Univ. Dept. Psychol., $Tokyo Univ. Facul. Med., Tokyo. Since an ablation of the i n f e r i o r temporal cortex (IT) results in a severe d e f i c i t in the visual pattern r e c o g n i t i o n and/or pattern memory, IT has been thought to be the center of pattern r e c o g n i t i o n . Gross and his colleagues (1972, 1984) found t h a t some of IT c e l l s responded s p e c i f i c a l l y to the sight of a face or hand, but l i t t l e is known f o r the coding of the other objects. The aim of the present study is to learn how the visual patterns are encoded by IT c e l l s . Immobilized ( F l a x e d i l ) monkeys anesthetized with N20 were used. The specific goal of t h i s paper is to e x t a b l i s h a functional mapping of IT, which is important because IT is a very wide area. For t h i s purpose, the monkeys were prepared f o r recording repeatedly (semichronic p r e p a r a t i o n ) , and 1636 penetrations were made over a wide A-P extent of the l a t e r a l surface of the IT gyrus in each of three hemispheres. A recorded c e l l was f i r s t tested using the whole range of white, black and colored disks and bars of various sizes, projected photographs of various regular patterns, and dozens of animal and plant 3D models. For c e l l s which were most e f f e c t i v e l y activated by some of the 3D models or complex 2D patterns, we attempted to c l a r i f y which component or combination of components of the s t i m u l i was crucial by using 2D models made of white, black and colored paper. The simplest 2D pattern t h a t f u l l y activated the c e l l was assumed to be the t r i g g e r f e a t u r e . There was a clear d i s c o n t i n u i t y at 4-10 mm a n t e r i o r to the t i p of the i n f e r i o r o c c i p i t a l sulcus in both the level of complexity of response s e l e c t i v i t y and the size of the receptive f i e l d . Most c e l l s in the p o s t e r i o r IT had small (<5deg) receptive f i e l d s , and they were maximally activated by disks or bars. They were s e l e c t i v e to the o r i e n t a t i o n , color and/or size of the s t i m u l i , but these properties have been found in the lower stages such as VI, V2 and V4. Most c e l l s in the a n t e r i o r IT had large (>lOdeg) receptive f i e l d s , and required more complex features f o r a c t i v a t i o n than the p o s t e r i o r IT c e l l s . The t r i g g e r feature of some c e l l s was a shape, such as disk, t r i a n g l e , s t a r , Tshape, disk with a p r o j e c t i o n and rounded tongue, while other c e l l s required a combination of a shape and a color and/or t e x t u r e . Thus we extablished the presence of a region in which most c e l l s require complex features of patterns f o r a c t i v a t i o n . This region occupies the a n t e r i o r 3/4 of IT.

VISUAL OBJECTS ARE REPRESLNTED BY A SET OF FEATURES OF MODERATECOMPLEXITY IN THE INFERIOR TEMPORAL CORTEX OF THE MACAQUEMONKEY. HIDE-AKI SAITO, KEIJI TANAKA, YOSHIRO FUKADA# and MADOKAFUKUMOTO*$, NHK Sci. & Tech. Res. Labs., #Teikyo Univ. Dept. Psychol., $ Tokyo Univ. Facul. Med., Tokyo There are two competitive hypotheses which attempt to explain how the brain accomplishes visual pattern r e c o g n i t i o n . One is a classical hypothesis called 'gnostic c e l l hypothesis' which assumes t h a t each object is represented in the brain by the a c t i v i t y of a specific c e l l . The other is a r e l a t i v e l y newly proposed hypothesis c a l l e d the ' p a r a l l e l d i s t r i b u t e d representation hypothesis' which assumes that each object is represented by an ensemble of the a c t i v i t i e s of a number of c e l l s , each of which represents a p a r t i a l feature of the object. We tested these hypotheses in the a n t e r i o r part of the macaque i n f e r i o r temporal cortex (AIT) which is thought to be the center of the object recognition system. The methods used have been described in the companion paper. Many c e l l s (227 c e l l s out of 428 recorded c e l l s ) in the area AIT were found to respond s e l e c t i v e l y to some complex 3D objects but were unresponsive to simple patterns such as a single bar and a spot. However, f o r h a l f of them (116 c e l l s ) the essential requirements f o r t h e i r a c t i v a t i o n were reduced to such r e l a t i v e l y simple 2D patterns as t e x t u r e s , contour-shapes, combinations of spatial parameters, or combinations of s p a t i a l parameters and color. For example, the t r i g g e r feature of a c e l l which responded s e l e c t i v e l y to the b e l l y of a zebra-model was found to be a p a i r of o r t h o g o n a l l y oriented stripes placed w i t h i n an e l l i p t i c frame. The stimulus requirement f o r a c e l l which responded s p e c i f i c a l l y to a banana-model was reduced to a combination of an elongated and tip-sharpened p r o f i l e , yellowish color and short p a r a l l e l lines (a kind of t e x t u r e ) on i t . The stimulus requirements of c e l l s which s e l e c t i v e l y responded to hands were reduced to the combinations of a c i r c u l a r disk and several bars extruding from i t r a d i a l l y with special angles. For the other h a l f of the c e l l s , we were unable to reduce the stimulus requirements to 2D patterns. However, the vast m a j o r i t y responded to several d i f f e r e n t objects comparably, suggesting t h a t these c e l l s were activated by some common features shared by the objects. In any type of c e l l , the magnitude of the response was restrained strongly by the o r i e n t a t i o n of the p a t t e r n . From these f a c t s , we suggest t h a t the area AIT, as a whole, encodes the visual patterns of objects not by gnostic units but by combinations of m u l t i p l e c e l l s each of which represents a feature of a moderate complexity.