Central fixation - peripheral attention - goal directed saccades: What determines the activation of prelunate cortical cells in the rhesus monkey?

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negative going component of the synaptic d e n d r i t i c potential and in the amplitude of the negative component of the antidromic response. A first

5 min perfusion of L-glutamate (LG I mM) induced a marked, revers-

i b l e decrease of both responses. However,successiveperfusions with the same concentration of LG were less and less e f f e c t i v e .

I t is worth noting that the

synaptic potential was not reduced at times when LG no longer showed any sign i f i c a n t e f f e c t s . Such a d e s e n s i t i z a t i o n was observed also with D-glutamate (DG I mM), L-aspartate (LA I mM) and N-methyl-D, L-asparate (NMDLA 0.02 mM), but not with D,L-homocysteate (DLH 0.05 mM) or kainate (K 0.01 mM). Thus, a first

c l a s s i f i c a t i o n is obtained by considering the amino acids showing an ap-

parent d e s e n s i t i z a t i o n (DG, LA, NMDLA) and those showing no apparent d e s e n s i t i zation (DLH, K, synaptic s i t e ) . A second c l a s s i f i c a t i o n is provided by the antagonistic e f f e c t of alphaaminoadipate (AA 6 mM) on the synaptic response and on the e x c i t a t o r y response to DG, NMDLA, DLH, but not those to LG or K.

CENTRAL FIXATION - PERIPHERAL ATTENTION - GOAL DIRECTED SACCADES: WHAT DETERMINES THE ACTIVATION OF PRELUNATE CORTICAL CELLS IN THE RHESUS MONKEY? FISCHER, B. AND BOCH, R. Department of Neurophysiology, U n i v e r s i t y of Freiburg, Hansastrasse 9, 7800 Freiburg, R.F.A. In natural l i f e a monkey f i x a t e s objects of i n t e r e s t f o r some time and saccades to other objects of i n t e r e s t , i . e . his d i r e c t i o n of gaze and a t t e n t i o n coincide. I t is during the preparation of saccades when the animal breaks his ac~al f i x a t i o n , changes his d i r e c t i o n of a t t e n t i o n , makes decisions, and computes the coordinates f o r the eye movement. To see the d i f f e r e n t i a l

e f f e c t s of d i r e c t i n g a t t e n t i o n versus preparing

eye movements we recorded from single c o r t i c a l c e l l s during the presence or absence of a central f i x a t i o n point when the animal executes or suppresses saccades to a peripheral target. Two animals were trained to f i x a t e a central f i x a t i o n point f o r randomly varying periods of time ( I - 6 s ) using the dimming paradigm and were prepared f o r chronic recording of eye movements and single c e l l s in the prelunate visual association cortex. I f the f i x a t i o n point was extinguished the animals could, on t h e i r own decision, execute or suppress a saccade to a peripheral target placed in the c e l l ' s

receptive f i e l d ,

u s u a l l y I-4 degrees from the fovea. In any case

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the peripheral target received the dimming impulse i f i t happened to occur at times the f i x a t i o n point was o f f . Note: animals were not rewarded for saccades but rather for detecting the dimming. Many prelunate cells (65%) exhibited a visual on-response that could be enhanced before saccades to the receptive f i e l d stimulus. In addition, 60% of the cells tested were activated separately when, in the presence of a receptive f i e l d stimulus, the f i x a t i o n point was extinguished irrespective of whether the animal saccaded to the stimulus or not. This a c t i v i t y was strong and lasted throughout the time the f i x a t i o n point remained o f f ( . 4 - I s ) . The activation did not occur i f the stimulus was placed outside a c e l l ' s receptive f i e l d . In some cells an on-response e l i c i t e d in the absence of the central f i x a t i o n spot and in the absence of a saccade was stronger than the on-response to the same stimulus delivered in the presence of the f i x a t i o n target. The other 40% of the cells exhibited only inconsistent or no activation related to the offset of f i x a t i o n point. In an "attention" task, where the peripheral target received the dimming while the f i x a t i o n point remained on we failed to demonstrate consistent changes of the on-responses obtained in the normal f i x a t i o n task. A simple connection of cells with foveal and parafoveal receptive f i e l d s cannot explain this type of cortical activation. I t rather seems as i f the absence of central f i x a t i o n f a c i l i t a t e s the presence of peripheral attention which is reflected in the a c t i v i t y of prelunate cortex. The results show, that the prelunate cortex representing parafoveal vision receives information about the presence or absence of a behaviourally relevant foveal stimulus, but usually not about the preparation of eye movements per se.

DIFFERENCES IN THE ORGANIZATIONOF VISUALLY GUIDED REACHINGTO IPSILATERAL AND CONTRALATERAL TARGETS FISK, J.D. AND GOODALE, M.A. Jniversity of Western Ontario, London, Ontario, Canada In an experiment designed to study the organization of limb movement during normal v i s u a l l y guided reaching, s i g n i f i c a n t differences were observed between i p s i l a t e r a l and contralateral reaches. Eight right-handed subjects (4 males, 4 females) were asked to point to a I degree target, back-projected on a tangent screen at 50 cm viewing distance. The target l i g h t was presented to the right or l e f t of a f i x a t i o n point at either 10 or 20 degrees eccentricity. Two strobeshuttered video cameras were used to record the limb movement. One camera