, Volume 6, Issue 4, pp 324-352

Functional synaptic organization of primary visual cortex neurones in the cat

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Summary

The spontaneous and light evoked post-synaptic activity of cells of the primary visual cortex was investigated with intracellular and quasi-intracellular records. The resting membrane potential fluctuated mostly between 3–10 mV below the firing threshold owing to spontaneous EPSP- and IPSP-activity. Discharge activity was therefore low. Forms and amplitudes of the visible EPSP's showed a large variability, the frequency was 150–300/sec. Discrete IPSP's were between 0.5–3 mV and were less frequent than EPSP's (about 1∶10). Their duration was only slightly longer than that of EPSP's. EPSP's and IPSP's could be elicited at on or off by appropriately positioned small light stimuli. During the initial reaction following a stimulus, single PSP's could be distinguished. Geniculate on-center- as well as off-center-afferents could lead to excitation or inhibition in different neurones. The receptive fields of cortical cells to monocular stimulation were analysed with averaged records. In each neurone 2–4 overlapping areas of on- or off-activation or -inhibition could be distinguished. Each of these activation or inhibition zones had the functional properties of a single geniculo-cortical onor off-center fibre with their receptive field centers separated by 1–3°. The variety of functional organizations of the cortical neurones to monocular stimulation was explained by variable combinations of 2–4 converging geniculate on- or off-center fibres with either excitatory or inhibitory action and variable overlap of their receptive fields. This was tested in a simple computer model. — Most neurones with pronounced reactions to movement or with direction specific movement sensitivity (about half of the neurones investigated) had an excitatory contact with an off-center fibre, which seemed to be mainly responsible for the movement reaction.- The findings suggest that from each eye less than 5 geniculo-cortical afferent converging fibres have a major effect on the activity of one cortical cell. Inhibitory afferents may be indirect and relayed through another cortical pyramidal cell.

This paper was partly supported by the Deutsche Forschungsgemeinschaft, which provided funds for the Lab computer IBM 1130 (Grant Cr. 30). — We are grateful to Dipl.-Ing. Probst for helping us in the computer programming.
When this manuscript was finished, a paper of Barlow et al. (1967) appeared which describes the effects of binocular stimulation on cortical neurones. Their findings of a variable disparity of the binocular inputs agree with our own observations which emphasized the variations of afferent monocular input and the variable distances of field centers of converging fibres in the different cortical neurones. — Barlow et al. also mention the fact, that stimulation of one part of a receptive area of a cortical neurone may “veto” the reaction to another stimulus. The demonstration of inhibitory areas in the receptive fields of cortical neurones is the synaptic basis for this observation.