[Receptive fields of cat visual cortex neurons when photic stimulation parameters are altered]

Spatial excitability reliefs of receptive fields of visual cortex units were investigated in acute experiments on immobilized cats by changing physical and physiological parameters of light stimulation under dark, mesopic and low photopic adaptation. Transition from dark to low mesopic adaptation results in no changes of the shape and size of the receptive fields, revealed by constant intensity test slits, whereas with transition to low photopic adaptation the receptive fields were narrowed in 72% of cases and acquired an elongated shape while the sharpness of their selectivity to direction of movement and orientation of stimuli increases. At a constant background the receptive fields narrowed with decreasing brightness of the test light slit. The excitability relief in the field estimated by the absolute threshold of the cell response and by the level of intensity needed to get a constant number of discharges in response, considerably narrowed with a rise in the threshold criterion and with transition from dark to light adaptation. At light adaptation, stimulation with physiologically equal intensity revealed narrower reactivity reliefs. than those at dark adaptation. The estimation of the receptive fields, taking into account a possible contribution of light scattering in the eye media and over the screen, shows that in most cases their size cannot be explained by this factor.