Structure and dynamics of receptive fields in the visual cortex of the cat(area 18) and the influence of GABAergic inhibition

Receptive fields (RFs) in the visual cortex are characterized by spatiotemp oral profiles that have been described in detail for area 17 simple cells. In this study, we analyse spatial and temporal RF properties of simple and complex cells in layer II/III of area 18 of the anaesthetized adult cat, us ing the reverse correlation method with brief 50 ms presentations of flashi ng bright and dark bars. Stimuli were presented with preferred orientation as previously determined by moving bars. Simple cell RFs were characterized by spatially and temporally separable ON and OFF subfields, while in compl ex cells ON and OFF subfields were superimposed. To discriminate possible c ontributions of GABAergic inhibition to RF structure and response dynamics in area 18, we have used three-barrelled micropipettes for single cell reco rdings and microiontophoresis, and have documented ON and OFF responses bef ore, during and after application of bicuculline methiodide for blockade of GABAA receptors. During blockade of GABAergic inhibition, the stimulus-ind uced and resting discharge frequency increased, and in about 50% of the cel ls both ON and OFF subfields changed significantly in space and/or time in a reversible manner. In space, blockade of inhibition widened RF subfields, whereas in time, it shortened the duration of the excitatory cell response in simple and complex cells. ON and OFF subfields separated in space and t ime (simple cells), or time (complex cells) became less isolated or even su perimposed. The results indicate substantial local inhibitory processing co ntributing to spatiotemporal RF properties in layers II/III of area 18 of t he cat.