CORTICAL POINT-SPREAD FUNCTION AND LONG-RANGE LATERAL INTERACTIONS REVEALED BY REAL-TIME OPTICAL IMAGING OF MACAQUE MONKEY PRIMARY VISUAL-CORTEX

Processing of retinal images is carried out in the myriad dendritic ar borizations of cortical neurons. Such processing involves complex dend ritic integration of numerous inputs, and the subsequent output is tra nsmitted to multiple targets by extensive axonal arbors. Thus far, det ails of this intricate processing remained unexaminable. This report d escribes the usefulness of real-time optical imaging in the study of p opulation activity and the exploration of cortical dendritic processin g. In contrast to single-unit recordings, optical signals primarily me asure the changes in transmembrane potential of a population of neuron al elements, including the often elusive subthreshold synaptic potenti als that impinge on the extensive arborization of cortical cells. By u sing small visual stimuli with sharp borders and realtime imaging of c ortical responses, we found that shortly after its onset, cortical act ivity spreads from its retinotopic site of initiation, covering an are a at least 10 times larger, in upper cortical layers. The activity spr eads at velocities from 100 to 250 mu m/msec. Near the V1/V2 border th e direct activation is anisotropic and we detected also anisotropic sp read; the ''space constant'' for the spread was similar to 2.7 mm para llel to the border and similar to 1.5 mm along the perpendicular axis. In addition, we found cortical interactions between cortical activiti es evoked by a small ''center stimulus'' and by large ''surround stimu li'' positioned outside the classical receptive field. All of the surr ound stimuli used suppressed the cortical response to the center stimu lus. Under some stimulus conditions iso-orientation suppression was mo re pronounced than orthogonal-orientation suppression. The orientation dependence of the suppression and its dependency on the size of some specific stimuli indicate that at least part of the center surround in hibitory interaction was of cortical origin. The findings reported her e raise the possibility that distributed processing over a very large cortical area plays a major role in the processing of visual informati on by the primary visual cortex of the primate.