The Contributions of the on and off Gain Difference to the Contextual Effect in Macaque Monkey V1

In macaque monkey V1, the response amplitudes of ON and OFF sub-regions of simple receptive fields are about equal for cells in the input layer 4c, but the OFF responses tend to be greater than the ON responses for layer-2/3 cells. This OFF-over-ON bias is evident when receptive fields are mapped with sparse noise (a series of dark or bright squares shown in different space and time) but is weaker with Hartleys (gratings shown briefly at different orientations and spatial frequencies). One possible mechanism for the receptive-field mismatch in layer 2/3 of V1 is the difference between ON and OFF response gains caused by different degrees of sparseness between Hartleys and sparse noise. Here we manipulated the relative strength of ON and OFF sparse-noise responses until the receptive field similarity (RFS, quantified as the pixel-by-pixel correlation between two maps) between sparse-noise and Hartley maps reached the maximum. On average, the relative strength of ON/OFF responses in sparse-noise maps should be increased to approximately two times of their origins in order to match closely their Hartley compartments for layer-2/3 neurons (1.93±0.19; mean±S.E.M.). The sole change in response gain largely increased the RFS between Hartley and sparse-noise maps (on average the RFS increases from 0.35 to 0.58), and it could account for approximately 81% of the overall differences between the two maps. In summary, the difference in ON and OFF gains under different visual stimulations is one critical mechanism underlying the contextual effect in the superficial layer of V1