Alan Freeman, Gloria Luo-Li, David Alais; Orientation discrimination depends on co-activation of on- and off-centre visual channels. Journal of Vision 2016;16(12):130. doi: 10.1167/16.12.130.
INTRODUCTION. It is becoming increasingly clear that visual responses to light increments and decrements are not symmetrical: both behavioural and neural data show that responses to darks are stronger and faster than to lights. Our aim was to see whether this asymmetry influences orientation discrimination. METHODS. We separated Gabor patches into light bars and dark bars, and presented the two components asynchronously. Bars were tilted 2° left or right of vertical but the tilt on any one trial was the same for light and dark bars. Adult human subjects indicated whether the tilt was leftward or rightward, and the probability of a correct response was plotted against stimulus contrast. RESULTS. Contrast sensitivity, obtained from the psychometric functions, was maximal when the light bars preceded the dark bars by 13 ms (1 video frame). Sensitivity fell sharply with stimulus onset asynchronies differing from this value. In particular, sensitivity was suboptimal when light and dark bars were presented simultaneously. CONCLUSION. It is commonly accepted that orientation-selective neurons receive convergent input from on- and off-centre subcortical inputs. Given the recent finding (Jin et al., J. Neuroscience, 31, 17471) that off-centre inputs usually precede on-centre inputs, our results are consistent with the idea that orientation-selective neurons require coincident on- and off-input for maximal sensitivity.