If it is assumed that the weighting function of early vision can be described by a spatiotemporal Gabor-like function (Manahilov, 1995 Vision Research 35 227 – 237), the spatial impulse response to flickering stimuli would consist of more alternating phases than that to a brief stimulus. To test this prediction, the Westheimer paradigm and the brightness matching technique were used. The effect of a flickering inducing disk (temporal frequency 7.7 Hz, duration 195 ms, zero starting phase) of variable radius on the apparent brightness of an incremental test stimulus (1.2 min arc radius, duration 14.25 ms) was measured in foveal photopic vision. The test stimulus was superimposed on the centre of the inducing disk and presented 142 ms after the onset of the inducing stimulus. When the radius of the inducing stimulus was increased, the test brightness was enhanced, reaching a maximum at a radius of 3 min arc, then diminished below the control level at radii of 9 – 11 min arc and was enhanced again at radii of 14 – 17 min arc. This finding differed from the unimodal effect of a brief inducing disk on test brightness and suggested a multiphasic profile of the spatial impulse response to a flickering stimulus. This suggestion was supported by the dependence of the apparent brightness of a test line on the distance between the test and two flanking inducing lines. The temporal conditions were similar to those of the previous experiment. The brightness of the test line was increased above the control level at distances of 0 – 2 min arc, decreased at distances of 3 – 6 min arc and again increased to a smaller extent at distances of 10 – 13 min arc. Brief inducing lines evoked only the initial enhancement of the test-line brightness and its decrease at lateral positions. The data obtained are in line with the predictions of the model of the visual weighting function.
