Spatiotemporal characteristics of thresholds adjacent to a luminance edge

Spatiotemporal incremental and decremental thresholds were measured for a thin vertical line (target) positioned adjacent to a briefly presented vertical edge. A significant difference between the stimuli used here and those used in previous studies is the background level against which the edge was presented. Here the edge was formed by briefly decreasing the luminance of the left side of a light background. This novel condition was compared with the more usual condition in which the edge is formed by briefly increasing the luminance of the right half of a dark background. In a further test the buildup of threshold after an edge was switched on was also measured. When the target was presented on the side of the edge that remained fixed in luminance, a small but reliable threshold change adjacent to the edge was measured. The effect was much larger for measurements made on the side of the edge that changed in luminance; however, the spread was comparable for the two conditions. For the target presented on the light side of the edge, decremental thresholds were much larger than incremental thresholds. This is attributed, at least in part, to the different types of tasks required of the observer. Maximum temporal threshold elevation occurs at or just before (i.e., less than 16 msec) 0 asynchrony between edge and target. The results are interpreted at a qualitative level as supporting a receptive-field type of model in which the edge, at various spatiotemporal locations relative to the target, inhibits or excites activity at the receptive field centered on the target.