Effects of Surround Luminance on Perceptual Latency*

The apparent brightness of a test field can be substantially reduced by an extended-duration surround. In experiment I, test-field luminance was adjusted to match the brightness of a reference stimulus. Over a wide range of surround luminance, latency was measured by comparing test-stimulus and surround-field onsets, using the perceived-order method, in which the temporal interval between the two stimuli is varied to obtain subjective simultaneity. A series of control observations was made using test stimuli covering the same range of test-stimulus luminance that was used in the main series of observations, but with no surround field. Latencies of test stimuli were found to be considerably shorter with high-luminance surround fields than with no surround. Apparently, high-luminance surrounds resulted in reduction of latency of response to the test stimulus (facilitation of response to the test stimulus). In experiment II, a constant test-stimulus luminance was used. Reduction of latency of response to the test stimulus was found to increase with increasing surround luminance when surround luminance exceeded test-stimulus luminance. Supplementary observations indicated that the reduction of test-stimulus latency was not an artifact of dark adaptation.