Increment and decrement detection on temporally modulated fields

Increment and decrement probe thresholds were measured during the presentation of two types of temporal masking stimuli. In Experiment 1, we measured thresholds for increment or decrement rectangular probes presented during the presentation of an increment or decrement Gaussian masking stimulus. We find that thresholds are higher when the probe and the Gaussian mask are of the same sign (e. g. both increments). However, both types of Gaussian mask raised increment and decrement probe thresholds above steady state conditions. In Experiment 2, we presented increment or decrement probes at one of eight possible phases of a 1 Hz luminance-modulated sine wave. For both increment and decrement probes, threshold variation with phase is non-sinusoidal in shape, but increment and decrement probe thresholds vary as a function of the sinusoid phase. These experiments show that increment and decrement thresholds vary as a function of the adaptation state of the visual system, and as a function of the direction of change in the adaptation state. Data from both experiments are discussed in terms of a recent neurophysiological model [Hood & Graham (1998). Threshold fluctuations on temporally modulated backgrounds: a possible physiological explanation based upon a recent computational model. Visual Neuroscience, 15 (5), 957-967]. We find that the predicted ON- and OFF-pathway responses do not correlate in a straightforward manner with the psychophysical thresholds, suggesting that detection of increment and decrement probes may not be performed exclusively by one pathway. Our data have implications for modeling visual performance under conditions where visual adaptation is dynamic, such as when scanning complex images or natural scenes.