Detection of Chromatic and Achromatic Patterns at Threshold: RT Evidence of Sustained and Transient Mechanisms

In primates, relatively slow sustained-like physiological mechanisms mediate chromatic processing and rapid transient-like mechanisms mediate low-spatial-frequency achromatic processing. Our objective was to investigate the role of transient and sustained mechanisms in the detection of chromatic and achromatic gratings with the use of a reaction time (RT) paradigm. Following Tolhurst (1975 Vision Research 15 1143 – 1149), we constructed RT histograms using near-threshold grating stimuli. Tolhurst showed that, for low-spatial-frequency achromatic patterns, the RT histograms cluster around the onset and the offset of the pattern, indicating transient detection. His onset and offset distributions were consistent with probability theory. With high spatial frequencies, RTs are evenly distributed over the duration of the stimulus presentation, showing the operation of a sustained mechanism.

When we used low-spatial-frequency (0.22 cycle deg⁻¹) isoluminant red - green gratings, the RT histograms were unimodal, revealing detection mechanisms with predominantly sustained characteristics, as expected. RT histograms obtained with low-spatial-frequency achromatic stimuli in our study were bimodal. However, when we compared small and large fields, the shape of the RT histogram varied in a manner which could not be accounted for by a simple probabilistic model. This can be rationalised if onsets and offsets are not equally detectable.