Dependency of reaction times to motion onset on luminance and chromatic contrast.
We measured reaction times for detecting the onset of motion of sinusoidal gratings of 1 c/deg, modulated in either luminance or chromatic contrast, caused to move abruptly at speeds ranging from 0.25 to 10 deg/s (0.25-10 Hz). At any given luminance or chromatic contrast, RTs varied linearly with temporal periodicity (r2 congruent with 0.97), yielding a Weber fraction of period. The value of the Weber fraction varied inversely with contrast, differently for luminance and chromatic contrast. The results were well simulated with a simple model that accumulated change in contrast over time until a critical threshold had been reached. Two crucial aspects of the model are a second-stage temporal integration mechanism, capable of accumulating information for periods of up to 2 s, and contrast gain control, different for luminance than for chromatic stimuli. The contrast response for luminance shows very low semi-saturating contrasts and high gain, similar to LGN M-cells and cells in MT; that for colour shows high semi-saturating contrasts and low gain, similar to LGN P-cells. The results suggest that motion onset for luminance and chromatic gratings are detected by different mechanisms, probably by the magno- and parvo-cellular systems.
Burr
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DC
Istituto di Neurofisiologia del CNR, Via S. Zeno 51, 56127, Pisa, Italy. dave@in.pi.cnr.it
Corsale
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B
D015350:Contrast Sensitivity / Q000502:physiology*; D004056:Differential Threshold / Q000502:physiology; D005260:Female; D006801:Humans; D008029:Lighting; D016014:Linear Models; D008297:Male; D008959:Models, Neurological; D009039:Motion Perception / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology*