The College of Optometrists

 

Abstract:           

 

Simple reaction times (RTs) were measured for the detection of motion-onset of 1c/deg sinusoidal luminance gratings and equiluminant chromatic gratings that were modulated in chromaticity along the L/M and S/(L+M) cardinal axes. 

 

The variation with contrast of motion-onset RTs is well described by a modified Pieron equation: RT = RT0 + k.1/C, where RT0 = asymptotic RT, C = RMS cone contrast. The constant k provides a measure of the contrast dependency of the RT to the different chromatic and luminance motion stimuli and can be measured over a range of velocities. RTs to the onset of S/(L+M) chromatic motion exhibit a much greater contrast dependency than those to either L/M chromatic or luminance-defined motion, particularly at low velocities.

 

Conclusions:

 

The results raise the possibility that these differences in contrast dependency reflect the fact that L/M and S/(L+M) motion signals have different physiological substrates; the former being mediated by the parvocellular system and the latter by the koniocellular system.  This is consistent with the view that the sensory component of RTs is dictated by the properties of pre-cortical processing pathways.