Neurophysiological interpretation of human visual reaction times: effect of contrast, spatial frequency and luminance.

Neurophysiological studies have demonstrated that in primates Magno and Parvo neurons have distinct contrast gain properties. Reaction Times (RTs) can be used to study supra-threshold contrast coding in humans over the same range of stimulus parameters. RTs to achromatic sinusoidal gratings were measured for a range of spatial frequencies (0.49-17.7 c/degree), stimulus luminances (0.005-20 cd/m2) and contrasts (from threshold to 0.5). The stimuli subtended an angle of 7.2 degrees at a viewing distance of 114 cm. RTs exhibit a linear relationship when plotted against the reciprocal of suprathreshold contrast. The slope of these functions reveals how contrast is linked to RT and can, therefore, be referred to as the RT-contrast factor with units of msec x contrast. A general equation is derived which accounts for all stimulus combinations. RT-based contrast functions resemble closely those obtained neurophysiologically for Magno (M) and Parvo (P) cells. Furthermore, the RT equivalent of contrast gain exhibits qualitatively similar gain characteristics to these neurons for a wide range of luminances and spatial frequencies. Our data support the notion that the sensory component of RTs is limited by the properties of pre-cortical neurons.