Temporal filtering of luminance and chromaticity in macaque visual cortex

The visibility of a periodic light modulation depends on its temporal frequency and spectral properties. Contrast sensitivity is highest at 8-10 Hz for modulations of luminance but is substantially lower for modulations between equiluminant lights. This difference between luminance and chromatic contrast sensitivity is rooted in retinal filtering, but additional filtering occurs in the cerebral cortex. To measure the cortical contributions to luminance and chromatic temporal contrast sensitivity, signals in the lateral geniculate nucleus (LGN) were compared to the behavioral contrast sensitivity of macaque monkeys. Long wavelength-sensitive (L) and medium wavelength-sensitive (M) cones were modulated in phase, to produce a luminance modulation (L+M), or in counterphase, to produce a chromatic modulation (L-M). The sensitivity of LGN neurons was well matched to behavioral sensitivity at low temporal frequencies but was approximately 7 times greater at high temporal frequencies. Similar results were obtained for L+M and L-M modulations. These results show that differences in the shapes of the luminance and chromatic temporal contrast sensitivity functions are due almost entirely to pre-cortical mechanisms. Simulations of cone photoreceptor currents show that temporal information loss in the retina and at the retinogeniculate synapse exceeds cortical information loss under most of the conditions tested.