Chromatic summation and receptive field properties of blue-on and blue-off cells in marmoset lateral geniculate nucleus.

The "blue-on" and "blue-off" receptive fields in retina and dorsal lateral geniculate nucleus (LGN) of diurnal primates combine signals from short-wavelength sensitive (S) cone photoreceptors with signals from medium/long wavelength sensitive (ML) photoreceptors. Three questions about this combination remain unresolved. Firstly, is the combination of S and ML signals in these cells linear or non-linear? Secondly, how does the timing of S and ML inputs to these cells influence their responses? Thirdly, is there spatial antagonism within S and ML subunits of the receptive field of these cells? We measured contrast sensitivity and spatial frequency tuning for four types of drifting sine gratings: S cone isolating, ML cone isolating, achromatic (S + ML), and counterphase chromatic (S - ML), in extracellular recordings from LGN of marmoset monkeys. We found that responses to stimuli which modulate both S and ML cones are well predicted by a linear sum of S and ML signals, followed by a saturating contrast-response relation. Differences in sensitivity and timing (i.e. vector combination) between S and ML inputs are needed to explain the amplitude and phase of responses to achromatic (S + ML) and counterphase chromatic (S - ML) stimuli. Best-fit spatial receptive fields for S and/or ML subunits in most cells (>80%) required antagonistic surrounds, usually in the S subunit. The surrounds were however generally weak and had little influence on spatial tuning. The sensitivity and size of S and ML subunits were correlated on a cell-by-cell basis, adding to evidence that blue-on and blue-off receptive fields are specialised to signal chromatic but not spatial contrast.

Temporal response properties of koniocellular (blue-on and blue-off) cells in marmoset lateral geniculate nucleus.

Visual perception requires integrating signals arriving at different times from parallel visual streams. For example, signals carried on the phasic-magnocellular (MC) pathway reach the cerebral cortex pathways some tens of milliseconds before signals traveling on the tonic-parvocellular (PC) pathway. Visual latencies of cells in the koniocellular (KC) pathway have not been specifically studied in simian primates. Here we compared MC and PC cells to "blue-on" (BON) and "blue-off" (BOF) KC cells; these cells carry visual signals originating in short-wavelength-sensitive (S) cones. We made extracellular recordings in the lateral geniculate nucleus (LGN) of anesthetized marmosets. We found that BON visual latencies are 10-20 ms longer than those of PC or MC cells. A small number of recorded BOF cells (n = 7) had latencies 10-20 ms longer than those of BON cells. Within all cell groups, latencies of foveal receptive fields (<10° eccentricity) were longer (by 3-8 ms) than latencies of peripheral receptive fields (>10°). Latencies of yellow-off inputs to BON cells lagged the blue-on inputs by up to 30 ms, but no differences in visual latency were seen on comparing marmosets expressing dichromatic ("red-green color-blind") or trichromatic color vision phenotype. We conclude that S-cone signals leaving the LGN on KC pathways are delayed with respect to signals traveling on PC and MC pathways. Cortical circuits serving color vision must therefore integrate across delays in (red-green) chromatic signals carried by PC cells and (blue-yellow) signals carried by KC cells.