LAUSR

Graphical abstract Figure. No Caption available. HighlightsBlue‐on and blue‐off receptive‐field properties were identified in primate LGN.Sensitivities to short‐wave (S) and medium/long‐wave (ML) cone contrast are similar.S and ML receptive field subunits have similar spatial extents.Vector sum model predicts responses to achromatic S + ML and chromatic S − ML contrast.Blue‐on and blue‐off receptive fields select more for chromatic than spatial contrast. Abstract 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.