Mesopic spectral responses and the Purkinje shift of macaque lateral geniculate nucleus cells.

Spectral responsiveness of different classes of macaque LGN cells at eccentricities smaller than 12 degrees was studied at low light intensities. Cone thresholds of cells varied from 1 to 10 td. Rod inputs were found in all classes of cells, including inhibitory inputs in some cells. Rod inputs were not apparent above 10-40 td, giving a total mesopic range of about 1-40 td. Strong rod-mediated responses could be evoked in broadband phasic cells and in spectrally opponent cells excited by short wavelengths. Only weak if any excitatory responses could be evoked by short wavelengths at scotopic levels in spectrally opponent long-wavelength excited cells. Hence, rod inputs do not confound the spectral responsiveness of cells because no spectrally opponent cell excited by long-wavelength stimuli at photopic levels became significantly responsive to short wavelength stimuli at mesopic or scotopic intensities. The so-called "rod color" may be blue. An increase in the dominance of wide-band cell responses that may explain the Bezold-Brücke hue shift was observed at higher stimulus radiances at wavelengths near 450 and 650 nm.