Maintained activity of monkey optic tract fibers and lateral geniculate nucleus cells

Abstract

The maintained discharge rates (MDRs) of monkey optic tract fibers and LGN cells were recorded to a wide range of different light intensities. “On” center optic tract fibers increase their firing almost linearly over 8 log units of background lights, while “off” center units show the opposite pattern. “On” center LGN cells show increases in rate up to 10 −2 cd/m 2 and decreases thereafter; “off” center units showed decreases in MDR up to 10 −2 cd/m 2 and increases in rate thereafter. The MDRs of tract fibers and LGN cells differ at high luminance levels because of a stronger tonic antagonism from the LGN receptive field surround than from optic tract receptive field surrounds. These differences in surround antagonism can be demonstrated with transient stimuli also. Results further suggest that while optic tract fibers can accurately convey absolute luminance signals, LGN cells cannot. Results are discussed in terms of receptive field organization.