Intensity coding in primate visual system

Abstract

The pupil reflex and the discharge of LGN cells of the awake macaque were measured under stimulus conditions that yielded evidence for wide-range intensity coding in human psychophysical experiments. Ganzfeld flashes of white light were delivered under dark-adapted conditions to the surgically immobilized eye of the monkey while the other eye was observed in the infrared. Three-sec flashes elicited a consensual pupil reflex that was graded from −8 to 0 log Lamberts (L), indicating that the optic nerve fibers are capable of coding at least an 8 log-unit range of light intensity. In the physiological experiments, shorter flashes (0.1–0.5 sec) but otherwise identical conditions elicited monotonically graded responses from one type of LGN cell over the photopic range of −5 to 0 log L. Responses from other types of LGN cells were also graded over wide ranges but had different thresholds and, in some cases, nonmonotonic intensity-response functions. Latency of the excitatory LGN responses decreased with increasing intensity according to a power function with slope of −0.08. The pupil reflex and the LGN cell excitatory responses approximate power functions of light intensity with exponents of 0.22 and 0.14–0.29 respectively. The range of intensity coding found for single LGN cells is the widest yet reported for diffuse stimuli.