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.