Further Analysis of the Responses of LGN Cells*

A previous report delineated the response characteristics of single cells in the macaque lateral geniculate nucleus to diffuse flashes of monochromatic light. This paper presents a further analysis of the response patterns from the four types of spectrally opponent cells. At least two of the macaque's cone photopigments are associated with a spectrally opponent cell in such a way that activation of one cone type increases the cell's response rate and the other decreases it. By equating stimuli for absorption by various combinations of cone types, a given opponent cell can be shown to receive inputs from only two of the three cone types; the various pairings of cone types determine the differences between the cells' response patterns; +R−G and +G−R cells are associated with cone pigments having their respective absorption maxima at 535 and 570 nm; the 570 cones excite the +R−G cells but inhibit the +G−R cells. +Y−B and +B−Y cells are associated with 445 and 570 cone types. Some observations are made on the process by which the two antagonistic inputs are summed to give the recorded responses; stimuli are equated for absorption by one of the cone types, thus ensuring a constant input from one of the two inputs, while at the same time the energy absorbed by the other subsystem is varied. Among other things there is a threshold-like phenomenon in the summation process, whereby excitation must exceed inhibition by some amount before the cell's responses are affected.