A COMPARISON OF THE 40-HZ RESPONSE IN MAN, AND THE PROPERTIES OF MACAQUE GANGLION-CELLS

Visually evoked field potentials in human subjects and single-cell res ponses from retinal ganglion cells in the macaque monkey were compared in closely similar stimulus situations. The classical heterochromatic flicker photometry (HFP) technique was used to measure spectral sensi tivity in man, both psychophysically and by recording the 40-Hz respon se, and to measure the spectral sensitivity of magnocellular (MC-) pat hway cells of the macaque. The three measures gave closely similar spe ctral-sensitivity curves. Close agreement between the three measures w as also found when the variable-modulation HFP technique was used to m easure spectral sensitivity. When the relative phase between red and g reen lights was varied, the point of minimum subjective flicker for hu man observers was close to a sharp minimum found in the amplitude of t he 40-Hz response in human and was also close to a minimum in the resp onse of MC-pathway neurons in the monkey. The human 40-Hz response sat urated at between 10 and 30% modulation depth, and so did the response of MC-pathway cells in the monkey. The 16-Hz response in human showed none of the above correlations with MC-pathway properties. On the oth er hand, parvocellular (PC-) pathway cells responded vigorously to con stant-luminance, chromatic modulation, at frequencies higher than can be detected by human observers. The human 16-Hz response also was stro ng in that stimulus situation. In addition, the response of PC-pathway cells on increasing modulation depth showed little saturation, and th is behaviour was paralleled by the human 16-Hz response. We conclude t hat the properties of MC-pathway neurons in macaque are closely simila r to the properties of the human 40-Hz response in the respects just d escribed. We suggest that the 40-Hz response may offer a means of obje ctively isolating and investigating the contribution of the magnocellu lar stream to cortical activity in human. In contrast, the properties of PC-pathway neurons in macaque are quite different from the properti es of the human 40-Hz response, and in several respects resemble the p roperties of the human 16-Hz response.