EVIDENCE FROM HUMAN ELECTRORETINOGRAM-A AND OFF RESPONSES THAT COLOR PROCESSING OCCURS IN THE CONES

Purpose. To investigate two apparent anomalies of the human electroret inogram: the ''on'' and ''off'' components of the cone based PIII are unequally sized, and transitions from red to green, which are electror etinographically silent, yield reverse transitions (green to red) in w hich a-waves develop. Methods. Ganzfeld electroretinograms were obtain ed with intense 100 msec flickering flashes from red and green light-e mitting diodes. Such stimuli light-adapt the retina, and the responses are caused by the excitation of long and medium wavelength cones. Res ults. In the 10-20 msec after the beginning of a flash (black to green or black to red) the beginning of rapid receptor-generated a-wave is seen. Ten to twenty milliseconds after the end of the flash, the begin ning of a rapid positive-going off response, also derived from recepto rs can be seen. If the retina is stimulated by the abrupt change from one wavelength of light to another (eg, from ''green'' to ''red''), at times > 20 msec after the change there are always slow changes in pot ential (presumably caused by postsynaptic activity) regardless of the relative intensities of red and green. However, if the two light inten sities are adjusted appropriately, 10-20 msec after the transition fro m green to red no electroretinographic a-wave (or off response) develo ps-the transition is ''silent.'' When the transition reverses (changes back from red to green), an a-wave occurs. In the same way if a red-t o-green transition is made silent by altering the relative light inten sities, the green-to-red reversal evokes an a-wave. This occurs for nu merous pairs of red and green intensities. Rod intrusion or minor elec troretinogram components do not explain this result. The relative red: green intensity in two color-anomalous subjects is different to that i n three normal subjects. The rule for a silent transition is that the decrease in excitation in one cone type should be twice the increase i n excitation in the second cone type. Conclusions. The most likely cau se is a reduction in the amplitude of cone receptor potentials 20-50 m sec after the onset of the stimulus, caused by a sign-reversing feedba ck mechanism such as that described in amphibians. This implies that t he chromatic signals for color vision required by theorists are partly generated in the cones.