Wavelength-discrimination functions were compared for three different stages in the processing of visual signals. Responses at an early retinal stage were recorded by use of a contact-lens electrode attached to the cornea of the eye. Occipital scalp electrodes were used to record potentials evoked at the cortical level. Finally, the observers made psychophysical judgments of the corresponding differences of hue. Electrical responses were evoked by rapid shifts from one wavelength to another within a set of vertical stripes that made up the visual field. In favorable regions of the spectrum (near 495 and 595 nm), small (4 to 7 nm) shifts of wavelength evoked measurable occipital potentials and produced noticeable changes of the perceived hue. Much larger differences (20 to 25 nm) were required, however, to elicit retinal responses. This suggests that an enhancement of wavelength-discriminating signals must take place at some stage between the bipolar layer of the retina and the striate area of the cortex.
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