The physiological basis of wavelength discrimination: evidence from dichoptic and Ganzfeld viewing

It has been suggested that human wavelength discrimination depends on the peripheral visual system outputting a signal which already indicates the difference between the two test wavelengths. In the present paper an alternative mechanism whereby wavelength discrimination is carried out by the visual cortex and not by the retina or the LGN is suggested. Wavelength discrimination was first tested under dichoptic viewing so that discrimination could, most likely, be performed by the visual cortex. Since performance was normal it is concluded that the visual cortex is capable of fine, normal level of wavelength discrimination. When the task was performed under ganzfeld viewing (alternating wavelengths) so that the LGN would output only the difference between the two wavelengths, discrimination was poor and sometimes, absent. This indicates that it is unlikely that wavelength discrimination is complete at the LGN level.