The dynamics of the Π1 colour mechanism: further evidence for two sites of adaptation

Abstract

The visual pathway that determines Stile''s II1 color mechanism was isolated by the auxiliary field technique and studied under dynamic conditions of light adaptation and recovery by threshold measurements in human observers. The time course of adaptation to II1-equated short wavelength (.mu. .ltoreq. 500 nm) and long wavelength (.mu. .gtoreq. 550 nm) fields are very distinct: a large and relatively long-enduring transient threshold elevation occurs at the onset of the long wavelength, but not of the short wavelength fields. Similarly, the time courses of recovery from II1-equated long and short wavelength fields are quite distinctive: a large and relatively long enduring transient (transient tritanopia) occurs at the offset of the long wavelength, but not of the short wavelength fields. The wavelengths of the fields which cause the adaptation transients coincide with those shown previously to combine nonadditively with .mu. = 430 nm fields in effecting II1 adaptation. The failure of the time course of II1 adaptation to be spectrally univariant combines with the failures of field-additivity to demonstrate that signals from the long and/or middle wavelength sensitive cones affect the adaptation state of the II1 pathway. The adaptation transients are not observed in the pathways that determine II4 and II5. Thus, instantaneous signals from the middle and/or long wavelength sensitive cones are not the cause of the transients. Rather the cause must lie in the path by which those cones transmit their signals to the II1 pathway or in the II1 pathway itself. The off-transient can be diminished by adding an adequately intense short wavelength field to a long wavelength field that would normally cause it. The II1 pathway must receive chromatically opponent signals.