The dynamics of the Π 1 colour mechanism: further evidence for two sites of adaptation

1. The visual pathway that determines Stiles's Π 1 colour mechanism was isolated by the auxiliary field technique and studied under dynamic conditions of light adaptation and recovery by threshold measurements. 2. The time courses of adaptation to Π 1 ‐equated short wave‐length (μ ≤ 500 nm) and long wave‐length (μ ≥ 550 nm) fields are very distinct: a large and relatively long‐enduring transient threshold elevation occurs at the onset of the long wave‐length, but not of the short wave‐length fields. 3. Similarly, the time courses of recovery from Π 1 ‐equated long and short wave‐length fields are quite distinctive: a large and relatively long enduring transient (‘transient tritanopia’) occurs at the offset of the long wave‐length, but not of the short wave‐length fields. 4. The wave‐lengths of the fields which cause the adaptation transients coincide with those shown previously (Pugh, 1976) to combine non‐additively with μ = 430 nm fields in effecting Π 1 adaptation. The failure of the time course of Π 1 adaptation to be spectrally ‘univariant’ combines with the failures of field‐additivity to demonstrate that signals from the long and/or middle wave‐length sensitive cones affect the adaptation state of the Π 1 pathway. 5. The adaptation transients are not observed in the pathways that determine Π 4 and Π 5 . Thus, instantaneous signals from the middle and/or long wave‐length 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 Π 1 pathway or in the Π 1 pathway itself. 6. The off‐transient can be diminished by adding an adequately intense short wave‐length field to a long wave‐length field that would normally cause it. The Π 1 pathway must receive chromatically opponent signals.