[A color after-effect: the McCollough effect]

Coloured after-effect parameters have already been studied but the process involved in these events remains unknown. In the case of the McCollough effect, Shute has shown that the induced colour decreases rapidly at first and then more slowly. In a normal subject the effect can no longer be detected after approximately 2 hours. Results have been used to determine a relationship between the purity of the induced colour and time: I = I0 e-bt, where b is a typical parameter of the subject related to his physiological state. A figure of b approximately 0.03 min-1 was obtained in the normal subject. The rate of decay is influenced by certain drugs. In particular, a greater decrease in induced colour is obtained when caffeine is taken. In this case, the value of the decay constant b is about 0.06 min-1. Results obtained with anticholinergic agents are contrasted with those obtained with caffeine: slowed decay occurs. Hence the value of the decay constant is about 0.01 min-1. Data show that the McCollough effect could be an inhibitory mechanism in a visual pathway and which tends to suppress neural connections. It should be noted that the geometrical structures of patterns play an important part in the perception of the induced colour. For example, with square-wave gratings, the strength of the induced colour depends on spatial frequency and orientation. The McCollough effect mainly concerns the ways in which we perceive both pattern and colour. It may be located either within the retina or more centrally in the visual system.