Keele Research Repository

A series of experiments are reported which have attempted to isolate some factors causing inter and intrasubject variability of the McCollough effect.* A number of such factors were found.
1. The initial strength of the OCCA is strongly influenced by sleep duration. Reduction of up to one third of a normal nights sleep caused a marked decrease in the aftereffect strength. Sleep periods of under a third of normal were found to have no further effect. Decay rates were not affected by prior sleep duration.
2. Both the strength and decay of the McCollough effect undergo diurnal changes late in the evening. These changes were linked with the sleep cycle and evidence is presented indicating that the effect of the time of day upon the initial strength may be linked with the effect of sleep duration.
3. Some visual defects result in abnormalities of binocular,' dichoptic and transferred McCollough effects.
4. Different visual stimuli presented after induction cause large variations in the rate of decay of the OCCA. Greatest decay was caused by those stimuli with identical characteristics to those of the induction stimuli.
5. Variation in the visual stimulation presented before induction strongly influences the initial strength of the aftereffect but does not affect subsequent decay rates. Chromatic fields of identical colours as the induction stimuli, and gratings of orientations of not more than 20 from those of the induction stimuli cause a large reduction in the strength of the OCCA.