Consistency of response during steady visual simulation.

The indication of subjective states during steady visual stimulation by means of key depression has been assumed to be free of systematic bias. However, it is shown that the reported visibility of simple stimuli may be determined by the level of categorisation employed before the initiation of an overt response. In particular, the duration of unitary disappearance of a stimulus is found to be greater when this is the only category of change to be reported than when the same measure is derived from concurrent reports of the disappearance of its parts. The correspondence of findings both with afterimages and with steady fixation shows that peripheral factors such as eye movements cannot provide an adequate explanation. Experiments comparing part/whole categorisation of the visibility of single lines suggest caution in the acceptance of previous studies designed to indicate underlying neurophysiological mechanisms in subjective disappearances.