Now you see it, now you don't : a case study of perceptual alternations

Under some conditions, what we are seeing or hearing can seem to change abruptly, radically, and apparently spontaneously even though the physical stimulus does not alter. Although these 'perceptual alternations' occur with a bewilderingly varied range of stimuli, the evidence suggests that they are all the result of a common central process, which periodically switches between different interpretations of the stimulus. The 'square-wave illusion' is one form of perceptual instability which has been claimed to be an exception to this rule,'with,not one but two completing explanations in terms of peripheral Mechanisms having been advanced. The explanation at the most peripheral level that in terms of established cortical mechanisms is not, it is argued, compatible with a more detailed analysis of the literature. Explanations in terms of central processes, however, are supported by experiments in which cues such as edge information, the actual or implicit position of a light source, and instructional set are manipulated. In addition the relative dominance of the percepts can be altered in opposite directions by the prior inspection of stimuli which are physically similar (but phenomenally dissimilar) implicates central processes in this effect. Nevertheless, it is argued that a full explanation of the effect must take into account the manner in which the peripheral mechanisms process the visual information. Support for this comes from the finding that movement relative to the retina (i.e. either movement of the stimulus with steady fixation or making eye movements across a stationary stimulus systematically distorts the perception of the illusion. Two possible explanations of this distortion are evaluated. On the basis of the experiments and analyses presented in this thesis, it is argued that the square-wave illusion should not be regarded as a 'special case', but rather as merely one further manifestation of a more general mental process.