The effects of abnormal stimulation on the judgement of visual direction

Abstract

Spatially oriented behaviour is to a considerable extent calibrated by reference to stimulus norms and invariant relationships between inputs from different stimulus channels. Three sorts of experimental disruption of these normal relationships in the field of direction perception are examined and experiments are reported which attempt to elucidate the mechanisms underlying the behavioural reaction to them in humans. The first two examples concern the visual perception of verticality and frontal plane tilt. The first is an examination of Gibson's concept of normalization and negative after-effect in spatial dimensions. A review is made of several attempts to subsume the behaviour which this theory was designed to explain under more elementary principles. The most serious of these attempts - that of Kȍhler and Wallach - is the subject of a series of experiments which are reported and from which it is concluded that the attempt must be considered a failure and that the postulation of some mechanism genuinely characteristic of the spatial dimension is required to explain the behaviour. The second issue is the role of non-visual gravitational cues in the visual judgment of the direction of gravity. The historical dispute about the relative importance of visual and postural cues is outlined. Then attention is focussed on the contribution of the various types of postural cue and it is concluded that some investigation have seriously misinterpreted the role of vestibular information. An experiment is reported in which the two main factors known to have disruptive effects on spatial behaviour - tilt of the visual field and tilt of the subject - are shown to have their effects considerably attenuated by the presence of vestibular cues. The third example concerns the disruption of azimuth-oriented behaviour by modification of the normal correlation between spatial inputs in two modalities. A critical scrutiny is made of theories concerning the location of the adaptive change in response to such disruption. A corollary of one of the theories - that active movement is necessary for adaptation - is tested and rejected in a series of experiments, and these in addition provide some evidence for an alternative theory.