No abstract

Theories of direct visual perception which claim that all the information for perception resides in the structure of the ambient optic array are only partial theories of perception. They run counter to evidence in neuropyschology and experimental psychology which suggests that perception may be a process from the inside out as well as from the outside in. The state of the organism at any moment in time, measured by feedback from the central nervous system itself, determines both what part of the structure of the optic array is relevant at a given moment and how it will be interpreted.

This study involved a partial replication of an experiment performed by Held & Rekosh (1963) on the relation between prism-induced changes in visuomotor feedback and shifts in the perception of straightness. The Held & Rekosh study is important because its methodology makes it possible to clearly separate changes in perception due to visuomotor feedback from those due to other perceptual factors. Thus it bears unambiguously on the hypothesis that the neurological processing of sensory input is dependent on the organization of motor activity. The present paper consists of a brief review of theory and experiment on sensorimotor phenomena in perception, followed by a detailed consideration of the Held & Rekosh study and the present replication. For greater effectiveness, Held & Rekosh should have used base-up and base-down prism orientations rather than the base-left and base-right orientations that they employed, but in any case our replication yielded negative results for both base-up and base-right. It is concluded that, given the minimally effective prism orientation and the attentional loads characterizing the Held & Rekosh study, there is need for further experimentation with these two variables. A more drastic suggestion is that there may be a need to confront anew the evidence for sensorimotor processes in perception.

A system is described which simulates some of the visual rearrangements and changes in sensorimotor relations that occur when experimental subjects move their eyes while looking through a contact lens with a prism attached. The simulated system is more convenient than a system based on the use of a contact lens. and it is based on measures of eye movement that are important in research on the perceptual effects of visual rearrangement. The effects that occur are reviewed in this paper. We also show in detail how the artificial system, which has an eye-movement monitor interfaced with a computer and display. simulates the former effects. The data show that experimental subjects who experience visual rearrangement by simulated means manifest the same kind of perceptual adaptation produced in studies in which visual rearrangement is generated by means of a prism mounted on a contact lens. The system. which simulates the effects produced by prisms. consists of an eye monitor. a PDP-9 computer. and a cathode ray tube (CRT). In order to describe system operation. it is necessary to review the effects produced by prisms as well as the experimental uses of prisms reported in the literature. USE OF PRISMS IN PAST RESEARCH IN PERCEPTION Optic devices like prisms have been employed typically when an investigator wants to study perceptual adaptation to visual rearrangement. and thus wants to displace the apparent location of objects relative to an observer or to alter the shape of contours by making. for example. straight lines look curved or curved lines look straight. For a recent review of much of the literature. see Gyr (1972). Visual rearrangements such as the latter-when produced by a contact lens with a prism mounted on it-lead to unusual visual effects when subjects start to scan the rearranged contours by moving their eyes. For example. suppose subjects scan an apparently straight but physically curved contour in the usual way. i.e .. bv means of a straight movement of the eye parallel to the contour; they expect to see a line which retains its orientation and is easily kept in focus. What the subjects see instead is a line that changes orientation as~function of their eve movements. and which therefore cannot be ti~ated by a straight eye movement. In fact. fixation can be maintained only by moving the eye in a parth similar to the true. curved. rather than the apparent. straight. shape of the object. Some perceptual adaptation theorists would say that subjects subjected to such experimental Requests for reprints should be sent to

Adaptation to visual distortion, simulated with a computer-generated display, was studied under conditions of (a) S falsely believing a real prism to be present and the nature of the distortion produced by the prism explained to him, and (b) S not given an optical rationale for the distortion but merely told about the nature of the distortion. The computer technique allowed measurements of visual adaptation, arm adaptation, and of motor learning in a manual guidance task. In (a) visual adaptation was different from zero but no difference between (a) and (b) was found. On one measure, arm adaptation was significantly different from zero in both conditions. On the same measure, arm adaptation in (b) was greater than that in (a). In (a) a positive correlation was found between motor learning and visual adaptation. For (a) and (b) combined there was a negative correlation between one measure of motor learning and one measure of arm adaptation.