Left-right and adaptation to reversal Little is known about the discrimination of right from left. Children can discriminate right from left on their own person by six years of age but they cannot discriminate right from left on other people until later (Piaget 1926 : Swanson & Benton 1995). Piaget considers this difference to be a development away from egocentric thought Howard and Templeton (1996) point out however that one always has the same relation to one own body,but not to other peoples bodies so that constancy of the former makes the discrimination a much simpler task.
Unlike devices that tilt the visual field which will normally offer good visual indication to the subject that the field has been tilted (p. 539) an optical device that reverses right for left will present little indication of that reversal to a passive subject. An active subject of course obtains visual feedback that reveals the nature of the optical rearrangement. As we have seen (p.533) Kohler reports that active subjects undergo a curious piecemeal adaptation to reversing prisms. Let us now consider Harris explanation of how this piecemeal adaption would be accounted for if is proprioception and not vision that is changing. Imagine a subject looking at the blackboard in Figure 13.37A . When he first puts on reversing prisms he feels his right hand to be near the side of the blackboard that he sees on the right-hand side on the visual field namely the side with the reversed L on it (Figure 13.37B). If he now looks at his hand adaption begins. When he moves his hand to the right he sees it moving to the left and he soon begins to feel it to be moving to the left. In fact this reversal of his proprioceptive perceptions may even make him write letters backwards while blindfolded as Harris and Harris found (sum-marized by Rock & Harris 1967).Similarly because his right hand looks nearer to the reversed R on the blackboard than to the L that hand feels nearer to the reversed R than to the L that is felt location of the limb comes to match its seen location as in the experiments on displacement. When the subject is asked at this stage which end of the blackboard appears to be on his right he should answer that the side with the reversed R does if he ask himself which side is nearest his right hand (which has changed its felt location). During this stage of adaption the reversed writing on the blackboard is illegible so that we get what looks to the experimenter like piecemeal visual adaptation. In time the subject also learns to read writing that is reversed as he would do even without mirrors or prisms if he were continually confronted by mirror writing. It is easy to see therefore why Kohler might conclude that mirrorwise seeing “ had been established as a result of the adaption even if the change were primarily proprioceptive and not visual.
The explanation based on proprioceptive change must still be considered a tentative one . For one thing we have seen that there are displacement – adaptation effects that do not seem to fit it (see Efstathiou et al. p 538). For another adaptation to curvature distortion of the retinal image presents a critical problem for a proprioceptive – change hypothesis.
Adaptation to curvature and distortion It is relatively easy to say of a change in the relationship between vision and proprioception that proprioception has changed. If the adaptation consists of a rearrangement of relationships within the visual field however it would be more difficult to escape the conclusion that it is vision that has changed