1636 



HVNDBOOK OF PHYSIOl < «."i 



M IROPHYSIOLOGY III 



environments, the adaptation is said to progress 

 more rapidly and to go further (77). [Curiously, the 

 distortions are rarely immediate on donning such 

 aniseikonic lenses; there is a noticeable latency of 

 seconds or minutes before the onset of the mcta- 

 morphopsiae (539).] 



•rf.afferent' stimulation as prerequisite for 



\i> aptation. Most of the studies reviewed thus far 

 indicate some partial reorganization of space per- 

 ception in man. Hardly any of the studies, however, 

 lend themselves to an experimental analysis of 

 stimulus conditions that might be necessary or 

 sufficient for such adaptation. The experiments by 

 Held on short-term exposure to "recombined' (atypi- 

 cal) stimulus patterns attempt to do just that. In- 

 fluenced by the earlier experiments of Wallach 

 (517) on auditory localization by a moving observer 

 (described above), and by the theories of von Hoist & 

 Mittclstaedt, (505), he proposed that one of the 

 crucial prerequisites for adaptation (e.g. to prisms) 

 is some orderly relation between sensation and the 

 sensory consequences of self-produced motion. 



In a typical experiment, prisms of equal power are 

 placed over the eyes, with bases either left or right. 

 This causes a lateral displacement of the visual 

 scene, accompanied by corresponding errors in 

 eye-hand coordination and in egocentric localization 

 (visual direction finding). The errors in eye-hand 

 coordination can be measured in a device (193) 

 which permits the subject to see his own hand as 

 well as the target pattern which he has to mark 

 manually while wearing the prisms; the device, 

 however, prevents him from seeing the marks he 

 makes, si) that he lacks any information about his 

 success or failure. In spite of this, the errors diminish 

 steadily through marking sessions lasting less than 

 .111 hour. The errors are likewise diminished if the 

 subject merely views his hand while moving it actively 

 to and fro. Conversely, viewing his hand while it is 

 being moved passively by .mother person is in- 

 effective in producing adaptation to the altered 



sensuiv input (194). Similarly, subjects who walk 

 about actively in a patterned environment while 

 wearing the prisms show significant reduction of 

 mislocalization (errors ol egocentric localization 1 

 at the end ol .1 i-hr. period. It, instead, the subjects 



are pushed about the same path in a wheelchair, no 

 significant adaptation takes place (Held & Bossom, 

 unpublished observations). Finally, il the static 

 prisms are replaced bv rotating prisms whose power 

 is changed continuously, adaptation is made im- 



possible (Cohen & Held, unpublished observa- 

 tions). 



The interpretation of these experiments is facili- 

 tated by introducing a distinction made by von 

 Hoist (504) between afferent stimulation (produced 

 by the environment) and rcafferent stimulation 

 (produced by the active motion of the perceiver). 

 Stability and orderliness of perceived space would 

 then depend on the existence of certain invariant 

 relations between afferent and reafferent activities. 

 Normally, head tilts are accompanied by converse 

 tilts of the visual vertical. If one assumes a corollary 

 discharge from motor to sensory systems ac- 

 companying the active tilt, then the sensory (e.g. 

 visual) apparatus is prepared to counteract the 

 ensuing optical tilt. Such a negative feed-back 

 mechanism, once established, turns into a mala- 

 daptive positive feedback on wearing spectacles 

 which reverse right and left. Yet the facts of (at least 

 partial) readaptation suggest that the nervous system 

 is capable of extracting new invariants as the atypical 

 stimulation continues. Removing the spectacles will 

 then, of course, lead again to maladaptive reactions 

 the previous adaptation turns into an illusion, 

 although the relatively briefer course of these after- 

 effects indicates either the preponderant weight of 

 the much longer 'normal' exposures, prior to the 

 experiment, or, perhaps, an innate bias of the system 

 ill favor of certain invariants. Experimental evidence 

 is much too fragmentary thus far to permit one to 

 assign relative weights to the role of exposure history 

 on the one hand and of prior structural determinants 

 on the other. 



DOUBL1 LOCALIZATION : Vl( IM 11 I I VR DIPLOPIA VXD 



DIPLOPHONIC EFFECTS. There may be important 

 limits in the plasticity of perception as here described. 

 The supposed adaptation of spatial perception to 

 unilateral squint is a ease in point. Not only is there 

 a curious reduction in pattern vision of the squinting 

 eye (see above, pp. [622 1623), bul there frequently 

 is monocular diplopia in that eve, this curious 

 disturbance of spatial organization becomes particu- 

 larly manifest after the uood eve is lost (as occasionally 

 happens) and only the squinting eve remains. Con- 

 sistent double vision with such a single eve has heen 

 described in several instances, the best studied case 

 is that of Bielschowsky (44) where the diplopia was 

 found to persist until the patient died. Hi years alter 

 the loss of his better eve. As noted above, a similar 

 monocular diplopia has been described In Kohlcr 



as a sequel ol protracted prism experiments (28b), 



