PERCEPTION 



1647 



alternate more rapidly for the occurrence of apparent 

 motion (39, 101, 273, 423, 472). 



RECOMBINATION (DISARRANGEMENT) STUDIES. Since mo- 

 tion perception is readily altered by injury to the 

 neural substrate and by exposure to unpatterned or 

 atypically patterned fields, it will be expected that 

 disarrangement experiments should likewise produce 

 abnormalities. Indeed, rotation of the eyes (produc- 

 ing an inverted visual field) leads to incessant forced 

 movements in several lower species where such ex- 

 periments have been performed. Thus, the fly 

 Enstalis moves about normally inside a stationary 

 drum with alternating vertical black and white 

 stripes; as soon, however, as the head has been 

 rotated 180° and fixed in this abnormal position (as 

 is possible in this species), ever) movement of the 

 animal produces paroxysms of circling (348, 505 

 The same forced circling is produced in fish with 

 rotated eyes (438) when placed in an optically 

 structured environment. This forced spinning is 

 abolished by covering the eyes or by destruction of 

 the optic lobes in the midbrain. By contrast, neither 

 bilateral labyrinthectomy nor destruction (singly or 

 together) of the forebrain, cerebellum or hypo- 

 thalamus abolish this abnormal optokinetic reaction. 

 In these fish, as well as in amphibians 1 456, 437, 439), 

 such maladaptive response to surgical rotation of the 

 eyeball seems to persist indefinitely without correc- 

 tion by re-education (439). 



Inversion of the visual field in man (by means "I 

 inverting mirrors or prismatic spectacles) results in 

 subjective motions of the visual scene, often with 

 giddiness and nausea. In children (but rarely in 

 adults) there may lie forced movements and loss of 

 postural control. However, in contrast to what has 

 been observed in lower forms, i.e. in invertebrates 

 (348), in fish (438) and in amphibians (436, 439), the 

 inversion by spectacles in man is eventually tolerated 

 with minimal motor disturbance (1 16-1 18, 431, 455- 

 457). In the most elaborate experiments of this type 

 by Erismann and Kohler [see especially Kohler's 

 monograph (285)], there are reports of gradual re- 

 adaptation of perception (taking several weeks), with 

 elimination of illusory object motions and of righting 

 of the scene. These perceptual adaptations are said 

 to take consistently longer than the readjustment of 

 motor performance (285). Following removal of the 

 spectacles, all observers again experience illusory 

 motions of the scene, especially on moving their eyes 

 or head, and these disturbances last for several hours 

 or days. 



Interpretation of all these effects is needed before 

 one can formulate a theory of perception. The prob- 

 lems of readaptation aside, abnormal (illusory) 

 motion with inverted fields recalls the common 

 observation that passive movement of the eyeball (by 

 tapping against it) leads to an apparent movement 

 of the visual scene. By contrast, normal ("voluntary') 

 eye movements leave the visual field immobile. 28 In 

 cases of paralysis of extraocular muscles, an intended 

 eye movement (which fails to be executed because of 

 the palsy) is accompanied by a subjective shift of 

 the visual scene in the direction of the intended 

 ocular movement. These curious illusory motions are 

 also present after complete experimental immobiliza- 

 tion of the eyeball by infiltration of the extrinsic eve 

 muscles with procaine 1288), a procedure which 

 presumably paralyzes and anesthetizes the extraocular 

 muscles, thus eliminating both movement and pro- 

 prioception. 



One attempt at interpreting all of these phenomena 

 would be to invoke again a corollary discharge; any 

 efferent discharge, leading to eve movement, is ac- 

 companied bv concurrent central discharges into tin- 

 visual system which anticipate and counteract those 

 changes in visual stimulation that arc most likelv to 

 result from the ocular movement. Such a self-regu- 

 lating circuit would yield a stationary scene, as long 

 as eye movement and shifting scene are in corre- 

 spondence with one another 1 sec Ji >_; . ; 1 i. If the eye is 

 moved passively 1 without any concurrent central dis- 

 charge), there is no compensation and the relative 

 motion of objects on the retina is perceived as (il- 

 lusory) movement of the surround. If the eve is 

 paralyzed, efferent and concurrent discharge produce 

 a compensatory shift of the visual scene which is 

 again in error, since there is no ocular movement 

 ili.it needs to be compensated. Inversion of the eyes, 

 or of the scene (as l>v spectacles), would turn the 

 normal negative feed-back arrangement into a mal- 

 adaptive positive feedback. The central compensatory 

 discharge no longer counteracts but augments the 

 apparent shift of the visual scene which accompanies 

 every movement of eves or head. This positive feed- 



28 In the normal subject, nystagmus of sudden onset (e.g. 

 after barbiturate medication) leads to a corresponding sub- 

 jective to-and-fro movement of the visual environment (oscil- 

 lopsia). By contrast, patients with congenital nystagmus 

 perceive a stable visual environment. These patients react to 

 barbiturates, paradoxically, with a temporary cessation of 

 their nystagmus. When the nystagmus returns (usually within 

 30 to 45 min. after medication), they complain, transiently, of 

 oscillopsia and even of polyopia (29). 



