i6_>8 



HANDBOOK OK PHYSIOLOGY 



NEUROPHYSIOLOGY III 



forged bank note, combined under a stereoscope, 

 will make the deviant features stand out by producing 

 an impression of relief in the combined view. There 

 are similar applications of stereoscopy for discovering 

 camouflaged features in aerial photographs taken at 

 slightly different angles. 



THE LOCUS OF BINOCULAR FUSION. The question 01 



how .irul where the activities of the two retinae 

 interact in the nervous system is only a special form 

 of the problem of patterning. Perceptions mediated 

 by different portions of a single eye, or a tactile 

 surface, raise actually the same unresolved issues. 

 Yei physiologists have long regarded binocular fusion 

 as a more accessible problem, even though the an- 

 swers differed. For Kepler in 1611 (255), "images' of 

 the two eyes were "projected" by the mind to the 

 single object on which the eyes converged. For Porta, 

 in 1 3<» 1 1 '}»7) there was no problem because the two 

 eyes alternated, he believed, in continual rivalry. 

 Newton in 1704 (361) rejected both views and in- 

 voked instead the decussation of the optic nerves and 

 their ultimate convergence at higher levels in the 

 nervous system as the structural basis of binocular 

 fusion. I here the problem remains. 



Anatomically, the optic tract fibers in mammals 

 (carrying impulses from ipsilateral and contralateral 

 eyes, respectively) terminate at separate lasers of 

 the lateral geniculate body. This segregation may 

 continue at the striate cortex. For man and monkey, 

 it has been proposed by Klcist (257) that the ipsi- 

 lateral elements of the optic radiation end in lamina 

 [Va of the striate cortex, and the (more numerous) 

 contralateral elements in lamina IVc. The inter- 

 vening lamina [Vb the stripe of Gennari has 

 been claimed to represent a binocular mixing zone 

 (257). [The arrangement was considered by Baranv 

 in 1924 hi but rejected in favor of a postulated 

 ending ol ipsilateral and contralateral elements at 

 differem depths of IVc, with the stripe of Gennari 

 again acting as a mixing zone.] These attributions 

 11111,1m i- ( 0niect11r.1l as Wilbrand's theory (j.lt) 



which postulated that ipsilateral and contralateral 

 elements were placed .it the same depths within tin' 

 visual Cortex in the manner of black and while 

 Squares on .1 checkerboard Minoeleetiode studies 

 ol cortical neurons apparently reveal a considerable 



nbei oi units responding to both eves (224); 



recording from differem depths may show whether 

 thru- is also some laminar segregation of monocular 

 elements or not Electrotonic interaction between 



I. num. 1 has been proved lor the lateral geniculate bv 



Bishop & Davis (45) so that anatomic separation of 

 layers would not preclude the interaction of ipsi- 

 lateral and contralateral lamina upon simultaneous 

 or closely successive stimulation. Binocular flicker 

 fusion differs only slightly from monocular (235, 

 426, 481). Sherrington (426), discounting the slight 

 differences [that have now been shown to be real 

 by Ireland (235) and by Thomas (481)], concluded 

 that binocular fusion was a "psychic" act super- 

 imposed on the physiologically separate monocular 

 mechanisms. Further physiologic study of central 

 interaction between the representations of the two 

 eyes may offer alternatives to Sherrington's con- 

 clusion. 



Auditory Space Perception 



Analysis of central interaction between paired 

 sense organs has been pushed further for the ears 

 than for the eves [see Chapter XIV in this Hand- 

 book, Vol. 1, p. 610]. In cats, dogs and monkeys, the 

 two ears are represented at each medial geniculate 

 body (and at the auditory cortex 1 by partly over- 

 lapping; populations of neurons. Stimulating both 

 ears simultaneously, or in rapid succession, produces 

 definite signs of interaction; the corresponding 

 electrical responses are not a simple summation of 

 those to separate stimulation of the two ears (405 

 The elegant analysis by Rosenzweig 1 to - ,) of evoked 

 responses in cat cortex suggests cortical (and sub- 

 cortical) correlates for various stereophonic effects 



BINAURAL parallax. In man, binaural localization 

 of sounds has long been known to utilize differences 

 in the relative intensity (396) and arrival lime (2) of 

 stimuli at the two ears. If two sounds (e.g. clicks) 

 delivered by earphones are balanced for loudness and 

 arrive simultaneously at the two ears, the listener 

 hears a single ('fused') sound in or near the mid-line 

 (inside or just above his head). As arrival times 01 

 intensitv relations are altered, the (subjectively 

 single sound shifts toward the leading ear, i.e. the 

 side of earlier arrival or greater intensitv ( |8o, ","- 

 If one varies intensity and time relations in directions 

 Opposite to each other, v erv large intensitv differences 



lover 10 dbi are needed to counterbalance differences 



in time ol less than loo M see. For loudness-l lalanced 

 clicks, time differences of less than ;o /jsec. can be 



effective in producing the impression ol .111 oil-center 



sound thresholds foi duality (two clicks heard, one 



in each earl are rarelv reached with intervals below 



