PERCEPTION 



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Sector of uncertainty 



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fig. 28. The diplophonic effect, a: Before exposure; observer (O) wears pseudophones with axis 

 normal; P, primary direction of 'straight ahead' sound source, .V.V,, sound source on left; SSg, 

 sound source on right h: Exposure condition: observer wears pseudophones with axis rotated by 

 22° so that the left ear 'leads.' Sounds emanating from .V, the original land anatomical 1 'straight 

 ahead' position, seem at first displaced by about 22° to the observer's left, his auditory straight 

 ahead' is at P. Walking forward or backward induces illusory motions of the sound source, c: Alter 

 exposure: observer has worn pseudophones as described under b for 7 hr. in his usual environment. 

 The axis of the pseudophone has now been reset to normal, and the observer's sound localizations 

 are tested in an anechoic room where he tries to locate concealed sound sources (SS L , SS R ) by 

 turning his head and body towards their supposed location. Although only one source is emitting 

 sounds at any one trial, the observer reports that he hears two sound sources simultaneously, or 

 that the source could be localized in two different directions A source located at P is perceived 

 simultaneously at P and at some point S, to the left of /' Sounds emanating from directions inter- 

 mediate between Sand /'all seem to be straight ahead from Meld 1 [02 



and there are analogous observations in the short- 

 term prism experiments reported by Held and his 

 co-workers. 



A direct analogy in binaural localization is the 

 diplophonic effect — the subjective doubling of an 

 objectively single sound source — in Field's studies 

 involving pseudophones (192). Figure 28 illustrates 

 one of these experiments in which the subjects wore 

 a pseudophone arrangement continuously tot several 

 hours during their regular working day. The device, 

 in effect, rotated their interaural axis by 22, thus 

 giving one ear an abnormal 'lead' over the other. 

 In subsequent experiments, it could be shown that 

 the gradual reduction in mislocalization of sounds 

 could be accelerated or retarded (and even reversed ) 

 by controlling the conditions of exposure while the 

 pseudophones were in place. Thus, making the 

 subjects walk along a prescribed path in an anechoic 

 room, while listening to concealed loudspeakers, 

 diminished the mislocalization if the subjects were 

 made to walk straight toward or away from the 

 hidden sound source. Under these conditions, the 

 difference in arrival time of the sound at their ears 

 remained invariant during self-induced motion. 

 Various other paths led to less correction or even to 

 deterioration of localizing performance. 



If one looks back upon these numerous studies on 



normal and abnormal space perception, we begin to 

 see some possible phylogenetic differences which we 

 failed to demonstrate convincingly in our review on 



perception of shape. The modifiability of spatial 

 organization in higher form- (even though in need of 

 further empirical delineation) is in sharp contrast 

 to the rigidity oi spatial organization in lower forms. 

 In time it should be possible to find those aspects of 

 perception thai are specific to man and thus correlated 

 with the fact that he alone among; animals has 

 developed language. 



It is also clear that all of the data on space per- 

 ception and its changes alter (rubral defects, or its 

 readjustments to radically altered inputs, suggest 

 that spatial projection in the nervous system — the 

 rctinotopical projection and its counterparts in the 

 somatosensory and auditory spheres — may represent 

 necessary but hardh sufficient neural conditions for 

 the tridimensional organization of perception. As 

 the organism changes its position relative to the en- 

 vironment, space has to be continuously reorganized 

 as the result of interaction between postural systems 

 and contact and distance receptor fields. How to 

 conceive of this physiologic achievement, we can 

 hardly guess. Anticipatory and facilitory discharges 

 from motor into sensory systems have been postulated 

 as one indispensable correlate by Lashley (304, 307), 



