PERCEPTION 



!°45 



and Woodworth (548), felt it made it unnecessary to 

 postulate a 'central anesthesia' of vision during such 

 movements (219). More recent disclosures of "sup- 

 pression' of incoming sensory information by means of 

 centrifugal control of sense organs [see Granit (168)] 

 suggest that the issue might deserve to be reopened. 

 There are other curious effects. A target moving 

 through a frame at certain speeds appears to 'jump' 

 into view, i.e. it is not seen at first as it enters the 

 opening, but 'further along,' at some distance from 

 the anterior edge of the frame. The phenomenon has 

 been studied by Frohlich (134) in the hopes of ob- 

 taining a measure of minimum time needed for any 

 perceptual act, but the stimulus situation is probably 

 too complex to provide a basis for estimating an 

 observer's Empfindungsrjit (sensation-time), as Froh- 

 lich (134) had hoped. Actually, there is not only the 

 delay (or rather displacement) in the appearance of a 

 target that traverses a diaphragm; an analogous effect 

 is noted when the target disappears — it is last seen at 

 some distance from the edge of the opening, rather 

 than right at it. More promising for intensive study 

 is the Hazclhoff effect (185). This effect is obtained 

 on looking at a moving target (e.g. a square traveling 

 across a screen); on sudden and momentary exposure 

 of a dot (projected by a tachistoscope onto the square), 

 the observer perceives the dot at some distance ahead 

 of (and outside of) the traveling square.- 7 



Abnormalities »/ Perception of Mot inn 



The phenomena of 'real' and 'apparent' motion 

 which we have reviewed demand a physiologic inter- 

 pretation, but such an interpretation is lacking Some 

 clues for physiology may be gained by surveying next 

 those alterations of motion perception that result 

 from defects in the neural substrate, from sensor) 

 isolation or from recombination (involving rearrange- 

 ment or disarrangement of relations between receptors 

 and central nervous system). 



ALTERED MOTION PERCEPTION AFTER CEREBRAL 



lesions. We can begin with evidence from defect 

 experiments. It is commonly thought that cerebral 

 lesions implicating central visual pathways tend to 

 produce greater impairment for pattern than for 

 motion perception (399). The evidence for such a 



27 There may be a curious auditory analogue of this effect 

 (Broadbrnt, personal communication to Dr. Richard Warren). 

 If a click is interposed somewhere within a tape recording of a 

 sentence, listeners err considerably in saying at what point 

 within the sentence the click has occurred. 



statement, however, is unconvincing. It is not unex- 

 pected to find areas in defective visual fields where 

 targets are perceived when they are in motion but 

 not when they are stationary. The movement takes 

 the target over a wider angular extent in the field and 

 thus produces more stimulation. Moreover, the move- 

 ment prevents the abnormally rapid fading that is 

 found in some (though not all) defective visual fields 

 (25, 26). Actual measurements of thresholds for ap- 

 parent (and real) motion in such impaired regions of 

 the visual field [e.g. after penetrating lesions of 

 man's geniculostriate system (473)] demonstrate that 

 motion perception is impaired pari passu with defects 

 in the forming of contours; a target has to move 

 through wider angles or with greater acceleration for 

 its motion to he perceived. Thresholds for apparent 

 motion are correspondingly raised in the same regions 

 of the visual field, i.e. the rate of alternation of two 

 (stationary) targets has to be higher before strobo- 

 scope effects are reported (473). Again, in the same 

 regions there is a characteristic decrease in the critical 

 rate at which .1 single intermittent light appears to be 

 fused (i.e. the critical flicker frequency is reduced). 

 These concomitant changes result in a loss of strobo- 

 scopie effects for particular stimulus conditions; the 

 patient with lesions of the central visual pathways 

 ma) perceive the succession of (slowly alternating) 

 stimuli, but their alternation seems to him abnormally 

 rapid. A slight increase in their rate of alternation 

 leads immediatel) into the stage of simultaneous 

 flicker, and a further increase leads to fusion of each 

 target without any intervening stages of apparent 

 motion. 



These abnormalities of motion perception are re- 

 markable because of the regular association of altera- 

 tions in the perception of apparent motion, of flicker 

 fusion and of real motion, suggesting that all of these 

 phenomena share a common physiologic substrate. 

 Furthermore, these threshold changes are accom- 

 panied by qualitative changes in the appearance of 

 true motion. In impaired portions of defective visual 

 fields, the perception of a continuous motion is fre- 

 quently dissected into a scries of multiple stationary 

 images, quite analogous to the phenomena obtained 

 for normal observers in Brown's experiments (68) 

 when the target speed exceeded certain values. (Thus, 

 one patient with a gunshot wound of the right oc- 

 cipitotemporal region complained that when a 

 motorcycle passed him to his left, he saw instead "a 

 string of motorcycles standing still.") There may be 

 a uniform basis for both the normal and abnormal 

 forms of polyopia (473). Analogous changes can be 



