396 



ll\M)BOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



Perception of 'Real' Motion 



Characteristics in Normal Subjects 



Minimal rates 



Differential thresholds 



Phenomenal stages 



Role of size and surround 



Velocity transposition 



'Paradoxes' of seen motion 

 \ I 'normalities of Perception of Motion 



Altered motion perception after cerebral lesions 



Isolation studies 



Recombination (disarrangement) studies 



Constancies, Illusions and Figural Aftereffects 



Constancies : Examples and Measurement 



Interpretations 



((instancy in animals and children 



The need for parametric studies 



Recent work on constancy of color and brightness 



Role of instruction and experimental setting 



Effects of 'reduction' 

 Loss of Constancies after Cerebral Lesions 

 Deprivation and Recombination Studies 

 Illusions: Phenomena and Interpretations 



Illusions as misapplied constancy effects 



Perceptual habituation; decrement of the Miillcr-Lyer 

 illusion on repeated trials 



Intermodal transfer of Miiller-Lyer decrement; 'haptic' 

 illusions 



Figural aftereffects 

 Conclusion 



it may seem strange to find a chapter on perception 

 in a handbook of neurophysiology, doubly strange if 

 the chapter begins with the claim that there is no 

 adequate definition of perception and ends with the 

 admission thai we lack a neurophysiology theory. 

 The two difficulties have a common source: physiolo- 

 gists distinguish sensation from perception and deal 

 with sensor) processes, in different modalities, as 

 dependent upon receptor mechanisms and subsequent 

 neural events. As a result, perception assumes (he 

 role ui sume supplementary higher process, super- 

 imposed upon these sensory capacities and devoid of 

 an) obvious neural correlate. After excluding per- 

 ceptual phenomena from sensory physiology, we are 

 thus hard pul to explain how different sense modalities 



inn i. M i in perceiving, how we apprehend shapes or 

 sizes, distance or depth, or, more broadly put, how 

 things manage to look, or feel or sound the way they 

 do (284). 



In the absence <>l an accepted definitionand theory 

 ui perception, any selection from iis vasl literature 

 will be arbitrary. Yci we shall try to stress those 

 phei una which reveal (lie inadequacy i>l tradi- 

 tional distinctions between sensation and perception, 



and those problems which seem most in need of 

 physiologic interpretation. We shall deal, first, with 

 subjective intensity and psychophysics in order to 

 show that, even here, where simple one-to-one rela- 

 tions between stimulus and sensation are assumed to 

 exist, there actually arise complex issues that require 

 a new approach to our search for physiologic corre- 

 lates. The same issues will be raised with regard to 

 the more traditional topics for students of perception : 

 the problems of perceived shape, depth and motion, 

 the constancies and the illusions. 



None of these topics can lie covered completely, 

 but we shall attempt to concentrate throughout on 

 those perceptual phenomena that can be demon- 

 strated not only in man, but in lower forms. Corre- 

 spondingly, we shall try to stress methods for studying 

 perception that might be applicable to a wide variety of 

 vertebrate and even invertebrate species. This is done 

 in spite of the risk of seeming capricious in our choice 

 of experimental evidence. The comparative approach 

 permits one to utilize naturally occurring differences 

 of neural structure in formulating guesses about tin' 

 way in which neural structures might determine 

 perceptual performance. Moreover, the problems of 

 pattern vision in monkey, octopus, or bee will seem 

 less remote if one realizes that (he nervous systems ol 

 infrahuman forms are available for experimental 

 intervention not permissible in man and that the 

 greater simplicity, especially of invertebrate neural 

 organization, may let us discover correlations between 

 structure and function (hat might go undetected in 

 the study of human subjects. 



In contrast to the usual picture drawn by surveys 

 ul comparative sensory physiology, most ol the phe- 

 nomena we shall note cannot be interpreted in terms 

 of known physiologic mechanisms. These unexplained 

 effects are presented, nevertheless, because they m.iv 

 suggest directions in which the search for neural bases 

 ul perception should proceed. 



Our effort .it reviewing these phenomena might be 



justified il we remember thai the facts observed in 

 any field are largely a function of the questions that 



are being asked. Understanding of neural activity 

 "is likelv to progress more rapidly il we keep before 



us the facts ol behavior which we hope eventually to 

 explain" (308). If perceptual phenomena continue 

 to be refractory io neurophy siology . one mighl wonder 



whether current approaches (o 'basic' sensory proc- 

 esses are correctlv conceived. 



The difficulty may be analogous to the one raised 



by phenomena of complex coordinated movements 

 (see Chapter LXVI1 by Paillard in this volume). As 



