SENSORY DISCRIMINATION 



1465 



Relation of the Primary Sensory Systems to Other Systems 



In considering the neural mechanisms of sensory 

 discrimination, our first emphasis has been on the 

 structure and function of the main afferent systems 

 leading from the peripheral sense organs to higher 

 centers of the brain. Without knowledge of the sensory 

 input, we cannot begin to talk about the neurophysiol- 

 ogy of sensory discrimination. On the other hand, 

 even with more exact and detailed knowledge of the 

 main afferent systems than we now possess, we would 

 still be far from an understanding of sensory discrimi- 

 nation. We must also consider the control of sensory 

 input, the effects of activity of nonsensory systems on 

 this input, and the manner in which the sensory input 

 leads to motor response. 



The living organism at any given moment i^ 

 subjected to a variety of external stimuli which may 

 act upon sense organs and arouse activity in sensory 

 pathways. For the higher animals, behavior does not 

 appear to be controlled simply by the total pattern 

 of stimuli acting upon the sense organs. Careful 

 observation of behavior leads to the inference that 

 there is some control exercised over the sensory 

 systems so that a systematic, not random, switching 

 occurs, thus allowing the input of one sensor) channel 

 to guide the organism's responses at one point in time 

 and another sensory channel to take over when 

 appropriate. Or, perhaps, two or more channels maj 

 be open simultaneously or in such rapid alternation 

 that the organism's actions are seemingly guided by .1 

 combined input. Psychologically, we talk about this 

 control of sensory input under the heading of atten- 

 tion. Introspective psychologists found it necessary to 

 add clearness or attensity as an attribute of sensation 

 along with such other attributes as intensity, qualiiv, 

 extensity and duration. Physiologists, psychologists 

 and neurologists studying brain function cither in the 

 experimental laboratory or in the clinic have also felt 

 impelled, or at least have found it convenient, to use 

 the concept of attention in describing the discrimina- 

 tory behavior of both normal subjects and of subjects 

 in which damage has been done to brain centers. 

 (Attention is considered in Chapter LXIY by Lindslcy 

 in this Handbook.) 



Any investigator who has tested the sensory dis- 

 criminations of animals in a multiple choice situation 

 is familiar with a pattern of behavior which often 

 occurs when the discrimination is made very difficult. 

 The experimental animal which has been making 

 appropriate responses in less difficult discriminations 

 suddenly appears to ignore the stimulus cues and 



adopts a position habit such as always selecting the 

 stimulus on the right. This behavior can be described 

 most readily by saying that the animal acts as if it 

 does not 'attend' to the stimulus cues which the experi- 

 menter is presenting. This and other kinds of behavior 

 which can, likewise, be conveniently described as 

 deficits in attention are seen in animals with brain 

 lesions in test situations in which the normal animal 

 makes appropriate responses. 



It is an old and well-substantiated finding that 

 animals with frontal lobe ablation are deficient in 

 ability to perform on a delayed response test (97, 98). 

 (See Chapter LIY by Pribram in this Handbook.) 

 Evidence from studies in which improvement in 

 performance has been shown to occur if animals are 

 tested in the dark or arc given mild sedation suggest 

 that the animal with frontal lobe lesions is more 

 distractablc; he is less able to maintain attention to the 

 relevant cues in the test situation (136, 212). 



Cats with lesions of the auditory cortex show a loss 

 in ability to perform well in a test which requires 

 localizing a sound in space and approaching the 

 source of the sound in order to obtain a food reward 

 (157). It has been suggested (hit the poor perform- 

 ance of these cats as compared with that of normal 

 animals ma) be due in part to an inability to 'main- 

 1.1111 attention' to the sound cues. 



Not only in the reports of animal experiments but 

 also in the clinical literature one finds man) studies in 

 which loss in some aspect of the ability to attend to 

 sensory cues is described in patients who have suffered 

 brain damage. Only a lew examples will be cited 

 here. 



In their studies of the somesthetic sensibilities of 

 patients with parietal lobe damage, Head & Holmes 

 (90) describe a number of phenomena which they 

 attribute to deficits in ability to attend to tactual 

 stimulation. When a series of von Frey hairs is used 

 to stimulate the skin in .1 region of the body affected 

 by damage to the parietal lobe of the opposite side, 

 the patient's responses are often quite erratic. He may 

 respond frequently to stimuli as weak as those per- 

 ceived on the normal side of the body but still fail to 

 respond consistently to much stronger stimuli. Head 

 offered the very interesting suggestion that such 

 phenomena may be explained as defects of local 

 attention. He thought of local attention, attention as 

 related to a given sensory modality, as being a function 

 of the cortical projection area of that modality. He 

 recognized that there might also be a more general 

 faculty of attention which would be altered by a 

 variety of conditions affecting the brain. 



