842 Comparative Animal Physiology 



ated with copulation is an example. Another example is facilitation and ex- 

 tinction in the cerebral cortex.^ "-•^' ^-^ 



Brief repetitive stimulation of a local area of the cortex may reduce the 

 electrical response to another brief stimulus applied seconds later at the 

 same point. In a monkey the extinction is maximal in the unanesthetized 

 state after 4 seconds, under moderate anesthesia after 13-15 seconds, and un- 

 der deep anesthesia after 1-3 minutes. The first stimuli may even be sub- 

 liminal for a cortical response, and the extinction effect depends on deep 

 layers of the cortex. A faster effect, facilitation, is an increase in cortical re- 

 sponse if preceded a few seconds earlier by cutaneous or adjacent cortical 

 stimulation. In sympathetic ganglia a single preganglionic volley may cause 

 facilitation for a few seconds, but after a series of volleys facilitation lasts 

 longer than five minutes.-^*^ The mechanism of these persistent effects must 

 be very different from synaptic excitability changes described above. 



Classification of Conditioned Responses, The literature of comparative 

 psychology is extensive and comprises observations of social behavior of ani- 

 mals and of habits of hunting, feeding, reproduction, and care of young, 

 and some experimental studies. An extensive bibliography of conditioning 

 (1111 references) is given by Razran,'^^^ and summaries of the literature 

 may be found in books by numerous authors, Maier and Schneirla,-'^" and 

 Warner,"*-^ among others. Modern theory of conditioning is well presented 

 by Hilgard and Marquis. ^''^ We are here concerned with adaptive behavior 

 only when it gives evidence concerning neural mechanisms. 



In classical conditioning one stimulus, the conditioned stimulus (CS), is 

 usually followed by another stimulus, the unconditioned stimulus (UnCS), 

 which alone elicits a given response (RUnCS); after sufficient presentations 

 of both stimuli in proper sequence, the first (CS) alone elicits the response 

 (RCS). If CS is then applied repeatedly, without reinforcement by UnCS, 

 the response to CS may become extinguished; recovery may, however, occur 

 spontaneously or as the result of some strong generalized stimulation, disin- 

 hibition. Many levels of conditioning exist, and methods for distinguishing 

 among different kinds of neural modification are needed before any func- 

 tional classification of types of conditioning can be made. A comparison of 

 the modifiability of different kinds of nerve centers is needed. The following 

 "levels of conditioning" may be recognized. 



1. Neural modification may result from repeated stimulation over one 

 sensory pathway, such that there occur changes in the quantitative relation- 

 ship between the intensity of a stimulus (S) and the size of response (R). 

 The effectiveness of the stimulus may be either increased or decreased so 

 that, for example, VzS-^R, or 2S-^R. Central modification must be distin- 

 guished from sensory adaptation by making certain that the repeated stimula- 

 tion does not alter the sense organ response. An example of decreased central 

 excitability is extinction or "fatigue" of a behavior response as the sense organ 

 is repeatedly stimulated. Also, animals may respond differently to different in- 

 tensities of stimulus, as of light, and the neural modification on repetition 

 may consist of raising the range in which the response to the low intensity 

 is elicited. Possiblv such modification occurs in sensorv centers. 



2. 1 he effect of one sensory pathway may be altered by messages over a 

 second sensory pathway, even though the two are not presented in a spe- 



