THE NEURAL BASIS OF LEARNING 



[481 



in an average of 11.1 trials in 8 monkeys with im- 

 planted cortical electrodes. When instead of sound 

 a second visual stimulus or mild shocks to the skin 

 were used as CS, averages of 13.2 and 9.5 trials, 

 respectively, were required. A remarkable increase in 

 trials is required when epileptogenic lesions (pro- 

 duced by alumina cream) exist in the cortical projec- 

 tion area specific for the CS or in the amygdala (166). 



In studies on man, Motokawa (168) paired sound 

 (CS) with light (US), recording, in addition to the 

 multiple scalp EEG, the galvanic skin response (GSR). 

 Both the alpha block and the GSR became conditioned 

 to the sound alone. This finding, besides portraying 

 the EEG details of a typical 'sensory-sensory' proce- 

 dure, shows that the alpha block CR may be highly 

 correlated with an autonomic CR. These observations 

 have been confirmed (103), and similar results were 

 found in a Type I situation where salivation instead 

 of GSR was conditioned to a sound (102). 



The alpha block CR may be preceded by a period 

 of enhanced alpha amplitude if the interval between 

 application of CS and US is sufficiently prolonged 

 (101). On the other hand, Motokawa & Iluzimori 

 ( 1 69) have shown that more than one alpha block 

 CR may occur during the conditioning procedure. 

 They used a bell (CS), and a shock to the fool (US) in 

 man, recording both the GSR and EEG. If the bell 

 was sounded for some 10 sec. before application of 

 the US, alpha block occurred both at the onset of 

 the CS and again just prior to its termination. A 

 GSR response coincided in time with each alpha 

 block when conditioning was well established. Both 

 periods of alpha block seem to meet the criteria 

 for CR's (102, p. 352). 



Kogan (118), finally, has used microelectrodes 

 implanted in cats to study the conical e\ cuts occurring 

 during the conditioning of a motor response to an 

 acoustic stimulus. Alpha block seems to be .1 constant 

 feature of the process, occurring first in the auditory 

 area and spreading later to the motor region. In 

 addition, beta waxes and 'special forms of electrical 

 activity' (not further defined in the brief English 

 report seen) appear. In the early stages of condition- 

 ins;, changes are prominent in cortical layers III 

 and IV, but later a shift to 'the deeper layers' is 

 noted. According to this report, very large oscilla- 

 tions of potential appear in the auditory cortex when 

 a differential CR is set up with auditory stimuli. The 

 use of microelectrodes is just beginning in non- 

 Russian hands, but it has already revealed some 

 valuable information (29, 202). 



The alpha-block CR obeys, more or less, the general 



rules established for conventional CR's. It appears 

 only after a number of pairings of CS and US, and 

 some stimuli are more effective than others (165). 

 Furthermore, when only one of two different sounds 

 is reinforced, the reinforced sound alone evokes the 

 CR, an example of differential conditioning (165, 

 207). Finally, if the CS is not reinforced, the CR 

 disappears; this is the phenomenon of extinction. 



Some attention has been given to the cortical regions 

 blocked in the response. Prior to its habituation, the 

 CS preferentially blocks the parietal region (165) 

 or the brain in general (70), but after establishment 

 of the CR it specifically blocks the occipital region 

 (165) or the cortical region to which the sensory 

 system of the US projects (70, p. 16). If the CS is 

 applied to one side of the body, the conditioned 

 EEG response may even be limited to the contralateral 

 cortex (70). 



The reliability with which alpha block appears 

 in conditioning experiments leaves much to be desired. 

 Certainly it docs not Invariably occur and in one 

 length) and important study, for example, 60 per 

 cent success is reported in a large group of human 

 subjects (70). Consequentlv the alpha block CR may 

 be of limited usefulness in understanding the basic 

 neural events in conditioning. The phenomenon is 

 related to action of the reticular formation discussed 

 in this Handbook in Chapter LI I by French and 

 elsewhere 1 ;, Ik), j .18 250 1 ; it appears when the 

 animal is merely alerted as well as during the learning 

 process. Nevertheless, .is we have seen, the alpha 

 block CR satisfies many of the criteria for a cortical 

 event that accompanies learning. 



Cortical Evoked Potentials 



In addition to its regular rhythmical activity, the 

 cortex generates brief electrical waves corresponding 

 to the arrival within it of impulses from the various 

 sense organs These so-called evoked responses follow- 

 sound, light, touch and similar stimuli. Their latency, 

 magnitude and duration have been examined during 

 the conditioning process. 



In an important variation of the sensory-sensory 

 conditioning procedure, for example, a flashing light 

 is substituted for the continuous one. Not only does 

 the regular cortical rhythm disappear, but evoked 

 cortical waves harmonically related to the frequency 

 of the flashing light are recorded ('photic driving'). 

 If a sound (CS) is now made to precede the flashing 

 light (US), the sound alone eventually produces an 

 EEG response remarkably similar to the characteristic 



