142 BRAIN MECHANISMS AND LEARNING 



CR was evoked from this pair with this pohirity three times in six stimula- 

 tions, never using the US. Stimulation in the right posterior parietal lobe 

 elicited eye and head movements which were almost the exact counterpart 

 of those elicited by the CS save they were to the left. No CRs were elicited 

 by this stimulation in nine attempts during two sessions. 



With four-minute intervals and the same US there was no CR to an 

 auditory CS despite no pairings in nine sessions. After this, giving the 

 auditory CS simultaneously with the CS to the occipital pole often pro- 

 duced 'external inhibition' of the CR to the latter stimulus. Seventeen 

 tests with the auditory CS alone (plus the US) elicited no CRs even though 

 the tests were given randomly throughout twenty-four presentations in 

 which simultaneous auditory and cortical CS (plus US) was used. 



The effect of coupling various stimuli with lever-pressing was then 

 studied. On July nth the animal pressed the lever forty-one times in a 

 fivc-minute period and with each press received a US of 0.4 niA. (double 

 the intensity used routinely) which produced violent movements, and 

 often convulsive after-discharges, with each press. There was no hesitation 

 whatever in lever-pressing behaviour beyond that attributable to the 

 physical handicap consequent to the induced movement (e.g. Fig. 5). 

 Coupling with cortical CS was similarly without effect. On the following 

 day, as seen in Fig. 5, a novel clicking stimulus completely disrupted the 

 behaviour when coupled with each lever-press. 



The animal was then taught in sessions of twenty trials per day to press 

 a lever to avoid a shock to its tail. The first response to a tonal CS occurred 

 on the thirty-third trial and a criterion of twelve avoidances in twenty CS 

 presentations was reached in 128 trials. Generalization to 20/sec. and 5/sec. 

 clicks as CS was immediate. The cortical CS, however, produced the 

 former CR, a flexion of the right hand towards the chin rather than the 

 extension to press the lever at the level of the abdomen. After twenty-five 

 combinations of this cortical CS with the tail-shock, the animal began 

 making lever presses to avoid this US. It took more than 100 trials, how- 

 ever, before the former CR was fully extinguished under these conditions. 

 The threshold for the shock-avoidance response to cortical CS was 0.2 

 mA. which is significantly less than the 0.55 niA. threshold at the same 

 electrodes for the flexion CR established without motivational context. 



Monkey 2. The CS and US elicited responses very similar to those seen 

 in Monkey 1 save that the seizure threshold was much lower in this animal 

 and the UR was frequently followed by a few clonic movements. The 

 animal struggled almost continuously in the experimental situation (even 

 when being given fruit juice) and no CRs were ever observed. Using 



