As already discussed, Pavlov's three formulations accounting for tempera-
ment differences were based on the view that the causal processes of
excitation and inhibition were located in the cerebral cortex. This conviction
derived from the knowledge, subsequently confirmed, that conditioned
responses cannot be retained or acquired by decerebrate animals.

Latterly, it is clear, he came to recognize the profound psychological
significance of differences in the relative influence of thalamocortical and
subcortical processes, and, it must be assumed, given a little more time, this
knowledge would have enabled him to reconcile the data obtained using the
collision and strength techniques. That is to say, he would have come to
recognize that the strength techniques indicated differences in thalamocor-
tical reactivity whereas the collision techniques indicated differences in the
degree of brain-stem inhibition. As noted earlier, the relationship between
these variables should reflect the fact that greater reactivity or excitability
of the thalamocortical system determines greater brain-stem inhibition and
greater inhibition of conditioned salivary responses.

The most relevant recent research concerning the global interaction of
thalamocortical and brain-stem arousal systems was reviewed by Magoun
( 1963). As already mentioned, these studies led Magoun to identify the
diffuse thalamocortical system with the neurological processes inferred by

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