Logan:  Basic Principles of Learning

              PRINCIPLES: EXTERNAL INHIBITION & DISINHIBITION

          If an unusual stimulus event occurs during or slightly before 
          the regular time of occurrence of a learned response, there 
          will result a temporary decrement in performance; the same 
          event following extinction results in a temporary increment 
          in performance.

     External inhibition was first observed in the classical conditioning
context when, for example, a noisy truck passed by outside Pavlov's lab.  
His interpretation was that such an unusual stimulus naturally elicits
investigatory responses which distract the dog from the CS and hence cause 
a decreased flow of conditioned saliva.  Whatever the reason for the 
effect, external inhibition applies to all conditioning situations.  A 
rat's speed of running down a runway or rate of pressing a bar will be
temporarily disrupted by almost any unusual event that occurs in the
environment.  It should be noted that the absence of a customary stimulus 
in the environment qualifies as a stimulus event that can lead to
external inhibition.

     External inhibition is probably not very surprising; we are all 
familiar with occasions when our well-practiced performance suffered 
because the conditions were somehow different from what was customary.
Disinhibition is less obvious but equally important.  Any response that 
has undergone some degree of extinction may reappear if the situation is 
changed by the occurrence of an unusual stimulus event.  Both effects 
depend on the intensity of the unusual stimulus; external inhibition is 
greater the more intense the stimulus, whereas disinhibition is greatest 
with a moderate level of intensity.  Both effects also depend on how long 
before observations are made that the unusual stimulus occurred; obviously, 
if a rather long time passes after the event occurred, it will have
little effect.

      From a strictly operational point of view, external inhibition and
disinhibition are the same; that is to say, both principles involve the
occurrence of an unusual stimulus event in the context of a learned
response.  This suggests that the same underlying processes should be
involved.  It is relatively straightforward to conceptualize external
inhibition as a special case of a stimulus generalization decrement.  An
unusual stimulus event necessarily changes the contextual or background
environment which, as a part of the total stimulus situation, means that
one is observing performance in a different situation from that in which
learning occurred.  Consistent with this interpretation is the fact that
the more rigidly the experimental environment has been controlled in the
past, the more disruptive will be the effect of any changes in the environment.

     It may not be immediately obvious, however, how this analysis could
generate an increase in performance as observed in disinhibition.  The 
approach suggested by Pavlov is that extinction involves the accumulation 
of an inhibitory process that suffers a greater stimulus generalization
decrement than the excitatory process acquired during conditioning.  
Logically, both processes must suffer a generalization decrement, but if 
the decrement is larger for inhibition than excitation, the net effect 
would be positive.  For this reason, many of the observations that appear 
to be external inhibition may actually be instances of disinhibition.  
If a skilled performance involves a substantial amount of inhibition of 
natural response tendencies, an unusual stimulus event may disinhibit
that inhibition and disrupt performance accordingly.  In the laboratory, for 
example, if an unusual stimulus is presented during the interstimulus 
interval in classical conditioning the CR that would normally not occur 
until the time of the US will appear too early.

     In view of these phenomena, it is highly desireable for learning to
occur under conditions designed to build up resistance to external
inhibition/disinhibition.  The wider the range of contexts in which a 
response has been practiced, the less subject it will be to disruption 
of either the excitatory of the inhibitory processes.  For example, the 
veteran stage performer for whom "the show must go on" has practiced
under such a variety of circumstances that even a recent tragedy will 
not destroy the act.  In contrast, the week-end golfer is distracted 
by the slightest noise while trying to make a shot.

TERMS: Disinhibition, external inhibition.