Journal of Neurophysiology, Vol 38, Issue 4 1015-1034, Copyright © 1975 by APS
"Stimulus generalization" between differentiated visual, auditory, and central stimuli
E. R. John and D. Kleinman
Cats were trained to discriminate between two different repetition rates of
flicker and of click. Both approach-approach and avoidance-avoidance
discriminations were used. After substantial overtraining, transfer of
frequency discrimination was initiated to stimulation of the reticular
formation using bursts of electrical pulses at the same two repetition
rates. Significant levels of discriminated performance were obtained in all
cats very quickly, indicating good cross-modal transfer between the
peripheral discriminanda and the central stimuli. The literature on
stimulus generalization and cross-modal transfer is reviewed and the
findings of this experiment are discussed in that context. Certain
conditions are defined which, if satisfied, justify the interpretation that
stimulus generalization or rapid cross-modal transfer indicate that
facilitation of subsequent tasks in a training sequence can be attributed
to mediation by a specific neuronal mechanism established by training on a
previous task. The present experiment was designed in view of such
criteria. The evidence of good cross-modal transfer is interpreted to mean
that brain mechanisms storing memories about discriminations between visual
or auditory stimuli with different repetition rates can be effectively
activated by gross electrical stimuli at the same repetition rates.
Conflict trials were then carried out in which flicker or click at either
frequency was contradicted by concurrent RF stimuli at the other frequency.
As the current level of RF stimuli was parametrically increased, it was
found that the central stimuli achieved almost complete control over the
behavioral outcome in most cases. Concurrent transfer of training, using a
counterbalanced training sequence, was then carried out to stimulation of
the visual cortex, lateral geniculate, medial geniculate, and the
intralaminar nuclei of the thalamus. In each case, rapid transfer was
displayed by at least one animal. Once performance to brain stimulation at
a given repition rate was established, little change was observed when the
fine structure of the stimulus was altered by changing parameters of the
stimulus burst. These findings are interpreted as providing support for a
statistical theory of memory, since they constitute evidence that
previously learned discriminative behavior can readily be elicited by
compelling large ensembles of neurons in various brain regions to discharge
with particular temporal patterns. It is difficult to reconcile these
results with theories which postulate that learning establishes new
synaptic pathways in which discharge must occur for memories to be
retrieved.
