A "visual" stimulus (V), either flashes or electrical stimulation of the optic chiasma, was reinforced in compound with a differentially reinforced (CS+) or nonreinforced (OS-) nonvisual stimulus. Visual stimulus control of conditioned eyeblink activity was acquired if V was reinforced in compound with CS-but was "blocked" when reinforced in compound with CS+. Both effects were demonstrable within subjects and were independent of the method of visual stimulation. Extinction and backward conditioning of chiasmio stimulation preceded retraining of 6 subjects. The establishment and blocking of visual stimulus control were again evident within subjects. The data were interpretable in terms of either attentional or associative theory.

Rabbit eyeblinks were conditioned to either the visual or the nonvisual element of a compound conditioned stimulus. Evoked potentials (EPs) were simultaneously recorded from the visual cortex. Changes in the late negative component of the EP were related to nonspecific arousal effects. The initial surface positive component, however, was significantly enhanced during visual stimulus control. This enhancement was observed whether the visual stimulus was presented alone or in compound and with either photic or optic chiasma stimulation as the visual stimulus. The effect could not be attributed to arousal, sensitization, or to peripheral mechanisms. Some implications of the data were assessed through a subsequent investigation of geniculostriate recovery cycles under the same conditions.

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