1. A burst of from one to four current pulses of 0.2 ms at 100 Hz was
administered bilaterally to medial temporal lobe areas while monkeys w
orked in a delayed matching-to-sample visual memory task. The brief el
ectrical stimulation was used as a probe to deter mine when, around th
e 20 or 50 ms sample presentation, the disruption was most severe. 2.
Stimulation within about 200 ms of the sample image onset severely per
turbed the animals' ability subsequently to recognize that image. Iden
tical stimulation at other times did not. 3. Thus, the processing duri
ng encoding, that is accessible to the implanted medial temporal lobe
electrodes, appears to occur only in a brief interval associated with
receipt of the sensory input.