Parallel neuronal mechanisms for short-term memory.

Although objects that have just been seen may persist in memory automatically for a time and interact passively with incoming stimulation, some tasks require that the memory be actively maintained and used. To test for the existence of separate automatic and volitional mechanisms of short-term memory, recordings were made from neurons in the inferior temporal cortex of monkeys while the monkeys held a sample picture "in mind" and signaled when it was repeated in a sequence of pictures, ignoring other stimulus repetitions. Some neurons were suppressed by any picture repetition, regardless of relevance, whereas others were enhanced, but only when a picture matched the sample. Short-term memory appears to reflect the parallel operation of these two mechanisms--one being automatic and the other active.