LONG-TERM POTENTIATION IN THE MONKEY INFERIOR TEMPORAL CORTEX

First  FAONS Congress

20-23 October, 1996 Thailand

Long-term potentiation (LTP) is a persistent increase in synaptic efficacy following repetitive electrical stimulation of neurons.  The natural equivalent of LTP is thought to modify neural circuits in the brain and underlie certain types of memory(e.g., spatial memory in rodents).  If this hypothesis holds for visual recognition memory, LTP should occur in the monkey inferior temporal cortex where long-term memory of visual stimuli is stored and can be reorganized throughout life.  LTP is indeed induced in field potentials evoked by electrical stimulation of intrinsic horizontal axons in area TE of the inferior temporal cortex of adult monkeys.  The potentiation grows slowly over 50-70 minutes and lasts more than 3hrs.  LTP occurs only in the conditioned pathway, and responses evoked from an unconditioned pathway are not changed in their amplitude and waveform (input specificity).  The slow time course of LTP in the TE contrasts with that found in rodent hippocampus where a maximal level of potentiation is induced immediately after a conditioning stimulus.  When a new memory is stored into a cortical area, patterned neural inputs are thought to cause changes in synaptic strength at distributed synapses.  It has been proposed that the neuronal network accumulates small changes caused by input activity at individual synapses, and that it gradually adjusts the distribution of synaptic strength to find a state where both new and existing memories can coexist.  The slow time course of LTP in the TE may reflect the properties required for this gradual adjustment.  When the same experiments are performed in the primary visual cortex (V1), an identical stimulus protocol does not potentiate field potentials in V1, but instead causes a depression.  This depression is also long-lasting and input-specific.  The success of evoking LTP in the TE in vivo will allow us to investigate in future studies whether and, if so, how LTP affects stimulus selectivity of TE neurons and the functional columnar organization of this area.  The results also suggest that visual cortical areas in the primate are diverse not only in their anatomical architecture, input/output organization, and receptive field properties of neurons, but also in their synaptic plasticity.  [Supported by Core Research for Evolutionary Science and Technology (Research Development Corporation of Japan), and by grants from the Uehara Memorial Foundation and the Inamori Foundation]


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