FUNCTIONAL ARCHITECTURE AND SYNAPTIC PLASTICITY IN THE MONKEY INFERIOR TEMPORAL CORTEX
UEHARA MEMORIAL FOUNDATION SYMPOSIUM ON gINTEGRATIVE
AND MOLECULAR APPROACH TO BRAIN FUNCTIONh
June25-27, 1996, Tokyo
Fujita,I., Kato,M., Murayama,Y. and Uka,T.
Systems-level analysis of cognitive functions
and cellular/subcellular-level analysis of synaptic plasticity represent two
major research fields in neuroscience today, yet there is only a limited
interface between them. We have
attempted to bridge these two research fields by starting our analysis at an
intermediate level of brain organization, i.e., at the neuronal circuit
level. We focused our efforts on area
TE of the monkey inferior temporal cortex which is the final or nearly final
stage of the visual cortical pathway crucial for object recognition.
The TE shares two organizational features with other cortices such as the primary visual cortex (V1): columnar organization and a network of intrinsic horizontal axons. TE neurons respond preferentially to visual features of objects such as particular shapes or combinations of shapes with colors or textures. Neurons with similar, but not identical, stimulus selectivities are clustered in multiple columns interspersed with columns selective for different object features. Evidence of columnar organization was first obtained by extracellular neuron(unit) recording in anesthetized monkeys, but has recently been obtained by unit recording in awake monkeys (our own results) and by optical imaging in anesthetized monkeys(results from K.Tanakafs laboratory). Horizontal axons run parallel to the cortical surface and produce patchy plexuses of terminals at certain intervals within the TE. Thus they connect nearby as well as distant sites in the TE. If their synapses are modifiable, they could provide a mechanism for changing response properties of neurons and, hence, neuronal representation of object features in columns of the TE.
Synapses formed or activated by horizontal axons are indeed plastic. After application of high-frequency electrical pulses to horizontal axons in the TE, the amplitude of field potentials evoked by a single pulse show slowly developing, long-lasting, and input-specific enhancement (long-term potentiation, LTP). In V1, on the other hand, an identical stimulus protocol does not potentiate field potentials, but instead causes long-term depression(LTD).
These experiments were performed on a whole animal preparation(i.e., in live monkeys). This provides us with a unique opportunity to investigate whether and how LTP and LTD alter functional properties of cortical neurons through examination of visual responses of neurons before and after LTP or LTD. The results also show that the TE and V1 differ from each other in terms of their synaptic plasticity. What cellular and molecular events explain this difference? How does this difference relate to functions of these two areas? These questions will be a key to linking studies on different levels of brain organization.