Effect of learning on receptive field shape and direction selectivity.

[Mayank R. Mehta]

The following two papers on effect of Hebbian learning, or temporally
asymmetric NMDA dependent LTP, on receptive field shape [1], and 
direction selectivity in V1 [2], are available at:

http://www.mit.edu/~mayank/

-Mayank 

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Mayank R. Mehta                          http://www.mit.edu/~mayank/
E25-236, 45 E Carleton Street                     Work: 617 252 1841
Massachusetts Institute of Technology              FAX: 617 258 7978
Cambridge, MA 02139                            Email: Mayank at MIT.edu
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Paper 1:

`Experience-dependent, asymmetric shape of hippocampal receptive fields'.
Mayank R. Mehta, Michael. C. Quirk & Matthew A. Wilson. Neuron (2000)
25:707-715.

Abstract: We propose a novel parameter, namely the skewness or the
asymmetry of the shape of a receptive field, and use this measure to
characterize two properties of hippocampal place fields that may reflect
the underlying mechanism of experience dependent plasticity. First, a
majority of hippocampal receptive fields on linear tracks are negatively
skewed, such that during a single pass the firing rate is low as the rat
enters the field, but high as it exits. Second, while the place fields are
symmetric at the beginning of a session, they become highly asymmetric
with experience. Further experiments suggest that these results are likely
to arise due to synaptic plasticity during behavior, and not due to other
non-specific mechanisms.  Using a purely feed forward neural network model
we show that following repeated directional activation, the temporally
asymmetric nature of NMDA dependent LTP/D could result in an experience
dependent asymmetrization of receptive fields.


Paper 2:

`From Hippocampus to V1: Effect of LTP on spatio-temporal dynamics of
receptive fields'.  Mayank R. Mehta & Matthew A. Wilson. To appear in
Neurocomputing, (2000).

Recent studies have revealed novel effects of patterns of neuronal
activity and synaptic plasticity on the size and specificity of receptive
fields. However, little has been done to quantify their effect on the
receptive field {\it shape}. It has been shown that place fields are
highly asymmetric such that, the firing rate of a place cell rises slowly
as a rat enters a place field but the firing rate drops off abruptly at
the end of the place field in an experience dependent fashion. Here we
present a computational model that can explain the results, based on NMDA
dependent LTP. Striking similarities between the hippocampal and striate
receptive field dynamics are pointed out. Our model suggests that LTP/D
could result in diverse phenomena such as phase precession in the
hippocampal neurons and the origin of directional receptive fields in the
striate cortex. It is suggested that the key feature underlying
directionality and inseparable spatio-temporal dynamics is the asymmetric
shape of the receptive field.