Receptive field expansion in adult visual cortex is linked to dynamic
changes in strength of cortical connections.
Das, A., and C. D. Gilbert.
The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399,
Phone: (212) 327-7670, FAX: (212) 327-7844, E-mail:
gilbert@rockvax.bitnet.
APStracts 2:0110N, 1995.
SUMMARY AND CONCLUSIONS
1. Receptive Field (RF) sizes of neurons in adult primary visual cortex are
dynamic, expanding and contracting in response to alternate stimulation
outside and within the RF over periods ranging from seconds to minutes. The
substrate for this dynamic expansion was shown to lie in cortex, as opposed to
subcortical parts of the visual pathway. The present study was designed to
examine changes in cortical connection strengths that could underlie this
observed plasticity by measuring the changes in cross-correlation histograms
between pairs of primary visual cortex neurons that are induced to dynamically
change their RF sizes. 2. Visually-driven neural activity was recorded from
single units in the superficial layers of primary visual cortex in adult cats,
with two independent electrodes separated by 0.1 - 5 mm at their tips, and
cross-correlated on-line. The neurons were then conditioned by stimulation
with an "artificial scotoma", a field of flashing random dots filling
the region of visual space around a blank rectangle enclosing the RFs of the
recorded neurons. The neuronal RFs were tested for expansion and their
visually-driven output again cross-correlated. Following this, the neurons
were stimulated vigorously through their RF centers to induce the field to
collapse, and the visually-driven output from the collapsed RFs was again
cross-correlated. Cross-correlograms obtained before and after conditioning,
and after RF collapse, were normalized by their flanks to control for changes
in peak size due solely to fluctuations in spike rate. 3. A total of 37 pairs
of neurons that showed distinct cross-correlogram peaks, and whose RF borders
were clearly discernible both before and after conditioning, were used in the
final analysis. Of these neuron pairs, conditioning lead to a clear expansion
of RF boundaries in 28 pairs, while in 9 pairs the RFs did not expand. RFs
that did expand showed no significant shifts in their orientation-preference,
orientation-selectivity, or ocularity. 4. When the RFs of a pair of neurons
expanded with conditioning, the area of the associated flank-normalized cross-
correlogram peaks also increased (by a factor ranging from 0.84 up to 3.5).
Correlograms returned to their pre-conditioning values when RFs collapsed.
Within the populaton of neurons that expanded with conditioning, the subset of
neurons starting with substantial RF overlap before conditioning showed
essentially no increase in cross-correlation peak area (peak areas increased
by factors ranging from 0.84 to 1.75 with a mean of 1.2 +/- 0.23) while the
complementary set of neuron pairs with low or no initial overlap showed a
significant increase in cross-correlation peak area (peak areas increased by
factors ranging from 1.1 to 3.5 with a mean of 1.72 +/- 0.53). RFs that did
not expand showed no increases in their flank-normalized cross-correlograms
(correlogram peak area changed by a factor of 0.81 +/- 0.23 with
conditioning). Neuron pairs that showed no significant cross-correlation
before conditioning (i.e. neurons whose orientation-preference differed by
more than 40 degrees , or were highly directional in opposite senses, or were
separated by more than about 3 mm on the surface of the cortex, or were highly
monocular with opposite ocularity) showed no significant cross-correlation
after conditioning. 5. Correlation strength did not change uniformly with
conditioning over the area of the RFs. We measured this inhomogeneity in the
effects of conditioning by obtaining correlograms with stimuli placed in
different positions within the RFs of each pair of neurons. In three neuron
pairs where the RF expansion was highly asymmetric, this comparison showed
that correlation strength increased significantly only in the RF subregions
where the expansion was greater.
Received 4 October 1994; accepted in final form 3 March 1995.
APS Manuscript Number J623-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 1 May 1995.