We have utilized an associative conditioning paradigm
to induce changes in the receptive field (RF) properties
of neurons in the adult cat striate cortex. During conditioning,
the presentation of particular visual stimuli were repeatedly
paired with the iontophoretic application of either GABA
or glutamate to control postsynaptic firing rates. Similar
paradigms have been used in kitten visual cortex to alter
RF properties (Fregnac et al., 1988, 1992; Greuel et al.,
1988; Shulz & Fregnac, 1992). Roughly half of the cells
that were subjected to conditioning with stimuli differing
in orientation were found to have orientation tuning curves
that were significantly altered. In general, the modification
in orientation tuning was not accompanied by a shift in
preferred orientation, but rather, responsiveness to stimuli
at or near the positively reinforced orientation was increased
relative to controls, and responsiveness to stimuli at
or near the negatively reinforced orientation was decreased
relative to controls. A similar proportion of cells that
were subjected to conditioning with stimuli differing in
spatial phase were found to have spatial-phase tuning curves
that were significantly modified. Conditioning stimuli
typically differed by 90 deg in spatial phase, but modifications
in spatial-phase angle were generally 30–40 deg.
An interesting phenomenon we encountered was that during
conditioning, cells often developed a modulated response
to counterphased grating stimuli presented at the null
spatial phase. We present an example of a simple cell for
which the shift in preferred spatial phase measured with
counterphased grating stimuli was comparable to the shift
in spatial phase computed from a one-dimensional Gabor
fit of the space-time RF profile. One of ten cells tested
had a significant change in direction selectivity following
associative conditioning. The specific and predictable
modifications of RF properties induced by our associative
conditioning procedure demonstrate the ability of mature
visual cortical neurons to alter their integrative properties.
Our results lend further support to models of synaptic
plasticity where temporal correlations between presynaptic
and postsynaptic activity levels control the efficiency
of transmission at existing synapses, and to the idea that
the mature visual cortex is, in some sense, dynamically
organized.
The velocity tuning of single units in cat striate cortex.
