Ocular dominance (OD) plasticity triggered by monocular eyelid suture
is a classic paradigm for studying experience-dependent changes in
neural connectivity. Recently, rodents have become the most popular
model for studies of OD plasticity. It is therefore important to
determine how OD is determined in the rodent primary visual cortex.
In particular, cortical cells receive considerable inputs from the
contralateral hemisphere via callosal axons, but the role of these
connections in controlling eye preference remains controversial.
Here we have examined the role of callosal connections in binocularity
of the visual cortex in na�ve young rats. We recorded cortical responses
evoked by stimulation of each eye before and after acute silencing,
via stereotaxic tetrodotoxin (TTX) injection, of the lateral geniculate
nucleus ipsilateral to the recording site. This protocol allowed
us to isolate visual responses transmitted via the corpus callosum.
Cortical binocularity was assessed by visual evoked potential (VEP)
and single-unit recordings. We found that acute silencing of afferent
geniculocortical input produced a very significant reduction in the
contralateral-to-ipsilateral (C/I) VEP ratio, and a marked shift
towards the ipsilateral eye in the OD distribution of cortical cells.
Analysis of absolute strength of each eye indicated a dramatic decrease
in contralateral eye responses following TTX, while those of the
ipsilateral eye were reduced but maintained a more evident input.
We conclude that callosal connections contribute to normal OD mainly
by carrying visual input from the ipsilateral eye. These data have
important implications for the interpretation of OD plasticity following
alterations of visual experience.
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