Orientation-specific relationship between populations of excitatory and inhibitory lateral connections in the visual cortex of the cat |
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Authors: | Kisvarday ZF; Toth E; Rausch M; Eysel UT |
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Institution: | Abteilung fur Neurophysiologie, Medizinische Fakultat, Ruhr-Universitat Bochum, Germany. |
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Abstract: | The topography of lateral excitatory and lateral inhibitory connections was
studied in relation to orientation maps obtained in areas 17 and 18. Small
iontophoretic injections of biocytin were delivered to the superficial
layers in regions where orientation selectivity had been mapped using
electrode recordings of single- and multi-unit activity from various
cortical depths. Biocytin revealed extensive patchy axonal projections of
up to 3.5 mm in both areas while labelled somata occurred chiefly at the
injection site, indicating that the labelling was primarily anterograde.
Two types of boutons could be clearly distinguished: (i) putative
excitatory boutons either en passant or having a short stalk and (ii)
inhibitory boutons which were invariably of the basket-type.
Three-dimensional reconstructions of all labelled boutons showed that the
excitatory and the inhibitory networks had a distinctively different
relationship to orientation maps. The overall distribution of connections
showed that 53-59% of excitatory and 46-48% of inhibitory connections were
at iso-orientation, +/-30 degrees; oblique-orientation, +/-(30-60) degrees,
was shown by 30% of excitatory and 28-39% of inhibitory connections;
cross-orientation was shown by 11- 17% of excitatory and 15-24% of
inhibitory connections. Although excitatory patches occupied mainly
iso-orientation locations, interpatch regions representing chiefly
non-iso-orientations (oblique + cross orientation) were also innervated.
There was considerable overlap between the excitatory and inhibitory
network. Nonetheless, inhibitory connections were more common than
excitatory connections with non-iso- orientation locations. There was no
significant difference between the orientation topography of area 17 and
area 18 projections. The results suggest that in general the lateral
connectivity system is not orientation specific, but shows a moderate
iso-orientation preference for excitation and an even weaker
iso-orientation preference for inhibition. The broad orientation spectrum
of lateral connections could provide the basis for mechanisms that
requiring different orientations, as for example in detecting orientation
discontinuities.
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