Journal of Neuroscience, Vol 12, 1816-1825, Copyright © 1992 by Society for Neuroscience
GABA-induced inactivation of functionally characterized sites in cat visual cortex (area 18): effects on orientation tuning
JM Crook and UT Eysel
Department of Neurophysiology, Faculty of Medicine, Ruhr University of Bochum, Germany.
Microiontophoresis of the inhibitory transmitter GABA was used to
reversibly inactivate small sites of defined orientation specificity at a
horizontal distance of some 600 microns from single cells recorded in area
18 of cat visual cortex, and the effects on orientation tuning were
studied. The receptive fields of cells at the recording and inactivation
sites overlapped extensively. During the inactivation of sites where the
orientation preference differed by 45 degrees or more from that of a
recorded cell ("cross-orientation" sites), 65% of 54 cells tested showed
significant broadening of orientation tuning, with a mean increase in
tuning width (measured at half the maximum response) of 93%, and an almost
fourfold increase in the relative response to the orientation orthogonal to
the optimum, compared with the response to the optimum; four cells
essentially lost their orientation tuning. Broadening of tuning reflected
an increase in response to nonoptimal orientations and was reversible upon
termination of GABA application. The effects on orientation tuning
typically peaked within 10-15 min of the onset of GABA iontophoresis with
50-100 nA ejecting currents, and could not be replicated by inactivating
sites where the orientation preference was similar to that of a recorded
cell; when the orientation preference at the inactivation sites was within
22.5 degrees of that of a recorded cell ("iso-orientation" sites), only 3
of 22 cells showed significant broadening of tuning, and in these cases,
the effects were relatively weak (mean increase in tuning width of 39% and
a negligible change in the relative response to the orientation orthogonal
to the optimum). The effect of inactivating "iso-orientation" sites
consisted primarily in an increase in response magnitude. The difference in
the magnitude of the effects on orientation tuning elicited by inactivating
"cross-orientation" and "iso-orientation" sites was highly statistically
significant. Additionally, inactivation of "cross- orientation" or
"iso-orientation" sites elicited differential effects on orientation tuning
in 10 of the 13 cells in which direct comparisons were made. It is argued
that the observed broadening of tuning was due to the loss of a
"cross-orientation" inhibitory input, which normally sharpens orientation
tuning by suppressing responses to nonoptimal orientations.
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