Mechanisms Underlying Direction Selectivity of Neurons in the Primary
Visual Cortex of the Macaque.
Sato, Hiromichi, Narumi Katsuyama, Hiroshi Tamura, Yoshio Hata & Tadaharu
Tsumoto.
Department of Neurophysiology, Biomedical Research Center, Osaka University
Medical School, Suita, Osaka 565, JAPAN, Present Address: Faculty of Health
and Sport Sciences, Osaka University, Machikaneyama, Toyonaka, Osaka, 560
Japan, and Laboratory for Neural Information Processing, Frontier Research
Program, The RIKEN Institute, Wako, Saitama, 351-01 JAPAN.
APStracts 2:0173N, 1995.
SUMMARY AND CONCLUSIONS
1. Effects of blocking intracortical inhibition by microiontophoretic
administration of bicuculline methiodide (BMI), a selective antagonist for
GABAA receptors, on direction sensitivity of 103 neurons were studied in the
primary visual cortex (V1) of anesthetized and paralyzed monkeys. 2. The
direction selectivity index (DSI) of each cell was calculated for the control
response and response during the BMI administration at the optimal stimulus
orientation to assess the directionality of an individual cell.
3. The averaged direction tuning of visual responses of cells was sharp in
layers IVa and IVb, moderate in both interblob and blob regions of layer
II/III and layers V and VI, and poor in layers IVca and IVcb. 4. Iontophoretic
administration of BMI uncovered or facilitated responses to stimuli moving in
the non-preferred direction, and reduced DSI of cells to a varying extent in
all the layers except those in layer VI. Responses to stimuli moving in the
preferred direction were also facilitated so that a slight bias of response
towards the originally preferred direction remained during the BMI
administration in most cells. 5. Most of the cells in layers II/III (both
blobs and interblobs) and IVb which receive inputs from layers IVca and IVcb
showed a clear reduction of the direction selectivity during the BMI
administration. This result suggests that intracortical inhibition plays an
important role in the elaboration of direction selectivity at the second stage
of information processing in V1. 6. The direction selectivity of cells in
layer VI was most resistant to the effects of BMI suggesting that it is
dependent on excitatory inputs which are already direction-selective, even
though the sample size of this layer was small. 7. In direction-selective
cells outside layer VI, responses to a stimulus moving in the preferred
direction were enhanced in a linearly related way with those in the non-
preferred direction as the BMI dose was increased. This suggests that various
amounts of inhibition interact linearly with directionally-biased excitatory
inputs to raise the firing threshold to various levels so as to produce
various degree of directionality. 8. These results suggest that, in most of
the directionally sensitive cells except for those in layer VI, there are
excitatory inputs which are bi-directional but slightly biased to one
direction and that the intracortical inhibition raises a threshold level of
responses to excitatory inputs so that the response become direction-
selective.
Received 31 October 1994; accepted in final form 22 May 1995.
APS Manuscript Number J685-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.