Journal of Neuroscience, Vol 14, 2732-2746, Copyright © 1994 by Society for Neuroscience
Functional organization of corticocortical projections from area 17 to area 18 in the cat's visual cortex
DJ Price, JM Ferrer, C Blakemore and N Kato
University Laboratory of Physiology, Oxford, United Kingdom.
We used anatomical and physiological methods to study the functional
organization of the association projection from area 17 to area 18 in the
cat's visual cortex. Neurons in area 17 projecting to area 18 (revealed by
retrograde transport of fluorescent tracer) tend to be clustered over
regions of layer 4 receiving input from the ipsilateral eye (visualized by
anterograde transneuronal tracing). Since the contralateral input overlaps
these ipsilateral patches, the association cells lie preferentially in
regions that are likely to be binocularly innervated. Indeed, almost all
cells recorded electrophysiologically within the association clusters were
strongly binocular, whereas between the clusters, many neurons were
dominated by the contralateral eye. There is sufficient jitter in the
retinotopic organization of area 17 for the discontinuous distribution of
association cells to provide a continuous representation of the visual
field. Cells in each association cluster in the rostral part of area 17
project divergently to innervate a zone extending up to 3 mm wide,
anteroposteriorly, in the superficial layers of area 18. The receptive
fields of cells at any point in area 18 are larger than for the
corresponding point in area 17. Neurons recorded at two points in area 18,
separated by a distance equal to the limit of anatomical divergence of the
projection from area 17, have receptive fields that overlap by an amount
similar to the region of visual field covered by the receptive fields of
cells in a single association cluster in area 17 at a similar retinotopic
position. Thus, area 18 receives a full and strongly binocular
representation of the visual field not only from the lateral geniculate
nucleus but also from area 17. The divergence of the area 17 to 18
projection compensates for the difference in receptive field size by
ensuring that the receptive fields of each cluster of projecting neurons
overlap fairly precisely those of the recipient neurons in area 18.