Functional organization of corticocortical projections from area 17 to area 18 in the cat's visual cortex.

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.