Termination patterns of individual X‐ and Y‐cell axons in the visual cortex of the cat: Projections to area 18, to the 17/18 border region, and to both areas 17 and 18

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Abstract
Horseradish peroxidase was injected intracellularly into single, physiologically identified X‐ and Y‐cell geniculocortical axons that projected to area 18, to the 17/18 border region, or to both areas 17 and 18 via branching axons. The axon terminal fields in cortex were labeled anterogradely, and the cell bodies of the axons in the A‐laminae, lamina C. and the medial interlaminar nucleus (MIN) of the dorsal lateral geniculate nucleus (LGN) were labeled retrogradely. The laminar projections in area 18 of eight Y‐cells and one geniculate, non‐Y‐cell were analyzed. Most of the cells arborized densely within layer IVa and the lower 200 to 400 m̈m of layer III. Most provided little or no input to layer IVb or layer VI. Thus, the laminar projections of Y‐cells to layer IV of area 18 were similar to those of their area 17 counterparts, although the input to layer III was greater and rose much higher in area 18 than in area 17. The terminal arbors in area 18 were two to three times larger in lateral extent than those in area 17. They spread over 2.0 to 2.8 mm2 of layer IV and occupied proportionately much greater regions of area 18 than the Y‐cell arbors in area 17. This may partially account for the large receptive fields of cortical cells in area 18, and it indicates that a small region of area 18 may receive converging inputs from a relatively wide retinotopic region of the LGN. The terminal arbors were also highly asymmetric, generally being two to four times longer anteroposteriorly than mediolaterally. These asymmetric arbors may form the structural basis for the anisotropic organization of the retinotopic map in area 18. We recovered three cells (two Y, one X) whose axons arborized in the border zone between areas 17 and 18. One Y‐cell axon had a receptive field located in the ipsilateral visual hemifield and it arborized in a small region restricted almost exclusively to the border zone. The other two cells had receptive fields on or adjacent to the vertical meridian, and they terminated on either side of the 17/18 border region as well as within it. Thus, geniculate afferents representing the ipsilateral hemifield or the vertical meridian appear to have different patterns of termination on and adjacent to the 17/ 18 border zone. Also, some X‐cell input may invade area 18 in the region immediately adjacent to the border zone. Some Y‐cell axons bifurcated in the white matter to innervate areas 17 and 18, but the majority innervated one area or the other. The bifurcations occurred at the level of the lateral sulcus or higher in cortex, within 1 to 5 mm of the termination zones in the gray matter. Of 12 well‐labeled Y‐cell axons arising from the A‐laminae of the LGN, two branched to areas 17 and 18, nine projected to area 17 only, and one projected to area 18 only. Of four lamina C Y‐cells, two branched and two projected only to area 18. Two Y‐ cells from the MIN projected to only area 18. Thus, within the A‐ and C‐ laminae there are substantial and dedicated populations of Y‐cells that project to one cortical area or the other, as well as some that project to both areas via branching axons. Finally, our data indicate little or no input of X‐cells to area 18, except across the 17/18 border zone. Of 23 axons projecting to area 18, 22 were Y‐ cells and one was unclassified. Of 35 X‐cell axons that were traced to their terminations, all projected to area 17 or the 17/18 border zone.