Tangential intracortical pathways and the development of iso-orientation bands in cat striate cortex.

Evidence is accumulating that tangential connections are a prominent feature of cortical organization. In the mammalian visual cortex, these connections appear to be related to columnar systems and theoretical considerations have suggested that they contribute to the development of regularly spaced orientation columns. We tested this assumption and examined whether early cortical lesions affected the formation and layout of the pattern of iso-orientation bands. In the striate cortex of 2-week-old kittens, tangential fiber paths were disrupted unilaterally along the representation of the horizontal meridian either by cuts, or by suction or by implanting pieces of Teflon. At this age, tangential fibers are still growing, orientation selectivity is only poorly developed and 2-deoxyglucose mapping does not yet reveal orientation columns. When the kittens were 7-10 weeks old, the organization of orientation bands was studied with the 2-deoxyglucose technique in flat-mounts of both visual cortices. In addition, kittens from the same litters as the experimental animals, having received the same lesions at the same postnatal day were subjected to neuroanatomical experiments to assess the effect of the lesion on the tangential fibers. These investigations revealed that a small fraction of tangential fibers had grown across the lesion when no mechanical barrier was implanted while disruption of tangential connections was complete in cases who had received Teflon implants. Apart from minor irregularities that were confined to the vicinity of the lesion, the 2-deoxyglucose experiments showed no differences in the pattern of orientation bands between the lesioned and intact hemispheres. In both, the bands extended throughout all cortical layers and their main trajectories were orthogonal to the representation of the vertical meridian. We conclude that at least from two weeks of age onwards, intracortical tangential connections are not necessary for the development of the regular pattern of iso-orientation bands in the striate cortex of cats.