Continuity or incontinuity of orientation columns in visual cortex: a critical evaluation of published and unpublished data.
Orientation columns in the visual cortex of cat and monkey were originally defined as small areas where all cells from layer 2 to 6 had the same preferred stimulus orientation. Large variation in preferred orientation, occasionally observed, were interpreted as biological scatter or artefacts. In contrast to this view, recent experiments revealed frequent abrupt shifts in preferred orientations at the transition from middle to lower layers. This controversial issue is of considerable relevance for models of cortical wiring. Therefore, in this report new and previously published data are quantitatively evaluated. The comparison shows that the large orientation shifts cannot be reconciled as mainly due to deviation of penetrations from the radial cell columns. The present data suggest that, in middle and lower layers of the cat's cortex, two groups of cells with approximately orthogonal orientations coexist. Comparison of results from different authors supports our evidence for the occurrence of orientation shifts. The controversy is reduced to the difference in the proportions of penetrations with and without shifts in different laboratories. A possible explanation for this remaining difference is the lower relative frequency of shifts in heavily sedated animals. Our own data were collected from awake, behaving or lightly anesthetized animals in chronic preparations.