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Abstract

“Optical imaging” maps of the visual cortex after systematic application of variously oriented visual stimuli provide an
opportunity to test different hypotheses on the distribution
of orientation sensitive neurons over the surface of the cortex.
Rectilinear “slabs” of uniform orientation, as postulated
in some earlier models, are not supported by the evidence.
What is compatible with the optical imaging maps is the
arrangement of neurons with different orientation around
centers, regularly spaced at distances of about 0.5mm in a
hexagonal array. According to the model proposed by [3],
the orientations to which the neurons are sensitive should
be arranged either radially, or, more likely, like the tangents [1] of circles around said centers, whereby in either case twice the same orientation occurs in opposite positions of the “hypercolumn” thus defined. The centers of the hypercolumns very likely coincide with the so-called cytochrome oxidase “blobs” which are spaced at the same distance. The fact that within these “blobs” orientation tuning of cortical neurons becomes undefined [4], makes the array of orientations around these centers less spectacular, and indeed other interpretations of the coloured maps produced by optical recording were put forward. So-called “pinwheels” stole the show, that is centers around which neurons with different orientation sensitivity crowd with the colours representing their orientation clashing without interposed indifferent regions.
In these pinwheels each of the different orientations occurs only once as you go full circle around their center.
They most likely correspond to the corners between the hypercolumns in their hexagonal array, and the different orientations within one “pinwheel” most likely belong to three different hypercolumns that meet there [2].
The distinction between the two entities, orientation hypercolumns and pinwheels may sound academic but becomes
crucial when one endeavours to underpin orientation
specificity of cortical neurons with schemes of neuronal interactions at the elementary level. The accompanying illustration should help the reader to partake in this discussion.