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Abstract

“Optical imaging” of the visual cortex after application of variously oriented visual stimuli
provides an opportunity to test different models of the distribution of orientation sensitive neurons
over the surface of the cortex. Rectilinear “slabs” of uniform orientation are not supported
by the evidence. What is compatible with the optical imaging is the arrangement of neurons
with different orientation around centers, regularly spaced at distances of about 0.5 mm in a
hexagonal array. According to a model proposed in 1979 [1], the orientations to which the
neurons are sensitive should be arranged either radially, or, more likely, like the tangents [2]
of circles around said centers, whereby in either case twice the same orientation occurs in opposite
positions of the “hypercolumn” thus defined. For this reason each colour, indicating a
certain orientation on the optical recording maps, should form a blotch the shape of two sectors
meeting at the center of the hypercolumn. We chose the term “bow tie” for this configuration,
to match the facetiousness of the competing term “pinwheel”. 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 rather undefined [3], makes the array of orientations around these centers less spectacular,
and indeed other interpretations of the coloured maps were put forward. “Pinwheels”
stole the show, i.e. 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 [4].
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. This is fairly
easy in the case of the hypercolumns under the assumption that in their centers are housed
special inhibitory neurons [2], while a similar elementary scheme was never found as an explanation
of the pinwheels.
On the coloured maps obtained with “optical recording” it is possible to discern both “pinwheels”
and “bow ties” as an aid to the localization of the two types of centers.