d2 – a new metric for characterising retinotopic areas
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Hans Strasburger, Ludwig-Maximilians-Universität München, Germany
The retino-cortical visual pathway through the thalamus up to area V4 is
retinotopically organized: Neighbourhood relationships on the retina are
preserved in the mapping. Local size relationships in that mapping are
inversely proportional to eccentricity in the visual field. Globally,
the locations in these maps are given by a logarithmic function
(Fischer, 1973) if certain premises are fulfilled. An individual map can
be characterised by the decrease of the cortical magnification factor
(CMF) with eccentricity, or by a set of iso-eccentricity contours drawn
on a flat map of the cortex. Here I propose, as a concise alternative, a
new structural parameter d2 to characterize these maps; it is defined as
the radial distance, in millimetres, from the retinotopic centre to
where the cortical magnification factor (CMF) is halved. It is a
neuroanatomical counterpart to E2, known from psychophysics, the
eccentricity where the foveal scale is doubled. d2 captures the map in a
single number along a radius, or a single contour on the map. It is the
scaled product of the central CMF, M0, and E2, and it is more stable
than either of these parameters. It can be used across studies, across
radii, across individuals, and even across species. It can further help
assess the retinotopic centre’s CMF, M0. Typical values for d2 and M0
are given for a number of well-known mapping studies. The comparison of
d2 and M0 across mapping studies shows considerable variation, and I
discuss possible reasons for this.
*Keywords:*Cortical map; logarithmic map; cortical magnification factor;
visual cortex; M-scaling; E2 value; retinotopy; M0; retinotopic centre;
visual field; local/global; neuroanatomy
