The primary visual cortex (V1) is organized into maps of ocular dominance
(OD) and orientation (OR). It is known that these maps are altered by
visual experience. However, the structures already exist before the newborn
cat, ferret or monkey opens its eyes. The mechanisms of this previsual map
development have been a controversial issue. Possible influences include
molecular signals and spontaneous neural activity, but their respective
contributions remain unclear.
With the help of the LISSOM model of the primary visual cortex the effect
of different patterns of spontaneous activity has been investigated.
The results suggest that previsual spontaneous activity alone is
sufficient for realistic OR and OD maps to develop. Maps of only one
feature, i.e. OR or OD alone, develop robustly with a wide range of
activity patterns. However, joint OR/OD maps depend crucially on how correlated
the patterns are between the eyes, even over brief initial
periods. Therefore the simultaneous measurement of joint maps and map
interactions rather than of maps of a single feature may
reveal more information about the biological processes
of previsual map development.
The figures that are referred to in the following are available
in a PDF
OR and OD maps and their interactions
Previsual maps in animals have four main properties: (i) smooth,
binocularly matching OR maps, (ii) an OD map with significant left or
right eye preferences and smooth transitions between them, (iii)
orthogonal intersections of OR and OD maps, and (iv) OR pinwheel centers
located within OD columns.
Various simulations tested how well the maps generated by different
neural activity patterns match the properties of animal maps. In the
extreme case of very strong input correlation between the eyes,
matching OR maps develop but no ocular dominance map (Fig. 3a). Conversely, in the
extreme case of very weak correlation between the input patterns, strong
OD maps develop, but the OR maps are different in each eye (Fig. 3b). Thus spatial
correlation of the input is required for property matching OR maps, yet some degree of
uncorrelation is simultaneously needed for the development of ocular
Maps that match properties (i)-(iv) developed with a
combination of correlated and uncorrelated activity patterns. After the
first 20% of the development, the ratio of correlation was decreased from
1:1 to 1:2 (Fig. 4).
The effect of noise on selectivity in OD map development
Prior LISSOM experiments demonstrated that uncorrelated patterns
without noise lead to very strong selectivity in OD maps, similar to maps of
adult strabismic animals. Those results suggest that uncorrelated
spontaneous activity patterns could result in the earliest maps being
strabismic even in normal animals. However, we found that including an
overall pattern of noise remedies the extreme selectivity. This
equalizing effect even persists long after the noise has disappeared. In
the maps presented in Figure 5, noise was only present during the first
tenth of the development.