Journal of Neuroscience, Vol 9, 3741-3752, Copyright © 1989 by Society for Neuroscience
Patterns of connections in rat visual cortex
R Malach
Center for Neurosciences and Behavioural Research, Weizmann Institute of Science, Rehovot, Israel.
The definition of visual areas is one of the central problems in visual
cortex research. Rodent extrastriate cortex offers a striking example of
the complexity of this issue, in that different parcelation schemes
identify within it from 2 to as many as 13 separate visual areas. In the
experiments reported here, patterns of connections within rat visual cortex
were studied in an effort to better define its organizational layout. The
experimental paradigm used consisted of the following steps: first, the
pattern of callosal connections was revealed in vivo with the fluorescent
tracer bisbenzimide. Then, using the callosal pattern as a landmark, single
injections of WGA-HRP were placed at various sites in striate and
extrastriate cortex. Subsequently, the relation between the tangential
distribution of ipsilateral corticocortical connections, the callosal
connections, and the borders of striate cortex were examined in the
flattened cortex preparation. The experiments revealed widespread, patchy
connections within rat visual cortex. These connections appeared to reflect
3 organizational trends. First, neighboring sites were more extensively
connected than distant ones. Second, extrastriate sites receiving common
striate cortex inputs tended to be interconnected. Finally, projections
from opposite poles in striate cortex tended to form interdigitating
patterns of connections in regions of overlap. Altogether these trends
suggest that the extrastriate band adjoining striate cortex has a single,
global map organization. However, within the global map, a clear modular
organization was evident, which appeared to correspond to the multiple
visuotopic representations reported for this region. Based on its location,
and some organizational similarities. it is suggested that the global map
may constitute the rat homolog of area V2 in cat and monkey.