Topography in the projections of lateral posterior thalamus with cingulate and medial agranular cortex in relation to circuitry for directed attention and neglect.

ABSTRACT

In the rat, the lateral posterior thalamic nucleus (LP) has reciprocal connections with areas of the cortex and the striatum involved in directed attention and its dysfunctional counterpart, contralateral neglect. It has also been shown that the medial portion of the mediorostral part of LP (mLPMR) is of special interest because it has connections with the dorsocentral striatum, a key node in this circuitry. In the present study we used neuroanatomical tracers to map the specific connections and topography of LP with the anterior cingulate cortex (ACC) and medial agranular cortex (AGm). We primarily used Alexa Fluor conjugates of the retrograde tracer cholera toxin subunit B, and injected two different colored conjugates into ACC and AGm in the same animal in order to directly compare the differential topography of the thalamocortical connections of mLPMR. The bidirectional tracer, dextran amine, was also used to examine anterograde corticothalamic projections of AGm and ACC. We found that mLPMR consists of two distinct groups of neurons, with the more dorsal group projecting to ACC and the more ventral group projecting to AGm. This is mirrored by a similar corticothalamic topography. These findings suggest that the ventral mLPMR is specifically associated with AGm and dorsocentral striatum, while dorsal mLPMR is associated with ACC. They also suggest that ACC may play a role in the circuitry for directed attention and contralateral neglect, as it is known to do in humans.