Figure 1.

Schematic diagram of HD circuit and model of proposed information flow. The DTN is depicted with a diagonal line transecting it to represent the pars ventralis and pars dorsalis subdivisions of the DTN (Hayakawa and Zyo, 1990). The DTN receives prominent projections from both the nucleus prepositus hypoglossi, ipsilaterally, and the supragenual nucleus, contralaterally (shown on left only). Both are innervated by the medial vestibular nucleus and are therefore likely sources of AHV information to the DTN. These projections to the DTN compose one of what could be multiple idiothetic pathways of information about self-generated movement into the HD system. The ADN is reciprocally connected to the retrosplenial cortex, and to the postsubiculum. These projections may constitute an allothetic pathway through which cortically processed visual, motor, and mnemonic information could enter the HD system. The interconnections of the DTN and LMN may be sufficient to generate the characteristic activity of HD cells, whereas the idiothetic pathway updates that activity from moment to moment based on the animal's movement, and the allothetic pathway relates the representation of directional heading to salient spatial cues from the environment (Sharp et al., 2001a; Taube and Bassett, 2003). In the case of a unilateral lesion (left DTN; gray shading), only information reaching the right DTN from the contralateral supragenual nucleus and ipsilateral nucleus prepositus hypoglossi can reach either ADN via the LMN (lost connections shown as dotted lines). AHV input might still reach the left LMN via a direct projection from the supragenual nucleus (Biazoli et al., 2006). MVN, Medial vestibular nucleus; NPH, nucleus prepositus hypoglossi; PoS, postsubiculum; RSC, retrosplenial cortex; SGN, supragenual nucleus.