The auditory cortex of the mouse: connections of the ultrasonic field.

The cortical and subcortical connections of the ultrasonic field (UF) of the auditory cortex of the house mouse (Mus musculus) were studied by using retrograde and anterograde transport of horseradish peroxidase (HRP). Small amounts of HRP were locally injected into the electrophysiologically defined UF. Superficial (layer I-IV) and deep (layer IV-VI) injections were prepared. Superficial injections led to labelling of both cells (retrograde) and terminals (anterograde) in areas of the ipsilateral primary and secondary auditory cortex and in its dorsoposterior field, in an ipsilateral dorsal association area (patches of label), probably in ipsilateral secondary somatosensory cortex, in the contralateral homotopic UF, and in the ipsilateral medial geniculate body (MGBv, MGBd, and MGBm) and caudal posterior nucleus complex. Deep injections showed the same connectivities as superficial ones and, in addition, terminals in the very caudal caudatoputamen, in the nucleus limitans and the nucleus reticularis of the thalamus, in the rostral pole, the dorsomedial, and lateral nucleus of the inferior colliculus, in the stratum griseum intermediale of the superior colliculus, and in a pontine nucleus ventromedial of the lateral lemniscus. All these projections occurred only ipsilaterally. The majority of connections, except those with the nucleus limitans, superior colliculus and pontine nucleus, suggest that UF is part of the primary anditory cortex (AI) and/or of the anterior anditory field (AAF) of the auditory cortex. Since UF has no regular tonotopy, this has important implications for the functional role that AI/AAF can have in communication-sound analysis.