Figure 1.

Schematic of thalamocortical pathways in adult mustached, horseshoe and pallid bats and young pallid bat. (A) In the horseshoe bat, the MGBd projects mainly to regions dorsal to A1 that contain the combination-sensitive neurons used in target distance computation. The MGBv projects to A1. In the mustached bat, the rostral pole nucleus projects to the delay-tuned areas. It is not clear if the rostral pole nucleus should be considered a part of the ventral or dorsal MGB. Therefore, the term “MGBd/MGBv-RP” is used in this schematic. In both species, the SG, considered a part of the MGBd, projects diffusely but more to the non-primary cortex than primary cortex. (B) In adult pallid bats, the HFR involved in echolocation behavior receives input from the SG and the MGBd, but not the MGBv. The LFR, involved in passive localization, receives input from the MGBv, but not the SG. Based on response selectivity in the auditory cortex, the SG → HFR pathway is involved in echolocation behavior while the MGBv → LFR pathway is involved in passive localization of prey-generated noise. (C) In a 2-week-old pallid bat pup, however, the pathways overlap. This is because the SG projects to both the LFR and HFR. The pup MGBv, as in adults, does not project to the HFR. Thus anatomically segregated pathways arise through postnatal refinement of initially overlapping connections.