Evidence for separate echolocation and communication processing areas in the inferior colliculus of bats

Although acoustic signals are used by bats for both echolocation and for communication little is known about how communication signals are represented in the bat's brain. Recently we have obtained data which strongly suggest that the inferior colliculus of Mexican free-tailed bats (Tadarida brasiliensis mexicana) is functionally partitioned into two major subdivisions: a medial region having neurons specialized for processing echolocation signals and a lateral region having neurons that appear to be well suited for analyzing communication sounds. The features which distinguish the two regions are marked differences in tonotopic organization as well as in tuning and discharge properties. The tonotopic organization of the medial region is mammalian in design. The characteristic frequencies (CF) of neurons in this region increase in an orderly manner with depth and encompass the full compliment of frequencies in the echolocation sounds emitted by this species. The neurons in the medial region are very sensitive to brief frequency-modulated (FM) signals which mimic the natural orientation cries and exhibit fast recovery. In striking contrast, the tonotopy of the lateral portion of the colliculus is characterized by a relative constancy of CFs with depth. The CFs of neurons situated dorsally are 20-22 kHz and only increase to 24-26 kHz in the deepest parts of the lateral region. In addition, the majority of neurons are either unresponsive to brief FM signals or have very high thresholds for responding to these stimuli. A final feature is that many neurons have two frequencies of maximal sensitivity in their tuning curves. Such multiply tuned neurons were only found in the lateral region. Many of the communication sounds emitted by Mexican free-tailed bats have long constant frequency portions. The fundamental frequencies range from about 20-25 kHz and are rich in harmonics, features which correlate closely with the tuning features of neurons in the lateral region. [Supported by NIH Grants NS13276 and NS00367.]