Neural coding of echo-envelope disparities in echolocating bats.

The effective use of echolocation requires not only measuring the delay between the emitted call and returning echo to estimate the distance of an ensonified object. To locate an object in azimuth and elevation, the bat's auditory system must analyze the returning echoes in terms of their binaural properties, i.e., the echoes' interaural intensity and time differences (IIDs and ITDs). The effectiveness of IIDs for echolocation is undisputed, but when bats ensonify complex objects, the temporal structure of echoes may facilitate the analysis of the echo envelope in terms of envelope ITDs. Using extracellular recordings from the auditory midbrain of the bat, Phyllostomus discolor, we found a population of neurons that are sensitive to envelope ITDs of echoes of their sonar calls. Moreover, the envelope-ITD sensitivity improved with increasing temporal fluctuations in the echo envelopes, a sonar parameter related to the spatial statistics of complex natural reflectors like vegetation. The data show that in bats envelope ITDs may be used not only to locate external, prey-generated rustling sounds but also in the context of echolocation. Specifically, the temporal fluctuations in the echo envelope, which are created when the sonar emission is reflected from a complex natural target, support ITD-mediated echolocation.

Representation of echo roughness and its relationship to amplitude-modulation processing in the bat auditory midbrain.

Bats use natural landmarks such as trees for orientation. Echoes reflected by a tree are stochastic and complex. The degree of irregular loudness fluctuations of perceived echoes, i.e. the echo roughness, may be used to classify natural objects reliably. Bats are able to discriminate and classify echoes of different roughness. A neural correlate of the psychophysical roughness sensitivity has been described in the auditory cortex of the bat Phyllostomus discolor. Here, the role of the inferior colliculus of P. discolor is explored in the neural representation of echo roughness. Using extracellular recording techniques, responses were obtained to simulated stochastic echoes of different roughness. The representation of these irregular loudness fluctuations in echoes is compared to the representation of periodic loudness fluctuations elicited by sinusoidal amplitude modulation (SAM) and to the shape of the peri-stimulus time histogram in response to pure tones. About half the recorded units responded significantly differently to echoes with different roughness. Roughness sensitivity was related to the units' sensitivity to the depth of an SAM: units that responded best to strong SAMs also responded best to echoes of high roughness. In response to pure tones, these units were typically characterized as Onset units. In contrast to the auditory cortex experiments, the responses of many units in the inferior colliculus decreased with increasing echo roughness. These units typically preferred weak SAMs and showed a sustained response to pure tones. The data show that auditory midbrain sensitivity to SAM is an important prerequisite for the neural representation of echo roughness as an ecologically important echo-acoustic parameter.

Psychophysical and neurophysiological hearing thresholds in the bat Phyllostomus discolor.

Absolute hearing thresholds in the spear-nosed bat Phyllostomus discolor have been determined both with psychophysical and neurophysiological methods. Neurophysiological data have been obtained from two different structures of the ascending auditory pathway, the inferior colliculus and the auditory cortex. Minimum auditory thresholds of neurons are very similar in both structures. Lowest absolute thresholds of 0 dB SPL are reached at frequencies from about 35 to 55 kHz in both cases. Overall behavioural sensitivity is roughly 20 dB better than neural sensitivity. The behavioural audiogram shows a first threshold dip around 23 kHz but threshold was lowest at 80 kHz (-10 dB SPL). This high sensitivity at 80 kHz is not reflected in the neural data. The data suggest that P. discolor has considerably better absolute auditory thresholds than estimated previously. The psychophysical and neurophysiological data are compared to other phyllostomid bats and differences are discussed.