Hearing in African pygmy hedgehogs (Atelerix albiventris): audiogram, sound localization, and ear anatomy.

The behavioral audiogram and sound localization performance, together with the middle and inner ear anatomy, were examined in African pygmy hedgehogs Atelerix albiventris. Their auditory sensitivity at 60 dB SPL extended from 2 to 46 kHz, revealing a relatively narrow hearing range of 4.6 octaves, with a best sensitivity of 21 dB at 8 kHz. Their noise-localization acuity around the midline (minimum audible angle) was 14°, matching the mean of terrestrial mammals. The African pygmy hedgehog was not able to localize low-frequency pure tones or a 3-kHz amplitude-modulated tone when forced to rely on the interaural phase-difference cue, a trait shared by at least nine other mammals. The middle ear of Atelerix has a configuration including an ectotympanic which is not fused to the surrounding bones, a substantial pars flaccida, a synostosed malleo-ectotympanic articulation and a 'microtype' malleus. The hearing and sound localization of A. albiventris is compared to that of a broad range of other mammals. It is shown that a malleus morphology like that of Atelerix, including a stiff articulation with the ectotympanic, is a consistent feature of other mammals that do not hear frequencies below 400 Hz.NIH Research Grant R01 DC/NS0296

Use of binaural cues for sound localization in large and small non-echolocating bats: Eidolon helvum and Cynopterus brachyotis

The authors determined the ability of two old-world non-echolocating bats, Eidolon helvum and Cynopterus brachyotis, to use binaural time and intensity difference cues for localization. The bats were trained to localize pure tones throughout most of their hearing range from loudspeakers located 30° to the left and right of midline. Both species easily localized high frequency tones, indicating they could use the interaural intensity difference cue. However, neither was able to localize low frequency tones even when the tones were amplitude modulated thereby indicating that they could not use ongoing phase difference cues. The authors now know of eight mammals that do not use binaural phase cues for localization, and some possible reasons for this inability are explored

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology, and behavior

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a signi®cant increase in the spontaneous ®ring rate of visual cortex cells. Further, auditory stimuli elicited ®eld potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63 % of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA±HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22°, with no signi®cant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typica