The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra, Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography.

The pink fairy armadillo Chlamyphorus truncatus is the smallest extant armadillo and one of the least-known fossorial mammals. The aim of this study was to establish if its middle ear is specially adapted to the subterranean environment, through comparison with more epigeic relatives of the groups Euphractinae (Chaetophractus villosus, Chaetophractus vellerosus, Zaedyus pichiy) and Dasypodinae (Dasypus hybridus). We examined the middle ears using micro-computed tomography and subsequent three-dimensional reconstructions. D. hybridus has a relatively small middle ear cavity, an incomplete bulla and 'ancestral' ossicular morphology. The other species, including Chlamyphorus, have fully ossified bullae and middle ear ossicles, with a morphology between 'transitional' and 'freely mobile', but in all armadillos the malleus retains a long anterior process. Unusual features of armadillo ears include the lack of a pedicellate lenticular apophysis and the presence, in some species, of an element of Paaw within the stapedius muscle. In common with many subterranean mammals, Chlamyphorus has a relatively flattened malleo-incudal articulation and appears to lack a functional tensor tympani muscle. Its middle ear cavity is not unusually enlarged, and its middle ear ossicles seem less robust than those of the other armadillos studied. In comparison with the euphractines, there is no reason to believe that the middle ear of this species is specially adapted to the subterranean environment; some aspects may even be indicative of degeneration. The screaming hairy armadillo, Chaetophractus vellerosus, has the most voluminous middle ear in both relative and absolute terms. Its hypertrophied middle ear cavity likely represents an adaptation to low-frequency hearing in arid rather than subterranean conditions.Argentinian research grants: Secretaría General de Ciencia y Tecnología, UNS (Project PGI 24/B243); Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) through a PhD fellowship to APB; Subsecretaría de Relaciones Internacionales, UNS, through a grant to APB

Effects of click polarity on brainstem auditory evoked potentials in cochlear hearing loss: a working hypothesis

The rarefaction-condensation differential potential (RCDP) obtained by subtracting brainstem auditory-evoked potentials (BAEPs) to C clicks from those to R clicks has been studied in 32 normal subjects and 31 cases of cochlear hearing loss. In normal subjects, no RCDP was recorded along the lower 30-55 dB of the JV latency-intensity function, thus defining the pre-RCDP range. The pre-RCDP range was always abolished in losses unmasking BAEPs from lower (<1 kHz) tonotopic regions. When the BAEP originated from higher (< 1 kHz) tonotopic regions, the pre-RCDP range was either reduced or abolished. These results led to a working hypothesis based on single-unit data and stating a dual dependence of polarity effects on variables distributed along the tonotopic and intensity dimensions, with respective break-points at 1 kHz, and at the junction of the tip and tail of unit frequency tuning curves. The 1 kHz break-point could represent the upper frequency limit for phase locking in man. © 1995 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.SCOPUS: ar.jinfo:eu-repo/semantics/publishe