Bethesda, MD (February 4, 2002) Ė Bats have inspired awe, fear, and even the inspiration for a world famous comic book character. But for a team of physiologists, the pallid bat can provide new clues into the structure and function of the auditory cortex.
This winged creature is unusual because it finds its prey by passively listening to prey-generated noise of short duration, while reserving high-frequency echolocation for obstacle avoidance. Echolocation is the method bats use to direct their flight and avoid solid objects. The creatures emit high-pitched cries, which are inaudible to human ears but are heard by bats as reflected echoes from objects in their path.
The primary auditory cortex is key to the pallid batís ability to perform frequency tuning, selectivity for behaviorally relevant sounds, and interaural intensity difference (IID) sensitivity. The auditory cortex is the region of the cerebral cortex that receives the auditory radiation from the medial geniculate body and has a spatial arrangement of structures such that certain tone frequencies are transmitted, as in the auditory pathway.
A new study from the University of Wyoming provides the first survey of the primary auditory cortex of a bat that relies heavily on both echolocation and passive sound localization. The objectives of the investigation were to identify insights into the organizational features present in both echolocators and passive listeners.
This study also attempted to map the cortex of the pallid bat in the dimensions of frequency representation, selectivity for behaviorally relevant sounds, and binaural response properties and describes a systematic organization of sensitivity to interaural intensity differences (IIDs) that may provide a substrate for passive sound localization.
The authors of the study, "Functional Organization of the Pallid Bat Auditory Cortex: Emphasis on Binaural Organization," are Khaleel A. Razak and Zoltan M. Fuzessery, both from the Department of Zoology and Physiology, University of Wyoming, Laramie, WY. Their findings appeared in the February 2002 edition of the Journal of Neurophysiology. Their research was supported by grants from the National Institute on Deafness and Other Communication Disorders and the National Science Foundation.
The pallid bat is a "gleaner" that uses high-frequency echolocation for general orientation and tunes in to low-frequency, prey-generated noise transients to detect and locate terrestrial prey. These two distinct sounds serve as physiological probes to identify regions that serve echolocation and passive sound localization. They have been used to demonstrate that a high degree of selectivity for these sounds is present at the level of the inferior colliculus (IC) and that these sounds are processed through two anatomically and functionally segregated regions of the IC.
Pallid bats were collected in Arizona and New Mexico and held in captivity in a 16 x 11 ft room, where they were given the freedom to fly and hunt crickets. The room was maintained at a reversed 12:12 light cycle. A few days prior to surgery, the bats were fed mealworms to increase body weight. All procedures adhered to federal animal welfare guidelines.
∑ Surgical procedures: Recordings were obtained from bats that were lightly anesthetized. To expose the auditory cortex, the head was held in a bite bar, a midline incision was made in the scalp, and the muscles over the dorsal surface of the skull were reflected to the sides. The front of the skull was scraped clean and a layer of glass microbeads applied, followed by a layer of dental cement. The bat was then placed in a Plexiglas restraining device. A cylindrical aluminum head pin was inserted through a cross bar over the batís head and cemented to the previously prepared region of the skull. This pin served to hold the batís head secure during the recording session.
∑ Recording procedures: Experiments were conducted in a heated (85Ė90įF), sound-proofed chamber which was lined with anechoic foam. Bats were kept lightly anesthetized throughout the course of the experiments; stimuli were generated using Modular Instruments and Tucker Davis Technologies digital hardware, and custom-written software.
Responses were quantified as the total number of spikes elicited over 30 stimulus presentations. Results are based on both single- and multiunit cluster recordings.
As in other species studied, neurons of the same binaural type are organized in homogeneous clusters. There is also an asymmetrical distribution of binaural types across the two regions serving echolocation and passive listening, suggesting that different binaural mechanisms may serve these behaviors. And, within the low-frequency, noise-preferring region that serves passive sound localization, neurons are systematically organized with respect to their sensitivity to IIDs, and this IID sensitivity remains stable with changes in intensity level. This finding suggests the presence of a topographically organized representation of a spatial cue in the mammalian auditory cortex.
A systematic representation of frequency is present in the primary auditory cortices of all mammals studied, suggesting that it provides a fundamental organizing substrate on which other attributes of sound are mapped.
Echolocating bats, with their highly specialized auditory behaviors, have provided some of the clearest examples of structure/function relationships in the auditory cortex. Previous studies of bat cortices have dealt with species that rely primarily, if not entirely, on echolocation to acquire information about their prey and immediate environment. In contrast, the pallid bat uses echolocation primarily for general orientation and relies on passive listening to detect and locate prey.
The organization of its auditory cortex appears to reflect a need to concurrently acquire information from what can be considered two auditory submodalities. A hunting pallid bat emits echolocation pulses at a rate of >5 pulses/s to maintain an acoustic image of its immediate environment, while passively listening for sounds generated by potential prey. It likely encounters situations in which these streams of information overlap in time. To what extent the pallid bat can attend to both streams of information is unclear in light of auditory scene analysis studies indicating that humans are able to fully attend to only one stream at a time.
Humans can however, rapidly shift attention between two streams, proportionate to the extent that they are distinct and coherent in spectrum, temporal features, and spatial origin. This may be the strategy employed by the pallid bat. Its auditory cortex seems to enhance contrast in multiple stimulus dimensions along at least part of the boundary between these two regions. Not only are there are abrupt changes in frequency tuning and response selectivity but in binaural processing as well. One way in which this organization of binaural processing can be interpreted in light of the need for dual-stream processing is that regions serving echolocation and passive listening have substrates for independent spatial representation. These independent substrates may further enhance the coherence of these streams, facilitate concurrent processing of echolocation and passive listening, and establish independent cortical representations of information acquired from these streams.
February 2002 edition of the Journal of Neurophysiology
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.
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