Steven P. Dear

The anatomical consequences of acoustic injury: A review and tutorial.

The anatomic consequences of acoustic overstimulation are explored in this presentation, and attention is directed toward issues where improvements in technology and empirical observation are needed before further advances in our understanding can be achieved. Gains have been made in the last decade in appreciating sound-induced cochlear injury, but there is now a need to evaluate not only cochlear pathology but also the functional state of the surviving structures. There is a wealth of information about the susceptibility of inner or outer hair cells to acoustic injury; however, the etiology of this injury is not yet fully understood. In addition, current ideas concerning the effects of noise on hair-cell stereocilia, hair-cell synapses, the cochlear vascular supply, and the central auditory pathways are in a state of flux and are either undergoing revision or emerging. Other issues, such as the basis of temporary or permanent threshold shift at the cellular level, and the individual differences in susceptibility to injury are in need of a fresh approach. It would seem that the time is now ripe to review our knowledge, recognize its gaps, and develop testable hypotheses concerning the mechanisms of acoustic injury to the ear.

A computational model of echo processing and acoustic imaging in frequency‐ modulated echolocating bats: The spectrogram correlation and transformation receiver

The spectrogram correlation and transformation (SCAT) model of the sonar receiver in the big brown bat (Eptesicus fuscus) consists of a cochlear component for encoding the bats frequency modulated (FM) sonar transmissions and multiple FM echoes in a spectrogram format, followed by two parallel pathways for processing temporal and spectral information in sonar echoes to reconstruct the absolute range and fine range structure of multiple targets from echo spectrograms. The outputs of computations taking place along these parallel pathways converge to be displayed along a computed image dimension of echo delay or target range. The resulting image depicts the location of various reflecting sources in different targets along the range axis. This series of transforms is equivalent to simultaneous, parallel forward and inverse transforms on sonar echoes, yielding the impulse responses of targets by deconvolution of the spectrograms. The performance of the model accurately reproduces the images perceived by Eptesicus in a variety of behavioral experiments on two-glint resolution in range, echo phase sensitivity, amplitude-latency trading of range estimates, dissociation of time- and frequency-domain image components, and ranging accuracy in noise.

Middle ear structure in the chinchilla: A quantitative study

The anatomic features of the chinchilla middle ear were identified and various aspects of the conductive apparatus were measured in a number of specimens by different methods. These aspects included area measures of the tympanic membrane, stapes footplate, oval window, and round window; middle-ear volume; dimensions of the ossicles, the length of their rotational axes as well as the malleus to incus lever ratio. We also weighed the ossicles. The findings are discussed with reference to their possible significance for auditory signal processing in the chinchilla.

Auditory Dimensions of Acoustic Images in Echolocation

Echolocation in bats is one of the most demanding adaptations of hearing to be found in any animal. Transforming the information carried by sounds into perceptual images depicting the location and identity of objects rapidly enough to control the decisions and reactions of a swiftly flying bat is a prodigious task for the auditory system to accomplish. The exaggeration of aspects of auditory function to achieve spatial imaging reflects the vital role of hearing in the lives of bats — for finding prey and perceiving obstacles to flight (Neuweiler 1990). It also highlights the mechanisms behind these functions to make echolocation a useful model for studying how the auditory system processes information and creates auditory perceptions in the most extreme circumstances.

The structure and function of actin in hair cells

Within the last 7 years, the protein actin has been identified in the stereocilia and cuticular plate region of the hair cell. An intensive effort has been mounted to describe the structural organization of this protein, to identify actin-associated proteins in these regions, and to identify the functional role of these proteins. The paracrystalline array of actin and its cross bridges imparts rigidity and stiffness to the stereocilia, which are important in determining their response properties. It also appears that these properties can be changed if the paracrystalline array is damaged by noise exposure. The functional implications of stereocilia rigidity and stiffness, as well as potential contractile mechanisms in the hair cell, are discussed. Finally, it is suggested that changes in the cross-sectional shape of the stereocilia caused by shearing of actin filaments during stereocilia deflections can be related to the mechano-electrical events in the plasma membrane of the cell. This may be the link between the transmission of vibrational energy through the sensory accessory structures of the peripheral ear, and the initiation of electrochemical events associated with transduction.

Auditory Computations for Biosonar Target Imaging in Bats

Bats are nocturnal flying mammals classified in the order Chiroptera. These animals have evolved a biological sonar, called echolocation, to orient in darkness—to guide their flight around obstacles and to detect their prey (Griffin 1958; Novick 1977; Neuweiler 1990; see Popper and Fay 1995). Echolocating bats broadcast ultrasonic sonar signals that travel outward into the environment, reflect or scatter off objects, and return to the bat’s ears as echoes. First the outgoing sonar signal and then the echoes impinge on the ears to act as stimuli, and the bat’s auditory system processes the information carried by these sounds to reconstruct images of targets (Schnitzler and Henson 1980; Simmons and Kick 1984; Suga 1988, 1990; Simmons 1989; Dear, Simmons, and Fritz 1993; Dear et al. 1993).

Biosonar signals impinging on the target during interception by big brown bats, Eptesicus fuscus

Big brown bats (Eptesicus fuscus) were videotaped in the dark with a night-vision lens and infrared illumination while flying repeatedly along the same straight course to seize a tethered mealworm or a small electret microphone used to record biosonar signals impinging on the target. Bats emitted frequency-modulated sounds with first to third harmonics covering frequencies from 23 to 105 kHz. As the bats neared the target, the first harmonic shifted lower in frequency while the third harmonic strengthened and the fourth harmonic, and sometimes the fifth harmonic, appeared. Incident-sound bandwidth remained broad throughout the maneuver, a feature not seen in field recordings of rapidly moving bats due to propagation losses and uncontrolled directional effects. Sound pressures at the microphone increased by about 20 dB during approach from 2.5 m down to 50 cm and then leveled off, indicating that emitted amplitudes were approximately constant until the terminal stage, when they progressively decreased for the remainder of the maneuver. Interpulse intervals decreased from 80-100 ms down to about 6-7 ms and then stabilized throughout the terminal stage, while durations decreased smoothly from 3-4 ms (limited by adjacent wall) down to 0.5 ms during the terminal stage, which ended with capture.

Representation of Perceptual Dimensions of Insect Prey During Terminal Pursuit by Echolocating Bats

The echolocating big brown bat, Eptesicus fuscus, broadcasts brief frequency-modulated (FM) ultrasonic sounds and perceives objects from echoes of these sounds returning to its ears. Eptesicus is an insectivorous species that uses sonar to locate and track flying prey. Although the bat normally hunts in open areas, it nevertheless is capable of chasing insects into cluttered environments such as vegetation, where it completes interceptions in much the same manner as in the open except that it has to avoid the obstacles as well as catch the insect. During pursuit, the bat shortens its sonar signals and increases their rate of emission as it closes in to seize the target, and it keeps its head pointed at the insect throughout the maneuver. In the terminal stage of interception, the bat makes rapid adjustments in its flight-path and body posture to capture the insect, and these reactions occur whether the bat is pursuing its prey in the open or close to obstacles such as vegetation. Insects can be distinguished from other objects by the spectrum and phase of their echoes, and Eptesicus is very good at discriminating these acoustic features. To identify the insect in the open, but especially to distinguish which object is the insect in clutter, the bat must have some means for representing these features throughout the interception maneuver. Moreover, continuity for perception of these features is necessary to keep track of the prey in complex surroundings, so the nature of the auditory representations for the spectrum and phase of echoes has to be conserved across the approach, tracking, and terminal stages. The first problem is that representation of changes in the phase of echoes requires neural responses in the bats auditory system to have temporal precision in the microsecond range, which seems implausible from conventional single-unit studies in the bats inferior colliculus, where the temporal jitter of responses typically is hundreds of microseconds. Another problem is that echoes do not explicitly evoke neural responses in the inferior colliculus distinct from responses evoked by the broadcast during the terminal stage because the delay of echoes is too short for responsiveness to recover from the emissions. In contrast, each emission and each echo evokes its own responses during the approach and tracking stages of pursuit. How does the bat consistently represent the phase of echoes in spite of these evident limitations in neural responses? Local multiunit responses recorded from the inferior colliculus of Eptesicus reveal a novel format for encoding the phase of echoes at all stages of interception. Changes in echo phase (0 degree or 180 degrees) produce shifts in the latency of responses to the emission by hundreds of microseconds, an unexpected finding that demonstrates the existence of expanded time scales in neural responses representing the target at all stages of pursuit.

Synchronized cortical potentials and wavelet packets: a potential mechanism for perceptual binding and conveying information.

Temporal synchronization in neuronal assemblies has been linked to the functional roles of perceptual binding, sensory-motor integration, attention, and information coding. We report new evidence for a common underlying mechanism that uses specific temporal patterns of synchronized neuronal activity as a basis for conveying information. The temporal patterns of stimulus-related synchronized neuronal discharges are structured to closely resemble specific members of the Symlet wavelet packet family employed in a computational framework. Together, these results suggest that temporal patterns of synchronized activity may act as a parallel, distributed code for information through a mechanism computationally equivalent to wavelet packet analysis.

Evidence for a resonant mechanism in auditory hair cell stereocilia

A mathematical analysis of stereocilia height and cochlear tonotopic organization along the length of the receptor epithelium in six species provided the basis for a resonant model of hair cell stereocilia. The analysis demonstrated an inverse power function between the distribution of frequency (f) and stereocilia height (h). The average empirical relation was: f∝h−2.67. These results are consistent with a resonant “clamped bar” model which has the relation: f∝h−2.00. The model provides conceptual insight into the mechanical properties of stereocilia by showing that Youngs Modulus remains constant for all stereocilia in a given cochlea. The mechanical properties of stereocilia can be understood further in terms of their macromolecular structure and the bending of crossbridge molecules. The uniform bending of every crossbridge molecule, coupled with the constant number of crossbridges per unit volume within the stereocilia, leads to a constant value for Youngs Modulus. Finally, the model raises the intr...