David C. Mountain

Biomimetic robot lobster performs chemo-orientation in turbulence using a pair of spatially separated sensors: Progress and challenges

Abstract Lobsters are capable of tracking turbulent plumes to their sources faster than can be accomplished by estimating a spatial gradient from time-averaging the concentration signal. We have used RoboLobster, a biomimetic robot lobster to investigate biologically scaled chemotaxis algorithms using two point concentration sampling to track a statistically characterized turbulent plume. Our results identify the range of effectiveness of these algorithms and, with studies of lobster behavior, suggest effective strategies beyond this range. They suggest that a lobster’s chemo-orientation strategy entails an unidentified means of dealing with the intermittency of the concentration signal. Extensions of these algorithms likely to improve are discussed.

Measurements of the stiffness map challenge a basic tenet of cochlear theories

The cochlear frequency map is believed to depend on the progressive decrease in partition stiffness from base to apex. Measurements on cochleae from human cadavers by von Békésy (1960) suggested that the elasticity of the partition increases by a factor of 100 from the stapes to the helicotrema. However, conventional models require a factor of nearly 10,000 to support the frequency range of normal hearing if entirely determined by partition stiffness. To test this assumption, we measured point stiffness along the width and length of the partition in the gerbil cochlea. Two major findings result from this study: (1) contrary to von Békésys results, both cellular and extracellular elements of the sensory epithelium exhibit stiffness gradients; and (2) the stiffness changes by only a factor of 100 over the whole cochlea. Our results imply that present ideas regarding partition vibration need to be significantly revised.

Progression of hair cell ejection and molecular markers of apoptosis in the avian cochlea following gentamicin treatment

Aminoglycoside treatment induces caspase‐dependent apoptotic death in inner ear sensory hair cells. The timing of apoptotic signaling in sensory hair cells following systemic aminoglycoside treatment has not been characterized in vivo. We administered a single subcutaneous injection of the aminoglycoside gentamicin (300 mg/kg) to 12–16‐day‐old chicks and used immunocytochemical techniques to document the following responses in affected hair cells: T‐cell restricted intracellular antigen‐related protein (TIAR) translocation from the nucleus to the cytoplasm, cytochrome c release from the mitochondria, caspase‐3 activation, nuclear condensation, and an orderly progression of hair cell ejection from the proximal end of the basilar papilla. Hair cells in the proximal tip exhibited TIAR translocation from the nucleus and aggregation into punctate granules in the cytoplasm 12 hours after injection and the response progressed distally. Cytochrome c release from the mitochondria into the cytoplasm and caspase‐3 activation were observed in affected hair cells immediately prior to and during ejection. Hair cell ejection occurred between 30 and 54 hours after injection, beginning in the proximal tip and progressing distally. Nuclear condensation accompanied ejection while the loss of: 1) membrane integrity; 2) phalloidin labeling of F‐actin; and 3) TO‐PRO‐1 labeling of nuclear contents occurred within 48 hours following ejection. Our results present a timeline of aminoglycoside‐induced inner ear sensory hair cell apoptotic death that includes an 18‐hour window between the initial apoptotic response and the later stages of programmed death signaling that accompany ejection and a gradual breakdown of hair cells following ejection. J. Comp. Neurol. 475:1–18, 2004.

Rapid force production in the cochlea.

Electrical stimulation of the mammalian cochlea causes a mechanical response which produces acoustic signals at the frequency of the electrical current. These electrically-evoked acoustic emissions can be as large as 34 dB SPL. Concurrent acoustic stimuli can enhance the emission response. Comparison of the enhancement effect with the cochlear microphonic (CM) suggests that the emissions originate from the outer hair cells (OHC). Frequency response measurements indicate a rate-limiting time constant for the force-generating process which is less than 35 microseconds.

Towards effective and rewarding data sharing.

Recently issued NIH policy statement and implementation guidelines (National Institutes of Health, 2003) promote the sharing of research data. While urging that “all data should be considered for data sharing” and “data should be made as widely and freely available as possible” the current policy requires only high-direct-cost (>US

A piezoelectric model of outer hair cell function

500,000/yr) grantees to share research data, starting 1 October 2003. Data sharing is central to science, and we agree that data should be made available.

The envelope following response: Scalp potentials elicited in the mongolian gerbil using sinusoidally AM acoustic signals

Mammalian outer hair cells (OHC) are believed to increase cochlear sensitivity and frequency selectivity via electromechanical feedback. A simple piezoelectric model of outer hair cell function is presented which integrates existing data from isolated OHC experiments. The model predicts maximum OHC force production to equal 1.25 nN/mV. The model also predicts that the maximum velocity of OHC contraction in situ to be 800 microns/s. These predictions are compared to available experimental data and are found to be in good agreement. The good agreement between the predicted and experimental results suggests that, at the characteristic frequency of a given cochlear location, the OHC receptor current is very efficiently converted into basilar membrane motion.

Electromechanical Processes in the Cochlea

Scalp potentials which follow the low frequency envelope of a sinusoidally amplitude modulated stimulus waveform were evoked and recorded in anesthetized gerbils. This envelope following response (EFR) is presumably due to the synchronized discharge of populations of neurons in the auditory pathway. The magnitude of the EFR increased and the latency decreased in a near monotonic fashion with increased stimulus intensity and modulation depth. The modulation rate transfer function (MRTF) was determined for modulation frequencies between 10 and 920 Hz imposed on carrier frequencies ranging from 1 to 7 kHz. The MRTF was low pass in character having a corner frequency of 100-120 Hz. Measurements of the group delay, determined from the phase of the response relative to the stimulus phase, indicate that the response is generated in at least three distinct regions within the auditory pathway.

In Vivo Measurement of Basilar Membrane Stiffness

We propose a feedback model for cochlear mechanics in which the hair cell cilia exert an active restoring force in response to displacement. The restoring force is described by first order kinetics and leads to an increase in sensitivity and frequency selectivity of the basilar membrane response. Several different experimental results from our laboratory suggest that hair cell membrane potential plays an important role in this feedback process. They include the measurement of acoustic emissions in response to passing sinusoidal electric current through the cochlea and the modulation of the sound pressure level (SPL) of single tones by electrical current. The latter effect saturates at levels of the acoustic stimulus similar to the level at which the cochlear microphonic saturates. The experimental data suggest that saturation of the transduction process at high SPL could account for the saturating nonlinearity observed by others in basilar membrane displacement. These electromechanical effects may also explain the generation of acoustic harmonics and distortion products as well as the positive cochlear summating potential.

Electrically evoked basilar membrane motion

Basilar membrane stiffness measurements were made in the base of the gerbil cochlea. Basilar membrane stiffness was determined by contacting the basilar membrane with a stainless steel needle (tip diameter 25 microns) attached to a force transducer, putting the needle/transducer structure through a low-frequency sinusoidal excursion with amplitude 5 or 25 nm, and measuring the restoring force exerted on the needle by the basilar membrane at the applied frequency. Stiffness was calculated as the amplitude of the restoring force divided by the amplitude of the excursion. Stiffness was measured over a 24-microns range of static displacements of the basilar membrane and is presented as stiffness versus static displacement. In cochleas that were not damaged during surgery the stiffness versus displacement characteristic usually had the following features: (1) an initial stiffness plateau with average stiffness 0.6 N/m; (2) a second plateau or level off with average stiffness 9.1 N/m; and (3) an increase in stiffness beyond the second plateau that was consistent with the theoretical stiffness-vs-displacement function of a beam. These features were present both pre- and post-mortem.