Acoustic echo sounders designed to map and discriminate organisms in the water column have primarily been deployed on ships. Because of acoustic attenuation of higher frequencies used to detect and discriminate micronekton and nekton, this has effectively restricted the range of this information to the upper water column. In an effort to overcome these range limitations by reducing the distance between the transducer and the targets of interest, dual-frequency (38 and 120 kHz) split-beam echo sounders were integrated into a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV), effectively doubling the range of quantitative acoustic data into the mesopelagic zone (600–1200 m). Data from the first set of missions in a range of conditions revealed that the AUV provided a stable platform for the echo sounders and improved vertical and horizontal positional accuracy over echo sounders towed by ships. In comparison to hull-mounted echo sounders, elimination of ship noise and surface bubbles provided a 17- and 19-dBW decrease in the noise floor for the 38- and 120-kHz echo sounders, respectively, effectively increasing the sampling range by 30%–40%. The extended depth range also increased the resolution of the acoustic horizontal footprint from 37–40 to 0.6–3.7 m, enabling discrimination of individual targets at depth. Also developed here is novel onboard echo sounder data processing and autonomy to allow sampling not feasible in a surface ship or towed configuration. Taken together, these data demonstrate an effective new tool for examining the biology of animals in the mesopelagic zone (600–1200 m) in ways previously only possible in the upper ocean.

At low stimulation rates, electrically stimulated auditory nerve fibers typically fire regularly, in lock-step to the applied stimulus. At high stimulation rates, however, these same fibers fire irregularly. Firing irregularity has been attributed to the random opening and closing of voltage-gated sodium channels at the spike generation site. We demonstrate, however, that the nonlinear dynamics of neural excitation and refractoriness embodied in the FitzHughNagumo (FN) model produce realistic firing irregularity at high stimulus rates, even in the complete absence of ongoing physiological noise. Indeed, we show that ongoing noise can actually regularize the response at low discharge rates. The degree of stimulus-dependent irregularity is determined not so much by the level of ongoing physiological noise as by the dynamical instability. Our work suggests that the dynamical instability, quantified by the Lyapunov exponent, controls neural sensitivity to input signals and to physiological noise, as well the amount of mutual desynchronization between similarly stimulated fibers. This instability, quantified by the value of the Lyapunov exponent, may play a critical role in determining modulation sensitivity and dynamic range in cochlear implants.Keywords: cochlear implant, electrical stimulation, chaos, firing irregularity, desynchronization, auditory nerve, Lyapunov exponent The firing irregularity exhibited by electrically stimulated auditory nerve fibers is thought to be produced primarily by ongoing physiological noise

The response of the auditory nerve to electrical stimulation is highly sensitive to small modulations (<0.5%). This report demonstrates that dynamical instability (i.e., a positive Lyapunov exponent) can account for this sensitivity in a modified FitzHugh-Nagumo model of spike generation, so long as the input noise is not too large. This finding suggests both that spike generator instability is necessary to account for auditory nerve sensitivity and that the amplitude of physiological noise, such as that produced by the random behavior of voltage-gated sodium channels, is small. Based on these results with direct electrical stimulation, it is hypothesized that spike generator instability may be the mechanism that reconciles high sensitivity with the cross-fiber independence observed under acoustic stimulation.