Dietz, M. | Search bioRxiv

bioRxiv2019

Brainstem mechanisms contributing to accurate source localisation are suited to the reverberant structure of natural acoustic scenes

Listeners perceive sound-energy as originating from the direction of its source, even as direct sound is followed milliseconds later by reflected sound from multiple different directions. Early-arriving sound is emphasised in the ascending auditory pathway, including the medial superior olive (MSO) where binaural neurons encode the interaural time difference (ITD) cue for spatial location. Behaviourally, weighting of ITD conveyed during rising sound-energy is stronger at 600 Hz, a frequency with higher reverberant energy, than at 200 Hz where reverberant energy is lower. Here we computationally explore the combined effectiveness of adaptation before ITD-encoding, and excitatory binaural coincidence detection within MSO neurons, in emphasising ITD conveyed in early-arriving sound. With excitatory inputs from adapting model spherical bushy cells (SBCs) of the bilateral cochlear nuclei, a Hodgkin-Huxley-type model MSO neuron reproduces the frequency-dependent emphasis of rising vs. peak sound-energy in ITD-encoding. Maintaining the adaptation in model SBCs, and adjusting membrane speed in model MSO neurons, hemispheric populations of model SBCs and MSO neurons, with simplified membranes for computational efficiency, also reproduce the stronger weighting of ITD information conveyed during rising sound-energy at 600 Hz compared to 200 Hz. This hemispheric model further demonstrates a link between strong weighting of spatial information during rising sound-energy, and correct unambiguous lateralisation of reverberant speech.

neuroscience↗

bioRxiv2019

Dietary polyunsaturated fatty acid supply improves Daphnia performance during vertical migration

O_LIDiel vertical migration (DVM) is a common predator avoidance strategy of zooplankton. Migration to deeper water layers during the day to escape visually hunting predators most likely requires physiological adaptations to periodically changing temperatures. Polyunsaturated fatty acids (PUFA) are essential nutrients that play crucial roles in membrane temperature acclimation. Exposure to cold temperatures typically results in an increase in the relative abundance of PUFA in cell membranes and PUFA requirements of Daphnia have been shown to increase with decreasing temperatures.\nC_LIO_LITo assess the significance of dietary PUFA for coping with temperature fluctuations experienced during DVM, we reared Daphnia magna at either constantly warm or fluctuating temperatures simulating DVM both with and without dietary PUFA supplementation.\nC_LIO_LIWe show that the well-known positive effect of dietary eicosapentaenoic acid (EPA) supplementation on offspring production and population growth rates of Daphnia is more pronounced at alternating temperatures than at constantly warm temperatures. Exposure to alternating temperatures caused modification in body PUFA concentrations and, consequently, increased lipid peroxidation. However, detrimental effects of lipid peroxidation were not evident.\nC_LIO_LIOur data demonstrate that the capacity to cope with the distinct temperature fluctuations experienced during DVM increases with dietary EPA supplementation, suggesting that an adequate dietary PUFA supply is crucial especially for migrating Daphnia populations. A dietary deficiency in long-chain PUFA may thus severely constrain predator evasion, potentially resulting in increased mortality and cascading effects on lower trophic levels.\nC_LI

physiology↗