Somatosensory Signaling for Flight Control in the Echolocating Bat Eptesicus fuscus

Abstract

Bats are the only mammals to have evolved powered flight. Their specialized hand-wings with elongated digits and a thin membrane spanning the digits not only enable flight, but give them unrivaled aerial maneuverability. Bat wing membrane is endowed with an

array of microscopic hairs that are hypothesized to monitor airflow and provide sensory feedback to guide rapid motor adjustments for flight control. The goal of this thesis is to contribute to a broader understanding of the response properties of wing-associated tactile

receptive fields, and the representation of aerodynamic feedback in the bat's nervous system.

Using the big brown bat, Eptesicus fuscus, a series of neurophysiological experiments were performed where the primary somatosensory cortical (S1) responses to tactile and airflow stimulation of the wings were analyzed. Results demonstrate that the body surface is organized topographically across the surface of S1, with an overrepresentation of wings, head and foot. The wings have an inverted orientation compared to hand representation of terrestrial mammals, with tactile thresholds that are remarkably close to human fingertips.

Airflow stimulation of the wings was achieved by brief puffs of air generated using a portable fluid dispensing system. By changing the intensity, duration and direction, airflow sensitive receptive fields were characterized based on responses of S1 neurons. Results

reveal that neuronal responses are rapidly adapting, encompassing relatively large and overlapping receptive fields with well-defined centers. S1 responses are directionally selective, with a majority preferring reversed airflow. The onset latency of evoked activity decreases as a function of airflow intensity, with no effect on response magnitude. Furthermore, when dorsal and ventral wings surfaces are stimulated simultaneously, S1 responses are either inhibited or facilitated compared to either wing surface stimulation alone. This finding suggests that outputs from the two wing surfaces are integrated in a manner that reflects the interplay of aerodynamic forces experienced by the wings.

To evaluate the central coding mechanisms of airflow sensing by bat wings, I applied an information theoretic framework to spike train data. Results indicate that the strength and direction of airflow can be encoded by the precise timing of spikes, where first post-stimulus spikes transmit bulk of the information, evidence for a latency code.