Analog VLSI Models of Range-Tuned Neurons in the Bat Echolocation System
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Analog VLSI Models of Range-Tuned Neurons in the Bat Echolocation System Matthew Cheely Neurosciences and Cognitive Sciences Program, University of Maryland, College Park, MD 20742, USA Email: [email protected]
Timothy Horiuchi Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742, USA Institute for Systems Research, University of Maryland, College Park, MD 20742, USA Email: [email protected] Received 3 May 2002 and in revised form 20 February 2003 Bat echolocation is a fascinating topic of research for both neuroscientists and engineers, due to the complex and extremely timeconstrained nature of the problem and its potential for application to engineered systems. In the bat’s brainstem and midbrain exist neural circuits that are sensitive to the specific difference in time between the outgoing sonar vocalization and the returning echo. While some of the details of the neural mechanisms are known to be species-specific, a basic model of reafference-triggered, postinhibitory rebound timing is reasonably well supported by available data. We have designed low-power, analog VLSI circuits to mimic this mechanism and have demonstrated range-dependent outputs for use in a real-time sonar system. These circuits are being used to implement range-dependent vocalization amplitude, vocalization rate, and closest target isolation. Keywords and phrases: neuromorphic engineering, echolocation, bat, biosonar, delay tuning.
1.
INTRODUCTION
Information about target range has many uses for bats during both prey capture and navigation tasks. Beyond the extraction of distance and velocity, it may be important for less obvious tasks, such as optimizing the parameters of the echolocation process. For example, as a bat approaches a target, it alters the repetition rate, duration, spectral content, and amplitude of its vocalizations [1]. Echolocation is not only used for insect capture but also provides information to the bat about obstacles, roosts, altitude, and other flying creatures. Neurons have been found in bats that show a “facilitated” response to paired sounds (a simulated vocalization and an echo) presented at particular delays. The cells’ responses to sounds presented at the appropriate delays are much greater than the sum of responses to the individual sounds presented alone. These cells are part of a larger class of neurons called “combination-sensitive” neurons, and are specifically referred to as delay-tuned cells. Delay-tuned cells are found at many levels in the bat auditory system. They have been found in the inferior colliculus (IC) [2], the medial geniculate body (MGB) [3], and the auditory cortex [4]. Disruption of cortical delay-tuned cells has been shown to impair a bat’s ability to discriminate artificial pulse-echo pair delays [5]. It is likely that delay-tuned neurons play a role in forming the
bat’s perception of range although delay-tuned cells have also been shown to respond to the social calls of other bats [6, 7]. The largest amount of information related to me
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