Event Abstract

Coherence for perception of target vs clutter in bat sonar: The role of neuroethology as pathfinder for neuroscience

  • 1 Brown University, Department of Neuroscience, United States
  • 2 Doshisha University, Neurosensing and Bionavigation Research Center, Japan

How is a perceptual “object” brought into existence by neuronal action? The temporal binding hypothesis is that responses of neurons representing different stimulus features become synchronized when these features belong to the same object. Coherence of responses binds the features together to form the percept. The crucial test is to demonstrate the dissolution of a percept by deliberately desynchronizing the spikes evoked in different feature-tuned cells. This is where neuroethology plays its pathfinding role by offering animal models that have better defined experimental examples than in conventional biomedical species. (Recall that both stimulus feature extraction and matched-filter reception were first revealed by research on frog neuroethology.) The ideal sensing system would have neurons tuned to different values of an easily manipulated stimulus feature, these neurons would produce only one spike per stimulus, and temporal coherence of spikes would be registered by coincidence-detecting cells that create the percept. Finally, to demonstrate that coherence detection is a deliberate process, the spikes being detected should occur at different absolute times, just at the same time relative to the synchronizing event. This ideal sensing system is echolocation. Big brown bats (Eptesicus fuscus) emit ~20-100 kHz FM sonar sounds containing harmonics FM1, FM2. They listen for echoes and define each object from echo delay across frequencies using pulse-echo coincidence-detecting neurons. The percept—sonar image—emerges from pooling delay estimates across frequency, as demonstrated by amplitude-latency trading of perceived delay and by the proportional decline in echo-delay accuracy as echo bandwidth is reduced. Bats perceive target shape from echo spectral nulls caused by interference between reflections from glints. Frequencies of nulls are used to estimate the separation of glints, which appear in the image. What happens to images when neuronal responses are desynchronized by manipulation of delays at different frequencies? Bats frequently fly in vegetation, and their broad transmit and receive beams preclude outright elimination of clutter echoes. To avoid clutter masking, they pay attention just to echoes from the region immediately to the front, on the beam axis. Clutter echoes are lowpass filtered by their off-axis locations, and amplitude-latency trading retards responses to the weakened FM2 relative to FM1. This disrupts coherence between harmonics, causing clutter percepts to become defocused, while the percept of an on-axis target remains sharply focused. Activation of the null-detecting system for all FM2 frequencies dramatically amplifies defocusing, which dissolves the images of clutter and renders them not capable of masking the target. Recasting the “sonar” question of clutter resistance into the cognitive question of “attention” vs “inattention” leads to the conclusion that focused object-selected attention is achieved through coherence of responses, while inattention is achieved through disruption of coherence. Furthermore, the bat’s amplification of defocusing, which occurs within the percept itself, not the response latencies, reveals that inattention is an active process, systematically imposed to isolate the object of attention from interference. Far from attention being a vague concept, these experiments identify its computational nature well enough to offer prospects for the design of truly novel biomimetic sonar.


Work supported by ONR, NSF, NIH, JSPS

Keywords: Attention, bat sonar, clutter rejection, echo delay, echo spectrum, Echolocation, inattention, target image

Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.

Presentation Type: Plenary Address (including special lectures) (Note, these individuals have already been invited)

Topic: Sensory: Audition

Citation: Simmons JA (2012). Coherence for perception of target vs clutter in bat sonar: The role of neuroethology as pathfinder for neuroscience. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00002

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Received: 28 Apr 2012; Published Online: 07 Jul 2012.

* Correspondence: Prof. James A Simmons, Brown University, Department of Neuroscience, Providence, RI, 02912, United States, James_Simmons@brown.edu