Discrimination of jittered sonar echoes by the echolocating bat, Eptesicus fuscus: the shape of target images in echolocation.

1. Behavioral experiments with jittering echoes examined acoustic images of sonar targets in the echolocating bat, Eptesicus fuscus, along the echo delay or target range axis. Echo phase, amplitude, bandwidth, and signal-to-noise ratio were manipulated to assess the underlying auditory processes for image formation. 2. Fine delay acuity is about 10 ns. Calibration and control procedures indicate that this represents temporal acuity rather than spectral discrimination. Jitter discrimination curves change in phase when the phase of one jittering echo is shifted by 180 degrees relative to the other, showing that echo phase is involved in delay estimation. At an echo detectability index of about 36 dB, fine acuity is 40 ns, which is approximately as predicted for the delay accuracy of an ideal receiver. 3. Compound performance curves for 0 degrees and 180 degrees phase conditions match the crosscorrelation function of the echoes. The locations of both 0 degrees and 180 degrees phase peaks in the performance curves shift along the time axis by an amount that matches neural amplitude-latency trading in Eptesicus, confirming a temporal basis for jitter discrimination.

An interpretation of cortical maps in echolocating bats.

Target parameters such as reflectivity, range, velocity, and angular position are represented by ordered maps of tuned cortical neurons in insectivorous bats. It is suggested that the response of each neuron in such a map is determined by a hypothesis test conditioned on a particular value of the mapped parameter. The excitation of each neuron is then interpreted as a sample value of a conditional log-likelihood ratio or a log-likelihood function. Interpolation between the samples, which is needed to find the parameter that maximizes the mapped function (e.g., the maximum likelihood parameter estimate), can be accomplished with overlapped tuning curves. An attempt to portray a sharp peak by a weighted sum of relatively broad neuronal tuning curves or interpolation functions results in excitatory center/inhibitory surround behavior. Facilitation or antifacilitation of neurons that are likely to be excited by succeeding observations can be used for sequential detection and tracking. Interpolation and pulse-to-pulse data storage capability are required to explain range jitter sensitivity and to allow for moving target indication in bat sonar. If a cortical map represents an ordered array of hypothesis tests, then many such tests are implemented in parallel when target parameters are unknown. Detection performance is then degraded relative to the idealized situation in which all parameters are specified. Performance in noise may thus appear to be much worse than that of an ideal detector, even if each hypothesis test is optimally implemented.

Ubiquity of hyperacuity.

A decision as to whether two line segments are colinear, as on a vernier scale (--/-- vs --/--), can be made with high sensitivity by the human visual system. Just-noticeable vernier displacement is much smaller than the separation required to resolve two parallel lines, i.e., to perceive them as two lines rather than one. Vernier acuity is thus also called "hyperacuity." Similar effects have been discovered in bat echolocation, for discrimination of a range-jittered point target from a nonjittered target, in the jamming avoidance response (JAR) of electric fish, and in differential pitch sensitivity experiments with human subjects. Are jitter sensitivity in echolocation, JAR in electroreception, differential pitch sensitivity in audition, and vernier acuity in vision based on the same general principle? The results in this article indicate that such phenomena are indeed similar from the viewpoint of detection theory, and that experimental performance can be used to behaviorally estimate auditory parameters such as bandwidth, beamwidth, and temporal resolution, as well as to test different signal processing models without resort to masking. Applications of the hyperacuity effect to radar, sonar, and medical ultrasound are suggested.

Phase sensitivity in electroreception.

The gymnotoid electric fish Hypopomus artedi discriminates between electric stimulus pulses with identical spectral amplitudes but different spectral phase functions. Behavioral results can be explained on the assumption that electroreception is based on a linear filter, approximately matched to the species' electric organ discharge. The impulse response of the appropriate matched filter, in fact, resembles the known impulse response of the electroreceptors involved.

Computer derivation of some dolphin echolocation signals.

Recent advances in radar theory have given rise to a straightforward method of sonar signal design. The method involves computer maximization of a signal-to-interference ratio. The procedure has been used to derive sonar signals that can accurately measure target velocity. When two dolphins were placed in a situation conducive to the utilization of such signals, their waveforms were similar to those that had been theoretically derived.