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At least 271 records · Page 15Linked to original sources
PubMed1989

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.

Animals↗
PubMed1989

Range estimation by echolocation in the bat Eptesicus fuscus: trading of phase versus time cues.

Bats of the species Eptesicus fuscus have been trained to discriminate a stationary simulated target from a target with a virtual distance that jitters from sound to sound. Similar to Simmons [Science 207, 1336-1338 (1979)], a jitter-detection threshold below 1 microsecond was found. However, Simmons' decreased performance at a time delay jitter of 30 microseconds could not be replicated, a critical feature used to postulate the idea that bats employ a coherent cross-correlation receiver for ranging. Such a receiver uses all phase information in the signal for delay estimation and therefore will be biased by phase manipulations. To test for such a bias, a phase jitter of +/- 45 degrees and a time jitter in the echo were overlaid. It was not found that there was a combination of both where their effects canceled. Full phase information is thus not used in delay estimation. However, bats were able to detect a pure phase jitter, e.g., polarity inversion of the signal. Bats could also detect phase jitter in the presence of randomized time jitter and vice versa. Phase jitter and time jitter, therefore, are separable features for a bat. The underlying physiological mechanism is not clear.

Amplifiers, Electronic↗
PubMed1992

Stabilization of perceived echo amplitudes in echolocating bats. I. Echo detection and automatic gain control in the big brown bat, Eptesicus fuscus, and the fishing bat, Noctilio leporinus.

Previous research on echo detection in bats has suggested that the effective threshold is a function of the acoustic clutter in the experimental environment, as might be expected given the low ambient noise levels typical of such psychophysical research. This paper demonstrates that theory of signal detectability (TSD) methodology is applicable to bats and uses it to show that an important element of clutter limiting in Eptesicus fuscus and Noctilio leporinus is backward masking of phantom targets by the real echo from the loudspeakers used to generate them. This information suggests that a previous estimate of the magnitude of automatic gain control (AGC) is too high, due to variable backward masking inherent in the experimental method used. A re-examination of gain control using a masking-free method shows that it reduces auditory sensitivity by 6 to 7 dB per halving of target range, rather than 11 dB as previously thought.

Animals↗
PubMed1993

A computational model of echo processing and acoustic imaging in frequency-modulated echolocating bats: the spectrogram correlation and transformation receiver.

The spectrogram correlation and transformation (SCAT) model of the sonar receiver in the big brown bat (Eptesicus fuscus) consists of a cochlear component for encoding the bat's frequency modulated (FM) sonar transmissions and multiple FM echoes in a spectrogram format, followed by two parallel pathways for processing temporal and spectral information in sonar echoes to reconstruct the absolute range and fine range structure of multiple targets from echo spectrograms. The outputs of computations taking place along these parallel pathways converge to be displayed along a computed image dimension of echo delay or target range. The resulting image depicts the location of various reflecting sources in different targets along the range axis. This series of transforms is equivalent to simultaneous, parallel forward and inverse transforms on sonar echoes, yielding the impulse responses of targets by deconvolution of the spectrograms. The performance of the model accurately reproduces the images perceived by Eptesicus in a variety of behavioral experiments on two-glint resolution in range, echo phase sensitivity, amplitude-latency trading of range estimates, dissociation of time- and frequency-domain image components, and ranging accuracy in noise.

Animals↗
PubMed2000

Foliage echoes: a probe into the ecological acoustics of bat echolocation.

The research reported here aims at understanding the biosonar system of bats based on the properties of its natural inputs (ecological acoustics). Echoes from foliages are studied as examples of ubiquitous, natural targets. The echo properties and their qualitative relationship to plant architecture are described. The echoes were found to be profoundly stochastic and in general neither Gaussian nor stationary. Consequently, features useful for discrimination of such target classes will be confined to estimated random process parameters. Several such statistical signal features which are sufficiently invariant to allow a classification of the used example plants were identified: the characteristic exponent and the dispersion of an alpha-stable model for the amplitude distribution, a crest factor defined as the ratio of maximum squared amplitude and signal energy, the dispersion of the first threshold passage distribution, the structure of the correlation matrix, and a nonstationarity in sound channel gain. Discrimination error probability could be reduced by combining features pairwise. The best combination was the crest factor and the correlation coefficient of a log-linear model of the time-variant sound channel gain; it yielded an estimated Bayes risk of 6.9% for data pooled from different views.

Acoustics↗
PubMed2007

Echolocation signals of wild harbour porpoises, Phocoena phocoena.

Field recordings of harbour porpoises (Phocoena phocoena) were made in the inner Danish waters with a vertical array of three or four hydrophones. The back-calculated source level ranged from 178 to 205 dB re 1 muPa pp @ 1 m with a mean source level of 191 dB re 1 muPa pp @ 1 m. The maximum source level was more than 30 dB above what has been measured from captive animals, while the spectral and temporal properties were comparable. Calculations based on the sonar equation indicate that harbour porpoises, using these high click intensities, should be capable of detecting fish and nets and should be detectable by porpoise detectors over significantly larger distances than had previously been assumed. Harbour porpoises in this study preferred a relatively constant inter-click interval of about 60 ms, but intervals up to 200 ms and down to 30 ms were also recorded.

Animals↗
PubMedDate not supplied

[Letter: Work of the impulse section of the horseshoe bat echolocator].

Peculiarities of location impulse of Rhinolophidae bring about a distinctive combination of dopler and impulse parts working almost independently, in its locator. In particular the range of the effect of impulse locator connected with the presence of LFM splash in the end of location cry is determined by the value of the period of its repetition. The existence of a long monofrequent part of locaion impulse providing the work of acholocator dopler part due to its narrow bands presents only a comparatively small obstacle to the impulse locator.

Animals↗
PubMedDate not supplied

[Effect of destruction of the auditory centers of the brain on the range of echolocation by bats Rhinolophus ferrum-equinum (the large horseshoe-nose bat)].

Effect of the auditory cortex, internal geniculate bodies and inferior colliculus removal on echo-ranging detection of immovable wire obstacle by bats has been studied in the behavioural procedure. It is shown that destruction of the auditory cortex results in an irreversible decrease (to 74.6% of control values) in limiting detection range of obstacle by animals. Extensive destructions of internal geniculate bodies decrease limiting detection range only in the first postoperative days. In the case of the inferior colliculi + removal the limiting detection range irreversibly decreases to values equaling 38.5% of control ones. A suggestion that the obtained effects are a consequence of the auditory system sensitivity deterioration in bats is substantiated.

Animals↗