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W M Masters

Publications and source records attributed to W M Masters.

7 recordsLinked to original sources
PubMed2000

Range discrimination by big brown bats (Eptesicus fuscus) using altered model echoes: implications for signal processing.

The sonar emissions of two big brown bats (Eptesicus fuscus) were modeled to create a "normal" echolocation signal for each bat which was then used as an artificial echo to synthesize a phantom target. The bat's task was to indicate which of two phantom targets (presented singly) was the "near" target and which the "far" target. Threshold range discrimination at a nominal target distance of 80 cm was about 0.6 cm for both bats. The normal signal was then modified to change the relative energy in each harmonic, the signal duration, the curvature of the frequency sweep, the absolute frequency, the phase of the second and third harmonics relative to the first, or the Doppler shift of the signal. To determine which modifications affected ranging performance, the altered models were used in tests of range discrimination that were interleaved on a day-to-day basis with tests using the normal model. Of the 12 modifications tested, only those changing the curvature of the frequency sweep affected performance. This result appears not to be predicted by current models of echo processing in FM bats. Eptesicus may be able to compensate for certain types of distortions of a returning echo, an ability possibly related to Doppler tolerance or to the characteristics of the natural variation in a bat's emissions.

Animals↗
PubMed1997

Detection of jitter in intertarget spacing by the big brown bat Eptesicus fuscus.

We trained bats to detect intertarget jitter, i.e., relative motion between two virtual (electronically synthesized) targets. Both targets were themselves moving with respect to nearby objects (e.g., the microphone and speaker used to create the virtual targets) so that the only reliable cue available to the bats was variation in intertarget spacing. Given a target at 80 cm and another at 95, 110 or 125 cm, the threshold for intertarget jitter (ITJ) of the two bats tested was < 10 microseconds, corresponding to < 1.7 mm of range. When, for one bat, we increased the range instability of the targets by adding varying amounts of random range shift to the target complex (while preserving the correct intertarget spacing), ITJ threshold worsened. When we presented three targets, one of which was jittering, the bat's threshold improved to 0.9 microsecond (equivalent to 0.16 mm). If no second target was presented, i.e., if the task was to detect jitter added to a single moving target, then bats' jitter threshold was very high (> 200 microseconds). Eptesicus fuscus appears to be very good at detecting changes in intertarget spacing, which might prove valuable for detecting targets moving relative to the background or for constructing a spatial image of a complex environment.

Animals↗
PubMed1996

The degradation of distance discrimination in big brown bats (Eptesicus fuscus) caused by different interference signals.

The ability of two big brown bats (Eptesicus fuscus) to discriminate the distance to an electronically synthesized "phantom" target by echolocation was tested in the presence of interfering signals presented slightly before the target echo. Interfering signals were chosen to have differing degrees of similarity to the typical echolocation emission used by the bat in this task (which was the signal used to create the phantom target), and we predicted that the degree of disruption of ranging would be proportional to the similarity of the interference to the target echo. This prediction was not confirmed; rather, all interference signals not identical to the target echo increased the threshold to about twice that found with no interference. When the interference was identical to the target echo, the threshold increased to about 4 times that with no interference. When each bat was presented with phantom target "echoes" appropriate for the other bat, its range discrimination threshold increased about ten fold, and in this case the degree of interference of different signals was related to their similarity to the target echo, not to their similarity to the bat's "normal" signal. We suggest that Eptesicus may suppress interference by a more sophisticated strategy than simple linear matched filtering.

Animals↗
PubMed1992

Sonar gain control and echo detection thresholds in the echolocating bat, Eptesicus fuscus.

The echolocating bat, Eptesicus fuscus, detects sonar echoes with a sensitivity that changes according to the time elapsed between broadcasting of each sonar signal and reception of echoes. When tested in an electronic target simulator on a two-choice echo-detection task, the bat's threshold improved by 11.5 dB as echo delay changed from 2.3 to 4.6 ms (target ranges of 40 and 80 cm). Earlier experiments measured the change in detection threshold for delays from 1 to 6.4 ms (target ranges from about 17 to 110 cm) and obtained about 11 dB of improvement per doubling of delay. The new experiments used electronic delay lines to simulate echo delay, thus avoiding movement of loudspeakers to different distances and the possible creation of delay-dependent backward masking between stimulus echoes and cluttering echoes from the loudspeaker surfaces. The slope of the threshold shift defines an echo gain control that keeps echoes from point targets at a fixed sensation level--reducing sensitivity by 11 to 12 dB as echo amplitude increases by 12 dB per halving of range during the bat's approach to the target. A recent experiment using loudness discrimination of echoes at 70 to 80 dB SPL (roughly 50 dB above threshold) found a slope of about 6 dB per halving of range, so the gain-control effect may be level dependent. The observed effect is operationally equivalent to forward masking of echoes by the transmission, but any events correlated with vocalization which impair hearing sensitivity for a short interval following transmissions could cause a decline in sensitivity to echoes. Contractions of the bat's middle-ear muscles synchronized to transmissions may account for the observed threshold shift, at least for a span of echo delays associated with the most critical portion of the approach stage of pursuit. Forward masking by the sonar transmissions may contribute to the threshold shift, too, but middle-ear muscle contractions do occur and must be a significant part of the cause.

Animals↗
PubMed1988

Clutter interference along the target range axis in the echolocating bat, Eptesicus fuscus.

The sensitivity of the echolocating bat, Eptesicus fuscus, for detection of a sonar target is impaired by the presence of additional targets located at similar distances. At a range of 54 cm, sensitivity to one target declines if the range separation to other targets is smaller than 8-9 cm. This loss of sensitivity is an example of clutter interference along the range axis, created by simultaneous masking of one set of echoes by another. Echoes that fall within an experimentally defined critical range band may sum together to contribute collectively to detection in that band. Echoes falling into separate bands may be detected independently. Acoustic glints within a band appear to be grouped together to be perceived as a single range-extended target of complex structure. Range bands may thus define what a "target" is by specifying within-target and between-target differences in range. The width of critical range bands appears to depend upon target range, with wider bands at greater ranges.

Animals↗
PubMed1985

Sonar tracking of horizontally moving targets by the big brown bat Eptesicus fuscus.

When following a moving target, echolocating bats (Eptesicus fuscus) keep their heads aimed at the target's position. This tracking behavior seems not to involve predicting the target's trajectory, but is achieved by the bat's pointing its head at the target's last known position. The bat obtains frequent position updates by emitting sonar signals at a high rate. After the lag between head and target positions and the nonunity tracking gain were corrected for, bats' tracking accuracy in the horizontal plane was +/- 1.6 degree.

Animals↗