Biomedical subjects

James A Simmons

Publications and source records attributed to James A Simmons.

7 recordsLinked to original sources

PubMed2005

Target representation of naturalistic echolocation sequences in single unit responses from the inferior colliculus of big brown bats.

Echolocating big brown bats (Eptesicus fuscus) emit trains of frequency-modulated (FM) biosonar signals whose duration, repetition rate, and sweep structure change systematically during interception of prey. When stimulated with a 2.5-s sequence of 54 FM pulse-echo pairs that mimic sounds received during search, approach, and terminal stages of pursuit, single neurons (N = 116) in the bat's inferior colliculus (IC) register the occurrence of a pulse or echo with an average of < 1 spike/sound. Individual IC neurons typically respond to only a segment of the search or approach stage of pursuit, with fewer neurons persisting to respond in the terminal stage. Composite peristimulus-time-histogram plots of responses assembled across the whole recorded population of IC neurons depict the delay of echoes and, hence, the existence and distance of the simulated biosonar target, entirely as on-response latencies distributed across time. Correlated changes in pulse duration, repetition rate, and pulse or echo amplitude do modulate the strength of responses (probability of the single spike actually occurring for each sound), but registration of the target itself remains confined exclusively to the latencies of single spikes across cells. Modeling of echo processing in FM biosonar should emphasize spike-time algorithms to explain the content of biosonar images.

Acoustic Stimulation↗

PubMed2004

Delay accuracy in bat sonar is related to the reciprocal of normalized echo bandwidth, or Q.

Big brown bats (Eptesicus fuscus) emit wideband, frequency-modulated biosonar sounds and perceive the distance to objects from the delay of echoes. Bats remember delays and patterns of delay from one broadcast to the next, and they may rely on delays to perceive target scenes. While emitting a series of broadcasts, they can detect very small changes in delay based on their estimates of delay for successive echoes, which are derived from an auditory time/frequency representation of frequency-modulated sounds. To understand how bats perceive objects, we need to know how information distributed across the time/frequency surface is brought together to estimate delay. To assess this transformation, we measured how alteration of the frequency content of echoes affects the sharpness of the bat's delay estimates from the distribution of errors in a psychophysical task for detecting changes in delay. For unrestricted echo frequency content and high echo signal-to-noise ratio, bats can detect extremely small changes in delay of about 10 ns. When echo bandwidth is restricted by filtering out low or high frequencies, the bat's delay acuity declines in relation to the reciprocal of relative echo bandwidth, expressed as Q, which also is the relative width of the target impulse response in cycles rather than time. This normalized-time dimension may be efficient for target classification if it leads to target shape being displayed independent of size. This relation may originate from cochlear transduction by parallel frequency channels with active amplification, which creates the auditory time/frequency representation itself.

Animals↗

PubMed2004

Interaction of vestibular, echolocation, and visual modalities guiding flight by the big brown bat, Eptesicus fuscus.

The big brown bat (Eptesicus fuscus) is an aerial-feeding insectivorous species that relies on echolocation to avoid obstacles and to detect flying insects. Spatial perception in the dark using echolocation challenges the vestibular system to function without substantial visual input for orientation. IR thermal video recordings show the complexity of bat flights in the field and suggest a highly dynamic role for the vestibular system in orientation and flight control. To examine this role, we carried out laboratory studies of flight behavior under illuminated and dark conditions in both static and rotating obstacle tests while administering heavy water (D2O) to impair vestibular inputs. Eptesicus carried out complex maneuvers through both fixed arrays of wires and a rotating obstacle array using both vision and echolocation, or when guided by echolocation alone. When treated with D2O in combination with lack of visual cues, bats showed considerable decrements in performance. These data indicate that big brown bats use both vision and echolocation to provide spatial registration for head position information generated by the vestibular system.

Animals↗

PubMed2003

Time-frequency model for echo-delay resolution in wideband biosonar.

A time/frequency model of the bat's auditory system was developed to examine the basis for the fine (approximately 2 micros) echo-delay resolution of big brown bats (Eptesicus fuscus), and its performance at resolving closely spaced FM sonar echoes in the bat's 20-100-kHz band at different signal-to-noise ratios was computed. The model uses parallel bandpass filters spaced over this band to generate envelopes that individually can have much lower bandwidth than the bat's ultrasonic sonar sounds and still achieve fine delay resolution. Because fine delay separations are inside the integration time of the model's filters (approximately 250-300 micros), resolving them means using interference patterns along the frequency dimension (spectral peaks and notches). The low bandwidth content of the filter outputs is suitable for relay of information to higher auditory areas that have intrinsically poor temporal response properties. If implemented in fully parallel analog-digital hardware, the model is computationally extremely efficient and would improve resolution in military and industrial sonar receivers.

Animals↗

PubMed2003

Evaluation of an auditory model for echo delay accuracy in wideband biosonar.

In a psychophysical task with echoes that jitter in delay, big brown bats can detect changes as small as 10-20 ns at an echo signal-to-noise ratio of approximately 49 dB and 40 ns at approximately 36 dB. This performance is possible to achieve with ideal coherent processing of the wideband echoes, but it is widely assumed that the bat's peripheral auditory system is incapable of encoding signal waveforms to represent delay with the requisite precision or phase at ultrasonic frequencies. This assumption was examined by modeling inner-ear transduction with a bank of parallel bandpass filters followed by low-pass smoothing. Several versions of the filterbank model were tested to learn how the smoothing filters, which are the most critical parameter for controlling the coherence of the representation, affect replication of the bat's performance. When tested at a signal-to-noise ratio of 36 dB, the model achieved a delay acuity of 83 ns using a second-order smoothing filter with a cutoff frequency of 8 kHz. The same model achieved a delay acuity of 17 ns when tested with a signal-to-noise ratio of 50 dB. Jitter detection thresholds were an order of magnitude worse than the bat for fifth-order smoothing or for lower cutoff frequencies. Most surprising is that effectively coherent reception is possible with filter cutoff frequencies well below any of the ultrasonic frequencies contained in the bat's sonar sounds. The results suggest that only a modest rise in the frequency response of smoothing in the bat's inner ear can confer full phase sensitivity on subsequent processing and account for the bat's fine acuity or delay.

Animals↗

PubMed2002

Transformation of external-ear spectral cues into perceived delays by the big brown bat, Eptesicus fuscus.

The external-ear transfer function for big brown bats (Eptesicus fuscus) contains two prominent notches that vary from 30 to 55 kHz and from 70 to 100 kHz, respectively, as sound-source elevation moves from -40 to +10 degrees. These notches resemble a higher-frequency version of external-ear cues for vertical localization in humans and other mammals. However, they also resemble interference notches created in echoes when reflected sounds overlap at short time separations of 30-50 micros. Psychophysical experiments have shown that bats actually perceive small time separations from interference notches, and here we used the same technique to test whether external-ear notches are recognized as a corresponding time separation, too. The bats' performance reveals the elevation dependence of a time-separation estimate at 25-45 micros in perceived delay. Convergence of target-shape and external-ear cues onto echo spectra creates ambiguity about whether a particular notch relates to the object or to its location, which the bat could resolve by ignoring the presence of notches at external-ear frequencies. Instead, the bat registers the frequencies of notches caused by the external ear along with notches caused by the target's structure and employs spectrogram correlation and transformation (SCAT) to convert them all into a family of delay estimates that includes elevation.

Animals↗

PubMed2002

Selectivity for echo spectral interference and delay in the auditory cortex of the big brown bat Eptesicus fuscus.

The acoustic environment for an echolocating bat can contain multiple objects that reflect echoes so closely separated in time that they are almost completely overlapping. This results in a single echo with a spectrum characterized by deep notches due to interference. The object of this study was to document the possible selectivity, or lack thereof, of auditory neurons to the temporal separation of biosonar signals on a coarse (ms) and fine (micros) temporal scale. We recorded single-unit activity from the auditory cortex of big brown bats while presenting four protocol designs using wideband FM signals. The protocols simulated a pair of partially overlapping echoes where the separation between the first and second echo varied between 0 and 72 micros, a pulse followed by a single echo at varying delay from 0 to 30 ms, a pulse followed at a fixed delay by a pair of partially overlapping echoes that had a varying temporal separation of 0-72 micros, and a pulse followed, with a varying delay between 0 and 30 ms, by a pair of echoes that themselves had a fixed temporal separation on a microsecond time scale. About half of the cortical units showed increased spike counts to pairs of partially overlapping echoes at particular separations (6-72 micros) compared with a baseline stimulus at 0-micros separation. For many neurons tested with a pulse followed by two overlapping echoes, we observed a sensitivity to the coarse delay between the pulse and pair of overlapping echoes and to the separation between the two echoes themselves. The sensitivity to the partial overlap between the two echoes was not tuned to a single temporal separation. For bats, this means that the absolute range to the closest reflector and range between reflectors may be jointly encoded across a small population of single units. There are several possible neuronal mechanisms for encoding the separation between two nearby echoes based on the sensitivity to spectral notches.

Acoustic Stimulation↗