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PubMed2005

Enhanced sensitivity to echo cues in blind subjects.

Many studies have reported that blind people compensate for their visual deficit by sharpening auditory processes. Here we compare the sensitivity to echo cues between blind and sighted subjects. In the first experiment, the blind subjects were more accurate than the sighted subjects in localizing an object on the basis of echo cues. To ensure that enhanced echolocalization abilities were not only due to the fact that blind individuals are more used to consciously paying attention to echo cues and are more familiar with this kind of tasks than sighted subjects, we tested both groups of subjects in a simple azimuthal localization task of auditory stimuli. In this second experiment, we evaluated the influence of irrelevant echo signals on auditory localization by placing the subjects and the sound sources at different positions in a sound reverberant room. Results revealed that blind subjects exhibit a higher sensitivity to echo signals than sighted subjects.

Acoustic Stimulation↗
PubMed2001

Guiding contact by coupling the taus of gaps.

Animals control contact with surfaces when locomoting, catching prey, etc. This requires sensorily guiding the rate of closure of gaps between effectors such as the hands, feet or jaws and destinations such as a ball, the ground and a prey. Control is generally rapid, reliable and robust, even with small nervous systems: the sensorimotor processes are therefore probably rather simple. We tested a hypothesis, based on general tau theory, that closing two gaps simultaneously, as required in many actions, might be achieved simply by keeping the taus of the gaps coupled in constant ratio. tau of a changing gap is defined as the time-to-closure of the gap at the current closure-rate. General tau theory shows that tau of a gap could, in principle, be directly sensed without needing to sense either the gap size or its rate of closure. In our experiment, subjects moved an effector (computer cursor) to a destination zone indicated on the computer monitor, to stop in the zone just as a moving target cursor reached it. The results indicated the subjects achieved the task by keeping tau of the gap between effector and target coupled to tau of the gap between the effector and the destination zone. Evidence of tau-coupling has also been found, for example, in bats guiding landing using echolocation. Thus, it appears that a sensorimotor process used by different species for coordinating the closure of two or more gaps between effectors and destinations entails constantly sensing the taus of the gaps and moving so as to keep the taus coupled in constant ratio.

Adult↗
PubMed2001

GABAergic and glycinergic neural inhibition in excitatory frequency tuning of bat inferior collicular neurons.

This study examined the effect of GABAergic and glycinergic inhibition on excitatory frequency tuning curves (FTCs) of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus. The excitatory FTCs of 70 IC neurons were either V-shaped (57, 81%), closed (11, 16%), or double-peaked (2, 3%). By means of a two-tone stimulation paradigm, inhibitory FTCs were obtained at one frequency flank only (low-frequency flank: 11, 16%; high-frequency flank: 7, 10%), at both frequency flanks (36, 51%) of excitatory FTCs, or between two excitatory FTCs (2, 3%). IC neurons that had inhibitory FTCs typically had larger Q(10) and Q(30) values (i.e., sharper excitatory FTCs) than neurons that did not have inhibitory FTCs. Neurons with inhibitory FTCs at both frequency flanks had larger Q(10) and Q(30) values than neurons with inhibitory FTCs at one frequency flank only. IC neurons with a small difference between excitatory and inhibitory best frequencies typically had sharper excitatory frequency tuning. Bicuculline (an antagonist for GABAA) application produced a greater degree of abolishing inhibitory FTCs than strychnine (an antagonist for glycine) application. Application of both drugs was most effective in abolishing the inhibitory FTCs of IC neurons. The implications of these findings for bat echolocation are discussed.

Acoustic Stimulation↗
PubMed2000

The sounds of silence: cessation of singing and song pausing are ultrasound-induced acoustic startle behaviors in the katydid Neoconocephalus ensiger (Orthoptera; Tettigoniidae).

Previous studies of acoustic startle in insects have dealt with behavioral and/or neural mechanisms employed in evading aerially hawking, echolocating bats; however, insects also face terrestrial predators. Here we describe an acoustic startle response of the nocturnal katydid, Neoconocephalus ensiger. Stridulating males disturbed in the field perform obvious antipredatory behaviors--cessation of singing, freezing, jumping, and evasive flight. Under controlled laboratory conditions we found that cessation of singing and song pausing are ultrasound-specific behaviors: when stimulated with pulsed ultrasound (20-100 kHz), but not audio-sound (< 20 kHz), males cease mate calling or insert pauses in their song. A second factor influencing acoustic startle is the phase of stimulation: an acoustic startle response occurs only when the pulse of ultrasound arrives during the window of silence between stridulatory syllables. The average startle threshold and response latency was 70 +/- 5 dB SPL and 34.2 +/- 6.0 ms, respectively. N. ensiger is particularly useful for examining acoustic startle responses of nonflying insects because (1) its calling song is broadband and contains ultrasound, thus the possibility exists of confusion over the biological meaning of ultrasound, and (2) this species shows the classic bat-avoidance response while flying, so a direct comparison between two types of acoustic startle is possible within the same species.

Acoustic Stimulation↗
PubMed1997

Bat-deafness in day-flying moths (Lepidoptera, Notodontidae, Dioptinae).

Assuming that bat-detection is the primary function of moth ears, the ears of moths that are no longer exposed to bats should be deaf to echolocation call frequencies. To test this, we compared the auditory threshold curves of 7 species of Venezuelan day-flying moths (Notodontidae: Dioptinae) to those of 12 sympatric species of nocturnal moths (Notodontidae: Dudusinae, Noctuidae and Arctiidae). Whereas 2 dioptines (Josia turgida, Zunacetha annulata) revealed normal ears, 2 (J. radians, J. gopala) had reduced hearing at bat-specific frequencies (20-80 kHz) and the remaining 3 (Thirmida discinota, Polypoetes circumfumata and Xenorma cytheris) revealed pronounced to complete levels of high-frequency deafness. Although the bat-deaf ears of dioptines could function in other purposes (e.g., social communication), the poor sensitivities of these species even at their best frequencies suggest that these moths represent a state of advanced auditory degeneration brought about by their diurnal life history. The phylogeny of the Notodontidae further suggests that this deafness is a derived (apomorphic) condition and not a retention of a primitive (pleisiomorphic), insensitive state.

Acoustic Stimulation↗
PubMed2000

Neural inhibition sharpens auditory spatial selectivity of bat inferior collicular neurons.

This study examines the role of neural inhibition in auditory spatial selectivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, using a two-tone inhibition paradigm. Two-tone inhibition decreases auditory spatial response areas but increases the slopes of directional sensitivity curves of inferior collicular neurons. Inferior collicular neurons have either directionally-selective or hemifield directional sensitivity curves. A directionally-selective curve always has a peak which is at least 50% larger than the minimum. A hemifield directional sensitivity curve rises from an ipsilateral angle by more than 50% and either reaches a plateau or declines by less than 50% over a range of contralateral angles. Two-tone inhibition does not change directionally-selective curves but changes most hemifield directional sensitivity curves into directionally-selective curves. Auditory spatial selectivity determined both with and without two-tone inhibition increases with increasing best-excitatory frequency. Sharpening of auditory spatial selectivity by two-tone inhibition is larger for neurons with smaller differences between excitatory and inhibitory best frequencies. The effect of two-tone inhibition on auditory spatial selectivity increases with increasing inhibitory tone intensity but decreases with increasing intertone interval. The implications of these findings in bat echolocation are discussed.

Animals↗
PubMed2003

High concentrations of isovaleric acid in the fats of odontocetes: variation and patterns of accumulation in blubber vs. stability in the melon.

Isovaleric acid (iso5:0) is an unusual fatty acid that is important for echolocation and hearing in acoustic tissues of some odontocetes, but its functional significance in blubber is unknown. We examined patterns of accumulation of this compound in blubber in 30 species of odontocetes ( n=299). Iso5:0 concentrations in blubber varied with phylogeny, ontogeny and body topography. Iso5:0 accumulated in greater quantities in superficial/outer blubber than in deep/inner blubber. In the outer blubber of northern right whale and Hector's dolphins, iso5:0 accounted for one-third to one-half of all fatty acids. Total blubber burden of iso5:0 in harbour porpoises represented up to 15 times the amount deposited in the melon. The composition of the melon does not change during starvation in harbour porpoises, supporting the hypothesis that lipids in melon are conserved for a specific function. Some odontocetes continually deposit iso5:0 in blubber after levels in melon have reached asymptotic levels, suggesting independent control of iso5:0 synthesis and storage in these compartments. Dolphins and porpoises inhabiting cold waters possess higher concentrations of iso5:0 in their outer blubber layers than species from warmer regions. We propose that this relationship represents an adaptive secondary role for iso5:0 in maintaining blubber flexibility in cold environments.

Acclimatization↗
PubMed2000

Computational models of temporal processing in the auditory thalamus.

Previous work has shown that neurons in the medial geniculate body (MGB) of the echolocating bat, Myotis lucifugus, display response properties that are distinguishable from those of their afferents in the inferior colliculus (IC). Specifically, MGB neurons display phasic temporal discharge patterns, poor entrainment to trains of constant-amplitude sound pulses, and facilitated responses to amplitude-modulated trains of sound pulses (Llano and Feng 1999). In this study we used a modeling approach to examine the relative contributions of different known sources of inhibition on the temporal response properties of auditory thalamocortical neurons. We found that GABA(A)-mediated post-excitatory inhibition resulting from coactivation of thalamocortical neurons and local inhibitory interneurons (in a triadic arrangement) is sufficient to reproduce many of the temporal response properties of MGB neurons. Addition of long-duration GABA(B)-mediated inhibition gave the thalamocortical neuron temporal response characteristics that more closely resemble those seen in the experimental data. Neither recurrent inhibition from the thalamic reticular nucleus nor postsynaptic nonlinear mechanisms were necessary to reproduce the temporal transformations between the IC and MGB. This work suggests that feed-forward inhibitory circuitry, coupled with slow GABA(B)-mediated inhibition, can emulate temporal information processing at the MGB. The transformation taking place in the MGB can be used to extract salient features from complex, time-varying stimuli, such as echoes returning from moving prey.

Animals↗
PubMed2001

Functional organization of the dorsal cochlear nucleus of the horseshoe bat (Rhinolophus rouxi) studied by GABA and glycine immunocytochemistry and electron microscopy.

Unique among mammals, the dorsal cochlear nucleus (DCN) of horseshoe bats consists of two functionally and anatomically distinct subdivisions: a laminated ventral portion that processes the frequency range below the constant frequency (CF) component of the echolocation signal and a nonlaminated dorsal portion that is specialized for processing the CF-signal range (76 kHz and higher). Using conventional transmission electron microscopy and postembedding immunocytochemistry for the inhibitory neurotransmitters GABA and glycine on semithin-alternating sections, we present further evidence that the ventral laminated subdivision of DCN conserves the main elements of microcircuitry and GABA/glycine labeling patterns typical for the mammalian DCN: (i) the main cell types and synaptic inventory of the granule cell/cartwheel cell system of the superficial layers are present as well as (ii) the tuberculoventral cell system of the deep layers. The nonlaminated dorsal subdivision lacks the granule cell/cartwheel cell system and is composed of a mixture of fusiform projection neurons with tuberculoventral cell analogues. Thus the inhibitory tuberculoventral system known to play an important role in temporal and spectral processing in VCN is conserved throughout the DCN of horseshoe bats, whereas functional components of cerebellar-like circuits are reduced in a specialized region that processes the dominant biosonar component.

Animals↗
PubMed2006

Genetic divergence and phylogeography in the genus Nyctalus (Mammalia, Chiroptera): implications for population history of the insular bat Nyctalus azoreum.

We used three mitochondrial DNA fragments with different substitution rates (ND1, Cyt b and the CR) to infer phylogenetic relationships among six species of the genus Nyctalus, and compare levels of genetic divergence between the insular, vulnerable Nyctalus azoreum and its continental counterpart to assess the origins of the Azorean bat. The larger species found throughout the Palaearctic region (N. lasiopterus, N. aviator and N. noctula) share a unique chromosome formula (2n=42) and form a monophyletic clade in our reconstructions. Nyctalus plancyi (=velutinus), a Chinese taxon with 2n=36 chromosomes, is sometimes included in N. noctula, but is genetically very divergent from the latter and deserves full species status. All Cyt b and CR haplotypes of N. azoreum are closely related and only found in the Azores archipelago, but when compared to continental sequences of N. leisleri, levels of mtDNA divergence are unusually low for mammalian species. This contrasts with the high level of differentiation that N. azoreum has attained in its morphology, ecology, and echolocation calls, suggesting a recent split followed by fast evolutionary change. The molecular data suggest that N. azoreum originated from a European population of N. leisleri, and that the colonisation of the Azores occurred at the end of the Pleistocene. The Madeiran populations of N. leisleri also appear to have a European origin, whereas those of the Canary Islands probably came from North Africa. In spite of its recent origin and low genetic divergence, the Azorean bat is well differentiated and consequently represents a unique evolutionary unit with great conservation value.

Animals↗
PubMed1983

Mapping of the auditory area in the cerebellar vermis and hemispheres of the mustache bat, Pteronotus parnellii parnellii.

Microelectrode mapping of the auditory areas in the cerebellar vermis and hemispheres of mustache bats, Pteronotus parnellii parnellii, reveals that a large area of the bat's cerebellum contains units responding to acoustic signals. A study of frequency tuning of isolated units shows that there are two large groups of auditory units. The units of one group are sharply tuned to a very narrow band of frequency with BFs between 60 and 64 kHz. The units of the other group are broadly tuned, with BFs between 47 and 59 kHz. These two groups of units are probably involved in processing the predominant CF and FM portions of the bat's orientation sounds during echolocation.

Acoustic Stimulation↗
PubMed1994

Pulse repetition rate increases the minimum threshold and latency of auditory neurons.

The effect of pulse repetition rate on auditory sensitivity of the big brown bat, Eptesicus fuscus, was studied by determining the minimum threshold, response latency and recovery cycle of inferior collicular neurons at different repetition rates under free field stimulation conditions. In general, collicular neurons shortened the response latency and increased the number of impulses monotonically or non-monotonically with stimulus intensity. They recovered at least 50% when the interpulse interval was 10-57 ms. In addition, they increased the minimum threshold, lengthened the response latency, and reduced the number of impulses discharged to each pulse with increasing repetition rate. The increase in minimum threshold with repetition rate is partly because the neuron can not recover from previous stimulation when the interpulse interval is shortened. This increase reduces a neuron's response sensitivity and thus diminishes its number of impulses to each presented pulse. This increase also reduces the effectiveness of a given stimulus intensity which contributes to the lengthening of the neuron's response latency. Data obtained from single neuron recordings are used to highlight these observations. Implications of present findings regarding the bat's echolocation are also discussed.

Acoustic Stimulation↗
PubMed1983

Unmasking in neurons of the inferior colliculus of Eptesicus fuscus with binaural stimulation.

171 single inferior colliculus neurons displaying basic auditory properties similar to those described previously were sampled, and 118 of those were tested to determine whether monaurally masked responses (band passed noise of +/- 5 kHz around the best frequency of the pure tone) could be recovered if the masking noise was presented binaurally. 26% of the units tested showed such an improvement in signal detection, i.e. what is called masking level difference (MLD) by psychoacousticians. Signal detection was improved by more than 20 dB in a few cases, but the usual improvement did not exceed 11 dB. The data suggest that MLD occurs only in units showing binaural facilitation in addition to inhibition (I, E/E type), in contrast to the more common binaural I/E types which may only provide basic azimuth information. The neurophysiological results are discussed in view of the findings described in the literature on psychoacoustical MLD and in terms of the biological importance these results have for the bat's acoustical space orientation (echolocation) and this system's excellent resistance to jamming. Since psychoacoustical explanations for the MLD effect appear to be of little relevance on a cellular level, possible neural mechanisms are discussed as well.

Animals↗
PubMed1992

Processing of pure-tone and FM stimuli in the auditory cortex of the FM bat, Myotis lucifugus.

FM bats perceive their surroundings during echolocation by analyzing frequency-modulated (FM) acoustic signals. Results from this study indicate a cortical organization in Myotis lucifugus which is largely made up of neurons sensitive to FM sounds (FM-sensitive neurons). Three types of neurons were distinguished by their responses to pure-tone and FM stimuli: (1) Type I FM-sensitive units (83%), Type II FM-sensitive units (13%) and pure-tone sensitive units (4%). Type I FM-sensitive units responded to pure tones, but exhibited greater response magnitudes to FM stimuli when the best FM swept through the BF. An orderly frequency representation was found when the frequencies of pure tones essential for response (EPTs) in Type I units were mapped along the cortical surface. The EPTs for Type I neurons were usually found within the last millisecond of a downward FM sweep. As outlined by two neuronal network models, both the responses of Type I and II units could likely result from the convergence of excitatory and inhibitory lower level neurons with slightly differing BFs. Type II units were selective for an FM sweep and showed negligible to no response to pure-tone stimuli. Pure-tone sensitive units exhibited weak or no responses to FM stimuli. These neurons were clustered in a small area located rostrodorsal to the tonotopic zone and had significantly lower best frequencies than adjacent EPT frequencies of Type I FM-sensitive neurons.

Acoustic Stimulation↗
PubMed1994

Otoacoustic emissions from the cochlea of the 'constant frequency' bats, Pteronotus parnellii and Rhinolophus rouxi.

During stimulation with continuous pure tones, the cochlea of each individual of the mustached bat, Pteronotus parnellii, produces a strong evoked stimulus-frequency otoacoustic emission (SFOAE) at about 62 kHz. The SFOAEs were on average 480 Hz above the dominant constant frequency component of the echolocation call (resting frequency, RF). In two out of nine individuals of Pteronotus the SFOAEs changed into spontaneous otoacoustic emissions of 25-40 dB SPL. In the rufuous horseshoe bat, Rhinolophus rouxi spontaneous emissions were not detected and only in two out of seven animals were there weak SFOAEs about 300 Hz above the RF of 78 kHz. This difference may be due to a stronger damping of underlying resonant processes in Rhinolophus (Henson et al., 1985a). Acoustic distortion products behaved quite similar in both species. The first lower sideband distortion 2f1-f2 was measurable over a wide frequency range between 10 and 100 kHz. The optimum frequency separation delta f of the two primary tones to evoke maximum 2f1-f2 distortion was 0.8 to 5.8 kHz in Pteronotus and 1 to 7 kHz in Rhinolophus for frequencies outside the range of the constant frequency components of the call. This corresponds to ratios f2/f1 of about 1.03 to 1.2. At the frequency of the SFOAE in Pteronotus (480 Hz above the RF) and about 300 Hz above the RF in Rhinolophus the optimum delta f decreased sharply to values of 31-63 Hz in Pteronotus (ratio f2/f1 of 1.0005-1.001), and to 39-590 Hz in Rhinolophus (ratio f2/f1 of 1.0005-1.007). In Pteronotus a second minimum of delta f was found at about 90 kHz (values of 180-620 Hz, ratios f2/f1 of 1.002-1.007). In both bat species, the respective minima of delta f are located at or close to frequencies where neuronal tuning sharpness is exceptionally high. This indicates a mechanical origin of enhanced tuning. After adjusting the frequency of f2 to match the optimum delta fs, 2f1-f2 threshold curves were obtained. The distortion product threshold approximately parallels neuronal data and is in both species characterized by a pronounced insensitivity at the RF followed by a steep threshold minimum at frequencies 0.3-3 kHz above the RF. These features may be involved in reducing the cochlear response to the call such that the bats are able to focus on the Doppler-shifted echos which are slightly higher in frequency and thus within the range of the threshold minimum.

Acoustic Stimulation↗
PubMed1994

Evidence for a mechanical filter in the cochlea of the 'constant frequency' bats, Rhinolophus rouxi and Pteronotus parnellii.

To investigate the function of basilar membrane (BM) thickenings in the cochlea of bats which use constant frequency (CF) echolocation calls, acoustic distortion products were measured while placing the primary stimuli f1 and f2 at frequencies which are represented in the thickened BM regions. In Rhinolophus, for primary stimuli between about 80-100 kHz, pronounced maxima of the level of distortion products (2f1-f2, 3f1-2f2, 4f1-3f2) can be measured if the frequency separation between the two primary tones is chosen so that the resulting distortion frequency matches the dominant CF frequency (resting frequency, RF). The distortion maxima extend from the individual RF down to frequencies which are 2-4 kHz lower. The data indicate that the thickened BM region in the basal halfturn of the cochlea strongly oscillates at the bats' RF and slightly below. The hearing threshold, however, is at a maximum at the RF (see Kössl 1993). Therefore, the RF oscillations are thought to be involved in absorbing the respective frequency such that the more apically located frequency place of the RF is insensitive and a mechanical notch filter is established. In Pteronotus, there are maxima of the level of 2f1-f2 at distortion frequencies around the RF of about 61.5 kHz up to the frequency of a strong stimulus-frequency otoacoustic emission (SFOAE) which is a few hundred Hz higher. Pronounced distortions in the RF range can only be elicited when the stimulus frequencies are between about 62 to 72 kHz. Similar to the situation in Rhinolophus, this frequency band is represented on a stretch of thickened BM.

Acoustic Stimulation↗
PubMed1996

Monaural and binaural spectral cues created by the external ears of the pallid bat.

The acoustic properties of external ears transform the spectra of incident sound in a location-dependent manner, and provide monaural and binaural spectral information used in 2-dimensional localization. Human studies suggest that binaural spectral differences, and spectral peaks and notches in monaural transfer functions, may all provide spatial information. This study examined the acoustic properties of the pallid bat ear to determine directionality, interaural intensity differences spectral peaks and notches in transfer functions, as well as acoustic gain. The pallid bat is a gleaning bat that uses passive sound localization to find prey, and echolocation for general orientation. It is capable of very accurate passive sound localization, and the primary focus of this study was to determine the spectral cues that might support this localization acuity. Results show that the external ears of this bat create spectral maxima and minima that vary systematically with azimuth and elevation. The monaural spectral cues resemble those reported in humans and cats and suggest that similar spectral cues are used across taxa. The ears also create robust interaural spectral differences that vary systematically with both sound azimuth and elevation. These monaural and binaural spectral cues may provide the basis for the 1 degrees angular resolution apparent in it this bat's passive sound localization performance.

Acoustic Stimulation↗
PubMed2005

Auditory compensation in myopic humans: involvement of binaural, monaural, or echo cues?

The purpose of this paper is to investigate the involvement of various auditory localization mechanisms in the improvement of auditory spatial sensitivity in the case of partial visual deprivation. We compared sensitivity to binaural, monaural, and echo cues between normal-sighted and myopic subjects. In an auditory task, which depended solely upon binaural processing, myopic and normal-sighted subjects showed an almost equal localization performance. We also found that myopic subjects were no more disturbed than normal-sighted subjects when spectral shape cues were removed. On the other hand, near-sighted subjects exhibited better echolocation skills and a higher sensitivity to echo cues than normal-sighted subjects. We can therefore conclude that an improved auditory spatial sensitivity in myopic subjects may result from the increased use of echo information.

Acoustic Stimulation↗