Echo frequency selectivity of duration-tuned inferior collicular neurons of the big brown bat, Eptesicus fuscus, determined with pulse-echo pairs

Bats are unusually insensitive to brief low-frequency tones

Bats use brief calls for echolocation, suggesting that they might be more sensitive to brief sounds than non-echolocating mammals. To investigate this possibility, absolute thresholds for brief tones were determined for four species of bats: The Common vampire bat (Desmodus rotundus) and the Greater spear-nosed bat (Phyllostomus hastatus), both of which use frequency-modulated calls, the Egyptian fruit bat (Rousettus aegyptiacus), an echolocator that uses tongue-clicks rather than laryngeal calls, and the Dog-faced fruit bat (Cynopterus brachyotis), a non-echolocating species. Norway rats and a human were tested for comparison using the same acoustic stimuli. Contrary to expectations, the echolocating bats were not superior to non-echolocating mammals in detecting brief tones in the frequency range of their echolocation calls. Instead, all four species of bats were remarkably less sensitive than non-bats to brief sounds of 10 kHz and below. This implies that temporal summation in the mammalian auditory system can show large species differences, and that the detection of brief sound is likely influenced by the selective pressures on each species as well as by the physical integration of energy in the auditory system. Such species differences in function are expected to be reflected in the physiology of their auditory systems.

The duration aftereffect does not reflect adaptation to perceived duration

Recent studies have provided evidence for a role of duration-tuned channels in the encoding of duration. Duration encoding in these channels is thought to reflect the time between responses to the onset and offset of an event. This notion is in apparent conflict with studies that demonstrate that the perceived duration of an event can vary independently from the time separating its perceived onset and offset. Instead, these studies suggest that duration encoding is sensitive to other temporal aspects of a sensory event. In the current study, we investigated whether duration-tuned channels encode duration based on the time between the on- and offset of an event (onset-offset duration), or if they encode a duration corresponding to the perceived duration of that event. We used a duration illusion to dissociate onset-offset duration and perceived duration and measured whether repeated exposure to illusion-inducing stimuli caused adaptation to the onset-offset duration or the perceived duration of these illusion-inducing stimuli. We report clear evidence for adaptation to the onset-offset duration of illusion-inducing stimuli. This finding supports the notion that duration-tuned mechanisms respond to the time between the onset and offset of an event, without necessarily reflecting the duration perceived, and eventually reported by the participant. Implications for the duration channel model and the mechanisms underlying duration illusions are discussed.

Frequency tuning of synaptic inhibition underlying duration-tuned neurons in the mammalian inferior colliculus

Inhibition plays an important role in creating the temporal response properties of duration-tuned neurons (DTNs) in the mammalian inferior colliculus (IC). Neurophysiological and computational studies indicate that duration selectivity in the IC is created through the convergence of excitatory and inhibitory synaptic inputs offset in time. We used paired-tone stimulation and extracellular recording to measure the frequency tuning of the inhibition acting on DTNs in the IC of the big brown bat (Eptesicus fuscus). We stimulated DTNs with pairs of tones differing in duration, onset time, and frequency. The onset time of a short, best-duration (BD), probe tone set to the best excitatory frequency (BEF) was varied relative to the onset of a longer-duration, nonexcitatory (NE) tone whose frequency was varied. When the NE tone frequency was near or within the cell's excitatory bandwidth (eBW), BD tone-evoked spikes were suppressed by an onset-evoked inhibition. The onset of the spike suppression was independent of stimulus frequency, but both the offset and duration of the suppression decreased as the NE tone frequency departed from the BEF. We measured the inhibitory frequency response area, best inhibitory frequency (BIF), and inhibitory bandwidth (iBW) of each cell. We found that the BIF closely matched the BEF, but the iBW was broader and usually overlapped the eBW measured from the same cell. These data suggest that temporal selectivity of midbrain DTNs is created and preserved by having cells receive an onset-evoked, constant-latency, broadband inhibition that largely overlaps the cell's excitatory receptive field. We conclude by discussing possible neural sources of the inhibition.NEW & NOTEWORTHY Duration-tuned neurons (DTNs) arise from temporally offset excitatory and inhibitory synaptic inputs. We used single-unit recording and paired-tone stimulation to measure the spectral tuning of the inhibitory inputs to DTNs. The onset of inhibition was independent of stimulus frequency; the offset and duration of inhibition systematically decreased as the stimulus departed from the cell's best excitatory frequency. Best inhibitory frequencies matched best excitatory frequencies; however, inhibitory bandwidths were more broadly tuned than excitatory bandwidths.

The aftereffect of perceived duration is contingent on auditory frequency but not visual orientation

Recent sensory history plays a critical role in duration perception. It has been established that after adapting to a particular duration, the test durations within a certain range appear to be distorted. To explore whether the aftereffect of perceived duration can be constrained by sensory modality and stimulus feature within a modality, the current study applied the technique of simultaneous sensory adaptation, by which observers were able to simultaneously adapt to two durations defined by two different stimuli. Using both simple visual and auditory stimuli, we found that the aftereffect of perceived duration is modality specific and contingent on auditory frequency but not visual orientation of the stimulus. These results demonstrate that there are independent timers responsible for the aftereffects of perceived duration in each sensory modality. Furthermore, the timer for the auditory modality may be located at a relatively earlier stage of sensory processing than the timer for the visual modality.

Sexual dimorphism in echolocation pulse parameters of the CF-FM bat, Hipposideros pratti

BACKGROUND

Previousstudies of sexual dimorphism in the echolocation pulses of the constant frequency-frequency modulating (CF-FM) bat have been mainly concentrated on the difference in the frequency of the CF component of the predominant second harmonic while neglected other pulse parameters. However, recent studies have shown that other pulse parameters of the predominant second harmonic are also biologically significant to the bat hunting. To complement and advance these studies, we have examined sexual dimorphism of multiple parameters (e.g., duration, frequency, bandwidth of the FM component, and repetition rate of emitted pulses) of the echolocation pulses of the CF-FM bat, Hipposideros pratti.

RESULTS

Our studies of the predominant second harmonic show that on average the male bat has higher frequency of the CF component, wider FM bandwidth, and higher pulse repetition rate while the female bat has longer duration of the CF and FM components.

CONCLUSIONS

Theseobservations suggest that bats may potentially use this sexual dimorphism in echolocation pulse parameters for social communication and species and sex identification.

Echo amplitude sensitivity of bat auditory neurons improves with decreasing pulse-echo gap

During hunting, insectivorous bats systematically vary the parameters of emitted pulses and analyze the returning echoes to extract prey features. As such, the duration of the pulse (P) and echo (E), the P-E gap, and the P-E amplitude difference progressively decrease throughout the prey-approach sequence. Our previous studies have shown that most inferior collicular neurons of bats discharge maximally to a best duration, and they have the sharpest echo frequency and amplitude sensitivity when stimulated with P-E pairs with duration the same as the best duration. Furthermore, their echo duration and frequency sensitivity improves with decreasing P-E duration and P-E gap. The present study shows that this is also true in the amplitude domain. Thus, all these data indicate that bats can better extract multiple parameters of expected rather than unexpected echo after pulse emission. They also support the hypothesis that a bat's inferior collicular neurons improve the response sensitivity in multiple parametric domains as the prey is approached to increase the success of hunting.

Organization and trade-off of spectro-temporal tuning properties of duration-tuned neurons in the mammalian inferior colliculus

Neurons throughout the mammalian central auditory pathway respond selectively to stimulus frequency and amplitude, and some are also selective for stimulus duration. First found in the auditory midbrain or inferior colliculus (IC), these duration-tuned neurons (DTNs) provide a potential neural mechanism for encoding temporal features of sound. In this study, we investigated how having an additional neural response filter, one selective to the duration of an auditory stimulus, influences frequency tuning and neural organization by recording single-unit responses and measuring the dorsal-ventral position and spectral-temporal tuning properties of auditory DTNs from the IC of the awake big brown bat (Eptesicus fuscus). Like other IC neurons, DTNs were tonotopically organized and had either V-shaped, U-shaped, or O-shaped frequency tuning curves (excitatory frequency response areas). We hypothesized there would be an interaction between frequency and duration tuning in DTNs, as electrical engineering theory for resonant filters dictates a trade-off in spectral-temporal resolution: sharp tuning in the frequency domain results in poorer resolution in the time domain and vice versa. While the IC is a more complex signal analyzer than an electrical filter, a similar operational trade-off could exist in the responses of DTNs. Our data revealed two patterns of spectro-temporal sensitivity and spatial organization within the IC: DTNs with sharp frequency tuning and broad duration tuning were located in the dorsal IC, whereas cells with wide spectral tuning and narrow temporal tuning were found in the ventral IC.

Duration tuning in the auditory midbrain of echolocating and non-echolocating vertebrates

Neurons tuned for stimulus duration were first discovered in the auditory midbrain of frogs. Duration-tuned neurons (DTNs) have since been reported from the central auditory system of both echolocating and non-echolocating mammals, and from the central visual system of cats. We hypothesize that the functional significance of auditory duration tuning likely varies between species with different evolutionary histories, sensory ecologies, and bioacoustic constraints. For example, in non-echolocating animals such as frogs and mice the temporal filtering properties of auditory DTNs may function to discriminate species-specific communication sounds. In echolocating bats duration tuning may also be used to create cells with highly selective responses for specific rates of frequency modulation and/or pulse-echo delays. The ability to echolocate appears to have selected for high temporal acuity in the duration tuning curves of inferior colliculus neurons in bats. Our understanding of the neural mechanisms underlying sound duration selectivity has improved substantially since DTNs were first discovered almost 50 years ago, but additional research is required for a comprehensive understanding of the functional role and the behavioral significance that duration tuning plays in sensory systems.

Bats and frogs and animals in between: evidence for a common central timing mechanism to extract periodicity pitch

Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down to 4 ms or less (about 20-300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons with different onset latencies from 4-5 to 20-50 ms. Midbrain latency distributions are wide enough to encompass two or more repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific times for the dispersed on-responses.