Processing of Natural Echolocation Sequences in the Inferior Colliculus of Seba's Fruit Eating Bat, Carollia perspicillata

Auditory processing of communication calls in interacting bats

There is strong evidence that social context plays a role in the processing of acoustic signals. Yet, the circuits and mechanisms that govern this process are still not fully understood. The insectivorous big brown bat, Eptesicus fuscus, emits a wide array of communication calls, including food-claiming calls, aggressive calls, and appeasement calls. We implemented a competitive foraging task to explore the influence of behavioral context on auditory midbrain responses to conspecific social calls. We recorded neural population responses from the inferior colliculus (IC) of freely interacting bats and analyzed data with respect to social context. Analysis of our neural recordings from the IC shows stronger population responses to individual calls during social events. For the first time, neural recordings from the IC of a copulating bat were obtained. Our results indicate that social context enhances neuronal population responses to social vocalizations in the bat IC.

A perspective on neuroethology: what the past teaches us about the future of neuroethology

For 100 years, the Journal of Comparative Physiology-A has significantly supported research in the field of neuroethology. The celebration of the journal's centennial is a great time point to appreciate the recent progress in neuroethology and to discuss possible avenues of the field. Animal behavior is the main source of inspiration for neuroethologists. This is illustrated by the huge diversity of investigated behaviors and species. To explain behavior at a mechanistic level, neuroethologists combine neuroscientific approaches with sophisticated behavioral analysis. The rapid technological progress in neuroscience makes neuroethology a highly dynamic and exciting field of research. To summarize the recent scientific progress in neuroethology, I went through all abstracts of the last six International Congresses for Neuroethology (ICNs 2010-2022) and categorized them based on the sensory modalities, experimental model species, and research topics. This highlights the diversity of neuroethology and gives us a perspective on the field's scientific future. At the end, I highlight three research topics that may, among others, influence the future of neuroethology. I hope that sharing my roots may inspire other scientists to follow neuroethological approaches.

Descending neurons of the hoverfly respond to pursuits of artificial targets

Many animals use motion vision information to control dynamic behaviors. Predatory animals, for example, show an exquisite ability to detect rapidly moving prey, followed by pursuit and capture. Such target detection is not only used by predators but is also important in conspecific interactions, such as for male hoverflies defending their territories against conspecific intruders. Visual target detection is believed to be subserved by specialized target-tuned neurons found in a range of species, including vertebrates and arthropods. However, how these target-tuned neurons respond to actual pursuit trajectories is currently not well understood. To redress this, we recorded extracellularly from target-selective descending neurons (TSDNs) in male Eristalis tenax hoverflies. We show that they have dorso-frontal receptive fields with a preferred direction up and away from the visual midline. We reconstructed visual flow fields as experienced during pursuits of artificial targets (black beads). We recorded TSDN responses to six reconstructed pursuits and found that each neuron responded consistently at remarkably specific time points but that these time points differed between neurons. We found that the observed spike probability was correlated with the spike probability predicted from each neuron's receptive field and size tuning. Interestingly, however, the overall response rate was low, with individual neurons responding to only a small part of each reconstructed pursuit. In contrast, the TSDN population responded to substantially larger proportions of the pursuits but with lower probability. This large variation between neurons could be useful if different neurons control different parts of the behavioral output.

The influence of stimulus history on directional coding in the monarch butterfly brain

The central complex is a brain region in the insect brain that houses a neural network specialized to encode directional information. Directional coding has traditionally been investigated with compass cues that revolve in full rotations and at constant angular velocities around the insect's head. However, these stimulus conditions do not fully simulate an insect's sensory perception of compass cues during navigation. In nature, an insect flight is characterized by abrupt changes in moving direction as well as constant changes in velocity. The influence of such varying cue dynamics on compass coding remains unclear. We performed long-term tetrode recordings from the brain of monarch butterflies to study how central complex neurons respond to different stimulus velocities and directions. As these butterflies derive directional information from the sun during migration, we measured the neural response to a virtual sun. The virtual sun was either presented as a spot that appeared at random angular positions or was rotated around the butterfly at different angular velocities and directions. By specifically manipulating the stimulus velocity and trajectory, we dissociated the influence of angular velocity and direction on compass coding. While the angular velocity substantially affected the tuning directedness, the stimulus trajectory influenced the shape of the angular tuning curve. Taken together, our results suggest that the central complex flexibly adjusts its directional coding to the current stimulus dynamics ensuring a precise compass even under highly demanding conditions such as during rapid flight maneuvers.

Oscillatory discharges in the auditory midbrain of the big brown bat contribute to coding of echo delay

Subsequent to his breakthrough discovery of delay-tuned neurons in the bat's auditory midbrain and cortex, Albert Feng proposed that neural computations for echo delay involve intrinsic oscillatory discharges generated in the inferior colliculus (IC). To explore further the presence of these neural oscillations, we recorded multiple unit activity with a novel annular low impedance electrode from the IC of anesthetized big brown bats and Seba's short-tailed fruit bats. In both species, responses to tones, noise bursts, and FM sweeps contain long latency components, extending up to 60 ms post-stimulus onset, organized in periodic, oscillatory-like patterns at frequencies of 360-740 Hz. Latencies of this oscillatory activity resemble the wide distributions of single neuron response latencies in the IC. In big brown bats, oscillations lasting up to 30 ms after pulse onset emerge in response to single FM pulse-echo pairs, at particular pulse-echo delays. Oscillatory responses to pulses and evoked responses to echoes overlap extensively at short echo delays (5-7 ms), creating interference-like patterns. At longer echo delays, responses are separately evident to both pulses and echoes, with less overlap. These results extend Feng's reports of IC oscillations, and point to different processing mechanisms underlying perception of short vs long echo delays.

Neural Processing of Naturalistic Echolocation Signals in Bats

Echolocation behavior, a navigation strategy based on acoustic signals, allows scientists to explore neural processing of behaviorally relevant stimuli. For the purpose of orientation, bats broadcast echolocation calls and extract spatial information from the echoes. Because bats control call emission and thus the availability of spatial information, the behavioral relevance of these signals is undiscussable. While most neurophysiological studies, conducted in the past, used synthesized acoustic stimuli that mimic portions of the echolocation signals, recent progress has been made to understand how naturalistic echolocation signals are encoded in the bat brain. Here, we review how does stimulus history affect neural processing, how spatial information from multiple objects and how echolocation signals embedded in a naturalistic, noisy environment are processed in the bat brain. We end our review by discussing the huge potential that state-of-the-art recording techniques provide to gain a more complete picture on the neuroethology of echolocation behavior.

Population registration of echo flow in the big brown bat's auditory midbrain

Echolocating big brown bats (Eptesicus fuscus) perceive their surroundings by broadcasting frequency-modulated (FM) ultrasonic pulses and processing returning echoes. Bats echolocate in acoustically cluttered environments containing multiple objects, where each broadcast is followed by multiple echoes at varying time delays. The bat must decipher this complex echo cascade to form a coherent picture of the entire acoustic scene. Neurons in the bat's inferior colliculus (IC) are selective for specific acoustic features of echoes and time delays between broadcasts and echoes. Because of this selectivity, different subpopulations of neurons are activated as the bat flies through its environment, while the physical scene itself remains unchanging. We asked how a neural representation based on variable single-neuron responses could underlie a cohesive perceptual representation of a complex scene. We recorded local field potentials from the IC of big brown bats to examine population coding of echo cascades similar to what the bat might encounter when flying alongside vegetation. We found that the temporal patterning of a simulated broadcast followed by an echo cascade is faithfully reproduced in the population response at multiple stimulus amplitudes and echo delays. Local field potentials to broadcasts and echo cascades undergo amplitude-latency trading consistent with single-neuron data but rarely show paradoxical latency shifts. Population responses to the entire echo cascade move as a unit coherently in time as broadcast-echo cascade delay changes, suggesting that these responses serve as an index for the formation of a cohesive perceptual representation of an acoustic scene.NEW & NOTEWORTHY Echolocating bats navigate through cluttered environments that return cascades of echoes in response to the bat's broadcasts. We show that local field potentials from the big brown bat's auditory midbrain have consistent responses to a simulated echo cascade varying across echo delays and stimulus amplitudes, despite different underlying individual neuronal selectivities. These results suggest that population activity in the midbrain can build a cohesive percept of an auditory scene by aggregating activity over neuronal subpopulations.

The frugivorous bat Carollia perspicillata dynamically changes echolocation parameters in response to acoustic playback

Animals extract behaviorally relevant signals from 'noisy' environments. Echolocation behavior provides a rich system testbed for investigating signal extraction. When echolocating in acoustically enriched environments, bats show many adaptations that are believed to facilitate signal extraction. Most studies to date focused on describing adaptations in insectivorous bats while frugivorous bats have rarely been tested. Here, we characterize how the frugivorous bat Carollia perspicillata adapts its echolocation behavior in response to acoustic playback. Since bats not only adapt their echolocation calls in response to acoustic interference but also with respect to target distances, we swung bats on a pendulum to control for distance-dependent call changes. Forward swings evoked consistent echolocation behavior similar to approach flights. By comparing the echolocation behavior recorded in the presence and absence of acoustic playback, we could precisely define the influence of the acoustic context on the bats' vocal behavior. Our results show that C. perspicillata decrease the terminal peak frequencies of their calls when echolocating in the presence of acoustic playback. When considering the results at an individual level, it became clear that each bat dynamically adjusts different echolocation parameters across and even within experimental days. Utilizing such dynamics, bats create unique echolocation streams that could facilitate signal extraction in noisy environments.

Enhanced representation of natural sound sequences in the ventral auditory midbrain

The auditory midbrain (inferior colliculus, IC) plays an important role in sound processing, acting as hub for acoustic information extraction and for the implementation of fast audio-motor behaviors. IC neurons are topographically organized according to their sound frequency preference: dorsal IC regions encode low frequencies while ventral areas respond best to high frequencies, a type of sensory map defined as tonotopy. Tonotopic maps have been studied extensively using artificial stimuli (pure tones) but our knowledge of how these maps represent information about sequences of natural, spectro-temporally rich sounds is sparse. We studied this question by conducting simultaneous extracellular recordings across IC depths in awake bats (Carollia perspicillata) that listened to sequences of natural communication and echolocation sounds. The hypothesis was that information about these two types of sound streams is represented at different IC depths since they exhibit large differences in spectral composition, i.e., echolocation covers the high-frequency portion of the bat soundscape (> 45 kHz), while communication sounds are broadband and carry most power at low frequencies (20–25 kHz). Our results showed that mutual information between neuronal responses and acoustic stimuli, as well as response redundancy in pairs of neurons recorded simultaneously, increase exponentially with IC depth. The latter occurs regardless of the sound type presented to the bats (echolocation or communication). Taken together, our results indicate the existence of mutual information and redundancy maps at the midbrain level whose response cannot be predicted based on the frequency composition of natural sounds and classic neuronal tuning curves.

The fully automated bat (FAB) flight room: A human-free environment for studying navigation in flying bats and its initial application to the retrosplenial cortex

BACKGROUND

Bats can offer important insight into the neural computations underlying complex forms of navigation. Up to now, this had been done with the confound of the human experimenter being present in the same environment the bat was navigating in.

NEW METHOD

We, therefore, developed a novel behavioral setup, the fully automated bat (FAB) flight room, to obtain a detailed and quantitative understanding of bat navigation flight behavior while studying its relevant neural circuits, but importantly without human intervention. As a demonstration of the FAB flight room utility we trained bats on a four-target, visually-guided, foraging task and recorded neural activity from the retrosplenial cortex (RSC).

RESULTS

We find that bats can be efficiently trained and engaged in complex, multi-target, visuospatial behavior in the FAB flight room. Wireless neural recordings from the bat RSC during the task confirm the multiplexed characteristics of single RSC neurons encoding spatial positional information, target selection, reward obtainment and the intensity of visual cues used to guide navigation.

COMPARISON WITH EXISTING METHODS

In contrast to the methods introduced in previous studies, we now can investigate spatial navigation in bats without potential experimental biases that can be easily introduced by active physical involvement and presence of experimenters in the room.

CONCLUSIONS

Combined, we describe a novel experimental approach for studying spatial navigation in freely flying bats and provide support for the involvement of bat RSC in aerial visuospatial foraging behavior.

Long-latency optical responses from the dorsal inferior colliculus of Seba's fruit bat

We used a novel microendoscope system to record simultaneously optical activity (fluorescence of a calcium indicator dye) and electrical activity (multi-unit activity and local field potentials) from the dorsal inferior colliculus of the echolocating bat, Carollia perspicillata. Optically recorded calcium responses to wide-band noise and to frequency-modulated bursts were recorded at probe depths down to 1300 µm, with the majority of active sites encountered at more shallow depths down to 800 µm. Calcium activity exhibited long latencies, within the time span of 50-100 ms after stimulus onset, significantly longer than onset latencies of either multi-unit activity or local field potentials. Latencies and amplitude/latency trading of these electrical responses were consistent with those seen in standard electrophysiological recordings, confirming that the microendoscope was able to record both neural and optical activity successfully. Optically recorded calcium responses rose and decayed slowly and were correlated in time with long-latency negative deflections in local field potentials. These data suggest that calcium-evoked responses may reflect known, sustained inhibitory interactions in the inferior colliculus.

Bats distress vocalizations carry fast amplitude modulations that could represent an acoustic correlate of roughness

Communication sounds are ubiquitous in the animal kingdom, where they play a role in advertising physiological states and/or socio-contextual scenarios. Human screams, for example, are typically uttered in fearful contexts and they have a distinctive feature termed as "roughness", which depicts amplitude fluctuations at rates from 30-150 Hz. In this article, we report that the occurrence of fast acoustic periodicities in harsh sounding vocalizations is not unique to humans. A roughness-like structure is also present in vocalizations emitted by bats (species Carollia perspicillata) in distressful contexts. We report that 47.7% of distress calls produced by bats carry amplitude fluctuations at rates ~1.7 kHz (>10 times faster than temporal modulations found in human screams). In bats, rough-like vocalizations entrain brain potentials and are more effective in accelerating the bats' heart rate than slow amplitude modulated sounds. Our results are consistent with a putative role of fast amplitude modulations (roughness in humans) for grabbing the listeners attention in situations in which the emitter is in distressful, potentially dangerous, contexts.

Adaptations in the call emission pattern of frugivorous bats when orienting under challenging conditions

Echolocating bats emit biosonar calls and use echoes arising from call reflections, for orientation. They often pattern their calls into groups which increases the rate of sensory feedback. Insectivorous bats emit call groups at a higher rate when orienting in cluttered compared to uncluttered environments. Frugivorous bats increase the rate of call group emission when they echolocate in noisy environments. In frugivorous bats, it remains unclear if call group emission represents an exclusive adaptation to avoid acoustic interference by signals of conspecifics or if it represents an adaptation that allows to orient under demanding environmental conditions. Here, we compared the emission pattern of the frugivorous bat Carolliaperspicillata when the bats were flying in narrow versus wide or cluttered versus non-cluttered corridors. The bats emitted larger call groups and they increased the call rate within call groups when navigating in narrow or cluttered environments. These adaptations resemble the ones shown when the bats navigate in noisy environments. Thus, call group emission represents an adaptive behavior when the bats orient in complex environments.

Behaviorally relevant frequency selectivity in single- and double-on neurons in the inferior colliculus of the Pratt's roundleaf bat, Hipposideros pratti

Frequency analysis is a fundamental function of the auditory system, and it is essential to study the auditory response properties using behavior-related sounds. Our previous study has shown that the inferior collicular (IC) neurons of CF-FM (constant frequency-frequency modulation) bats could be classified into single-on (SO) and double-on (DO) neurons under CF-FM stimulation. Here, we employed Pratt's roundleaf bats, Hipposideros pratti, to investigate the frequency selectivity of SO and DO neurons in response to CF and behavior-related CF-FM sounds using in vivo extracellular recordings. The results demonstrated that the bandwidths (BWs) of iso-frequency tuning curves had no significant differences between the SO and the DO neurons when stimulated by CF sounds. However, the SO neurons had significant narrower BWs than DO neurons when stimulated with CF-FM sounds. In vivo intracellular recordings showed that both SO and DO neurons had significantly shorter post-spike hyperpolarization latency and excitatory duration in response to CF-FM in comparison to CF stimuli, suggesting that the FM component had an inhibitory effect on the responses to the CF component. These results suggested that SO neurons had higher frequency selectivity than DO neurons under behavior-related CF-FM stimulation, making them suitable for detecting frequency changes during echolocation.

Echo interval and not echo intensity drives bat flight behavior in structured corridors

To navigate in the natural environment, animals must adapt their locomotion in response to environmental stimuli. The echolocating bat relies on auditory processing of echo returns to represent its surroundings. Recent studies have shown that echo flow patterns influence bat navigation, but the acoustic basis for flight path selection remains unknown. To investigate this problem, we released bats in a flight corridor with walls constructed of adjacent individual wooden poles, which returned cascades of echoes to the flying bat. We manipulated the spacing and echo strength of the poles comprising each corridor side, and predicted that bats would adapt their flight paths to deviate toward the corridor side returning weaker echo cascades. Our results show that the bat's trajectory through the corridor was not affected by the intensity of echo cascades. Instead, bats deviated toward the corridor wall with more sparsely spaced, highly reflective poles, suggesting that pole spacing, rather than echo intensity, influenced bat flight path selection. This result motivated investigation of the neural processing of echo cascades. We measured local evoked auditory responses in the bat inferior colliculus to echo playback recordings from corridor walls constructed of sparsely and densely spaced poles. We predicted that evoked neural responses would be discretely modulated by temporally distinct echoes recorded from the sparsely spaced pole corridor wall, but not by echoes from the more densely spaced corridor wall. The data confirm this prediction and suggest that the bat's temporal resolution of echo cascades may drive its flight behavior in the corridor.

Neuronal coding of multiscale temporal features in communication sequences within the bat auditory cortex

Experimental evidence supports that cortical oscillations represent multiscale temporal modulations existent in natural stimuli, yet little is known about the processing of these multiple timescales at a neuronal level. Here, using extracellular recordings from the auditory cortex (AC) of awake bats (Carollia perspicillata), we show the existence of three neuronal types which represent different levels of the temporal structure of conspecific vocalizations, and therefore constitute direct evidence of multiscale temporal processing of naturalistic stimuli by neurons in the AC. These neuronal subpopulations synchronize differently to local-field potentials, particularly in theta- and high frequency bands, and are informative to a different degree in terms of their spike rate. Interestingly, we also observed that both low and high frequency cortical oscillations can be highly informative about the listened calls. Our results suggest that multiscale neuronal processing allows for the precise and non-redundant representation of natural vocalizations in the AC.

Natural echolocation sequences evoke echo-delay selectivity in the auditory midbrain of the FM bat, Eptesicus fuscus

Echolocating bats must process temporal streams of sonar sounds to represent objects along the range axis. Neuronal echo-delay tuning, the putative mechanism of sonar ranging, has been characterized in the inferior colliculus (IC) of the mustached bat, an insectivorous species that produces echolocation calls consisting of constant frequency and frequency modulated (FM) components, but not in species that use FM signals alone. This raises questions about the mechanisms that give rise to echo-delay tuning in insectivorous bats that use different signal designs. To investigate whether stimulus context may account for species differences in echo-delay selectivity, we characterized single-unit responses in the IC of awake passively listening FM bats, Eptesicus fuscus, to broadcasts of natural sonar call-echo sequences, which contained dynamic changes in signal duration, interval, spectrotemporal structure, and echo-delay. In E. fuscus, neural selectivity to call-echo delay emerges in a population of IC neurons when stimulated with call-echo pairs presented at intervals mimicking those in a natural sonar sequence. To determine whether echo-delay selectivity also depends on the spectrotemporal features of individual sounds within natural sonar sequences, we studied responses to computer-generated echolocation signals that controlled for call interval, duration, bandwidth, sweep rate, and echo-delay. A subpopulation of IC neurons responded selectively to the combination of the spectrotemporal structure of natural call-echo pairs and their temporal patterning within a dynamic sonar sequence. These new findings suggest that the FM bat's fine control over biosonar signal parameters may modulate IC neuronal selectivity to the dimension of echo-delay. NEW & NOTEWORTHY Echolocating bats perform precise auditory temporal computations to estimate their distance to objects. Here, we report that response selectivity of neurons in the inferior colliculus of a frequency modulated bat to call-echo delay, or target range tuning, depends on the temporal patterning and spectrotemporal features of sound elements in a natural echolocation sequence. We suggest that echo responses to objects at different distances are gated by the bat's active control over the spectrotemporal patterning of its sonar emissions.

Robustness of cortical and subcortical processing in the presence of natural masking sounds

Processing of ethologically relevant stimuli could be interfered by non-relevant stimuli. Animals have behavioral adaptations to reduce signal interference. It is largely unexplored whether the behavioral adaptations facilitate neuronal processing of relevant stimuli. Here, we characterize behavioral adaptations in the presence of biotic noise in the echolocating bat Carollia perspicillata and we show that the behavioral adaptations could facilitate neuronal processing of biosonar information. According to the echolocation behavior, bats need to extract their own signals in the presence of vocalizations from conspecifics. With playback experiments, we demonstrate that C. perspicillata increases the sensory acquisition rate by emitting groups of echolocation calls when flying in noisy environments. Our neurophysiological results from the auditory midbrain and cortex show that the high sensory acquisition rate does not vastly increase neuronal suppression and that the response to an echolocation sequence is partially preserved in the presence of biosonar signals from conspecifics.