Neural Processing of Naturalistic Echolocation Signals in Bats

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.

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.

Object specific adaptation in the auditory cortex of bats

To identify behaviourally relevant sounds is an important function of the auditory system. Echolocating bats have to negotiate a wealth of sounds in the context of navigation and foraging. They must be able to detect relatively rare but behaviourally important echoes and segregate them from a large number of unimportant background echoes. For this, the bat auditory system might rely on neural deviance detection, a process influencing the excitability of a neuron depending on the frequency of occurrence of a stimulus. To investigate neural deviance detection in the auditory cortex (AC) of anaesthetised bats (Phyllostomus discolor), we designed sequences of repetitive naturalistic virtual echoes differing in spectro-temporal envelope, resembling those bats might perceive in their natural environment. In these sequences, one echo (standard) was repeated ten times and another echo (deviant) was presented at the end. Temporal intervals between echoes within the sequences varied. Our results show, that neurons in the AC of the bat P. discolor are sensitive to novel virtual echoes presented at the end of these repetitive sequences: In 49 % (62/126) of cortical neurons, extracellularly recorded responses adapted to the standard echo, but showed a strong response to the finally presented deviant echo. This effect depended strongly on the temporal intervals between echoes, with stronger adaptation at shorter intervals. This type of response behavior might represent a form of neuronal deviance detection in the AC that could help the bats to detect echoes of novel and potentially important objects within a stream of homogeneous background echoes.