Dr. Susanne HoffmannResearch Scientist
MPI for Ornithology Seewiesen
In a nutshell, my research interests can be described as being of sensory neuroethological nature. I have always been fascinated by specialized sensory systems and their underlying neural mechanisms that enable animals to survive in ecological niches. During the last few years, my research was mainly focused on the biosonar and visual system of echolocating bats but has recently been expanded to the auditory and vocal-motor system of songbirds. For my research, I mainly use in-vivo electrophysiological techniques, but I try as often as possible to combine this method with behavioral or anatomical approaches to tackle scientific questions from different directions. Novel telemetric miniature devices that have been developed at the Max-Planck-Institute for Ornithology, for example, allow the combination of individual bioacoustic monitoring with neural recordings in groups of freely-behaving birds.
Neural Control of Synchronized Social Behavior in Songbirds
Social interactions often include coordinated behavioral patterns, which are based on simultaneous precision and flexibility of motor actions between individuals. Many animal species coordinate their behavioral displays to defend common resources or to signal social bonds. The vocal duets of songbirds are in this context the signals studied best. The function and evolution of vocal duets have been repeatedly investigated, whereas knowledge about the neural substrate of duet-singing is fragmentary.
To investigate the rhythmic pattern of vocal duets, we synchronously record the individual vocal activity of wild-living colonies of duetting birds by equipping each bird with a miniature microphone that is connected to a miniature radiotelemetric transmitter. We find a highly precise temporal alignment of song syllables that are sung in alternation by male and female duet partners. Individuals of the same sex, however, exactly synchronize the production of song syllables when singing in concert. This precisely coordinated group behavior results in a chorus song that is almost indistinguishable from a duet song produced by only two individuals. To elucidate the neural mechanisms that enable this highly precise temporal coordination of vocal production between different individuals, we currently equip duetting birds with an additional miniature transmitter. This transmitter is connected to electrodes, which are implanted in the bird’s song control system and record extracellular neural activity while the bird is duetting.
The Neural Basis of Dim-light Vision in Echolocating Bats
How do sensory systems function adequately under limited conditions? Which adaptations do exist that enable sensory organs to achieve high sensitivity during restricted sensory input? The visual system of nocturnal animals, for example, possesses various adaptations allowing them to use vision in dim light. Even though echolocating bats mainly rely on their biosonar system for orienting during twilight, biosonar can be supported by vision, as many bat species have large eyes that are well adjusted to low light levels. Thus the question arises, what neural mechanisms that further facilitate the use of visual information under dim-light conditions may exist in these nocturnal animals? Surprisingly, the neural capabilities of the bat visual system have not been investigated, so far.
To investigate the peripheral visual system (visual field and retinal topography) and the representation of visual space in the midbrain of echolocating bats, we used optical, anatomical, histological and neurophysiological methods. We found that in accordance with the biosonar system, high acuity vision is directed frontally. Monocular visual fields, however, covered a much larger peripheral spatial area than what could be probed by the bat with a single echolocation call. Furthermore, for the first time ever in echolocating bats, we could show that visual space is represented in a map-like fashion in a sensory-motor nucleus of the midbrain. The sensory information provided by the visual space map may represent an adequate input for motor neurons to mediate the adjustment of the biosonar system towards the direction of a visually detected target. In addition, we discovered three neural features in the midbrain that most likely contribute to the ability of echolocating bats to exploit visual information under dim-light conditions: 1) extremely wide spatial receptive fields of visual neurons enable spatial summation of visual information, which improves the sensitivity of the visual system. 2) comparably short neural response latencies provide good temporal resolution of visual information, which is important for a fast flying animal. 3) strong oscillations in neural responses may mediate synchronization of activity within and between different levels of the bat’s visual system and thus enhance the effectiveness of visual input. With these results, we demonstrate that omnivorous bats represent a so far unknown but promising animal model to study the neurophysiological aspects of dim-light vision in nocturnal mammals.
The Neural Basis of Spatial Orienting by Echolocation
Most animals rely on their spatially extremely precise visual system for orienting. Information gained by the spatially less precise auditory system is rather used as an alerting signal to redirect the visual system towards the approximate location of a sound source. In echolocating bats the situation is typically assumed to be different: these animals exploit their highly directional biosonar system for spatial navigation and target detection. They emit ultrasonic calls and analyze returning echoes to estimate the position, distance, size, shape and surface structure of nearby objects. In this context, I am specifically interested in how the spatial information contained in the echoes is represented by auditory neurons in the bat’s brain.
Whereas neural maps for target distance have already been found in different structures of the bat’s auditory system, we recently discovered a topographic representation of target azimuth in a sensory-motor nucleus of the bat’s midbrain. Especially for bats navigating at high speed in densely structured environments, it is vitally important to transfer and coordinate spatial information between sensors and motor systems. The midbrain neural space map may mediate the transformation process of biosonar spatial information into goal-directed orienting movements. Since the sensory world of a bat is rarely static but highly dynamic, we investigated how the neural coding of spatial echo-information changes over time. We found that the pattern of spatiotemporal response characteristics of auditory neurons differs considerably between the midbrain and the cortex and that spatiotemporal processing of echo information in these structures serves very different purposes: Whereas the spatiotemporal contrast enhancement provided by the midbrain contributes to echo-feature extraction, the cortex reflects the result of this processing in terms of a high selectivity and task-oriented recombination of the extracted features.
Since 2016: Research Scientist, Department of Behavioural Neurobiology, Max-Planck-Institute for Ornithology
2015 – 2016: Maternity Leave
2013 – 2015: PostDoc, Chair of Zoology, Technische Universität München
2011 – 2013: Maternity Leave
2009 – 2011: PostDoc, Division of Neurobiology, LMU München
2005 – 2009: PhD Candidate, Division of Neurobiology, LMU München