A Review of Neurologgers for Extracellular Recording of Neuronal Activity in the Brain of Freely Behaving Wild Animals

Mapping spatial memory in teleosts: a new Frontier in neural logging techniques

Recent advancements in microelectromechanical system technology have significantly enhanced our ability to monitor neuronal activity in free-swimming fish without disrupting their natural movement, thereby greatly improving the capabilities of neural logging using "neurologger" technology. In this review, we compiled the findings from studies applying neurologgers to teleost fish, emphasizing the discovery of various spatial-cognition cells in regions of the telencephalon analogous to the mammalian hippocampus that are deeply involved in spatial navigation. We detailed how different fish species, such as goldfish and salmonids, correlate their neural activity with environmental boundaries, head direction, speed, and other navigational cues for spatial memory and navigation strategies. We critically analyzed the similarities and differences in these mechanisms to provide insights into the evolutionary aspects of spatial cognition. We also identified gaps in current methodologies and suggest directions for future research, emphasizing the need for further exploration of spatial encoding in aquatic environments. The insights gained herein suggest the existence of a complex and evolutionarily conserved substrate for navigation and memory in vertebrates, highlighting the potential of neurologgers to expand our understanding of spatial cognition.

Mediodorsal thalamus and ventral pallidum contribute to subcortical regulation of the default mode network

Humans and other animals readily transition from externally to internally focused attention, and these transitions are accompanied by activation of the default mode network (DMN). The DMN was considered a cortical network, yet recent evidence suggests subcortical structures are also involved. We investigated the role of ventral pallidum (VP) and mediodorsal thalamus (MD) in DMN regulation in tree shrew, a close relative of primates. Electrophysiology and deep learning-based classification of behavioral states revealed gamma oscillations in VP and MD coordinated with gamma in anterior cingulate (AC) cortex during DMN states. Cross-frequency coupling between gamma and delta oscillations was higher during DMN than other behaviors, underscoring the engagement of MD, VP and AC. Our findings highlight the importance of VP and MD in DMN regulation, extend homologies in DMN regulation among mammals, and underline the importance of thalamus and basal forebrain to the regulation of DMN.

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.

Emergence of the Hippocampus as a Vector for Goal-Directed Spatial Navigation

The hippocampus, which is deeply involved in episodic memory, plays a pivotal role in spatial navigation, an essential animal behavior. Spatial navigation requires the calculation of the distance and direction from a current to the final position, i.e., a vector to a goal. Place cells in the mammalian hippocampus maximally increase their firing rates when the animal passes a particular location and then encode the animal's current location. The entorhinal cortex, one synapse upstream of the hippocampus, contains both grid and head direction cells that encode distance and direction information, respectively. However, the question of whether the hippocampus generates a vector for goal-directed navigation during the integration of distance and direction to the destination remains unclear. Mounting evidence of the cell types involved in spatial navigation has been obtained mainly in mammalian model animals such as rats and mice. Recent advances in wireless and miniaturized neural activity monitoring devices have begun to yield results not only in model organisms but also in wild mammals, birds, fish, and insects. A scrutiny of the literature examining neural correlates of spatial navigation across multiple animal species reveals that few place cells or grid cells have been found, but that head direction cells are commonly present in multiple animal species. Exceptionally, rodent-like place cells were only found in the medial pallium of tufted titmice, a food-caching bird. The medial pallium is an avian brain region homologous to the mammalian hippocampus. By contrast, rodent-like head direction cells are found in the medial pallium of quails. Head direction cells are also found in the medial pallium of streaked shearwaters, a migratory bird. The avian hippocampus contains information about the animal's current location or direction, but the neural encoding may differ depending on the ecological characteristics of the bird species. The place cells of bats, which are mammals, fly in three-dimensional space and encode vectorial information toward the goal. Training rats with an ingenious task that required them to choose a direction for each run in a maze suggested that place cells encode a vector for goal-directed spatial navigation. Thus, the scrutiny of the literature on spatial navigation-related neuronal activity across multiple animal species suggests that depending on a combination of external conditions such as the context in which the animal is situated (e.g., the context or the framework composed of landmarks in the environment) and internal conditions such as the ecological and behavioral characteristics of the animal, hippocampal neurons can be identified as place cells or head direction cells. We thus propose a conjecture that primitively, the hippocampus, or its homolog, contains information about the travel direction and that the emergence of the hippocampus during evolution has enabled the generation of vector information to the goal for advanced spatial navigation such as the search for the shortcut path and episodic memory capacity.

Adding the neuro to cognition: from food storing to nest building

Typically, investigations of animal cognition couple careful experimental manipulations with examination of the animal's behavioural responses. Sometimes those questions have included attempts to describe the neural underpinnings of the behavioural outputs. Over the past 25 years, behaviours that involve spatial learning and memory (such as navigation and food storing) has been one context in which such dual or correlated investigations have been both accessible and productive. Here I review some of that work and where it has led. Because of the wealth of data and insights gained from that work and song learning before it, it seems that it might also be useful to try to add some neurobiology to other systems in animal cognition. I finish then, with a description of recent work on the cognition and neurobiology of avian nest building. It is still relatively early days but asking questions about the cognition of nest building has already shown both neural correlates of nest building and that learning and memory play a much greater role in this behaviour than previously considered. While it is not yet clear how putting these components together will be synergistic, the examples of song learning and food storing provide encouragement. Perhaps this might be true for other behaviours too?