Recent studies of the neurobiology of the dorsal frontal cortex (FC) of the ferret have illuminated its key role in the attention network, top-down cognitive control of sensory processing, and goal directed behavior. To elucidate the neuroanatomical regions of the dorsal FC, and delineate the boundary between premotor cortex (PMC) and dorsal prefrontal cortex (dPFC), we placed retrograde tracers in adult ferret dorsal FC anterior to primary motor cortex and analyzed thalamo-cortical connectivity. Cyto- and myeloarchitectural differences across dorsal FC and the distinctive projection patterns from thalamic nuclei, especially from the subnuclei of the medial dorsal (MD) nucleus and the ventral thalamic nuclear group, make it possible to clearly differentiate three separate dorsal FC fields anterior to primary motor cortex: polar dPFC (dPFCpol), dPFC, and PMC. Based on the thalamic connectivity, there is a striking similarity of the ferret’s dorsal FC fields with other species. This possible homology opens up new questions for future comparative neuroanatomical and functional studies.

The pale spear-nosed bat Phyllostomus discolor, a microchiropteran bat, is well established as an animal model for research on the auditory system, echolocation and social communication of species-specific vocalizations. We have created a brain atlas of Phyllostomus discolor that provides high-quality histological material for identification of brain structures in reliable stereotaxic coordinates to strengthen neurobiological studies of this key species. The new atlas combines high-resolution images of frontal sections alternately stained for cell bodies (Nissl) and myelinated fibers (Gallyas) at 49 rostrocaudal levels, at intervals of 350 µm. To facilitate comparisons with other species, brain structures were named according to the widely accepted Paxinos nomenclature and previous neuroanatomical studies of other bat species. Outlines of auditory cortical fields, as defined in earlier studies, were mapped onto atlas sections and onto the brain surface, together with the architectonic subdivisions of the neocortex. X-ray computerized tomography (CT) of the bat's head was used to establish the relationship between coordinates of brain structures and the skull. We used profile lines and the occipital crest as skull landmarks to line up skull and brain in standard atlas coordinates. An easily reproducible protocol allows sectioning of experimental brains in the standard frontal plane of the atlas. An electronic version of the atlas plates and supplementary material is available from https://doi.org/10.12751/g-node.8bbcxy.

A new stereotaxic brain atlas of the Mongolian gerbil (Meriones unguiculatus), an important animal model in neurosciences, is presented. It combines high-quality histological material for identification of brain structures with reliable stereotaxic coordinates. The atlas consists of high-resolution images of frontal sections alternately stained for cell bodies (Nissl) and myelinated fibers (Gallyas) of 62 rostro-caudal levels at intervals of 350 μm. Brain structures were named according to the Paxinos nomenclature for rodents. The accuracy of the stereotaxic coordinate system was improved substantially by comparing and matching the series of histological sections to in vivo brain images of the gerbil obtained by magnetic resonance imaging (MRI). The skull outlines corresponding to the MR images were acquired using X-ray computerized tomography (CT) and were used to establish the relationship between coordinates of brain structures and skull. Landmarks such as lambda, bregma, ear canals and occipital crest can be used to line up skull and brain in standard atlas coordinates. An easily reproducible protocol allows sectioning of experimental brains in the standard frontal plane of the atlas.

Echolocation comprises an animal sending out signals and listening for the echoes bouncing off objects in the environment. Given the ease of recording outgoing clicks and cries, more information is known about the outgoing signals than what animals hear of the echoes. Loud outgoing signals produce pronounced echoes but also create problems for hearing when the animal immediately attempts to listen for the quiet returning echoes. Recent work measuring hearing with auditory evoked potentials has allowed a new look at what animals hear while they echolocate. Hearing sensation changes due to middle ear muscle contractions during bat vocalization production protect the auditory system from overstimulation and allow better echo hearing. Whale and dolphin hearing sensation levels also change to maximize the hearing of echoes but may operate differently. A number of mechanisms come into play to maximize hearing during echolocation.

Bats can orient and hunt for prey in complete darkness using echolocation. Due to the pulse-like character of call emission they receive a stroboscopic view of their environment. During target approach, bats adjust their emitted echolocation calls to the specific requirements of the dynamically changing environmental and behavioral context. In addition to changes of the spectro-temporal call features, the spatial focusing of the beam of the sonar emissions onto the target is a conspicuous feature during target tracking. The neural processes underlying the complex sensory-motor interactions during target tracking are not well understood. In this study, we used a two-tone-pulse paradigm with 81 combinations of inter-aural intensity differences and six inter-pulse intervals in a passive hearing task to tackle the question of how transient changes in the azimuthal position of successive sounds are encoded by neurons in the auditory cortex of the bat Phyllostomus discolor. In a population of cortical neurons (11%, 24 of 217), spatial receptive fields were focused to a small region of frontal azimuthal positions during dynamic stimulation with tone-pulse pairs at short inter-pulse intervals. The response of these neurons might be important for the behaviorally observed locking of the sonar beam onto a selected target during the later stages of target tracking. Most interestingly, the majority of these neurons (88%, 21 of 24) were located in the posterior dorsal part of the auditory cortex. This cortical subfield might thus be specifically involved in the analysis of dynamic acoustic scenes.

The Old World horseshoe bats are speculated to comprise of enormous cryptic diversity. The only Rhinolophid that has been studied with some detail in the subcontinent is the rufous horseshoe bat Rhinolophus rouxii. This bat has shown some extent of acoustic diversity between allopatric population of Peninsular India and Sri Lanka. As part of a long-term study of cryptic diversity we discovered a new phonic type of this bat in Southern India. Bats sampled from Yercaud, Tamil Nadu have principal frequencies above 90 kHz, whereas previously reported principal frequencies from bats of allopatric populations of Mahabaleswar and Srirangapattana are below 85 kHz. Interestingly, the difference between the principal frequencies of the Srirangapattana and Yercaud populations are more than those between Mahabaleswar and Srirangapattana populations, indicating the possible presence of cryptic lineages with in this species in Southern India.

Call production in the bat larynx follows the general mammalian pattern. However, several adaptations in the design of the larynx and vocal tract enable bats to produce precisely timed, high-intensity ultrasonic echolocation calls. Whereas the laryngeal innervation by the vocal motor nucleus, the nucleus ambiguus, also follows the common mammalian scheme, certain brainstem areas feeding into the nucleus ambiguus play a peculiar role in bats and operate in parallel to the descending connections from the periaqueductal gray to the final common vocal motor pathway commonly described in other mammals. Some brainstem areas exclusively control echolocation pulses, but not social calls. A similar separated involvement in either echolocation or communication may also occur at higher levels of vocal control, such as in the anterior cingulate cortex, and involve differential gene expression. Most brainstem areas involved in vocal control also receive auditory inputs, providing the audio-vocal feedback quintessential for bat echolocation.

Bats use natural landmarks such as trees for orientation. Echoes reflected by a tree are stochastic and complex. The degree of irregular loudness fluctuations of perceived echoes, i.e. the echo roughness, may be used to classify natural objects reliably. Bats are able to discriminate and classify echoes of different roughness. A neural correlate of the psychophysical roughness sensitivity has been described in the auditory cortex of the bat Phyllostomus discolor. Here, the role of the inferior colliculus of P. discolor is explored in the neural representation of echo roughness. Using extracellular recording techniques, responses were obtained to simulated stochastic echoes of different roughness. The representation of these irregular loudness fluctuations in echoes is compared to the representation of periodic loudness fluctuations elicited by sinusoidal amplitude modulation (SAM) and to the shape of the peri-stimulus time histogram in response to pure tones. About half the recorded units responded significantly differently to echoes with different roughness. Roughness sensitivity was related to the units' sensitivity to the depth of an SAM: units that responded best to strong SAMs also responded best to echoes of high roughness. In response to pure tones, these units were typically characterized as Onset units. In contrast to the auditory cortex experiments, the responses of many units in the inferior colliculus decreased with increasing echo roughness. These units typically preferred weak SAMs and showed a sustained response to pure tones. The data show that auditory midbrain sensitivity to SAM is an important prerequisite for the neural representation of echo roughness as an ecologically important echo-acoustic parameter.

Echolocating bats can identify three-dimensional objects exclusively through the analysis of acoustic echoes of their ultrasonic emissions. However, objects of the same structure can differ in size, and the auditory system must achieve a size-invariant, normalized object representation for reliable object recognition. This study describes both the behavioral classification and the cortical neural representation of echoes of complex virtual objects that vary in object size. In a phantom-target playback experiment, it is shown that the bat Phyllostomus discolor spontaneously classifies most scaled versions of objects according to trained standards. This psychophysical performance is reflected in the electrophysiological responses of a population of cortical units that showed an object-size invariant response (14/109 units, 13%). These units respond preferentially to echoes from objects in which echo duration (encoding object depth) and echo amplitude (encoding object surface area) co-varies in a meaningful manner. These results indicate that at the level of the bat's auditory cortex, an object-oriented rather than a stimulus-parameter-oriented representation of echoes is achieved.

The processing of apparent acoustic motion was investigated in neurons in the auditory cortex of anaesthetized bats (Phyllostomus discolor). Apparent motion in the horizontal plane was generated by presenting pairs of pure tones with different interaural intensity differences (IIDs) and temporal separations, i.e. inter-pulse intervals (IPIs). Thus the spatial extent, motion direction and velocity of motion changed for each stimulus pair. A complete stimulus matrix consisted of 81 IID-combinations in the range between -40 to +40 dB IID and was randomly presented via earphones with ten repetitions for up to five different IPIs (6.25 to 150 ms). Thirty percent (71) of the 236 extracellularly recorded cortical single cells or small neuronal clusters showed facilitatory responses to acoustic motion compared to static stimulation and were classified as motion sensitive. With decreasing IPI, all motion sensitive neurons changed their azimuthal receptive field in size or spatial position. Twenty two percent (15) of them preferred small movements in the frontal area at very short IPIs. Most interestingly, the motion sensitive neurons were almost exclusively (97 %) found in the dorsal area of the caudal part of the auditory cortex indicating that this cortical area is specifically involved in the processing of acoustic motion.

Background: The mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. Besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. This paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat Phyllostomus discolor (family: Phyllostomidae).Results: The auditory cortical area of P. discolor is located at parieto-temporal portions of the neocortex. It covers a rostro-caudal range of about 4800 mu m and a medio-lateral distance of about 7000 mu m on the flattened cortical surface.The auditory cortices of ten adult P. discolor were electrophysiologically mapped in detail. Responses of 849 units (single neurons and neuronal clusters up to three neurons) to pure tone stimulation were recorded extracellularly. Cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions.Based on neurophysiological and neuroanatomical criteria, the auditory cortex of P. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. The representation of response properties within the different auditory cortical fields was analyzed in detail. The two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. The dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.Conclusion: The auditory cortex of P. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. The tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. As most energy of the echolocation pulse of P. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation.

Echolocating bats can recognize 3-D objects exclusively through the analysis of the reflections of their ultrasonic emissions. For objects of small size, the spectral interference pattern of the acoustic echoes encodes information about the structure of an object. For some naturally occurring objects such as, e.g., flowers, the interference pattern as well as the echo amplitude can regularly change with the object's size, and bats should be able to compensate for both of these changes for reliable, size-invariant object recognition. In this study, electrophysiological responses of units in the auditory cortex of the bat Phyllostomus discolor were investigated using extracellular recording techniques. Acoustical stimuli consisted of echoes of virtual two-front objects that varied in size. Thus, the echoes changed systematically in amplitude and spectral envelope pattern. Whereas 30% of units simply encoded echo loudness, a considerable number of units (20%) encoded a specific spectral envelope shape independent of stimulus amplitude. In addition, a small number of cortical units (3%) were found that showed response-invariance for a covariation of echo amplitude and echo spectral envelope. The response of these two classes of units could not be simply predicted from the excitatory frequency response areas. The results show that units in the bat auditory cortex exist that might serve for the recognition of characteristic object-specific spectral echo patterns created by, e.g., flowers or other objects independent of object size or echo amplitude.

Bats, like other mammals, use communication calls for social interaction, but rely at the same time on sophisticated echolocation systems for orientation and prey capture. Both call types are of laryngeal origin, but can be distinguished on the basis of their spectral and temporal features and apparently their functional involvement as well. Although they share a common final motor pathway, there is evidence that separate vocally active brainstem areas are involved in the functional control of communication and echolocation calls. This review summarizes findings that support the above assumption, and focusses on the functional involvement of the periaqueductal gray, the paralemniscal area, and the nucleus of the brachium of the inferior colliculus, in differentiated vocal control.