Cortical depression in the mouse auditory cortex after sound discrimination learning

Auditory cortical activity elicited by infrared laser irradiation from the outer ear in Mongolian gerbils

Infrared neural stimulation has been studied for its potential to replace an electrical stimulation of a cochlear implant. No studies, however, revealed how the technic reliably evoke auditory cortical activities. This research investigated the effects of cochlear laser stimulation from the outer ear on auditory cortex using brain imaging of activity-dependent changes in mitochondrial flavoprotein fluorescence signal. An optic fiber was inserted into the gerbil’s ear canal to stimulate the lateral side of the cochlea with an infrared laser. Laser stimulation was found to activate the identified primary auditory cortex. In addition, the temporal profile of the laser-evoked responses was comparable to that of the auditory responses. Our results indicate that infrared laser irradiation from the outer ear has the capacity to evoke, and possibly manipulate, the neural activities of the auditory cortex and may substitute for the present cochlear implants in future.

Associative responses to visual shape stimuli in the mouse auditory cortex

Humans can recall various aspects of a characteristic sound as a whole when they see a visual shape stimulus that has been intimately associated with the sound. In subjects with audio-visual associative memory, auditory responses that code the associated sound may be induced in the auditory cortex in response to presentation of the associated visual shape stimulus. To test this possibility, mice were pre-exposed to a combination of an artificial sound mimicking a cat’s “meow” and a visual shape stimulus of concentric circles or stars for more than two weeks, since such passive exposure is known to be sufficient for inducing audio-visual associative memory in mice. After the exposure, we anesthetized the mice, and presented them with the associated visual shape stimulus. We found that associative responses in the auditory cortex were induced in response to the visual stimulus. The associative auditory responses were observed when complex sounds such as “meow” were used for formation of audio-visual associative memory, but not when a pure tone was used. These results suggest that associative auditory responses in the auditory cortex represent the characteristics of the complex sound stimulus as a whole.

ON-OFF receptive fields in auditory cortex diverge during development and contribute to directional sweep selectivity

Neurons in the auditory cortex exhibit distinct frequency tuning to the onset and offset of sounds, but the cause and significance of ON and OFF receptive field (RF) organisation are not understood. Here we demonstrate that distinct ON and OFF frequency tuning is largely absent in immature mouse auditory cortex and is thus a consequence of cortical development. Simulations using a novel implementation of a standard Hebbian plasticity model show that the natural alternation of sound onset and offset is sufficient for the formation of non-overlapping adjacent ON and OFF RFs in cortical neurons. Our model predicts that ON/OFF RF arrangement contributes towards direction selectivity to frequency-modulated tone sweeps, which we confirm by neuronal recordings. These data reveal that a simple and universally accepted learning rule can explain the organisation of ON and OFF RFs and direction selectivity in the developing auditory cortex.

Auditory cortex neurons exhibit distinct frequency tuning to sound onset and offset. Here the authors demonstrate that during development ON-OFF receptive fields diverge to occupy adjacent frequency ranges that may underlie their direction selective responses to frequency modulated sweeps.

Reconsidering Tonotopic Maps in the Auditory Cortex and Lemniscal Auditory Thalamus in Mice

The auditory thalamus and auditory cortex (AC) are pivotal structures in the central auditory system. However, the thalamocortical mechanisms of processing sounds are largely unknown. Investigation of this process benefits greatly from the use of mice because the mouse is a powerful animal model in which various experimental techniques, especially genetic tools, can be applied. However, the use of mice has been limited in auditory research, and thus even basic anatomical knowledge of the mouse central auditory system has not been sufficiently collected. Recently, optical imaging combined with morphological analyses has enabled the elucidation of detailed anatomical properties of the mouse auditory system. These techniques have uncovered fine AC maps with multiple frequency-organized regions, each of which receives point-to-point thalamocortical projections from different origins inside the lemniscal auditory thalamus, the ventral division of the medial geniculate body (MGv). This precise anatomy now provides a platform for physiological research. In this mini review article, we summarize these recent achievements that will facilitate physiological investigations in the mouse auditory system.

Specific distribution of non-phosphorylated neurofilaments characterizing each subfield in the mouse auditory cortex

Recent imaging studies revealed the presence of functional subfields in the mouse auditory cortex. However, little is known regarding the morphological basis underlying the functional differentiation. Distribution of particular molecules is the key information that may be applicable for identifying auditory subfields in the post-mortem brain. Immunoreactive patterns using SMI-32 monoclonal antibody against non-phosphorylated neurofilament (NNF) have already been used to identify or parcellate various brain regions in various animals. In the present study, we investigated whether distribution of NNF is a reliable marker for identifying functional subfields in the mouse auditory cortex, and found that each auditory subfield has region-specific cellular and laminar patterns of immunoreactivity for NNF.

Delineation of a frequency-organized region isolated from the mouse primary auditory cortex

The primary auditory cortex (AI) is the representative recipient of information from the ears in the mammalian cortex. However, the delineation of the AI is still controversial in a mouse. Recently, it was reported, using optical imaging, that two distinct areas of the AI, located ventrally and dorsally, are activated by high-frequency tones, whereas only one area is activated by low-frequency tones. Here, we show that the dorsal high-frequency area is an independent region that is separated from the rest of the AI. We could visualize the two distinct high-frequency areas using flavoprotein fluorescence imaging, as reported previously. SMI-32 immunolabeling revealed that the dorsal region had a different cytoarchitectural pattern from the rest of the AI. Specifically, the ratio of SMI-32-positive pyramidal neurons to nonpyramidal neurons was larger in the dorsal high-frequency area than the rest of the AI. We named this new region the dorsomedial field (DM). Retrograde tracing showed that neurons projecting to the DM were localized in the rostral part of the ventral division of the medial geniculate body with a distinct frequency organization, where few neurons projected to the AI. Furthermore, the responses of the DM to ultrasonic courtship songs presented by males were significantly greater in females than in males; in contrast, there was no sex difference in response to artificial pure tones. Our findings offer a basic outline on the processing of ultrasonic vocal information on the basis of the precisely subdivided, multiple frequency-organized auditory cortex map in mice.

Age-related deterioration of cortical responses to slow FM sounds in the auditory belt region of adult C57BL/6 mice

To compare age-related deterioration of neural responses in each subfield of the auditory cortex in C57BL/6 mice, we evaluated amplitudes of tonal responses in young (5-11 weeks old) and adult (16-23 weeks old) groups using transcranial flavoprotein fluorescence imaging. Cortical responses to 20-kHz amplitude-modulated (AM) sounds, which were mainly found in the anterior auditory field (AAF) and the primary auditory cortex (AI) of the core region, were not markedly different between the two groups. In contrast, cortical responses to direction reversal of slow frequency-modulated (FM) sounds, which were mainly found in the ultrasonic field (UF), were significantly disrupted in the adult group compared with those in the young group. To investigate the mechanisms underlying such age-related deterioration, biotinylated dextran amine (BDA) was injected into UF. The number of retrograde labeled neurons in the dorsal division of the medial geniculate body (MGd) was markedly reduced in the adult group compared with that in the young group. These results strongly suggest that cortical responses to FM direction reversal in UF of adult C57BL/6 mice are mainly deteriorated by loss of non-lemniscal thalamic inputs from MGd to UF due to aging.

Auditory cortical areas activated by slow frequency-modulated sounds in mice

Species-specific vocalizations in mice have frequency-modulated (FM) components slower than the lower limit of FM direction selectivity in the core region of the mouse auditory cortex. To identify cortical areas selective to slow frequency modulation, we investigated tonal responses in the mouse auditory cortex using transcranial flavoprotein fluorescence imaging. For differentiating responses to frequency modulation from those to stimuli at constant frequencies, we focused on transient fluorescence changes after direction reversal of temporally repeated and superimposed FM sweeps. We found that the ultrasonic field (UF) in the belt cortical region selectively responded to the direction reversal. The dorsoposterior field (DP) also responded weakly to the reversal. Regarding the responses in UF, no apparent tonotopic map was found, and the right UF responses were significantly larger in amplitude than the left UF responses. The half-max latency in responses to FM sweeps was shorter in UF compared with that in the primary auditory cortex (A1) or anterior auditory field (AAF). Tracer injection experiments in the functionally identified UF and DP confirmed that these two areas receive afferent inputs from the dorsal part of the medial geniculate nucleus (MG). Calcium imaging of UF neurons stained with fura-2 were performed using a two-photon microscope, and the presence of UF neurons that were selective to both direction and direction reversal of slow frequency modulation was demonstrated. These results strongly suggest a role for UF, and possibly DP, as cortical areas specialized for processing slow frequency modulation in mice.

Detection of virtual pitch up to 5kHz by mice

Natural sounds consist of a component at the fundamental frequency (f0) and its overtones. Pitch is perceived at f0, even when spectral energy at f0 is missing. This missing f0, or 'virtual pitch', is thought to be detected in the auditory cortex and related cortical areas, but the precise neural mechanisms are unknown. One possibility is that virtual pitch can be retrieved from the periodicity of sound waveforms. However, this mechanism requires the temporal accuracy in periodicity detection, and so far the detection of virtual pitch has only been demonstrated at frequencies lower than 1kHz. We investigated the ability of mice to detect virtual pitch up to 5kHz using a two-step sound discrimination test. In the first step of this test, mice were trained to discriminate between tone bursts at 2.5 and 5kHz. In the second step, we tested the ability of mice to discriminate between virtual pitches at 2.5kHz and at 5kHz. It was demonstrated that the performance of mice to discriminate between virtual pitches at 2.5 and 5kHz was significantly affected by previous discrimination learning between tone bursts, indicating that mice can detect virtual pitch up to 5kHz.