Shunji Sugimoto

The columnar and layer-specific response properties of neurons in the primary auditory cortex of Mongolian gerbils

The columnar and layer-specific response properties of neurons in the primary auditory cortex (AI) of Mongolian gerbils were studied using single-unit recordings of responses to tone-burst stimuli presented to the ear contralateral to the recording side. During near-radial microelectrode penetrations of the AI in 100-microm steps, the best frequency (BF), best threshold (BT), best amplitude (BA), latency, tuning curve and Q10dB were recorded. Neurons encountered during single penetrations showed similar BFs, indicating a columnar frequency organization, but their latencies and Q10dBs differed. The BAs and BTs recorded within single penetrations often showed a similar value in the middle cortical layers. The latencies and Q10dBs of these neurons exhibited a tendency toward a layer-specific distribution. The latencies of neurons located in layers I-V were longer than those located in layer VI. The Q10dBs of neurons located in layers III and IV were higher than those located in layers I and VI. These results are almost consistent with those of previous studies on frequency representation, and indicated the existence of an integrative mechanism of frequency processing in the AI. This is the first study in which a layer-specific, partially columnar organization for stimulus amplitude is described.

Optical imaging of dynamic horizontal spread of excitation in rat auditory cortex slices.

Optical recordings using a voltage-sensitive dye (RH482) were conducted in brain slice preparations to investigate spatiotemporal patterns of excitation in the rat auditory cortex. Electrical stimulation of the border between the white matter and layer VI evoked vertical as well as horizontal spreading responses. While velocities of vertical and horizontal propagation of excitation were similar to those reported in non-disinhibited preparations, the horizontal propagation was widespread and strong especially in layers II/III in auditory cortex slices. This horizontal spread was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not by D-2-amino-5-phosphonopentanoic acid (D-AP5). These results suggest that the horizontal responses, especially in layers II/III, are prominent and are mediated primarily by non-N-methyl-D-aspartic acid (NMDA) receptors in the auditory cortex.

Spatiotemporal dynamics of neural activity related to auditory induction in the core and belt fields of guinea-pig auditory cortex.

Auditory induction is a continuity illusion in which missing sounds are perceived under appropriate conditions, for example, when noise is inserted during silent gaps in the sound. To elucidate the neural mechanisms underlying auditory induction, neural responses to tones interrupted by a silent gap or noise were examined in the core and belt fields of the auditory cortex using real-time optical imaging with a voltage-sensitive dye. Tone stimuli interrupted by a silent gap elicited responses to the second tone following the gap as well as early phasic responses to the first tone. Tone stimuli interrupted by broad-band noise (BN), considered to cause auditory induction, considerably reduced or eliminated responses to the tone following the noise. This reduction was stronger in the dorsocaudal field (field DC) and belt fields compared with the anterior field (the primary auditory cortex of guinea pig). Tone stimuli interrupted by notched (band-stopped) noise centered at the tone frequency, considered to decrease the strength of auditory induction, partially restored the second responses from the suppression caused by BN. These results suggest that substantial changes between responses to silent gap-inserted tones and those to BN-inserted tones emerged in field DC and belt fields. Moreover, the findings indicate that field DC is the first area in which these changes emerge, suggesting that it may be an important region for auditory induction of simple sounds.

Optical imaging of binaural interaction in multiple fields of the guinea pig auditory cortex.

Locating the source of a sound is an important function of the auditory system and interaural intensity differences are one of the most important cues. To study the functional pathways of sound localisation processing in the auditory cortex, activity in multiple fields of the guinea pig auditory cortex during stimulation with interaural intensity differences was studied using optical imaging with a voltage-sensitive dye. Of the auditory core (primary and dorsocaudal) and the belt fields which surround them, the posterior and ventroposterior belt fields were the most sensitive to interaural intensity differences. This suggests that the caudal pathway of the auditory cortex is involved in sound localisation.

Dynamic spatiotemporal inhibition in the guinea pig auditory cortex

Real-time optical imaging was conducted in the guinea pig auditory cortex to study spatiotemporal interrelations of excitation and inhibition in response to tone stimulation. Tone stimulation elicited responses consisting of three phases in the anterior field (the primary auditory cortex of guinea pig) and in the dorsocaudal field of the auditory cortex. An early depolarization was followed by a late hyperpolarization and an even later depolarization both in the maximum excitatory regions and in the lateral regions beside and/or between them. The late hyperpolarization began significantly earlier and was stronger in the lateral regions than in the maximum excitatory regions. These results show that inhibition is dynamic, both in time and in space, in the auditory cortex.

Anisotropic neural interaction in the primary auditory cortex of guinea pigs with sound stimulation

NEURAL interaction in the primary auditory cortex of guinea pigs anesthetized with sodium pentobarbital was studied using a single line multi-electrode (4 × 1) aligned across and along the isofrequency band. Under the spontaneous condition, the neural interaction was isotropic; the amplitude of cross-correlogram peaks decreased as the electrode separation increased both across and along the isofrequency band. Under tone stimulation, the neural interaction was anisotropic; the amplitude of peaks was decreased rapidly beyond 400 μm across the isofrequency band, while it decreased little up to 700 μm along the isofrequency band. This anisotropic interaction was dependent on the stimulus intensity.

Optical imaging of neural activity to vocalized sounds in the guinea‐pig auditory cortex

Spatiotemporal neural activity in response to vocalized sounds (calls) was investigated in the auditory cortex of anesthetized guinea pigs using an optical imaging technique with a voltage‐sensitive dye (RH795). Guinea‐pig calls were recorded digitally and presented to the ear from a loudspeaker located contralaterally to the recording cortex. Pure tone (PT), click (CLK), and white noise (WN) stimuli were used for comparison. The response in the primary (AI) and dorsocaudal (DC) fields of the auditory cortex to a PT appeared at the stimulus onset and in the corresponding frequency band, followed by nonsimultaneous lateral inhibition in the adjacent frequency bands. The responses to CLK and WN stimuli appeared in a wide area of the fields corresponding to the wide frequency components of the stimuli. The response to calls changed spatially and temporally because the components’ frequencies changed over time. Strong responses occurred in the frequency bands corresponding to prominent instantaneous frequency...

Extraction of sounds’ time‐frequency features in the auditory cortex

Although ‘‘time‐frequency feature extraction’’ is a fundamental cue that is used to recognize speech sounds, its physiological mechanisms remain largely unknown. This research is aimed at understanding what time‐frequency features of sounds are extracted by auditory cortical neurons. We investigated spatiotemporal response patterns in the guinea‐pig auditory cortex, using an optical recording method (MiCAM Ultima; Brain Vision) with a voltage‐sensitive dye (RH795). Experiments were performed under anesthesia (ketamine, 100 mg/kg, and xylazine, 25 mg/kg). Pure tones (PTs), frequency‐modulated (FM) sounds with various modulation rates, and their combinations were presented to the animals. Cortical activation was induced at the onset and offset of PTs along the isofrequency contours. The onset response was followed by strong inhibition. In contrast, FM sounds evoked an additional activation moving across the isofrequency contours in the cortex, with locations corresponding to the instantaneous frequency inpu...

1707 Optical recordings of spatio-temporal patterns of excitation in the rat auditory cortex in slice prepatations

HE, JUFANG The present study examined neurons in the dorsal zone (DZ) of cat auditory cortex whose responses depended on the duration of noise bursts. Of 150 long-latency neurons, 78 were classified as selective for long durations; 41 responded selectively to noise bursts of short duration; 13 showed maximal responses to noise bursts of a particular duration. We suggest that the duration-selectivity results from integration along the time domain of a stimulus during the latent period. Wheat germ agglutinin-horseradish peroxidase was injected to the DZ after physiological study. The main projection to the target region was the dorsal medial geniculate body (MGB) and posterior complex of the thalamus. Together with previous physiological and anatomical results from monkey, cat, and bat, the present results may lead to a hypothesis that the auditory pathway via the dorsal MGB is related to temporal information processing while the ventral MGB processes frequency information.

Response characteristics of neurons in columnar organization of the primary auditory cortex of Mongolian gerbils

Response characteristics of neurons in each column of the primary auditory cortex (Al) of Mongolian gerbils were studied by single‐unit recording. Tone‐burst stimuli were presented to the ear contralateral to the recording side. During a near‐radial microelectrode penetration of Al in steps of 100 μm, best frequencies (BFs), best thresholds, best amplitudes (BAs), latencies, tuning curves, and Q10 dB’s, defined as BF/(frequency bandwidth 10 dB above the best threshold), were measured. Neurons in single penetrations had similar BFs, showing columnar organization of frequency, but had various values of best thresholds (19.1–81.3 dB SPL), BAs (44.9–102.1 dB SPL), latencies (5–80 ms), and Q10 dB’s (0.34–17.53). There was a tendency for high Q10 dB’s to be located around layer IV, while there was no tendency in the distribution of low Q10 dB’s, best thresholds, and BAs. These results provide a layer specificity of tuning properties within a single column of frequency.