Optical imaging of neural activity in multiple auditory cortical fields of guinea pigs.

Neural activity of multiple fields in the auditory cortex of anesthetized guinea pigs in response to pure tones was visualized by optical recording using a voltage-sensitive dye (RH795). Ten auditory fields were identified based on the tonotopic organization and response latency: the core fields consisting of the primary (AI) and secondary (AII) fields and surrounding belt areas consisting of dorso-anterior (DA), dorsal (D), dorso-posterior (DP), posterior (P), ventro-posterior (VP), ventro-medial (VM), ventro-anterior (VA) and ventral (V) fields. Tonotopic organization was observed in all the fields apart from DA, D, DP and V. Spatio-temporal displays suggest that the auditory information spreads from the core fields of AI and AII to belt fields via three distinct (dorsocaudal, caudal and ventrorostral) pathways.

Statistical procedures for spatiotemporal neuronal data with applications to optical recording of the auditory cortex.

This article presents new procedures for multisite spatiotemporal neuronal data analysis. A new statistical model - the diffusion model - is considered, whose parameters can be estimated from experimental data thanks to mean-field approximations. This work has been applied to optical recording of the guinea pig's auditory cortex (layers II-III). The rates of innovation and internal diffusion inside the stimulated area have been estimated. The results suggest that the activity of the layer balances between the alternate predominance of its innovation process and its internal process.

Spatiotemporal representation of binaural difference in time and intensity of sound in the guinea pig auditory cortex.

Neural activity of the auditory cortex (AC) in response to a change of interaural intensity difference (IID) and interaural time difference (ITD) of sound stimuli was observed by optical recording with a 12 x 12 photodiode array and the voltage-sensitive dye, RH795. Guinea pigs (280-450 g) were anesthetized with sodium pentobarbital (30 mg/kg) and supplemental doses of neuroleptic solutions. When both ears were stimulated dichotically by tone bursts (14 kHz, 75 dB SPL), excitatory optical signals appeared in both anterior (A) and dorsocaudal (DC) fields of AC. An increase of intensity of ipsilateral stimulation from 65 to 95 dB SPL caused a decrease of neural activity of isofrequency bands in both fields. An increase of ipsilateral leads from -2.5 to 10 ms resulted in a gradual decrease of the amplitude of the excitatory responses. A strong inhibition was observed in field DC and the ventral portion of field A. These results show the different spatiotemporal representation of IID and ITD sensitivities in AC. However, the ipsilateral lead inducing a large inhibition was much longer than the time difference (80 micros) calculated from the interaural distance of the guinea pig. This indicates that the longer binaural inhibition observed in AC would have a different functional significance from that of the neural system of ITD detection in the guinea pig.

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 x 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 microm across the isofrequency band, while it decreased little up to 700 microm along the isofrequency band. This anisotropic interaction was dependent on the stimulus intensity.

Optical recording of responses to frequency-modulated sounds in the auditory cortex.

Using an optical recording method with a voltage-sensitive dye, we recorded activities in the primary auditory cortex (AI) of anesthetized guinea pigs in response to frequency-modulated (FM) sounds and sounds with stepwise changes in frequency (SF). Responses to the FM sound showed a spatiotemporal pattern in which a localized active spot traversed the isofrequency bands in the AI, and they differed from the band-like responses to the SF sound. These results indicate that time-varying sounds are represented as spatiotemporal activation of tonotopic organization in the AI by spectral cues with interactions between frequency bands.

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.

NMDA-mediated facilitation in the echo-delay tuned areas of the auditory cortex of the mustached bat.

We recorded the responses of single delay-tuned neurons in the dorsal fringe (DF) area and the FM-FM area of the auditory cortex of the mustached bat using multi-barreled carbon-fiber electrodes. An iontophoretic application of N-methyl-D-aspartate (NMDA) or kainate (KA) to a DF neuron evoked a burst of discharges from the neuron. The burst of discharges evoked by NMDA was always smaller than that evoked by KA. Simultaneous application of D-2-Amino-5-phosphonovalerate (APV) with NMDA and KA abolished the NMDA-evoked but not the KA-evoked discharges. APV did not evoke any significant changes in the auditory responses of 43 out of the 47 delay-tuned neurons studied in the DF area, and in all 20 neurons studied in the FM-FM area. In the remaining four DF neurons, however, APV either increased the initial discharges of their auditory response or decreased the late discharges of their response. These results indicate that in the majority of neurons in the DF and FM-FM areas NMDA receptors do not play a significant role in the processing of target-distance information, and that their facilitative auditory responses are basically created by synaptic interactions occurring in the subcortical auditory nuclei.

Optical imaging of neural activity in auditory cortex induced by intracochlear electrical stimulation.

Little is known about the representation of electrically evoked activity in the auditory cortex. We observed evoked activity in guinea pig auditory cortex evoked by acoustical and electrical stimulation to the cochlea by optical imaging with the aid of a voltage-sensitive dye. Light signals from the cortex were recorded with a 12 x 12 array of photodiodes, and transferred to the spatio-temporal images by every 0.57 ms. The activity by pure tones was shown spatio-temporally through tonotopical organization in the cortex according to the sound frequencies. The tonotopic responses were dynamically changed. When the cochlea was stimulated with single electrical pulses, focal activities were observed in the cortex as spatio-temporal patterns. Activated cortical regions were not sharply localized, but varied with stimulating positions of the cochlea. The curves of response magnitude versus stimulus intensity showed the narrow dynamic range, and that of latency was almost constant. These results were significantly different from those for normal sound stimulation.

Optical recording of azimuth representation in guinea pig auditory cortex.

The frequency dependency of directional sensitivity of the auditory cortex of barbiturate-anesthetized guinea pigs in response to tone bursts was studied by an optical equipment and a voltage sensitive dye (RH795). Tone bursts at 4, 8 and 14 kHz were presented from loudspeakers placed at the azimuths from contralateral 90 degrees to ipsilateral 90 degrees in 30 degrees steps on a front-horizontal semi-circular board (100 cm in radius). The maximum responses were observed at around contralateral 30 degrees (best azimuth). The onset and peak latencies were the shortest at the best azimuth. The best azimuth was the same for all these frequencies, but the azimuth selectivity became broader for the lower frequencies. These results indicate that the azimuth selectivity observed in the guinea pig auditory cortex is mainly associated with pinna characteristics.

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.

Real-time imaging of neural activity during binaural interaction in the guinea pig auditory cortex.

Spatio-temporal patterns of binaural interaction in the guinea pig auditory cortex (AC) were observed using optical recording with a 12 x 12 photodiode array and a voltage-sensitive dye. The amplitudes of the sound-induced light signals from the cortex were transformed into sequential two-dimensional images every 0.58 ms. Binaural sound stimuli evoked an excitatory response followed by a strong inhibition, and contralateral stimuli evoked a strong excitatory response followed by a weak inhibition. Ipsilateral sound stimuli evoked a weak response. Binaural stimulation induced two types of ipsilateral inhibition: a fast binaural inhibition which was detected only after the contralateral and ipsilateral responses were subtracted from the binaural responses, and which appeared 12-25 ms after the onset of stimulation, and a slow binaural inhibitory effect which was clearly observed in the binaural responses themselves, appearing 70-95 ms after the onset of stimulation. The fast binaural inhibition was observed in the same area as the contralateral excitatory response. The inhibited area became stronger and more widespread with increasing intensity of ipsilateral stimulation. We did not observe the specialized organization of binaural neurons as electrophysiologically found in the cat AC, in which binaural neurons of the same binaural response type are clustered together and alternate with clusters of other response types.

Optical study of spatiotemporal inhibition evoked by two-tone sequences in the guinea pig auditory cortex.

Spatiotemporal response patterns in the anterior and dorsocaudal fields of the guinea pig auditory cortex after two-tone sequences were studied in anesthetized animals (Nembutal 30 mg kg-1) using an optical recording method (voltage-sensitive dye RH795, 12 x 12 photodiode array). Each first (masker) and second (probe) tone was 30 ms long with a 10-ms rise-fall time. Masker-probe pair combinations of the same or different frequencies with probe delays of 30-150 ms were presented to the ear contralateral to the recording side. With same-frequency pairs, responses to the probe were inhibited completely after probe delays of less than 50 ms and the inhibition lasted for more than 150 ms, and the inhibition magnitudes in different isofrequency bands of the anterior field were essentially the same. With different-frequency (octave-separated) pairs, responses to the probe were not inhibited completely even after probe delays as short as 30 ms, and the inhibition lasted only for 110-130 ms. Inhibition magnitudes were different from location to location.

Optical imaging of spatiotemporal patterns of glutamatergic excitation and GABAergic inhibition in the guinea-pig auditory cortex in vivo.

1. Glutamatergic excitation and gamma-aminobutyric acid (GABA)-ergic inhibition in layers II and III of the auditory cortex of anaesthetized guinea-pigs were recorded optically using a voltage-sensitive dye RH795 and a 12 x 12 photodiode array. 2. After contralateral ear stimulation with pure tones, transient excitatory responses followed by inhibitory responses were observed in fields A (primary) and DC of the auditory cortex. The area of the excitatory responses was sandwiched or surrounded by the areas of the inhibitory responses. 3. Optically recorded excitatory responses to pure tones had two components: a component sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, and a component sensitive to 2-amino-5-phosphono-valerate (APV), an NMDA receptor antagonist. Application of CNQX (5 microM) to the auditory cortex suppressed an early, but not a late, phase of the excitation; application of APV (100 microM) had the opposite effect. Concomitant application of CNQX and bicuculline methiodide (BMI, 4 microM), a GABAA receptor antagonist, increased the amplitude of the late phase 4-fold. This enhanced response was suppressed by APV. 4. These results indicate that (i) auditory cortical excitatory responses are mediated by both non-NMDA and NMDA receptors, (ii) inhibition is mediated by GABAA receptors, (ii) the excitatory bands are sandwiched or surrounded by GABAA receptor-mediated inhibitory areas and (iv) GABAA receptors effectively inhibit the NMDA, but not the non-NMDA, receptor-mediated excitation.

A modified fluid percussion device.

This report examines a modified fluid percussion device with specific improvements made to address deficiencies found in previously reported devices. These improvements include the use of a cylindrical saline reservoir made of stainless steel, placement of the reservoir in a 15-degree head-up position for the easy release of air bubbles, placement of the fluid flushing outlet and the pressure transducer close to the piston on the same plane, with both perpendicular to the direction of the piston, and adjustable reservoir volume to vary the waveform of the pressure pulse, and a metallic central injury screw secured to the animal's skull over the exposed dura. Using this device, midline fluid percussion (MFP) and lateral fluid percussion (LFP) injuries were performed in 70 rats. Histopathologic findings included diffuse axonal injury in the MFP model and cortical contusion in the LFP model. Survival rate was 41.4% in MFP animals and 100% in LFM animals when the device settings were 178 mm3 of the cylindrical reservoir and 50 degrees-60 degrees in height of the pendulum. Our results suggest that this modified fluid percussion device may offer significant improvements over previously reported fluid percussion models for use in experimental head injury.

After-discharges in the auditory cortex of the mustached bat: No oscillatory discharges for binding auditory information.

Action potentials of single or multi-neurons were recorded from the DSCF, FM-FM and DF areas in the auditory cortex of the mustached bat to study stimulus-induced neural oscillation in the auditory system. Out of 125 neurons 120 recorded in these three areas showed after-discharges to a best stimulus. Durations of after-discharges of 120 neurons ranged between 4.8 and 217 ms. In the remaining 5 neurons, the duration of the discharges was shorter that of the stimulus. The PST histograms displaying responses of these 125 neurons showed no oscillatory component locked to the stimulus. 98% of the autocorrelograms of responses (122/125) showed no sign of oscillation, but the remaining two percent showed a very weak oscillatory component that was not stimulus-locked. The duration of the after-discharges had no correlation with the best delay or cortical depth of neurons. After-discharges are common in the auditory cortex of the mustached bat, but oscillatory discharges are very rare, so that neural oscillations play no role in binding various types of biosonar information processed in the different 'specialized' areas in the auditory cortex.

Spatio-temporal pattern of frequency representation in the auditory cortex of guinea pigs.

The spatio-temporal pattern of sound-evoked neural activity in the guinea pig auditory cortex was studied by optical recording with the aid of voltage-sensitive dye. Changes in light intensity induced by sounds at various frequencies and pressure levels were recorded with a 12 x 12 array of photodiodes. The amplitudes of the responses were displayed as sequential two-dimensional images. Tonotopical organization was found in two subdivisions of the auditory cortex, the anterior field (field A) and the dorsocaudal field (field DC). The frequency gradients in fields A and DC had a mirror-image relationship. This agrees with results obtained by the microelectrode technique. However, the tonotopic response observed in our study was transient. The focal activity that began in field A propagated in two directions; dorsally along the iso-frequency bands in field A, and caudally toward field DC. This suggests that the sound information processing initiates at field A, and its outputs are transferred to field DC, which is probably a hierarchically higher center.

Frequency thresholds of rat cochlear nerve fibers.

Activities of single cochlear nerve fibers of Wistar rats were recorded extracellularly. Best frequencies (BF) distributed from 0.50 to 62.6 kHz. The audiogram was made as the minimum boundary of the BF threshold distribution. The range of audible frequency was 0.54-63 kHz at 60 dB SPL and 0.15-67 kHz at 100 dB SPL. The lowest trough of the audiogram was 5 dB SPL at 41.2 kHz. There was the second trough of 10 dB SPL at 7.01 kHz leaving a notch between the two troughs. The shapes of the frequency-threshold curves (FTCs) of fibers were evaluated quantitatively and typical FTCs were shown as a function of BF.

[Activities of single cochlear nerve fibers in rats].

Activities of 325 single cochlear nerve fibers in response to pure tones at various frequencies including ultrasonic sounds were observed in rats. The stimulus sounds were measured with a probe tube inserted into an ear canal and a microphone and expressed in dB SPL. The best frequency (BF, characteristic frequency) and the threshold at BF were determined in every fiber. The measured BF ranged from 0.58 kHz to 62.6 kHz. BF and the threshold at BF of a fiber with the lowest BF-threshold were 27.49kHz and 6dB SPL respectively. The highest sensitivity was found in fibers with BF from 20 to 50 kHz. The tuning curve rose from BF and its slope was steeper for the high side than the low one as observed in other animals. The sharpness of the tuning curve was expressed by QN values, where QN is the ratio of BF to the band width at N dB above the BF threshold. Mean and the variance of Q10dB, Q20dB, Q30dB, Q40dB, Q50dB, all increased as the BF did. Almost all fibers had spontaneous discharge. The discharge rate increased with the increase in sound intensity and saturated at about 30 dB above the threshold. The auditory threshold curve obtained from distribution of the lowest BF-threshold of 325 fibers was consistent with the audiograms determined behaviorally.

Biosonar signals and cerebellar auditory neurons of the mustached bat.

In the vermis (VIp, VIIa, VIIp, and VIII), crus, and paraflocculus of unanesthetized mustached bats Pteronotus parnellii parnellii, responses of single neurons to acoustic stimuli were studied. The stimuli delivered were constant-frequency (CF) tones, frequency-modulated (FM) sounds, noise bursts (NBs), and sounds similar to the orientation sounds (pulses) of the species and echoes. The effect of ablation of the cerebellar cortex on vocalization was also investigated to explore whether the cerebellum was involved in sound emission. In the cerebellum of the mustached bat, auditory neurons are predominantly tuned to frequencies within the bands between 23 and 30, 55 and 63, or 85 and 94 kHz, which are found in the first, second, and third harmonics of bat's biosonar signals, respectively. The first harmonic is represented in the paraflocculus. The second harmonic is represented in vermis VIp and VIIa and crus I and IIa. The third harmonic is mainly represented in vermis VIIp and crus IIp. Different lobules represent different frequencies, but there is no systematic tonotopic representation in each lobule. The resting frequency of the CF component of the second harmonic (CF2) of the pulse differs among bats. The majority of auditory neurons in vermis VIp and VIIa and crus IIa are tuned to the CF2 frequency of the bat's own pulse. The frequency-tuning curves of cerebellar neurons are broader than those of peripheral neurons, reflected in significantly lower quality factors of Q-10, -30, and -50 dBs. In vermis VIp and VIIa, there are tiny clusters of FM-FM and CF/CF combination-sensitive neurons. They show strong facilitation of responses when two FM or CF sounds are delivered with particular relationships in the frequency, amplitude, and time domains. Because the clusters of these combination-sensitive neurons in the cerebellum are so small, we found no sign of a systematic representation of certain acoustic parameters, unlike that found in the auditory cortex. In vermis VIp and VIIa, there is a large cluster of NB-sensitive neurons that are more sensitive to NBs than to CF tones. The wider the bandwidth of the NBs, the better are the responses of these NB-sensitive neurons. The ablation of the vermis (VIp, VIIa, and VIIp), crus, and paraflocculus increases the variation of the CF frequency of the pulse. The ablation of the crus and paraflocculus causes a clear increase in the variation of CF frequency. The ablation of vermis (VIp, VIIa, and VIIp) has only a small effect on the variation. Any of the above ablations has little effect on the repetition rate of the pulse emission and the duration of pulses.(ABSTRACT TRUNCATED AT 400 WORDS)