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PubMed1985

Inhibition and level-tolerant frequency tuning in the auditory cortex of the mustached bat.

For echolocation the mustached bat, Pteronotus parnellii, emits complex orientation sounds (pulses), each consisting of four harmonics with long constant-frequency components (CF1-4) followed by short frequency-modulated components (FM1-4). The CF signals are best suited for target detection and measurement of target velocity. The CF/CF area of the auditory cortex of this species contains neurons sensitive to pulse-echo pairs. These CF/CF combination-sensitive neurons extract velocity information from Doppler-shifted echoes. In this study we electrophysiologically investigated the frequency tuning of CF/CF neurons for excitation, facilitation, and inhibition. CF1/CF2 and CF1/CF3 combination-sensitive neurons responded poorly to individual signal elements in pulse-echo pairs but showed strong facilitation of responses to pulse-echo pairs. The essential components in the pairs were CF1 of the pulse and CF2 or CF3 of the echo. In 68% of CF/CF neurons, the frequency-tuning curves for facilitation were extremely sharp for CF2 or CF3 and were "level-tolerant" so that the bandwidths of the tuning curves were less than 5.0% of best frequencies even at high stimulus levels. Facilitative tuning curves for CF1 were level tolerant only in 6% of the neurons studied. CF/CF neurons were specialized for fine analysis of the frequency relationship between two CF sounds regardless of sound pressure levels. Some CF/CF neurons responded to single-tone stimuli. Frequency-tuning curves for excitation (responses to single-tone stimuli) were extremely sharp and level tolerant for CF2 or CF3 in 59% of CF1/CF2 neurons and 70% of CF1/CF3 neurons. Tuning to CF1 was level tolerant in only 9% of these neurons. Sharp level-tolerant tuning may be the neural basis for small difference limens in frequency at high stimulus levels. Sharp level-tolerant tuning curves were sandwiched between broad inhibitory areas. Best frequencies for inhibition were slightly higher or lower than the best frequencies for facilitation and excitation. We thus conclude that sharp level-tolerant tuning curves are produced by inhibition. The extent to which neural sharpening occurred differed among groups of neurons tuned to different frequencies. The more important the frequency analysis of a particular component in biosonar signals, the more pronounced the neural sharpening. This was in addition to the peripheral specialization for fine frequency analysis of that component. The difference in bandwidth or quality factor between the excitatory tuning curves of peripheral neurons and the facilitative and excitatory tuning curves of CF/CF neurons was larger at higher stimulus levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed2005

Frequency map variations in squirrel monkey primary auditory cortex.

OBJECTIVE: The goal of this work is to understand the neural basis for cortical representation of hearing in highly vocal primates to gain insights into the substrates for communication. Variation patterns in frequency representation among animals are incorporated into an explanatory model to reconcile heterogeneous observations. STUDY DESIGN: Prospective. METHODS: Thirty-four squirrel monkeys underwent microelectrode mapping experiments in primary auditory cortex (AI) using tone pip stimuli. Characteristic frequency (CF) was extracted from the excitatory frequency receptive field. Frequency maps were reconstructed using Voronoi-Dirichlet tessellation. The spatial locations (rostral vs. caudal) of highest CF isofrequency contours (minimum length 1 mm) and highest CF neuronal clusters on the temporal gyral surface were analyzed. RESULTS: Isofrequency contours at least 1 mm long with CFs greater than 2.9 kHz (75% cases) are accessible on the temporal gyrus. Variability of the highest CF isofrequency contours accessible on the temporal gyrus has an interquartile range from 2.9 to 5.1 (mean 4.3) kHz. The highest CF isofrequency contours are located mainly in rostral AI, whereas the highest CF neuronal clusters flanking fully expressed isofrequency contours are equally distributed in rostral and caudal locations. CONCLUSIONS: Squirrel monkey AI frequency map variations are sizeable across animals and small within single animals (interhemispheric comparison). AI frequency map variations, modeled as translations and rotations relative to the lateral sulcus, are independent transfers. Caution must be exercised when interpreting nominal frequency map changes that are attributed to hearing loss and auditory learning effects.

Animals↗
PubMedDate not supplied

Functional magnetic resonance imaging of auditory cortex: with special reference to the side of aural stimulation.

PURPOSE: To determine whether left- or right-side uniaural stimulation produces different fMRI activation patterns. METHODS: Subjects were 12 volunteers (8 right-handed, 4 left-handed) with normal hearing. Functional imaging using FE-type multishot echo planar imaging was obtained in the axial plane during pure-tone and pseudoword tasks. Auditory stimuli were presented to each ear individually. In pure-tone tasks, subjects heard clustered sequences at 2000 Hz. In pseudoword tasks, subjects heard spoken Japanese syllables. The numbers of activated pixels in the auditory cortex were counted and compared for pure-tone and pseudoword tasks, as was activation according to the side of aural stimulation. RESULTS: In right-handed subjects, prominent activation in pure-tone tasks was noted in the dominant hemisphere in 100% of cases and was unrelated to the side of the stimulation. In pseudoword tasks, prominent activation was noted on the side contralateral to the stimulus in 62.5-100% of cases. In left-handed subjects, prominent activation was noted on the side contralateral to the stimulus in both pure-tone and pseudoword tasks. CONCLUSION: Left- and right-side stimulation produced differences in fMRI responses, especially between pure-tone and pseudoword tasks. Moreover, right-handedness and left-handedness affected results. This type of auditory fMRI may be a noninvasive indicator of language lateralization.

Acoustic Stimulation↗
PubMed2006

Effect of auditory cortex lesions on the discrimination of frequency-modulated tones in rats.

The lateralization of functions to individual hemispheres of the mammalian brain remains, with the exception of the human brain, unresolved. The aim of this work was to investigate the ability to discriminate between falling and rising frequency-modulated (FM) stimuli in rats with unilateral or bilateral lesions of the auditory cortex (AC). Using an avoidance conditioning procedure, thirsty rats were trained to drink in the presence of a rising FM tone and to stop drinking when a falling FM tone was presented. Rats with a lesion of the AC were able to learn to discriminate between rising and falling FM tones; however, they performed significantly worse than did control rats. A greater deficit in the ability to discriminate the direction of frequency modulation was observed in rats with a right or bilateral AC lesion. The discrimination performance (DP) in these rats was significantly worse than the DP in rats with a left AC lesion. Animals with a right or bilateral AC lesion improved their DP mainly by recognizing the pitch at the beginning of the stimuli. The lesioning of the AC in trained animals caused a significant decrease in DP, down to chance levels. Retraining resulted in a significant increase in DP in rats with a left AC lesion; animals with a right lesion improved only slightly. The results demonstrate a hemispheric asymmetry of the rat AC in the recognition of FM stimuli and indicate the dominance of the right AC in the discrimination of the direction of frequency modulation.

Acoustic Stimulation↗
PubMed2005

Spatio-temporal analysis of auditory cortex activation as detected with silent event related fMRI.

Functional magnetic resonance imaging (fMRI) allows neuroscientists to assess brain function by evaluating haemodynamic activity (blood flow) when a stimulus is present or absent. In clinical practice, the hearing levels of individuals are determined using an audiometer that allows presentation of a pure-tone of specific intensity and frequency. Functional images of the auditory nervous system have been obtained using stimuli such as pure-tone, speech, noise, etc. However, the observed activation evoked by the stimulus is confounded with the neuronal response evoked by scanner noise generated during imaging. Hence, researchers have been developing fMRI techniques to overcome the inadvertent effect of scanner noise on fMRI studies of the auditory cortex. Silent event related fMRI is a recently reported fMRI technique diminishing the confounding effects of background scanner noise. A drawback of sfMRI is that it requires long acquisition times (30-40 min) to achieve statistically significant activation. An additional complication associated with all fMRI data is that measurements obtained at consecutive times tend to exhibit substantial temporal correlation. Such correlation structure complicates the identification of brain locations (voxels) demonstrating statistically significant activation. We propose an approach for detecting activation with high statistical power and low false-positive rate. To accomplish these goals of high power and low type I error rate in sfMRI with shorter acquisition times, we describe a statistical model that accounts for the spatial and temporal correlation structure of the haemodynamic response. Temporal dependence within each voxel's measurements is modelled, and a regional measurement-error-free kriging predictor is used to combine information from neighbouring voxels when assessing voxel activation. Instead of simply applying a post hoc smoothing to thevoxelwise test statistics (e.g. t statistics), we attempt to make optimal use of information in the locality of each voxel when estimating the voxel's mean, variance, and temporal dependence parameters. The primary advantage to this spatial modelling approach is that the degree to which voxel parameters are smoothed is driven by the data. Thus, we are not subjectively smoothing noisy data, but objectively estimating the noise-free version of the spatial processes associated with the response. The resulting voxel activation maps exhibit substantially more spatial continuity than other currently used approaches, while exhibiting desirable inferential properties including a lower false-positive rate and high power for detection of activated regions. Minimal computational resources are necessary to carry out the approach, which yielded voxel activation maps for our experiment in only minutes.

Auditory Cortex↗
PubMed2006

Microtopography of the dual corticothalamic projections originating from domains along the frequency axis of the cat primary auditory cortex.

Spatial relationships between clusters of corticothalamic (CT) large terminals originating from cortical domains tuned to different frequencies were examined by pair-injecting two different anterograde tracers. Large-terminal CT projection originating from layer 5 was highly divergent with each injection site producing, on average, 15 local clusters distributing throughout non-lemniscal thalamic nuclei following a single anterograde tracer injection in the cat primary auditory cortex. Paired injections in higher- and lower-frequency cortical domains, resulting in labeling of two independent sets of terminal clusters, showed five recognizable patterns of spatial interaction between them. (1) In the ventral division of the medial geniculate complex (vMGC), sheet-like plexuses of small terminals of different origins were situated in parallel, with minimal overlap. (2) Extensive overlap of two low-density plexuses of differently labeled small terminals was observed in the medial division of the medial geniculate complex (MGC). (3) At the transition zones between the vMGC and the superficial dorsal nucleus of the MGC dorsal division, and between the vMGC and the ventrolateral nucleus, there were relatively broad clusters of a high density of large-terminal structures from the two cortical domains, which overlapped extensively. (4) At multiple loci in the nonlemniscal nuclei, pairing of two small clusters of differently labeled large terminals was observed. (5) Small unpaired clusters of large terminals were also found in the nonlemniscal nuclei. For large terminals, approximately 14%, 59%, and 27% clusters per injection demonstrated patterns 3, 4, and 5, respectively. The results provide evidence for the precise topographical organization for the large-terminal CT system at the microscopic level despite its highly divergent projection. This microtopographical projection from the tonotopic cortical field to non-tonotopic thalamic nuclei may raise the possibility of presence of a map that has not been defined in auditory non-lemniscal thalamic nuclei yet.

Animals↗
PubMed1984

Functional anatomy of the inferior colliculus and the auditory cortex: current source density analyses of click-evoked potentials.

In the auditory midbrain (inferior colliculus) and cortex (superior temporal gyrus) of awake squirrel monkeys profiles of click-evoked field potentials were recorded. The recording tracks were reconstructed anatomically. From the field potentials the one-dimensional current source density (CSD) distributions were calculated. By comparing the CSD profiles with the anatomical features of the reconstructed recording paths, the components of the CSDs could be attributed to certain anatomical sites. Thus a physiological method for the functional identification of recording sites was obtained. It permits the identification of depth locations of specific laminae in cortex. In the inferior colliculus it permits distinction between central and peripheral regions and between three depth segments. The CSDs in the central nucleus of the inferior colliculus lend functional support to the anatomical division into three distinct parts and, in addition, provide the temporal aspects of the main groups of synaptic activities. The CSDs in the auditory cortex permit determination of five different groups of excitatory synaptic activations. The spatio-temporal distributions of these components are very similar to those obtained in other neocortical areas and thus corroborate the hypothesis that afferent activity is relayed very similarly in all sensory areas of neocortex.

Acoustic Stimulation↗
PubMed1999

Imaging functional activation of the auditory cortex during focal repetitive transcranial magnetic stimulation of the primary motor cortex in normal subjects.

Positron emission tomography (PET) during focal repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising approach to study cortical connectivity in awake humans. However, the noise caused by the discharging magnetic coil might have confounding effects on the rTMS-related cortical activation pattern. In twelve healthy volunteers, 18-fluoro-2-deoxy-D-glucose (18FDG) PET was employed to visualize the functional activation of the primary auditory cortex (PAC) during 2 Hz rTMS of the left primary sensorimotor hand area. Magnetic stimuli (1800) were applied at an intensity of 140% of motor resting threshold during the uptake period of 18FDG. Though all subjects wore earplugs, rTMS-related noise induced a consistent bilateral increase of regional glucose utilization in the PAC (P < 0.05, corrected). Thus, rTMS-related acoustic input needs to be taken into account in combined rTMS/PET studies.

Adult↗
PubMed2000

[Studies of directional sensitivity of neurons in the cat primary auditory cortex].

A set of impulsive transient signals has been synthesized for earphone delivery whose waveform and amplitude spectra, measured at the eardrum, mimic those of sounds arriving from a free-field source. The complete stimulus set forms a "virtual acoustic space" (VAS) for the cat. VAS stimuli are delivered via calibrated earphones sealed into the external meatus in cats under barbiturate anesthesia. Neurons recorded extracellularly in primary (AI) auditory cortex exhibit sensitivity to the direction of sound in VAS. The aggregation of effective sound directions forms a virtual space receptive field (VSRF). At about 20 dB above minimal threshold, VSRFs recorded in otherwise quiet and anechoic space fall into categories based on spatial dimension and location. The size, shape and location of VSRFs remain stable over many hours of recording and are found to be shaped by excitatory and inhibitory interactions of activity arriving from the two ears. Within the VSRF response latency and strength vary systematically with stimulus direction. In an ensemble of such neurons these functional gradients provide information about stimulus direction, which closely accounts for a human listener's spatial acuity. Raising stimulus intensity, introducing continuous background noise or presenting a conditioning stimulus all influence the extent of the VSRF but leave intact the gradient structure of the field. These and other findings suggest that such functional gradients in VSRFs of ensembles of AI neurons are instrumental in coding sound direction and robust enough to overcome interference from competing environmental sounds.

Acoustic Stimulation↗
PubMed1985

[Characteristics of the responses of auditory cortex neurons in the cat to tonal stimulation during nembutal anesthesia and after recovery from it].

Responses of auditory (AI) cortex neurons to tonal (1-25 kHz) stimulation were studied in cats under and after nembutal anaesthesia. Patterns of responses recorded during first hours and 10-30 hours after nembutal injection differed essentially. Most (89%) of neurons of anaesthetized cat had no background activity and produced stereotypical on-responses to tonal stimulation with the best frequency. Off-effects were usually absent. Variability of responses increased substantially after cessation of anesthesia. On-, on-off- and off-responses were obvious in 76% of neurons, approximately 21% of neurons tonically increased or decreased spike activity during the stimulus action. Duration of the stimulus action was reflected in response characteristics of the overwhelming majority of AI cortical neurons.

Anesthesia, General↗
PubMed1997

Influence of experience on the representation of the "mothering call" in frontoparietal and auditory cortex of pups of the rodent Octodon degus: FDG mapping.

Several types and subtypes of vocalizations which have a behavioral impact on degu pups were identified. Among these the complex "mothering call" which is exclusively uttered by females and first during extensive nursing periods in the nest is a candidate for filial learning. In 14C-2-fluoro-2-deoxyglucose (FDG) experiments two-weeks-old pups raised by normal mothers showed higher metabolic activity in somatosensory frontoparietal and frontal cortex upon play back of a mothering call than pups raised by muted mothers. It is suggested that pups learn to associate the mothering call with close body contact with their mother early in life. In addition, FDG representation of the call, of its components and of tone and noise stimuli were studied in degu auditory cortex. Five fields and some aspects of tonotopic organization were identified. The mothering call activated all fields, but with more spatial extent of labeling in normally raised pups. A rostral field was activated by play-back of the mothering call, noise, and two-tone sequences, but hardly by single-frequency tones and the narrow-band component of the mothering call.

Acoustic Stimulation↗
PubMed1986

Response properties of FM-FM combination-sensitive neurons in the auditory cortex of the mustached bat.

For echolocation, the mustached bat, Pteronotus parnellii rubiginosus, emits orientation sounds (pulses) and listens to echoes. Each pulse is made up of 8 components, of which 4 are constant frequencies (Cf 1.4) and 4 are frequency-modulated (FM 1-4). Target-range information, conveyed by the time delay of the echo FM from the pulse FM, is processed in this species by specialized neurons in a part of the auditory cortex known as the FM-FM area. These cortical neurons are responsive to pulse-echo pairs at specific echo delays. The essential components in the sound pair include the pulse FM1 followed by an echo FMn (n = 2, 3 or 4). Downward sweeping FM1-FMn sounds that are similar to those the animal naturally hears during echolocation are the most effective in evoking facilitative responses. Most FM-FM neurons, however, still exhibit facilitative responses to stimulus pairs consisting of upward sweeping FM sounds and/or pure tones at frequencies found in FM sweeps. The magnitude of facilitation is altered by changes in echo rather than pulse amplitude. Neurons characterized by shorter best delays (or echoes from closer targets) do not require larger best echo amplitudes for facilitation.

Animals↗
PubMed2003

Biphasic changes in tissue partial pressure of oxygen closely related to localized neural activity in guinea pig auditory cortex.

An understanding of the local changes in cerebral oxygen content accompanying functional brain activation is critical for making a valid signal interpretation of hemodynamic-based functional brain imaging. However, spatiotemporal relations between changes in tissue partial pressure of oxygen (Po2) and induced neural activity remain incompletely understood. To characterize the local Po2 response to the given neural activity, the authors simultaneously measured tissue Po2 and neural activity in the identical region of guinea pig auditory cortex with an oxygen microelectrode (tip < 10 microm) and optical recording with voltage-sensitive dye (RH 795). In addition, a laser displacement gauge and a laser-Doppler flowmeter were used to monitor the spatial displacement and regional cerebral blood flow, respectively, in the Po2 measurement region. In the activated region, tissue Po2 initially decreased during the approximately 3-seconds after the onset of acoustic stimuli, and then increased during the next approximately 5 seconds. Such biphasic changes are consistently found in cortical layers I to IV. In addition, amplitude of the biphasic change was closely related to detected peak height of the optical signal changes. The results suggest that the initial decrease in tissue Po2 is coupled to the induced neural activity and depends on response time of local increase in cerebral blood flow.

Acoustic Stimulation↗
PubMed1975

Single cell activity in the auditory cortex of the unanesthetized, behaving monkey: correlation with stimulus controlled behavior.

The neural activity of 60 cells in the auditory cortices of two rhesus monkeys was examined in relation to systematic variations in cued reinforcement conditions. Subjects were trained on a variant of the auditory reaction time (RT) task. In the final behavioral paradigm monkeys were rewarded for rapid key releases to all tonal stimuli in one reinforcement condition (frequency irrelevant = FI), while in the other stimulus-cued condition (frequency discrimination = FD) releases to certain tonal test frequencies were unrewarded. Upon completion of behavioral training, RTs to identical tonal test stimuli were longer and more variable when presented in the unrewarded (FD) condition. Following neurosurgery it was possible to observe the effects of reinforcement condition on auditory RT performance and the activity of single auditory cortical cells simultaneously. Of the auditory cortical cells sampled, 25% showed definite and repeatable alterations in evoked activity to the same tonal stimulus which were correlated with reinforcement condition. For nearly all cells examined the influence of reinforcement condition was much the same: on excitatory responses were increased in the FD condition. A few of the cells also showed alterations in latency and/or pattern of evoked discharge. Importantly, none of the units examined showed changes in their spontaneous discharge rates as a function of reinforcement condition. both peripheral mechanical and central neural theories were considered as a basis for the observed neural alterations. The specificity and latency of the alterations as well as the absence of tonic effects seemed to indicate that the neural changes observed were mediated by central mechanisms. Our results strongly suggest that the activity of a sample of auditory cortical neurons depends on the behavioral state of the preparation. We propose that 'behavioral state", appropriately defined, can be a useful concept for neurophysiologists.

Action Potentials↗
PubMed1982

Postnatal development of evoked responses in the auditory cortex of the cat.

Postnatal development of the auditory evoked cortical response was investigated in the cat by laminar field potential analysis. Responses were induced by electrical stimulation of the cochlear nuclear complex and by sonic stimulation under sodium pentobarbital anesthesia. As is well known, the auditory evoked cortical response in adult cats is a diphasic positive-negative wave in the superficial cortical layers and a negative-positive wave in the deeper cortical layers. By contrast, the cortical response in neonatal kittens was a monophasic negative wave in the superficial cortical layers and a positive wave in the deeper cortical layers (sN-dP wave). After 1 week, the sN-dP wave was preceded by a small wave which was positive in the superficial cortical layers and negative in the deeper cortical layers (sP-dN wave). As animals grew older, the sP-dN wave became dominant over the sN-dP wave to take an adult-like configuration of responses at 3 to 4 weeks of age. The sN-dP wave was separable from the preceding sP-dN wave by double shock stimulation at a certain time interval. Therefore, these two wave components are presumably mediated by two different types of thalamocortical projections. The level of potential reversal in each wave component shifted from the deeper cortical layers to the more superficial cortical layers during maturation. The latency of the response decreased sharply from 44 msec at birth to 11 msec by 3 weeks, and thereafter gradually to 5 msec in adulthood.

Acoustic Stimulation↗