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Isofrequency labelling revealed by a combined [14C]-2-deoxyglucose, electrophysiological, and horseradish peroxidase study of the inferior colliculus of the cat.

Tonotopic sequences of single units were recorded in the central nucleus (ICC) of the inferior colliculus of cats. The sites of units with a particular characteristic frequency (CF) were marked histologically. After injection with [14C]-2-deoxyglucose (2-DG), each animal was stimulated with tone bursts, which corresponded to the CFs of the selected units. The position of a selected unit was found to correspond with the position of a band of discrete 2-DG labelling. In some animals, multiple penetrations revealed that more than one unit with the same CF was located on the same band, indicating that the 2-DG bands represent isofrequency contours. Single units were recorded in the pars lateralis (LV) of the medial geniculate body of cats and areas of consistent CF were found, which contained the same frequencies as some of those employed with 2-DG. Horseradish peroxidase (HRP) was ejected into these selected areas and labelled cells were identified in ICC after retrograde transport. The labelled cells were found to lie in a band which had a similar orientation to the 2-DG bands produced by the same frequencies. Both sets of data indicate that ICC is organized into three-dimensional isofrequency sheets, the orientation of which do not match the concentric model of frequency organization inferred from some Golgi studies. The isofrequency sheets slope from medial to lateral throughout ICC with high-frequency contours found more medially and caudally and low-frequency contours more laterally and rostrally. The organization of the isofrequency contours is largely in agreement with a recent Golgi model of the laminations in ICC put forward by Oliver and Morest ('84). The 2-DG contours do not agree with the orientation of laminae in the lateral part of ICC as described by this model. The 2-DG contours also extend further into the dorsal cortex than previous degeneration evidence would suggest. The functional isofrequency contours are often quite narrow and could represent two or three overlapping cellular laminae. The results show clearly that there is only one tonotopic organization extending throughout ICC and the dorsal cortex.

Animals↗

Hearing loss in dogs after lesions of the brachium of the inferior colliculus and medial geniculate.

Seven dogs were tested for their sensitivity to pure tones following lesions of the brachium of the inferior colliculus and medial geniculate body. Bilateral section of the brachium of the inferior colliculus consistently resulted in an average hearing loss of as much as 37 dB in the midrange of the animals' audiograms. Lesions of the medial geniculate appear to produce a similar hearing loss if the ventral division of the medial geniculate is completely destroyed.

Animals↗

Projections from the anteroventral cochlear nucleus to the lateral and medial superior olivary nuclei.

The projections from the cochlear nucleus to the lateral and medial superior olivary nuclei were studied in the cat by use of retrograde transport of horseradish peroxidase to demonstrate the connections. The medial superior olivary nucleus receives input only from the anterior and posterodorsal subdivisions of the anterior division of the anteroventral cochlear nucleus (AA and APD, respectively; Brawer, Morest, and Kane: J. Comp. Neurol. 155: 251-300, 1974). These two subdivisions are populated almost exclusively by spherical bushy cells. Like the medial superior olivary nucleus, the lateral superior olivary nucleus receives inputs from AA and APD. In addition, the lateral superior olivary nucleus receives projections from the posterior subdivision (AP) of the anterior division and also from the posterior division of the anteroventral cochlear nucleus. The projections to the medial superior olivary nucleus are bilateral, whereas the projections to the lateral superior olivary nucleus are almost entirely ipsilateral. One implication of the results is that the medial superior olivary nucleus receives inputs from only one cell type--the spherical bushy cell--but that, at the least, two cell types project to the lateral superior olivary nucleus. Both the olivary nuclei receive input from most, if not all, of the dorsoventral extent of the anteroventral cochlear nucleus, implying that both receive input from neurons arrayed across the entire frequency representation of the anteroventral cochlear nucleus. All of the projections appear to be organized topographically such that frequency representation is preserved.

Animals↗

The central projections of intracellularly labeled auditory nerve fibers in cats: an analysis of terminal morphology.

The axons of physiologically characterized spiral ganglion neurons (type I) were stained throughout their arborizations in the cochlear nucleus by the intracellular injection of horseradish peroxidase (HRP). The tips of the axonal branches were marked by distinct swellings, ranging in size and shape from small boutons to large perisomatic ramifications. Electron microscopic analysis of such swellings revealed ultrastructural features characteristic of primary auditory synapses, consistent with the hypothesis that terminal swellings identifiable in the light microscope represent presynaptic endings. On the basis of light microscopic differences in size, these endings were organized into three categories. Endings of relatively small size (terminal boutons, free endings, boutons with filopodia, string endings, and small complex endings) composed 94% of all terminal endings. Within this category of small endings, there were predictable variations in relative size and regional distribution that related to the spontaneous discharge rate (SR) of the fiber. The endings of low and medium SR fibers (SR less than or equal to 18 spikes/second) were smaller on average than those of high SR fibers (SR greater than 18 spikes/second). Furthermore, there were more endings arising from the ascending branch than from the descending branch when comparing fibers of the low and medium SR group with those of the high SR group. There were not, however, obvious morphological features of this ending category that correlated with the characteristic frequency (CF, the pure tone frequency to which the neuron is most sensitive). A second category contained medium-sized complex endings, most of which formed axosomatic contacts. This category composed 4% of the population and was found in close proximity to the perikarya of globular, octopus, and spherical cells. The endings from low and medium SR fibers were smaller on average than those from high SR fibers. These endings did not vary in their parent branch distribution with respect to fiber SR, nor did they exhibit morphological features that correlated with fiber CF. The third category contained large complex endings (endbulbs of Held) and composed 2% of the ending population. Within the anteroventral cochlear nucleus, these large, complex endings made axosomatic contact with spherical cells in the anterior division and with globular cells in the posterior division. There were no systematic variations in ending size or branch distribution that correlated with fiber SR. There was, however, a relationship between ending size and fiber CF such that fibers having CFs below 4 kHz gave rise to the largest endbulbs.

Action Potentials↗

Frequency representation in auditory cortex of the common marmoset (Callithrix jacchus jacchus).

The location and characteristics of the primary auditory cortex of the common marmoset, Callithrix jacchus jacchus, were determined in five anesthetized male adult animals by mapping the responses of cortical units and unit clusters to pure tone stimuli presented to the contralateral ear. The primary auditory cortex lies largely ventral to the lateral sulcus, the only major fissure on the lateral cortex of this smooth-brained primate, but in some animals it may extend significantly down the ventral bank of this sulcus. Responses are distributed such that low best frequencies are found rostroventrally whereas high best frequencies occur caudally. The disposition of frequency-band contours is fan-shaped, with contours separating low-frequency octaves nearly parallel to the lateral sulcus and high-frequency (greater than 8 kHz) contours perpendicular to that sulcus. Best frequencies range from 0.6 to 30 kHz across the primary field, but there is a disproportionate representation of the three octaves between 2 and 16 kHz. The most sensitive thresholds (as low as -2 dB SPL) are found between 7 and 9 kHz. The primary auditory cortex is similar in cytoarchitecture to that reported for the cat, showing a blurring of lamination in the middle layers (II-IV) and a preponderance of small cells in these merged layers, giving a highly granular appearance. The accessibility of the cochlear representation on the gyral surface makes the marmoset an attractive animal for studies of primate auditory cortex.

Animals↗

Convergence of ascending pathways at the inferior colliculus of the mustache bat, Pteronotus parnellii.

To compare patterns of projections to the inferior colliculus from different sources, injections of [3H]-leucine were placed in the cochlear nuclei, superior olivary complex, and nuclei of the lateral lemniscus of Pteronotus parnellii. The results show that the target of the anteroventral cochlear nucleus (AVCN) is the ventral and lateral two thirds of the central nucleus of the inferior colliculus. The binaural pathways from the medial and lateral superior olives (MSO and LSO) project to the same target. The dorsal cochlear nucleus (DCN) projects to the entire central nucleus of the inferior colliculus and does so in a more diffuse manner than does the AVCN. The DCN also sends sparse projections beyond the central nucleus into dorsal parts of the pericentral area. The intermediate (INLL) and ventral (VNLL) nuclei of the lateral lemniscus are relays in pathways that originate in the cochlear nucleus and terminate in the contralateral inferior colliculus. These nuclei also receive indirect input from the contralateral AVCN via the medial nucleus of the trapezoid body. Although nuclei of the lateral lemniscus project most densely to those areas of the inferior colliculus that are also the targets of the AVCN, MSO, and LSO, the nuclei of the lateral lemniscus also send spare projections outside these areas. Many of the pathways just described project in bands, a finding that raises the possibility that the projections parallel the orientation of disk-shaped cells in the inferior colliculus and raises the question of whether the bands from one source overlap or interdigitate with the bands from another source.

Animals↗

Cochlear microphonics and action potentials mature and decline at different rates in the normal and pathologic mouse cochlea.

A technique is described that can nontraumatically assess cochlear microphonic (CM) and cochlear action potential (AP) thresholds to pure tones in the intact mouse. This method was applied to the normal hearing CBA/J, and to the C57BL/6J, which has early-onset hearing loss. CM thresholds are most sensitive in both mice by 23 days of age, and the C57BL/6J CM responses degenerate rapidly thereafter, especially at the high-frequency end. The CMs of the CBA/J continue to change past 23 days, developing a threshold dip at 24 kHz, and becoming slightly less sensitive at 32 kHz. The AP of the C57BL/6J is also most sensitive at 23 days, and becomes gradually less sensitive to 32- and 64-kHz tones over the next 36 days. The CBA/J AP thresholds are not maximally sensitive until 41 days of age.

Age Factors↗

Contour variables in vocal communication between squirrel monkey mothers and infants.

This study investigates the occurrence and structure of the caregiver call, a form of communication in squirrel monkeys that is almost exclusively infant-directed. Caregiver calls exhibit variability in several acoustic features, most notably pitch range and contour, which are significantly associated with particular contexts of infant care; e.g., nursing or retrieval. The use of caregiver calls by squirrel monkeys is compared to the use of motherese (infant-directed speech) by humans. The possible functional and motivational basis for specific acoustic variables in the caregiver call is discussed.

Animals↗

Activation of the left planum temporale in pitch processing is shaped by language experience.

Implicit, abstract knowledge acquired through language experience can alter cortical processing of complex auditory signals. To isolate prelexical processing of linguistic tones (i.e., pitch variations that convey part of word meaning), a novel design was used in which hybrid stimuli were created by superimposing Thai tones onto Chinese syllables (tonal chimeras) and Chinese tones onto the same syllables (Chinese words). Native speakers of tone languages (Chinese, Thai) underwent fMRI scans as they judged tones from both stimulus sets. In a comparison of native vs. non-native tones, overlapping activity was identified in the left planum temporale (PT). In this area a double dissociation between language experience and neural representation of pitch occurred such that stronger activity was elicited in response to native as compared to non-native tones. This finding suggests that cortical processing of pitch information can be shaped by language experience and, moreover, that lateralized PT activation can be driven by top-down cognitive processing.

Acoustic Stimulation↗

Neural characteristics of successful and less successful speech and word learning in adults.

A remarkable characteristic of the human nervous system is its ability to learn to integrate novel (foreign) complex sounds into words. However, the neural changes involved in how adults learn to integrate novel sounds into words and the associated individual differences are largely unknown. Unlike English, most languages of the world use pitch patterns to mark individual word meaning. We report a study assessing the neural correlates of learning to use these pitch patterns in words by English-speaking adults who had no previous exposure to such usage. Before and after training, subjects discriminated pitch patterns of the words they learned while blood oxygenation levels were measured using fMRI. Subjects who mastered the learning program showed increased activation in the left posterior superior temporal region after training, while subjects who plateaued at lower levels showed increased activation in the right superior temporal region and right inferior frontal gyrus, which are associated with nonlinguistic pitch processing, and prefrontal and medial frontal areas, which are associated with increased working memory and attentional efforts. Furthermore, we found brain activation differences even before training between the two subject groups, including the superior temporal region. These results demonstrate an association between range of neural changes and degrees of language learning, specifically implicating the physiologic contribution of the left dorsal auditory cortex in learning success.

Acoustic Stimulation↗

Dysprosody following acquired neurogenic impairment.

Dysprosody is typically associated with the dysarthrias following acquired neurogenic disturbance but it has also been associated with impairment to the cortex of the right cerebral hemisphere (Behrens, 1985). Currently there is little agreement in the clinical literature as to the locus or nature of processes involved in production and reception of prosody. This paper investigates the usefulness of two psycholinguistic models of prosody--involving "abstract" and "concrete" processes (Ladd & Cutler, 1983)--in accounting for dysprosody following motor pathway as well as cortical lesions. Four groups of 10 adult patients each participated in the study; the first group had sustained a right cerebral hemisphere cortical stroke, the second group had hypokinetic dysarthria, the third group had ataxic dysarthria, and the fourth group was normal controls. Acoustic analysis using the Kay VisiPitch/PC was conducted on pairs of matched noun phrase and noun compounds (e.g., greenhouse, green house), interrogative and declarative versions of sentences, and sentences spoken in each of four different emotional tones. Right cerebral hemisphere damaged subjects produced shorter durations than the other groups for each stimulus class. However, all of the subjects were able to use duration to signal temporal information for the lexical distinction between noun compounds and noun phrases. The results are consistent with a model of prosodic control which involves both cortical as well as physiological control processes. Current treatment for dysprosody is discussed in light of this new emphasis on cognitive control processes.

Cerebrovascular Disorders↗

Gender and the integration of acoustic dimensions of prosody: implications for clinical studies.

This study was conducted to detect the existence of a relationship between spectral and temporal prosodic cues and to examine gender differences in any such relationship. The rationale for the investigation was to gain a greater understanding of normal prosody and the requirements for control groups in clinical studies of prosody. Ten male and 10 female speakers with no known speech or neurological deficits participated in the study. They performed a reading task which involved delivering 10 sentences first with a declarative and then repeated with an interrogative intonation (20 sentences per speaker). Intrasubject and intersubject analyses of the speech data revealed a dependence of pitch on duration that differed between male and female speakers. Significant differences between the genders were also found in speech rate, pitch range, and pitch slope. The findings suggest that an integrated treatment of acoustic cues may provide a more invariant feature of normal prosody against which clinical groups may be compared. The data also imply that in clinical studies of the production of prosody gender should be carefully controlled.

Cues↗

The development of selective attention as reflected by event-related brain potentials.

The development of auditory selective attention was assessed using event-related brain potentials (ERPs) and behavioral measures, with children (mean age = 8.1), adolescents (14.4), and young adults (23.8) as subjects. During separate blocks, subjects heard two sequences of pure tones (low- and high-pitched) or consonant-vowels (CVs; for example, ba vs da). Subjects were required to attend to one of the two stimuli in order to detect a deviant target embedded within the attended sequence, while ignoring the sequence comprised of the other stimulus (which also contained standard and deviant stimuli). The frequent standards were always 100 ms in duration, whereas the infrequent targets were of longer duration. The effect of selective attention was operationalized by computing the Nd difference waveform (ERP elicited by the unattended standard subtracted from that elicited by the attended standard). There was an increase in early Nd amplitude (measured from about 200 to 400 ms) and a decrease in its latency for both pure tones and CVs from childhood through young adulthood. For the amplitude measure, this effect was much more marked for CVs. Additional analyses indicated that the major effect of age involved reduction of negative-going ERP amplitude elicited by stimuli in the unattended channel, suggesting that with age there is an improvement in the narrowing of the attentional focus, with the major change taking the form of greater facility in rejecting stimuli in the unattended channel. Age-related shifts in scalp distribution of both early and late Nd were seen as consistent with, respectively, age-related changes in the way attention was allocated to the two input channels, and in the way in which the attentional trace was maintained by selective rehearsal.

Adolescent↗

A landscape parametric profile approach for rat brain image analysis.

This paper develops the use of the landscape parametric profile as a descriptive and interpretative method of analyzing metabolic activity data obtained by neuroimaging techniques such as 2-deoxyglucose autoradiography. The method is suggested as an initial survey-type approach that can be used to map regional activity throughout the coronal planes of a rat brain and to compare relative differences in the patterns of activity between different brains. Images are acquired using preexisting image analysis software and then transformed to landscape parametric profiles derived from each matrix array. The image profile analysis tasks are done using commercially available software for matrix computations such as MATLAB. An effective way of interpreting activity differences between brains is presented by plotting the results from a randomized blocks ANOVA significance test combined with the average profiles of different subjects within the same experimental group. The method greatly reduces the computational burden associated with image averaging and statistical comparison of brains from different subjects while preserving the richness in topographic metabolic information. One significant advantage of the method is that it results in figures that are readily depicted and interpretable in black and white.

Analysis of Variance↗

Differential fMRI responses in the left posterior superior temporal gyrus and left supramarginal gyrus to habituation and change detection in syllables and tones.

Using a habituation-recovery paradigm adapted to functional magnetic resonance imaging, we investigated the brain responses to syllables and tones in six right-handed male subjects. We opposed a standard condition (STD) in which the subjects were listening to homogeneous sequences of four identical stimuli, to a deviant condition (DEV) in which the fourth stimulus of the sequence differed in pitch or spectral content for tones and in the initial stop consonant for syllables. The corresponding runs alternated four rest periods with two STD and two DEV conditions. In addition to a marked rightward asymmetry in the primary and secondary auditory cortex for tones and a right inferior frontal activation for the tone condition where the deviant had increased spectral content, the experiment revealed differential activations in the left posterior superior temporal gyrus and in the left supramarginal gyrus. Activations within the left posterior superior temporal gyrus were observed for the DEV condition with tones and for the STD and DEV conditions with syllables. Activation within the inferior part of the left supramarginal gyrus was only observed for the DEV condition with syllables. The analysis of the decreases and increases in the BOLD signal across the STD, DEV, and rest conditions suggests that the left posterior superior temporal gyrus is implicated in the preattentive change detection of acoustic changes in speech as well as nonspeech stimuli, whereas the left supramarginal gyrus is more specifically engaged in the detection of changes in phonological units.

Adult↗

Three distinct auditory areas of cortex (AI, AII, and AAF) defined by optical imaging of intrinsic signals.

Using pure-tone sound stimulation, three separate auditory areas are revealed by optical imaging of intrinsic signals in the temporal cortex of the chinchilla (Chinchilla laniger). These areas correlate with primary auditory cortex (AI) and two secondary areas, AII and the anterior auditory field (AAF). We have distinguished AI on the basis of concurrent single-unit electrophysiological recording; neurons within the AI intrinsic signal region have short (<15 ms) onset-response latencies compared with neurons recorded in AII and the AAF. Within AI, AII, and AAF we have been able to define cochleotopic or tonotopic organization from the differences in intrinsic signal areas evoked by pure tones at octave-spaced frequencies from 500 Hz to 16 kHz. The maps in AI and AII are arranged orthogonal to each other.

Animals↗

Auditory sensory memory and the cholinergic system: implications for Alzheimer's disease.

Auditory sensory memory represents one of the simplest types of short-term memory that can be studied electrophysiologically with mismatch negativity (MMN); a specific auditory event-related potential indexing automatic comparison of incoming stimuli to an existing memory trace. Previous results suggest that auditory sensory memory deteriorates in aging and especially in Alzheimer's disease (AD). It has remained unsettled, however, whether MMN is regulated by the cholinergic system, which is deteriorated in AD contributing to cognitive impairments. We recorded cortical auditory responses with a magnetometer from 13 healthy subjects after intravenous injection of scopolamine, centrally acting cholinergic antagonist, or glycopyrrolate, a drug with a peripheral anticholinergic properties without penetrating the blood-brain barrier, using a double-blind protocol. Scopolamine reduced MMNm amplitude in response to frequency, but not duration, change, increased P50m amplitude, and delayed N100m latency. These findings suggest that the cholinergic system regulates the frequency-specific comparison of incoming stimuli to existing memory trace and modulates the preattentive processing related to stimulus detection. Further, neural mechanisms responsible for cortical frequency- and duration-specific discrimination appear to have different sensitivities to cholinergic modulation. Auditory evoked potentials might be suitable to monitor cholinergic activity in AD.

Adult↗

Detecting and correcting for head movements in neuromagnetic measurements.

Head movements during neuromagnetic measurements may cause a significant error in the estimated locations of active brain areas. In this study we present a fast method for measuring the head position while neuromagnetic data are acquired. We then compare two methods for removing the effect of the movements in neuromagnetic source estimation and magnetic field alignment: minimum-norm estimate alignment and forward calculation correction. Simulations and a test measurement show that the proposed head position measurement method works with millimeter precision and that incorporating the effect of the head movements into the magnetic field forward calculations is an efficient and a sufficiently accurate method for correcting for the head movements.

Adult↗