Auditory Association Areas Research Articles

Dynamics of thalamocortical circuits for sound processing revealed by magnetoencephalography.

Music perception and cognition involves multi‐modal processing within a wide range of neural networks working in concert. Rhythmic brain activities, or neural oscillations, are thought to play an important role in such long‐range communication. How are related networks established and dynamically reconfigured in order to adapt to the ever changing auditory environment? Oscillations in the 40‐Hz range (gamma band) in thalamocortical connections are proposed as a key mechanism. A common problem to delineate the behavior of 40‐Hz oscillatory activity, however, is the small effect size and unknown time, courses when using noninvasive megnetoencephalography (MEG) recording. To overcome this problem, auditory stimulation with sounds containing a strong 40‐Hz rhythm can be used to drive neural networks into a state of high synchrony. These areas are successfully identified by beamforming techniques, which transform MEG signals to voxel‐based source images. Phase lags between primary auditory cortices and thalamus and auditory association areas suggest the information flow across the regions. Moreover, changes in the sound stimulus were observed as temporal changes of synchrony reflecting dynamic reconfiguration of neural networks. The relevance of these observations for detecting changes in sound localization will be demonstrated.

Auditory sensory gating to the human voice: A preliminary MEG study

The ability of the brain to suppress incoming irrelevant sensory input is termed ‘sensory gating,' and auditory sensory gating is often indexed by the auditory evoked response. We recorded the auditory evoked magnetic fields to the human voice, using the conditioning–testing paradigm, to investigate whether or not healthy subjects show less activation to the second voice stimulus. Seventeen healthy adults (mean age 27.9 ± 4.8 years, 9 males and 8 females) participated in the experiment. The auditory stimuli were presented monaurally as a series of 120 paired voices, with 500-ms interstimulus intervals and 6-s interpaired stimulus intervals. The P50m and the N100m responses were investigated, and dipole source localization was performed. Root mean squares of both P50m and N100m were significantly suppressed to the second stimulus bilaterally, and the suppression was more significant in N100m. The N100m was located significantly more laterally than the P50m for both hemispheres. These results therefore demonstrate the presence of sensory gating for auditory inputs of the human voice in the primary auditory cortex and the auditory association area.

Cross-modal enhancement of the MMN to speech-sounds indicates early and automatic integration of letters and speech-sounds

Recently brain imaging evidence indicated that letter/speech-sound integration, necessary for establishing fluent reading, takes place in auditory association areas and that the integration is influenced by stimulus onset asynchrony (SOA) between the letter and the speech-sound. In the present study, we used a specific ERP measure known for its automatic character, the mismatch negativity (MMN), to investigate the time course and automaticity of letter/speech-sound integration. We studied the effect of visual letters and SOA on the MMN elicited by a deviant speech-sound. We found a clear enhancement of the MMN by simultaneously presenting a letter, but without changing the auditory stimulation. This enhancement diminishes linearly with increasing SOA. These results suggest that letters and speech-sounds are processed as compound stimuli early and automatically in the auditory association cortex of fluent readers and that this processing is strongly dependent on timing.

Convergence of unimodal and polymodal sensory input to the entorhinal cortex in the fascicularis monkey

The hippocampal formation is a key structure in memory formation and consolidation. The hippocampus receives information from different cortical and subcortical sources. Cortical information is mostly funneled to the hippocampus through the entorhinal cortex (EC) in a bi-directional way that ultimately ends in the cortex. Retrograde tracing studies in the nonhuman primate indicate that more than two-thirds of the cortical afferents to the EC come from polymodal sensory association areas. Although some evidence for the projection from visual unimodal cortex to the EC exists, inputs from other visual and auditory unimodal association areas, and the possibility of their convergence with polymodal input in the EC remains largely undisclosed. We studied 10 Macaca fascicularis monkeys in which cortical deposits of the anterograde tracer biotinylated dextran-amine were made into different portions of visual and auditory unimodal association cortices in the temporal lobe, and in polymodal association cortex at the upper bank of the superior temporal sulcus. Visual and auditory unimodal as well as polymodal cortical areas projected to the EC. Both visual unimodal and polymodal association cortices presented dense projections, while those from unimodal auditory association cortex were more patchy and less dense. In all instances, the projection distributed in both the superficial and deep layers of the EC. However, while polymodal cortex projected to all layers (including layer I), visual unimodal cortex did not project to layer I, and auditory unimodal cortex projected less densely, scattered through all layers. Topographically, convergence from the three cortical areas studied can be observed in the lateral rostral and lateral caudal subfields. The present study suggests that unimodal and polymodal association cortical inputs converge in the lateral EC, thereby providing the possibility for the integration of complex stimuli for internal representations in declarative memory elaboration.

Processing Prosodic Boundaries in Natural and Hummed Speech: An fMRI Study

Speech contains prosodic cues such as pauses between different phrases of a sentence. These intonational phrase boundaries (IPBs) elicit a specific component in event-related brain potential studies, the so-called closure positive shift. The aim of the present functional magnetic resonance imaging study is to identify the neural correlates of this prosody-related component in sentences containing segmental and prosodic information (natural speech) and hummed sentences only containing prosodic information. Sentences with 2 IPBs both in normal and hummed speech activated the middle superior temporal gyrus, the rolandic operculum, and the gyrus of Heschl more strongly than sentences with 1 IPB. The results from a region of interest analysis of auditory cortex and auditory association areas suggest that the posterior rolandic operculum, in particular, supports the processing of prosodic information. A comparison of natural speech and hummed sentences revealed a number of left-hemispheric areas within the temporal lobe as well as in the frontal and parietal lobe that were activated more strongly for natural speech than for hummed sentences. These areas constitute the neural network for the processing of natural speech. The finding that no area was activated more strongly for hummed sentences compared with natural speech suggests that prosody is an integrated part of natural speech.

A functional MRI study of happy and sad affective states induced by classical music.

The present study investigated the functional neuroanatomy of transient mood changes in response to Western classical music. In a pilot experiment, 53 healthy volunteers (mean age: 32.0; SD = 9.6) evaluated their emotional responses to 60 classical musical pieces using a visual analogue scale (VAS) ranging from 0 (sad) through 50 (neutral) to 100 (happy). Twenty pieces were found to accurately induce the intended emotional states with good reliability, consisting of 5 happy, 5 sad, and 10 emotionally unevocative, neutral musical pieces. In a subsequent functional magnetic resonance imaging (fMRI) study, the blood oxygenation level dependent (BOLD) signal contrast was measured in response to the mood state induced by each musical stimulus in a separate group of 16 healthy participants (mean age: 29.5; SD = 5.5). Mood state ratings during scanning were made by a VAS, which confirmed the emotional valence of the selected stimuli. Increased BOLD signal contrast during presentation of happy music was found in the ventral and dorsal striatum, anterior cingulate, parahippocampal gyrus, and auditory association areas. With sad music, increased BOLD signal responses were noted in the hippocampus/amygdala and auditory association areas. Presentation of neutral music was associated with increased BOLD signal responses in the insula and auditory association areas. Our findings suggest that an emotion processing network in response to music integrates the ventral and dorsal striatum, areas involved in reward experience and movement; the anterior cingulate, which is important for targeting attention; and medial temporal areas, traditionally found in the appraisal and processing of emotions.

Hemodynamic responses in human multisensory and auditory association cortex to purely visual stimulation

BackgroundRecent findings of a tight coupling between visual and auditory association cortices during multisensory perception in monkeys and humans raise the question whether consistent paired presentation of simple visual and auditory stimuli prompts conditioned responses in unimodal auditory regions or multimodal association cortex once visual stimuli are presented in isolation in a post-conditioning run. To address this issue fifteen healthy participants partook in a "silent" sparse temporal event-related fMRI study. In the first (visual control) habituation phase they were presented with briefly red flashing visual stimuli. In the second (auditory control) habituation phase they heard brief telephone ringing. In the third (conditioning) phase we coincidently presented the visual stimulus (CS) paired with the auditory stimulus (UCS). In the fourth phase participants either viewed flashes paired with the auditory stimulus (maintenance, CS-) or viewed the visual stimulus in isolation (extinction, CS+) according to a 5:10 partial reinforcement schedule. The participants had no other task than attending to the stimuli and indicating the end of each trial by pressing a button.ResultsDuring unpaired visual presentations (preceding and following the paired presentation) we observed significant brain responses beyond primary visual cortex in the bilateral posterior auditory association cortex (planum temporale, planum parietale) and in the right superior temporal sulcus whereas the primary auditory regions were not involved. By contrast, the activity in auditory core regions was markedly larger when participants were presented with auditory stimuli.ConclusionThese results demonstrate involvement of multisensory and auditory association areas in perception of unimodal visual stimulation which may reflect the instantaneous forming of multisensory associations and cannot be attributed to sensation of an auditory event. More importantly, we are able to show that brain responses in multisensory cortices do not necessarily emerge from associative learning but even occur spontaneously to simple visual stimulation.

Auditory cortex and anterior cingulate cortex sources of the early evoked gamma-band response: Relationship to task difficulty and mental effort

High frequency oscillations in the 40Hz (gamma-band)-range are involved in the synchronization of brain regions, e.g., in cognitive functions. It has been suggested that the auditory evoked gamma-band response (GBR) is affected by attention and apart from auditory cortex activity a frontal or anterior cingulate cortex (ACC) generator could be involved. It was the aim of the present study to address three questions: (1) is there a neural generator of the early evoked GBR in the dorsal (d)ACC? (2) Are there different activation patterns in the dACC and the auditory cortex areas in response to task difficulty? (3) Is it possible to detect an influence of early ACC-gamma-band activity (GBR timeframe) to later auditory information processing (N1 timeframe)? In the present EEG/ERP-study we have investigated 30 healthy subjects using six auditory reaction tasks with increasing difficulty and mental effort demands. In the MANOVA analysis we found a significant main effect of task difficulty on both the GBR amplitude (F=7.75; p<0.001) and the auditory evoked N1 potential (F=7.00; p<0.001) with higher amplitudes in the more difficult tasks. In the LORETA region of interest (ROI) analysis, this effect was only due to increased dACC-activity during the GBR-timeframe. For the ROI analysis during the N1 timeframe, in addition to a strong effect of task difficulty in the dACC a similar main effect was found in the auditory association area 22. These findings are in line with a top-down influence of dACC-activity to the auditory association area 22 during the early evoked GBR.

Neural correlates of intelligibility in speech investigated with noise vocoded speech—A positron emission tomography study

Functional imaging studies of speech perception in the human brain have identified a key role for auditory association areas in the temporal lobes (bilateral superior temporal gyri and sulci) in the perceptual processing of the speech signal. This is extended to suggest some functional specialization within this bilateral system, with a particular role for the left anterior superior temporal sulcus (STS) in processing intelligible speech. In the current study, noise-vocoded speech was used to vary the intelligibility of speech parametrically. This replicated the finding of a selective response to intelligibility in speech in the left anterior superior temporal sulcus, in contrast to the posterior superior temporal sulcus, which showed a response profile insensitive to the degree of intelligibility. These results are related to theories of functional organization in the human auditory system, which have indicated that there are separate processing streams, with different functional roles, running anterior and posterior to primary auditory cortex. Specifically, it is suggested that an anterior stream processing intelligibility can be distinguished from a posterior stream associated with transient representations, important in spoken repetition and working memory.

Regional specificity of chandelier neuron axon terminal alterations in schizophrenia

Background: The axon terminals of GABAergic chandelier cells form linear arrays, termed cartridges, that synapse on the axon initial segment of neocortical pyramidal cells. These cartridges are immunoreactive for the GABA membrane transporter-1, and the density of GABA membrane transporter-1-immunoreactive cartridges in the prefrontal cortex has been reported to be reduced in schizophrenia. The goal of this study was to determine if reductions in the density of GABA membrane transporter-1-immunoreactive cartridges in schizophrenia are restricted to the prefrontal cortex. Methods: Relative GABA membrane transporter-1-immunoreactive cartridge density was determined in auditory association area 42, a region previously implicated in the pathophysiology of schizophrenia, in 14 matched pairs of subjects with schizophrenia and normal comparison subjects. The results were compared with similar data from prefrontal area 46 in the same subjects. Results: Mean GABA membrane transporter-1-immunoreactive cartridge density in area 42 was decreased by 9.8% in layers II–IIIa, and by 11.9% in layer VI in subjects with schizophrenia, although these differences did not achieve statistical significance. However, the magnitude of the reductions in the density of GABA membrane transporter-1-immunoreactive cartridges in area 42 of the subjects with schizophrenia was not significantly smaller than those in area 46. Conclusions: In subjects with schizophrenia, alterations in chandelier neuron axon cartridges appear to be more marked in the prefrontal cortex than in another cortical region implicated in the illness, although such changes might not be restricted to the prefrontal cortex.

Sound level dependence of the primary auditory cortex: Simultaneous measurement with 61-channel EEG and fMRI

Sound level dependence has been investigated for years with event-related potentials (ERP). A serotonergic modulation of the sound level dependence only of the primary auditory cortex but not of the auditory association cortex has been suggested by a number of clinical and preclinical studies. Therefore, a precise covering of the activity of the primary auditory cortex seems necessary if sound level dependence is used as an indicator of the central serotonergic system. Recent fMRI studies described a pronounced sound level dependence only in the Heschl gyrus/primary auditory cortex but not in auditory association areas. In the present simultaneous 61-channel EEG and fMRI study investigating fourteen healthy subjects, we found a high correlation between the loudness-dependent change of the extent of fMRI activation (number of activated voxels) and the corresponding changes of the mean current source density within the same region of interest covering the primary auditory cortex (r = 0.84, P < 0.001). Our findings suggest a close relationship between the fMRI signal and event-related potential activity. In addition, the correspondence of the ERP-based data and the fMRI results further supports the validity of the ERP localization approach.

Age-dependent changes of glyoxalase I expression in human brain

Increased modification and crosslinking of proteins by advanced glycation end products (AGEs) is a characteristic feature of aging, and contributes to the formation of many of the lesions of neurodegenerative diseases including neurofibrillary tangles and amyloid plaques in Alzheimer's disease. Therefore, defense mechanisms against AGE formation or detoxification of their precursors such as the glyoxalase system are of particular interest in aging research. Thus, we investigated the age-dependent protein expression, the activity as well as the RNA level of glyoxalase I in Brodmann area 22 (auditory association area of superior temporal gyrus) of the human cerebral cortex. Our immunohistochemical results demonstrate the localization of glyoxalase I in neurons, predominantly pyramidal cells, as well as in astroglia, located predominantly in the subpial region. The number of glyoxalase I expressing neurons and astroglia increases with age, with a peak at approximately 55 years, and progressively decreases thereafter. These results were confirmed by biochemical investigations in total brain tissue, where the RNA, the protein level as well as the activity of glyoxalase I enzyme were analyzed in different age groups. In conclusion, the increase in glyoxalase I expression up to the age of 55 may be a compensatory mechanism against high oxoaldyde levels and the accumulation of AGEs. However, the decline of glyoxalase expression and activity in old age, possibly caused by impairment in transcription or/and translation, may subsequently lead to increased levels of reactive carbonyl compounds, followed by protein crosslinking, inflammation, oxidative stress and neuronal degeneration.

Hemispheric asymmetry, modular variability and age-related changes in the human entorhinal cortex

The verrucae areae entorhinalis (VAE) are a characteristic feature of the human brain that occupy the anterior and posterolateral parts of the parahippocampal gyri and correspond to the islands of layer II neurons. We analyzed VAE in 60 neurologically normal subjects ranging from 23 to 85 years of age using a casting method. In 10 of these subjects the total number of neurons in the entorhinal islands was estimated stereologically using the optical fractionator. The number and surface area of VAE were higher in the left hemisphere compared with the right, and this leftward asymmetry was highly significant. Regression analysis showed a negative correlation between average VAE area and age in both hemispheres, representing a rate loss of about 800 μm 2 per year. The estimated number of neurons obtained with the optical fractionator showed no significant difference between the left and the right hemisphere (468,000±144,000 vs. 405,000±117,000). There was a highly significant negative correlation between neuron numbers and age in both sides. In addition, clusters of small, undifferentiated layer II neurons (‘heterotopias’) were frequently observed in the rostral part of the entorhinal cortex in young and elderly adults. Layer II entorhinal neurons are among the first to show neurofibrillary changes during normal aging. The present data confirm the occurrence of age-related neuron loss in the entorhinal cortex. Considering the consistent projections from ipsilateral auditory association areas that, together with Broca’s motor-speech area (Brodmann areas 44 and 45), show leftward asymmetry from early infancy (such as Brodmann area 22, planum temporale, and area 52 in the long insular gyrus), we speculate that functional lateralization of the human entorhinal cortex may be associated with specialization for memory processing related to language. Due to the dependence of hippocampal formation on entorhinal projections, this finding is also consistent with the greater capacity of the left hippocampus for verbal episodic memory.

Grabbing Your Ear: Rapid Auditory–Somatosensory Multisensory Interactions in Low-level Sensory Cortices Are Not Constrained by Stimulus Alignment

Multisensory interactions are observed in species from single-cell organisms to humans. Important early work was primarily carried out in the cat superior colliculus and a set of critical parameters for their occurrence were defined. Primary among these were temporal synchrony and spatial alignment of bisensory inputs. Here, we assessed whether spatial alignment was also a critical parameter for the temporally earliest multisensory interactions that are observed in lower-level sensory cortices of the human. While multisensory interactions in humans have been shown behaviorally for spatially disparate stimuli (e.g. the ventriloquist effect), it is not clear if such effects are due to early sensory level integration or later perceptual level processing. In the present study, we used psychophysical and electrophysiological indices to show that auditory-somatosensory interactions in humans occur via the same early sensory mechanism both when stimuli are in and out of spatial register. Subjects more rapidly detected multisensory than unisensory events. At just 50 ms post-stimulus, neural responses to the multisensory 'whole' were greater than the summed responses from the constituent unisensory 'parts'. For all spatial configurations, this effect followed from a modulation of the strength of brain responses, rather than the activation of regions specifically responsive to multisensory pairs. Using the local auto-regressive average source estimation, we localized the initial auditory-somatosensory interactions to auditory association areas contralateral to the side of somatosensory stimulation. Thus, multisensory interactions can occur across wide peripersonal spatial separations remarkably early in sensory processing and in cortical regions traditionally considered unisensory.

Age- and stage-dependent accumulation of advanced glycation end products in intracellular deposits in normal and Alzheimer's disease brains.

In this immunohistochemical study, the age- and stage-dependent accumulation of advanced glycation end-products (AGEs) in Alzheimer's disease (AD) and their relation to the formation of neurofibrillary tangles and neuronal cell death was investigated. For this purpose, the distribution of AGEs in neurons and glia was analyzed in the auditory association area of superior temporal gyrus (Brodmann area 22) of young and old non-demented controls and compared with early- and late-stage AD. A possible co-localization of AGEs with typical hallmarks of AD, such as hyperphosphorylated tau (as a marker for disturbed kinase/phosphatase activity), nNOS (as a marker for nitroxidative stress) and caspase-3 (as a marker of apoptotic cell death), was also investigated. Our results show that the percentage of AGE-positive neurons (and astroglia) increase both with age and, in AD patients, with the progression of the disease (Braak stages). Interestingly, nearly all if those neurons which show diffuse cytosolic AGE immunoreactivity also contain hyperphosphoryated tau, suggesting a link between AGE accumulation and the formation of early neurofibrillary tangles. Many, but not all, neurons show a co-localization of AGEs with other markers of neurodegeneration, such as nNOS and caspase-3.

Right temporal cortex is critical for utilization of melodic contextual cues in a pitch constancy task.

Pitch constancy, perceiving the same pitch from tones with differing spectral shapes, requires one to extract the fundamental frequency from two sets of harmonics and compare them. We previously showed this difficult task to be easier when tonal context is present, presumably because the context creates a tonal reference point from which to judge the test tone. The present study assessed the role of the right auditory cortex in using tonal context for pitch judgements. Thirty-six patients with focal brain excisions of the right or left anterior temporal lobe (RT, LT) and 12 matched control participants (NC) made pitch judgements on complex tones that could differ in fundamental frequency and/or spectral shape. This task was performed in isolation and within a melodic context. The RT group showed impairments both on trials in which extraction of pitch from differing spectral shapes was required (different-timbre trials) and when this was not required (same-timbre trials). All groups performed poorly in the isolated condition, but improved with melodic context. Degree of improvement varied in that the LT group performed normally, whereas the RT group was not able to obtain the same amount of facilitation from the melodic context. In particular, melodic context did not facilitate the RT group's performance on different-timbre trials. Excisions within Heschl's gyrus did not affect these results, suggesting that the impairments were due to the removal of the anterior temporal cortex. The results of this study therefore implicate right anterior auditory cortical areas in making pitch judgements relative to tones that were heard previously. We propose that auditory association areas located on the anterior portion of the superior temporal gyrus, an area with connections to frontal regions implicated in working memory, could be involved in holding and integrating tonal information.

Common features of fluency-evoking conditions studied in stuttering subjects and controls: an [formula omitted] PET study

We used H 2 15 O PET to characterize the common features of two successful but markedly different fluency-evoking conditions — paced speech and singing — in order to identify brain mechanisms that enable fluent speech in people who stutter. To do so, we compared responses under fluency-evoking conditions with responses elicited by tasks that typically elicit dysfluent speech (quantifying the degree of stuttering and using this measure as a confounding covariate in our analyses). We evaluated task-related activations in both stuttering subjects and age- and gender-matched controls. Areas that were either uniquely activated during fluency-evoking conditions, or in which the magnitude of activation was significantly greater during fluency-evoking than dysfluency-evoking tasks included auditory association areas that process speech and voice and motor regions related to control of the larynx and oral articulators. This suggests that a common fluency-evoking mechanism might relate to more effective coupling of auditory and motor systems — that is, more efficient self-monitoring, allowing motor areas to more effectively modify speech. These effects were seen in both PWS and controls, suggesting that they are due to the sensorimotor or cognitive demands of the fluency-evoking tasks themselves. While responses seen in both groups were bilateral, however, the fluency-evoking tasks elicited more robust activation of auditory and motor regions within the left hemisphere of stuttering subjects, suggesting a role for the left hemisphere in compensatory processes that enable fluency. Educational objectives: The reader will learn about and be able to: (1) compare brain activation patterns under fluency- and dysfluency-evoking conditions in stuttering and control subjects; (2) appraise the common features, both central and peripheral, of fluency-evoking conditions; and (3) discuss ways in which neuroimaging methods can be used to understand the pathophysiology of stuttering.

Parcellation of cortical afferents to three distinct sectors in the parahippocampal gyrus of the rhesus monkey: an anatomical and neurophysiological study.

The posterior parahippocampal gyrus (PHG) of the rhesus monkey (Macaca mulatta) is comprised of three distinct cortical areas based on cytoarchitecture, connectivity, and neurophysiological response properties. Fluorescent retrograde tracers placed in each PHG area demonstrated unique patterns of cortical afferent input to areas TH, TL, and TF. Area TF receives inputs from the multimodal cortices of the superior temporal sulcus including areas PGa, TPO, and MST, from the visuospatial parietal area PG-Opt, and from visual areas V3A and dorsal V4. Area TL receives afferents from the inferotemporal region including visual areas TE1 and TE2 as well as from areas TEa, IPa, and FST in the lower bank and depth of the superior temporal sulcus. In contrast, the input to area TH is from the rostral part of superior temporal gyrus, including the auditory association areas TS1-3, and from the middle sector of area TPO in the superior temporal sulcus. Frontal and cingulate areas also project to the PHG in largely differential patterns. To further investigate this a correlative electrophysiological study of the three PHG areas resulted in a confirmation of these differential cortical inputs such that visually responsive neurons were found in areas TF and TL, auditory responsive neurons or bimodal auditory/visual-responsive neurons in area TH, and somatosensory-responsive neurons at the TF/TL border. Since each PHG area also receives differential hippocampal input, these data suggest that the processing of unimodal or multimodal information may be related to memory processing functions that are largely segregated within areas TH, TL, and TF.

Multisensory convergence in calcarine visual areas in macaque monkey

The neural substrates of multisensory perception and integration are still obscure, especially at the cortical level. Alternative viewpoints emphasize (1) ‘bottom–up’ processes, where different modalities converge in higher order multisensory areas, or (2) ‘top–down’ projections from multimodal to unimodal areas. In this anatomic study, we use anterograde tracer injections in parietal (8 monkeys) and auditory (3 monkeys) association areas, and demonstrate direct projections to areas V1 and V2 in the calcarine fissure (i.e. the peripheral visual field representation). The laminar signature, with terminations in layers 1 and/or 6, could be consistent with feedback-type connections. A subset of connections from parietal areas, however, branch to both V1 and a ventral extrastriate area (TEO or TEp). Thus, the direct connections to early visual areas V1 and V2 may well operate in conjunction with polysynaptic pathways in a densely parallel network.

Effect of prenatal sound stimulation on medio-rostral neostriatum/hyperstriatum ventrale region of chick forebrain: a morphometric and immunohistochemical study

The higher auditory association area in chick forebrain, i.e. medio-rostral neostriatum/hyperstriatum ventrale region (MNH), is involved in juvenile auditory filial imprinting. Studies show that neuronal size as well as expression of calcium-binding proteins, parvalbumin (PV) and calbindin D28K (CaBP) are regulated by neuronal activation. In the present study, we have determined the effect of extra auditory stimulation, given as a prenatal sound enrichment protocol, on MNH neurons of posthatch day 1 chicks. Patterned species-specific or musical (sitar) sounds were provided in a graded manner from embryonic day 10 through hatching. Thionin and immunohistochemically stained (PV and CaBP) neurons were evaluated by morphometric methods. The thionin-stained MNH neurons of both the auditory stimulated groups showed a significant increase in nuclear area compared to controls. The change in nuclear dimension was greater in the music-stimulated than in the species-specific sounds-stimulated group. These observations indicate a positive influence of prenatal sound stimulation on MNH neurons. The auditory stimulated groups also demonstrated an increase in the proportion of PV- and CaBP-neurons compared to controls, with the species-specific sounds-stimulated group showing a significantly higher percentage of immunostained cells than the music-stimulated group. However, immunostained cells of both the auditory stimulated groups did not show a significant change in size. These cytoplasmic proteins, by acting as intracellular buffers, enable neurons to display high electrical activity without calcium overload. The influx of Ca 2+ ions is essential for long-term potentiation, a phenomenon important for learning and memory. The increase in percentage of the neurons containing calcium-binding proteins may provide a morphological basis for enhancement of auditory imprinting and learning.