Cortical Response Patterns Research Articles

Distinct cortical responses evoked by electrical stimulation of the thalamic ventral intermediate nucleus and of the subthalamic nucleus.

ObjectiveTo investigate the spatial and temporal pattern of cortical responses evoked by deep brain stimulation (DBS) of the subthalamic nucleus (STN) and ventral intermediate nucleus of the thalamus (VIM).MethodsWe investigated 7 patients suffering from Essential tremor (ET) and 7 patients with Parkinson's Disease (PD) following the implantation of DBS electrodes (VIM for ET patients, STN for PD patients). Magnetoencephalography (MEG) was used to record cortical responses evoked by electric stimuli that were applied via the DBS electrode in trains of 5 Hz. Dipole fitting was applied to reconstruct the origin of evoked responses.ResultsBoth VIM and STN DBS led to short latency cortical responses at about 1 ms. The pattern of medium and long latency cortical responses following VIM DBS consisted of peaks at 13, 40, 77, and 116 ms. The associated equivalent dipoles were localized within the central sulcus, 3 patients showed an additional response in the cerebellum at 56 ms. STN DBS evoked cortical responses peaking at 4 ms, 11 ms, and 27 ms, respectively. While most dipoles were localized in the pre- or postcentral gyrus, the distribution was less homogenous compared to VIM stimulation and partially included prefrontal brain areas.ConclusionMEG enables localization of cortical responses evoked by DBS of the VIM and the STN, especially in the sensorimotor cortex. Short latency responses of 1 ms suggest cortical modulation which bypasses synaptic transmission, i.e. antidromic activation of corticofugal fiber pathways.

Differences in Click and Speech Auditory Brainstem Responses and Cortical Response Patterns: A Pilot Study

Hearing loss has adverse developmental, cognitive and social impacts on individual’s lives. To improve its diagnosis and remediation outcomes, development of objective hearing methods offer new ways of optimizing care strategy. Complementary to the classical click-evoked brainstem and cortical responses, interest grows regarding the speech auditory brainstem responses, whose components, namely the onset and frequency following responses, are proposed as biomarkers of speech encoding at the brainstem level. To improve the understanding of auditory processing in the human brainstem and its effect on cortical processing, we used a multimodal set-up and recorded brainstem and cortical potentials in response to click stimuli and speech stimuli presented at different stimulation intensities in normal hearing adults. We hypothesized that even though click- and onset responses of the speechauditory brainstem responses share some similarities; their underlying mechanisms are in some point distinct. We also presumed that using a noninvasive method we could assess the generators underlying click, onset and frequency following responses and the time wise influence of brainstem encoding on cortical processing of click and speech stimuli. Results showed evidence regarding mechanisms underlying onset response and frequency following response components at the brainstem level. A direct time wise relationship between subcortical encoding and cortical encoding was revealed: cortical activity in the left cortex was related to the onset response latency at the brainstem level. These results highlight the potential use of new methods in speech processing electroencephalographic studies and provide qualitative and topographical data regarding subcortical and cortical auditory processing network.

PCI & auditory ERPs for the quantification of the level of consciousness: an EEG-based methods comparison study applied to disorders of consciousness.

Event Abstract Back to Event PCI & auditory ERPs for the quantification of the level of consciousness: an EEG-based methods comparison study applied to disorders of consciousness. Federico Raimondo1, 2, 3, 4*, Audrey Wolff1, Leandro R. Sanz1, Silvia Casarotto5, Matteo Fecchio5, Séverine Blandiaux1, Olivier Bodart1, Alice Barra1, Angela Comanducci5, Renate Rutiku5, Jitka Annen1, Mario Rosanova5, Marcello Massimini5, Jacobo D. Sitt2, 3, Steven Laureys1 and Olivia Gosseries1 1 GIGA Consciousness, University of Liège, Belgium 2 INSERM U1127 Institut du Cerveau et de la Moelle épinière, France 3 Sorbonne Université, France 4 Applied Artificial Intelligence Lab, Department of Computer Science, University of Buenos Aires, Argentina 5 Dipartimento di Scienze biomediche e cliniche Luigi Sacco, Università degli Studi di Milano, Italy Aims Patients with disorders of consciousness (DOC) suffer from lack of awareness at different levels[1]. Clinical categories range from reflexive behaviour (Unresponsive Wakefulness Syndrome, UWS), to more complex purposeful interaction with the environment such as visual pursuit (Minimally Conscious State minus, MCS-), response to command (MCS+) or capacity to communicate and/or functionally use objects (Emergence from the Minimally Conscious State, EMCS). A challenge exists when diagnosing DOC patients. The gold standard diagnosis is performed by repeated and standardised behavioural assessments[2] (Coma Recovery Scale-Revised). Objective neuroimaging techniques are utilized to support clinical diagnosis. Compared to neuroimaging, EEG-based systems have the advantage of being repeatable and cheap. Two of the most reliable methods are the Perturbational Complexity Index[3] (PCI), driven by the Integrated Information Theory[4] (IIT), and a synergy of EEG-extracted markers[5], [6] from a standardised auditory oddball paradigm (EEG-ERP) that combines information content, information sharing and global workspace theory[7] (GWT) markers. This study, which is part of the Human Brain Project (SP3), aims to confront the results from these two methods when applied to the diagnosis of Disorders of Consciousness. Methods A multi-centre group of 25 DOC patients (i.e., UWS, MCS and EMCS) were subject to both EEG recordings. The PCI value was computed by compressing the spatiotemporal pattern of cortical responses to the perturbation of the cortex with Transcranial Magnetic Stimulation. We then extracted 120 markers, corresponding to quantification of power spectrum and complexity in individual EEG sensors and information sharing between EEG sensors. Finally, we contrasted the obtained PCI values, the values of the EEG-extracted markers and the predicted individual probability of being (minimally) conscious using machine learning. Results When we analysed the relationship between PCI and the most informative EEG-extracted markers, we found a significant correlation with Weighted Symbolic Mutual Information (r=0.42, p=0.034) but not with Alpha Power (r=0.37, p=0.06) and Kolmogorov Complexity (r=0.1, p=0.63). For the multivariate approach, PCI and EEG markers provided a consistent diagnosis for 19 patients (76%) and correlated positively (r=0.59, p=0.002). All UWS (N=5), all EMCS (N=4) and 9 MCS patients were correctly diagnosed by both methods. However, 1 MCS patient was misdiagnosed by both methods, and 6 MCS patients had inconsistent results between the measures. Conclusions PCI correlated positively with the combination of EEG markers in severely brain-injured patients, but not with all the markers independently. Although the EEG-ERP method was designed to probe the GWT, the combination of EEG markers used with machine learning is driven by multiple theories, including IIT and GWT. Case mismatches can be explained by the diverging purposes of the methods: while the PCI is designed to probe capacity for consciousness, EEG-ERP characterises the current state of consciousness. These findings suggest that the shared common background is also evident in the results, providing a validation of the methods and a link between these theories.

Task-Modulated Cortical Representations of Natural Sound Source Categories.

In everyday sound environments, we recognize sound sources and events by attending to relevant aspects of an acoustic input. Evidence about the cortical mechanisms involved in extracting relevant category information from natural sounds is, however, limited to speech. Here, we used functional MRI to measure cortical response patterns while human listeners categorized real-world sounds created by objects of different solid materials (glass, metal, wood) manipulated by different sound-producing actions (striking, rattling, dropping). In different sessions, subjects had to identify either material or action categories in the same sound stimuli. The sound-producing action and the material of the sound source could be decoded from multivoxel activity patterns in auditory cortex, including Heschl's gyrus and planum temporale. Importantly, decoding success depended on task relevance and category discriminability. Action categories were more accurately decoded in auditory cortex when subjects identified action information. Conversely, the material of the same sound sources was decoded with higher accuracy in the inferior frontal cortex during material identification. Representational similarity analyses indicated that both early and higher-order auditory cortex selectively enhanced spectrotemporal features relevant to the target category. Together, the results indicate a cortical selection mechanism that favors task-relevant information in the processing of nonvocal sound categories.

Mapping the fine structure of cortical activity with different micro-ECoG electrode array geometries

Objective. Innovations in micro-electrocorticography (µECoG) electrode array manufacturing now allow for intricate designs with smaller contact diameters and/or pitch (i.e. inter-contact distance) down to the sub-mm range. The aims of the present study were: (i) to investigate whether frequency ranges up to 400 Hz can be reproducibly observed in µECoG recordings and (ii) to examine how differences in topographical substructure between these frequency bands and electrode array geometries can be quantified. We also investigated, for the first time, the influence of blood vessels on signal properties and assessed the influence of cortical vasculature on topographic mapping. Approach. The present study employed two µECoG electrode arrays with different contact diameters and inter-contact distances, which were used to characterize neural activity from the somatosensory cortex of minipigs in a broad frequency range up to 400 Hz. The analysed neural data were recorded in acute experiments under anaesthesia during peripheral electrical stimulation. Main results. We observed that µECoG recordings reliably revealed multi-focal cortical somatosensory response patterns, in which response peaks were often less than 1 cm apart and would thus not have been resolvable with conventional ECoG. The response patterns differed by stimulation site and intensity, they were distinct for different frequency bands, and the results of functional mapping proved independent of cortical vascular. Our analysis of different frequency bands exhibited differences in the number of activation peaks in topographical substructures. Notably, signal strength and signal-to-noise ratios differed between the two electrode arrays, possibly due to their different sensitivity for variations in spatial patterns and signal strengths. Significance. Our findings that the geometry of µECoG electrode arrays can strongly influence their recording performance can help to make informed decisions that maybe important in number of clinical contexts, including high-resolution brain mapping, advanced epilepsy diagnostics or brain–machine interfacing.

A Near-Infrared Spectroscopy Study on Cortical Hemodynamic Responses to Normal and Whispered Speech in 3- to 7-Year-Old Children.

The purpose of this study was to assess cortical hemodynamic response patterns in 3- to 7-year-old children listening to two speech modes: normally vocalized and whispered speech. Understanding whispered speech requires processing of the relatively weak, noisy signal, as well as the cognitive ability to understand the speaker's reason for whispering. Near-infrared spectroscopy (NIRS) was used to assess changes in cortical oxygenated hemoglobin from 16 typically developing children. A profound difference in oxygenated hemoglobin levels between the speech modes was found over left ventral sensorimotor cortex. In particular, over areas that represent speech articulatory body parts and motion, such as the larynx, lips, and jaw, oxygenated hemoglobin was higher for whisper than for normal speech. The weaker stimulus, in terms of sound energy, thus induced the more profound hemodynamic response. This, moreover, occurred over areas involved in speech articulation, even though the children did not overtly articulate speech during measurements. Because whisper is a special form of communication not often used in daily life, we suggest that the hemodynamic response difference over left ventral sensorimotor cortex resulted from inner (covert) practice or imagination of the different articulatory actions necessary to produce whisper as opposed to normal speech.

EPV 8. Thalamic and subthalamic deep brain stimulation evoke distinct and consistent patterns of cortical responses

Recent research revealed that cortical areas are substantially modulated by deep brain stimulation (DBS). To better understand the spatial and temporal pattern of cortical modulation, we recorded and localized responses evoked by low-frequency DBS using magnetoencephalography (MEG). We investigated nine patients suffering from Essential Tremor (ET) and 11 patients with Parkinson’s disease (PD) following DBS surgery. The DBS target area was the subthalamic nucleus (STN) in PD patients and the ventral intermediate nucleus of the thalamus (VIM) in ET patients. We applied unilateral, bipolar DBS with a frequency of 5 Hz at clinically effective amplitude, using an external stimulator and externalized leads. Dipole fitting was applied to MEG data to estimate the anatomical origin of evoked responses. Evoked responses were observed in 7 of 9 ET patients, and 7 of 11 PD patients. The temporal profile of responses was very consistent within groups. The response to VIM stimulation was best explained by a dipole which alternated between anterior and posterior orientation periodically (peaks around 13, 40, 77, and 116 ms). It localized to the central sulcus ipsilateral to DBS in all cases. Three patients showed an additional response in the contralateral cerebellum. STN DBS evoked responses around 4, 11 and 27 ms in sensorimotor channels, which were an order of magnitude smaller than those evoked by VIM DBS. Localization results were variable across subjects. Ipsilateral precentral gyrus was the most frequently estimated origin among a number of cortical and subcortical areas. Finally, we observed very short latency responses around 1 ms in individual ET and PD patients. We conclude that MEG enables localization of cortical responses evoked by DBS. Our results suggest that sensorimotor cortex is primarily modulated. The response in sensorimotor cortex is larger for VIM than for STN stimulation, which is characterized by a distinct spatio-temporal profile of responses. Deeper source estimates, lower sensor amplitudes and earlier peaks point towards stronger subcortical contributions compared to VIM DBS.

Mapping the cortical representation of speech sounds in a syllable repetition task

Speech repetition relies on a series of distributed cortical representations and functional pathways. A speaker must map auditory representations of incoming sounds onto learned speech items, maintain an accurate representation of those items in short-term memory, interface that representation with the motor output system, and fluently articulate the target sequence. A “dorsal stream” consisting of posterior temporal, inferior parietal and premotor regions is thought to mediate auditory-motor representations and transformations, but the nature and activation of these representations for different portions of speech repetition tasks remains unclear. Here we mapped the correlates of phonetic and/or phonological information related to the specific phonemes and syllables that were heard, remembered, and produced using a series of cortical searchlight multi-voxel pattern analyses trained on estimates of BOLD responses from individual trials. Based on responses linked to input events (auditory syllable presentation), predictive vowel-level information was found in the left inferior frontal sulcus, while syllable prediction revealed significant clusters in the left ventral premotor cortex and central sulcus and the left mid superior temporal sulcus. Responses linked to output events (the GO signal cueing overt production) revealed strong clusters of vowel-related information bilaterally in the mid to posterior superior temporal sulcus. For the prediction of onset and coda consonants, input-linked responses yielded distributed clusters in the superior temporal cortices, which were further informative for classifiers trained on output-linked responses. Output-linked responses in the Rolandic cortex made strong predictions for the syllables and consonants produced, but their predictive power was reduced for vowels. The results of this study provide a systematic survey of how cortical response patterns covary with the identity of speech sounds, which will help to constrain and guide theoretical models of speech perception, speech production, and phonological working memory.

Neurobiology of Bipolar Disorder: Abnormalities on Cognitive and Cortical Functioning and Biomarker Levels.

Bipolar disorder (BD) affects 1 to 1.5% of the world population and consists of at least one manic episode (or hypomanic) associated with depressive episodes, interspersed with periods of euthymic mood. Recurrent crises lead to significant disability in BD patients, and correlates negatively to social and occupational adjustment. Such disability can be explained by a series of events, such as cortical and altered metabolic activity, impairments in cognitive functions, and in core anatomical structures involved in mood modulation. Therefore, our review aims to provide information on the current research related to the pathophysiology of BD. We will review the cognitive and brain functioning, and biomarkers of BD. The current literature shows that cognitive deficits are commonly observed in all phases in BD patients, independent of a remissive state. These deficits are assigned to functional, structural and metabolic changes, particularly in the pre-frontal cortex region, hippocampus and amygdala, along with the connections between them, as well as decreased baseline brain-derived neurotrophic factor levels or imbalance between pro- and anti-inflammatory cytokines, implying a lower physical ability to reestablish from a stressful stimulus. BD patients effectively present a differentiated pattern of cortical, neuroanatomical and functional responses. It is suggested that physiological processes occur differently in bipolar subjects compared to healthy individuals, affecting behavior and brain function in such patients. Future directions are yet necessary to establish the best way to neutralize or reverse these events.

Velocity Selective Networks in Human Cortex Reveal Two Functionally Distinct Auditory Motion Systems.

The auditory system encounters motion cues through an acoustic object’s movement or rotation of the listener’s head in a stationary sound field, generating a wide range of naturally occurring velocities from a few to several hundred degrees per second. The angular velocity of moving acoustic objects relative to a listener is typically slow and does not exceed tens of degrees per second, whereas head rotations in a stationary acoustic field may generate fast-changing spatial cues in the order of several hundred degrees per second. We hypothesized that these two types of systems (i.e., encoding slow movements of an object or fast head rotations) may engage functionally distinct substrates in processing spatially dynamic auditory cues, with the latter potentially involved in maintaining perceptual constancy in a stationary field during head rotations and therefore possibly involving corollary-discharge mechanisms in premotor cortex. Using fMRI, we examined cortical response patterns to sound sources moving at a wide range of velocities in 3D virtual auditory space. We found a significant categorical difference between fast and slow moving sounds, with stronger activations in response to higher velocities in the posterior superior temporal regions, the planum temporale, and notably the premotor ventral-rostral (PMVr) area implicated in planning neck and head motor functions.

Fish Oil Supplementation Increases Event-Related Posterior Cingulate Activation in Older Adults with Subjective Memory Impairment.

To determine the effects of long-chain omega-3 (LCn-3) fatty acids found in fish oil, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), on cortical blood oxygen level-dependent (BOLD) activity during a working memory task in older adults with subjective memory impairment. Randomized, double-blind, placebo-controlled study. Academic medical center. Healthy older adults (62-80 years) with subjective memory impairment, but not meeting criteria for mild cognitive impairment or dementia. Fish oil (EPA+DHA: 2.4 g/d, n=11) or placebo (corn oil, n=10) for 24 weeks. Cortical BOLD response patterns during performance of a sequential letter n-back working memory task were determined at baseline and week 24 by functional magnetic resonance imaging (fMRI). At 24 weeks erythrocyte membrane EPA+DHA composition increased significantly from baseline in participants receiving fish oil (+31%, p ≤ 0.0001) but not placebo (-17%, p=0.06). Multivariate modeling of fMRI data identified a significant interaction among treatment, visit, and memory loading in the right cingulate (BA 23/24), and in the right sensorimotor area (BA 3/4). In the fish oil group, BOLD increases at 24 weeks were observed in the right posterior cingulate and left superior frontal regions during memory loading. A region-of-interest analysis indicated that the baseline to endpoint change in posterior cingulate cortex BOLD activity signal was significantly greater in the fish oil group compared with the placebo group during the 1-back (p=0.0003) and 2-back (p=0.0005) conditions. Among all participants, the change in erythrocyte EPA+DHA during the intervention was associated with performance in the 2-back working memory task (p = 0.01), and with cingulate BOLD signal during the 1-back (p = 0.005) with a trend during the 2-back (p = 0.09). Further, cingulate BOLD activity was related to performance in the 2-back condition. Dietary fish oil supplementation increases red blood cell omega-3 content, working memory performance, and BOLD signal in the posterior cingulate cortex during greater working memory load in older adults with subjective memory impairment suggesting enhanced neuronal response to working memory challenge.

Diverse cortical codes for scene segmentation in primate auditory cortex.

The temporal coherence of amplitude fluctuations is a critical cue for segmentation of complex auditory scenes. The auditory system must accurately demarcate the onsets and offsets of acoustic signals. We explored how and how well the timing of onsets and offsets of gated tones are encoded by auditory cortical neurons in awake rhesus macaques. Temporal features of this representation were isolated by presenting otherwise identical pure tones of differing durations. Cortical response patterns were diverse, including selective encoding of onset and offset transients, tonic firing, and sustained suppression. Spike train classification methods revealed that many neurons robustly encoded tone duration despite substantial diversity in the encoding process. Excellent discrimination performance was achieved by neurons whose responses were primarily phasic at tone offset and by those that responded robustly while the tone persisted. Although diverse cortical response patterns converged on effective duration discrimination, this diversity significantly constrained the utility of decoding models referenced to a spiking pattern averaged across all responses or averaged within the same response category. Using maximum likelihood-based decoding models, we demonstrated that the spike train recorded in a single trial could support direct estimation of stimulus onset and offset. Comparisons between different decoding models established the substantial contribution of bursts of activity at sound onset and offset to demarcating the temporal boundaries of gated tones. Our results indicate that relatively few neurons suffice to provide temporally precise estimates of such auditory "edges," particularly for models that assume and exploit the heterogeneity of neural responses in awake cortex.

Effects of spectral degradation on attentional modulation of auditory responses to continuous speech

Auditory attention enhances phase-locked responses to the temporal envelope of the attended speech stream in a competing background. This study investigates how cortical responses are affected by spectral degradation and the extent to which low-frequency fine-structure cues facilitate sound segregation. Two competing speech streams were presented diotically to normal-hearing subjects. Each subject was tested with unprocessed speech, vocoder speech (8 to 64 channels), and six-channel high-frequency vocoder + lowpass filtered speech simulating electric-acoustic stimulation (EAS). Ongoing EEG responses were measured in each condition. Cross-correlation between speech envelope and EEG responses was calculated at different time lags. For unprocessed speech, the cross-correlation function showed opposite signs of correlations between the attended and unattended speech, supporting the neural mechanism of suppression of the competing speech stream. This suppression mechanism, however, was only evident for the 32- and 64-channel vocoder conditions, but not for the 8- and 16-channel conditions. This indicates that greater frequency resolution is required for sound segregation. For EAS, the pattern of cortical responses was similar to that of the eight-channel vocoder condition, although speech intelligibility was higher in EAS than vocoder speech. Consistent with previous psychoacoustic evidence, these neural results further argue against the segregation mechanism for EAS benefit.

Task-Dependent Decoding of Speaker and Vowel Identity from Auditory Cortical Response Patterns

Selective attention to relevant sound properties is essential for everyday listening situations. It enables the formation of different perceptual representations of the same acoustic input and is at the basis of flexible and goal-dependent behavior. Here, we investigated the role of the human auditory cortex in forming behavior-dependent representations of sounds. We used single-trial fMRI and analyzed cortical responses collected while subjects listened to the same speech sounds (vowels /a/, /i/, and /u/) spoken by different speakers (boy, girl, male) and performed a delayed-match-to-sample task on either speech sound or speaker identity. Univariate analyses showed a task-specific activation increase in the right superior temporal gyrus/sulcus (STG/STS) during speaker categorization and in the right posterior temporal cortex during vowel categorization. Beyond regional differences in activation levels, multivariate classification of single trial responses demonstrated that the success with which single speakers and vowels can be decoded from auditory cortical activation patterns depends on task demands and subject's behavioral performance. Speaker/vowel classification relied on distinct but overlapping regions across the (right) mid-anterior STG/STS (speakers) and bilateral mid-posterior STG/STS (vowels), as well as the superior temporal plane including Heschl's gyrus/sulcus. The task dependency of speaker/vowel classification demonstrates that the informative fMRI response patterns reflect the top-down enhancement of behaviorally relevant sound representations. Furthermore, our findings suggest that successful selection, processing, and retention of task-relevant sound properties relies on the joint encoding of information across early and higher-order regions of the auditory cortex.

Neuroimaging-aided differential diagnosis of the depressive state

A serious problem in psychiatric practice is the lack of specific, objective biomarker-based assessments to guide diagnosis and treatment. The use of such biomarkers could assist clinicians in establishing differential diagnosis, which may improve specific individualised treatment. This multi-site study sought to develop a clinically suitable neuroimaging-guided diagnostic support system for differential diagnosis at the single-subject level among multiple psychiatric disorders with depressive symptoms using near-infrared spectroscopy, which is a compact and portable neuroimaging method. We conducted a multi-site, case–control replication study using two cohorts, which included seven hospitals in Japan. The study included 673 patients (women/men: 315/358) with psychiatric disorders (major depressive disorder, bipolar disorder, or schizophrenia) who manifested depressive symptoms, and 1007 healthy volunteers (530/477). We measured the accuracy of the single-subject classification in differential diagnosis among major psychiatric disorders, based on spatiotemporal characteristics of fronto-temporal cortical haemodynamic response patterns induced by a brief (<3min) verbal fluency task. Data from the initial site were used to determine an optimal threshold, based on receiver-operator characteristics analysis, and to generate the simplest and most significant algorithm, which was validated using data from the remaining six sites. The frontal haemodynamic patterns detected by the near-infrared spectroscopy method accurately distinguished between patients with major depressive disorder (74.6%) and those with the two other disorders (85.5%; bipolar disorder or schizophrenia) that presented with depressive symptoms. These results suggest that neuroimaging-guided differential diagnosis of major psychiatric disorders developed using the near-infrared spectroscopy method can be a promising biomarker that should aid in personalised care in real clinical settings. Potential confounding effects of clinical (e.g., age, sex) and systemic (e.g., autonomic nervous system indices) variables on brain signals will need to be clarified to improve classification accuracy.

Cortical hemodynamic response patterns to normal and whispered speech

Whispered speech is often used in direct person-to-person communication as a means to confidentiality. Compared with normally vocalized speech, whispered speech is predominantly unvoiced, i.e., produced without vocal fold vibration, and has no clear fundamental frequency. By using near-infrared spectroscopy (NIRS), we assessed cortical hemodynamic response patterns to normally vocalized and whispered speech in adult listeners (n = 13). Stimuli consisted of 20-s strings of Japanese word associations spoken by a female voice. Average oxygenated hemoglobin values (oxy-Hb) were obtained over two regions of interest (ROIs). Results showed that oxy-Hb values during the perception of normally vocalized speech were highest over the left temporal ROI, but not significantly different from values measured over other ROIs. Oxy-Hb values during whispered speech were highest over the right temporal ROI and significantly higher (p &amp;lt; 0.05) than those obtained over the left temporal ROI. No significant differences, however, were found in oxy-Hb comparisons between normally vocalized and whispered speech, although the right temporal ROI comparison bordered on significance, with whisper inducing the higher value. Together, the results seem to suggest that whispered speech is a potent catalyst of cortical hemodynamic activity, especially over the right temporal cortex, in spite of its relatively modest sound level as compared to normal speech.

Cortical speech-evoked response patterns in multiple auditory fields are correlated with behavioral discrimination ability

Different speech sounds evoke unique patterns of activity in primary auditory cortex (A1). Behavioral discrimination by rats is well correlated with the distinctness of the A1 patterns evoked by individual consonants, but only when precise spike timing is preserved. In this study we recorded the speech-evoked responses in the primary, anterior, ventral, and posterior auditory fields of the rat and evaluated whether activity in these fields is better correlated with speech discrimination ability when spike timing information is included or eliminated. Spike timing information improved consonant discrimination in all four of the auditory fields examined. Behavioral discrimination was significantly correlated with neural discrimination in all four auditory fields. The diversity of speech responses across recordings sites was greater in posterior and ventral auditory fields compared with A1 and anterior auditor fields. These results suggest that, while the various auditory fields of the rat process speech sounds differently, neural activity in each field could be used to distinguish between consonant sounds with accuracy that closely parallels behavioral discrimination. Earlier observations in the visual and somatosensory systems that cortical neurons do not rely on spike timing should be reevaluated with more complex natural stimuli to determine whether spike timing contributes to sensory encoding.

Frequency-dependent patterns of somatosensory cortical responses to vibrotactile stimulation in humans: A fMRI study

In the human mechanosensation system, rapidly adapting afferents project sensory signals of flutter (5–50Hz) to the contralateral primary somatosensory cortex (S1) and bilateral secondary somatosensory cortex (S2) whereas Pacinian afferents project sensory signals of vibration (50–400Hz) to bilateral S2. However, it remains largely unknown how somatosensory cortical activity changes as a function of vibrotactile frequency. This functional magnetic resonance imaging (fMRI) study investigated frequency dependency of somatosensory cortical activity in humans by applying vibrotactile stimulation with various frequencies (20–200Hz) to the index finger. We found more frequency-dependent voxels in the upper bank of the lateral sulcus (LS) of S2 than in S1 and the posterior parietal cortex of S2. Our statistical spatial clustering analysis showed that two groups of positively or negatively frequency-dependent voxels formed distinct clusters, most clearly in the LS. Using a cortical separability index, we reaffirmed that somatosensory cortical activity was most separable at 50Hz, previously known to demarcate flutter and vibration. Our results suggest that the LS (S2) may play an important role in processing vibrotactile frequency information and that the somatosensory cortex may include spatially localized neural assemblies specialized to higher or lower vibrotactile frequency.

Functional Signalers of Changes in Visual Stimuli: Cortical Responses to Increments and Decrements in Motion Coherence

How does our brain detect changes in a natural scene? While changes by increments of specific visual attributes, such as contrast or motion coherence, can be signaled by an increase in neuronal activity in early visual areas, like the primary visual cortex (V1) or the human middle temporal complex (hMT+), respectively, the mechanisms for signaling changes resulting from decrements in a stimulus attribute are largely unknown. We have discovered opposing patterns of cortical responses to changes in motion coherence: unlike areas hMT+, V3A and parieto-occipital complex (V6+) that respond to changes in the level of motion coherence monotonically, human areas V4 (hV4), V3B, and ventral occipital always respond positively to both transient increments and decrements. This pattern of responding always positively to stimulus changes can emerge in the presence of either coherence-selective neuron populations, or neurons that are not tuned to particular coherences but adapt to a particular coherence level in a stimulus-selective manner. Our findings provide evidence that these areas possess physiological properties suited for signaling increments and decrements in a stimulus and may form a part of cortical vigilance system for detecting salient changes in the environment.

Learning of New Sound Categories Shapes Neural Response Patterns in Human Auditory Cortex

The formation of new sound categories is fundamental to everyday goal-directed behavior. Categorization requires the abstraction of discrete classes from continuous physical features as required by context and task. Electrophysiology in animals has shown that learning to categorize novel sounds alters their spatiotemporal neural representation at the level of early auditory cortex. However, functional magnetic resonance imaging (fMRI) studies so far did not yield insight into the effects of category learning on sound representations in human auditory cortex. This may be due to the use of overlearned speech-like categories and fMRI subtraction paradigms, leading to insufficient sensitivity to distinguish the responses to learning-induced, novel sound categories. Here, we used fMRI pattern analysis to investigate changes in human auditory cortical response patterns induced by category learning. We created complex novel sound categories and analyzed distributed activation patterns during passive listening to a sound continuum before and after category learning. We show that only after training, sound categories could be successfully decoded from early auditory areas and that learning-induced pattern changes were specific to the category-distinctive sound feature (i.e., pitch). Notably, the similarity between fMRI response patterns for the sound continuum mirrored the sigmoid shape of the behavioral category identification function. Our results indicate that perceptual representations of novel sound categories emerge from neural changes at early levels of the human auditory processing hierarchy.