Spatio temporal dynamics of face recognition

To better understand face recognition, it is necessary to identify not only which brain structures are implicated but also the dynamics of the neuronal activity in these structures. Latencies can then be compared to unravel the temporal dynamics of information processing at the distributed network level. To achieve high spatial and temporal resolution, we used intracerebral recordings in epileptic subjects while they performed a famous/unfamiliar face recognition task. The first components peaked at 110 ms in the fusiform gyrus (FG) and simultaneously in the inferior frontal gyrus, suggesting the early establishment of a large-scale network. This was followed by components peaking at 160 ms in 2 areas along the FG. Important stages of distributed parallel processes ensued at 240 and 360 ms involving up to 6 regions along the ventral visual pathway. The final components peaked at 480 ms in the hippocampus. These stages largely overlapped. Importantly, event-related potentials to famous faces differed from unfamiliar faces and control stimuli in all medial temporal lobe structures. The network was bilateral but more right sided. Thus, recognition of famous faces takes place through the establishment of a complex set of local and distributed processes that interact dynamically and may be an emergent property of these interactions.     

The neural basis of visual skill learning: an fMRI study of mirror reading

The learning of perceptual skills is thought to rely upon multiple regions in the cerebral cortex, but imaging studies have not yet provided evidence about the changes in neural activity that accompany visual skill learning. Functional magnetic resonance imaging (fMRI) was used to examine changes in activation of posterior brain regions associated with the acquisition of mirror-reading skill for novel and practiced stimuli. Multiple regions in the occipital lobe, inferior temporal cortex, superior parietal cortex and cerebellum were involved in the reading of mirror-reversed compared to normally oriented text. For novel stimuli, skilled mirror-reading was associated with decreased activation in the right superior parietal cortex and posterior occipital regions and increased activation in the left inferior temporal lobe. These results suggest that learning to read mirror- reversed text involves a progression from visuospatial transformation to direct recognition of transformed letters. Reading practiced, relative to unpracticed, stimuli was associated with decreased activation in occipital visual cortices, inferior temporal cortex and superior parietal cortex and increased activation in occipito-parietal and lateral temporal regions. By examining skill learning and item- specific repetition priming in the same task, this study demonstrates that both of these forms of learning exhibit shifts in the set of neural structures that contribute to performance.      

Spatiotemporal interaction between sound form and meaning during spoken word perception

Cortical dynamics of spoken word perception is not well understood. The possible interplay between analysis of sound form and meaning, in particular, remains elusive. We used magnetoencephalography to study cortical manifestation of phonological and semantic priming. Ten subjects listened to lists of 4 words. The first 3 words set a semantic or phonological context, and the list-final word was congruent or incongruent with this context. Attenuation of activation by priming during the first 3 words and increase of activation to semantic or phonological mismatch in the list-final word provided converging evidence: The superior temporal cortex bilaterally was involved in both analysis of sound form and meaning but the role of each hemisphere varied over time. Sensitivity to sound form was observed at approximately 100 ms after word onset, followed by sensitivity to semantic aspects from approximately 250 ms onwards, in the left hemisphere. From approximately 450 ms onwards, the picture was changed, with semantic effects now present bilaterally, accompanied by a subtle late effect of sound form in the right hemisphere. Present MEG data provide a detailed spatiotemporal account of neural mechanisms during speech perception that may underlie characterizations obtained with other neuroimaging methods less sensitive in temporal or spatial domain.     

Object and Spatial Visual Working Memory Activate Separate Neural Systems in Human Cortex

Human and nonhuman primate visual systems are divided into objectand spatial information processing pathways. In the macaque,it has been shown that these pathways project to separate areasin the frontal lobe and that the ventral and dorsal frontalareas are, respectively, involved in working memory for objectsand spatial locations. A positron emission tomography (PET)study was done to determine if a similar anatomical segregationexists in humans for object and spatial visual working memory.Face working memory demonstrated significant increases in regionalcerebral blood flow (rCBF), relative to location working memory,in fusiform, parahippocampal, inferior frontal, and anteriorcingulate cortices, and in right thalamus and midline cerebellum.Location working memory demonstrated significant increases inrCBF, relative to face working memory, in superior and inferiorparietal cortex, and in the superior frontal sulcus. Our resultsshow that the neural systems involved in working memory forfaces and for spatial location are functionally segregated,with different areas recruited in both extrastriate and frontalcortices for processing the two types of visual information.     

Cerebral blood flow relationships associated with a difficult tone recognition task in trained normal volunteers

Tone recognition is partially subserved by neural activity in the right frontal and primary auditory cortices. First we determined the brain areas associated with tone perception and recognition. This study then examined how regional cerebral blood flow (rCBF) in these and other brain regions correlates with the behavioral characteristics of a difficult tone recognition task. rCBF changes were assessed using H2(15)O positron emission tomography. Subtraction procedures were used to localize significant change regions and correlational analyses were applied to determine how response times (RT) predicted rCBF patterns. Twelve trained normal volunteers were studied in three conditions: REST, sensory motor control (SMC) and decision (DEC). The SMC-REST contrast revealed bilateral activation of primary auditory cortices, cerebellum and bilateral inferior frontal gyri. DEC-SMC produced significant clusters in the right middle and inferior frontal gyri, insula and claustrum; the anterior cingulate gyrus and supplementary motor area; the left insula/claustrum; and the left cerebellum. Correlational analyses, RT versus rCBF from DEC scans, showed a positive correlation in right inferior and middle frontal cortex; rCBF in bilateral auditory cortices and cerebellum exhibited significant negative correlations with RT These changes suggest that neural activity in the right frontal, superior temporal and cerebellar regions shifts back and forth in magnitude depending on whether tone recognition RT is relatively fast or slow, during a difficult, accurate assessment.      

Changes in auditory cortex parallel rapid perceptual learning

Learning perceptual skills is characterized by rapid improvements in performance within the first hour of training (fast perceptual learning) followed by more gradual improvements that take place over several daily practice sessions (slow perceptual learning). Although it is widely accepted that slow perceptual learning is accompanied by enhanced stimulus representation in sensory cortices, there is considerable controversy about the neural substrates underlying early and rapid improvements in learning perceptual skills. Here we measured event-related brain potentials while listeners were presented with 2 phonetically different vowels. Listeners' ability to identify both vowels improved gradually during the first hour of testing and was paralleled by enhancements in an early evoked response ( approximately 130 ms) localized in the right auditory cortex and a late evoked response ( approximately 340 ms) localized in the right anterior superior temporal gyrus and/or inferior prefrontal cortex. These neuroplastic changes depended on listeners' attention and were preserved only if practice was continued; familiarity with the task structure (procedural learning) was not sufficient. We propose that the early increases in cortical responsiveness reflect goal-directed changes in the tuning properties of auditory neurons involved in parsing concurrent speech signals. Importantly, the neuroplastic changes occurred rapidly, demonstrating the flexibility of human speech segregation mechanisms.     

Neurodevelopmental correlates of true and false recognition

The Deese/Roediger-McDermott (DRM) false-memory effect has been extensively documented in psychological research. People falsely recognize critical lures or nonstudied items that are semantically associated with studied items. Behavioral research has provided evidence for age-related increases in the DRM false-recognition effect. The present event-related functional magnetic resonance imaging study was aimed at investigating neurodevelopmental changes in brain regions associated with true- and false-memory recognition in 8-year olds, 12-year olds, and adults. Relative to 8-year olds, adults correctly endorsed more studied items as "old" but also mistakenly endorsed more critical lures. Age-related increases in recollection were associated with changes in the medial temporal lobe (MTL) activation profile. Additionally, age-related increases in false alarms (FAs) to semantically related lures were associated with changes in the activation profile of left ventrolateral prefrontal cortex, a region associated with semantic processing. Additional regions exhibiting age-related changes include posterior parietal and anterior prefrontal cortices. In summary, concomitant changes in the MTL, prefrontal cortex, and parietal cortex underlie developmental increases in true and false recognition during childhood and adolescence.     

Selective aging of the human cerebral cortex observed in vivo: differential vulnerability of the prefrontal gray matter

In a prospective cross-sectional study, we used computerized volumetry of magnetic resonance images to examine the patterns of brain aging in 148 healthy volunteers. The most substantial age-related decline was found in the volume of the prefrontal gray matter. Smaller age-related differences were observed in the volume of the fusiform, inferior temporal and superior parietal cortices. The effects of age on the hippocampal formation, the postcentral gyrus, prefrontal white matter and superior parietal white matter were even weaker. No significant age- related differences were observed in the parahippocampal and anterior cingulate gyri, inferior parietal lobule, pericalcarine gray matter, the precentral gray and white matter, postcentral white matter and inferior parietal white matter. The volume of the total brain volume and the hippocampal formation was larger in men than in women even after adjustment for height. Inferior temporal cortex showed steeper aging trend in men. Small but consistent rightward asymmetry was found in the whole cerebral hemispheres, superior parietal, fusiform and orbito-frontal cortices, postcentral and prefrontal white matter. The left side was larger than the right in the dorsolateral prefrontal, parahippocampal, inferior parietal and pericalcarine cortices, and in the parietal white matter. However, there were no significant differences in age trends between the hemispheres.      

Identification and differential vulnerability of a neural network in sleep deprivation

The study aimed to identify task-related brain activation networks whose change in expression exhibits subject differences as a function of differential susceptibility to sleep deprivation. Brain activity during a non-verbal recognition memory task was investigated in an event-related functional MRI paradigm both prior to and after 48 h of sleep deprivation. Nineteen healthy subjects participated. Regional covariance analysis was applied to data. An activation network pattern was identified whose expression decreased from pre- to post-sleep deprivation in 15 out 19 subjects (P < 0.05). Differential decrease in expression correlated with worsening performance in recognition accuracy (P < 0.05). Sites of de-activation were found in the posterior cerebellum, right fusiform gyrus and precuneus, and left lingual and inferior temporal gyri; increased activation was found in the bilateral insula, claustrum and right putamen. A network whose expression decreased after sleep deprivation and correlated with memory performance was identified. We conclude that this activation network plays a role in cognitive function during sleep deprivation.     

An fMRI study of verbal self-monitoring: neural correlates of auditory verbal feedback

The ability to recognize one's own inner speech is essential for a sense of self. The verbal self-monitoring model proposes that this process entails a communication from neural regions involved in speech production to areas of speech perception. According to the model, if the expected verbal feedback matches the perceived feedback, then there would be no change in activation in the lateral temporal cortices. We investigated the neural correlates of verbal self-monitoring in a functional magnetic resonance (fMRI) study. Thirteen healthy male volunteers read aloud presented adjectives and heard their auditory feedback which was experimentally modified. Decisions about the source of the feedback were made with a button-press response. We used a 'clustered' fMRI acquisition sequence, consisting of periods of relative silence in which subjects could speak aloud and hear the feedback in the absence of scanner noise, and an event-related design which allowed separate analysis of trials associated with correct attributions and misattributions. Subjects made more misattribution responses when the feedback was a distorted version of their voice. This condition showed increased superior temporal activation relative to the conditions of hearing their own voice undistorted and hearing another person's voice. Furthermore, correct attributions during this condition were associated with greater temporal activation than misattributions. These findings support the self-monitoring model as mismatches between expected and actual auditory feedback were associated with greater temporal activation.     

A distributed left hemisphere network active during planning of everyday tool use skills

Determining the relationship between mechanisms involved in action planning and/or execution is critical to understanding the neural bases of skilled behaviors, including tool use. Here we report findings from two fMRI studies of healthy, right-handed adults in which an event-related design was used to distinguish regions involved in planning (i.e. identifying, retrieving and preparing actions associated with a familiar tools' uses) versus executing tool use gestures with the dominant right (experiment 1) and non-dominant left (experiment 2) hands. For either limb, planning tool use actions activates a distributed network in the left cerebral hemisphere consisting of: (i) posterior superior temporal sulcus, along with proximal regions of the middle and superior temporal gyri; (ii) inferior frontal and ventral premotor cortices; (iii) two distinct parietal areas, one located in the anterior supramarginal gyrus (SMG) and another in posterior SMG and angular gyrus; and (iv) dorsolateral prefrontal cortex (DLFPC). With the exception of left DLFPC, adjacent and partially overlapping sub-regions of left parietal, frontal and temporal cortex are also engaged during action execution. We suggest that this left lateralized network constitutes a neural substrate for the interaction of semantic and motoric representations upon which meaningful skills depend.     

Spatiotemporal analysis of auditory "what" and "where" working memory

Goal-directed attention to sound identity (what) and sound location (where) has been associated with increased neural activity in ventral and dorsal brain regions, respectively. In order to ascertain when such segregation occurs, we measured event-related potentials during an n-back (n = 1, 2) working memory task for sound identity or location, where stimuli selected randomly from 3 semantic categories (human, animal, music) were presented at 3 possible virtual locations. Accuracy and reaction times were comparable in both "what" and "where" tasks, albeit worse for the 2-back than for the 1-back condition. The partial least squares analysis of scalp-recorded and source waveform data revealed domain-specific activity beginning at about 200-ms poststimulus onset, which was best expressed as changes in source activity near Heschl's gyrus, and in central medial, occipital medial, right frontal and right parietal cortex. The effect of working memory load emerged at about 400-ms poststimulus and was expressed maximally over frontocentral scalp region and in sources located in the right temporal, frontal and parietal cortices. The results show that for identical sounds, top-down effects on processing "what" and "where" information is observable at about 200 ms after sound onset and involves a widely distributed neural network.     

Sequential auditory scene analysis is preserved in normal aging adults

Normal aging is accompanied by speech perception difficulties, especially in adverse listening situations such as a cocktail party. To assess whether such difficulties might be related to impairments in sequential auditory scene analysis, event-related brain potentials were recorded from normal-hearing young, middle-aged, and older adults during presentation of low (A) tones, high (B) tones, and silences (--) in repeating 3 tone triplets (ABA--). The likelihood of reporting hearing 2 streams increased as a function of the frequency difference between A and B tones (Delta f) to the same extent for all 3 age groups and was paralleled by enhanced sensory-evoked responses over the frontocentral scalp regions. In all 3 age groups, there was also a progressive buildup in brain activity from the beginning to the end of the sequence of triplets, which was characterized by an enhanced positivity that peaked at about 200 ms after the onset of each ABA--triplet. Similar Delta f- and buildup-related activity also occurred over the right temporal cortex, but only for young adults. We conclude that age-related difficulties in separating competing speakers are unlikely to arise from deficits in streaming and might instead reflect less efficient concurrent sound segregation.     

Gradual recovery from dyslexia and related serial magnetoencephalographic changes in the lexicosemantic centers after resection of a mesial temporal astrocytoma. Case report

Letter-perception centers are not held in as high regard as motor- and language-related cortices during planning of neurosurgical procedures, and there have been no reports suggesting cortical reorganization of reading ability. The authors describe a patient with a left mesial temporal glioma in whom two letter-perception centers (the anterior portion of the left superior temporal gyrus and the left fusiform gyrus) were successfully localized before surgery by performing magnetoencephalography (MEG) during reading tasks. Control MEG examinations of 15 healthy volunteers were also performed to assist in a careful interpretation of patient results. Although a radical resection of the mesial temporal glioma, which involved the left fusiform gyrus, caused severe dyslexia, the patient's impaired reading skills improved gradually during a 1-year postoperative period. In the meantime, the spared left superior temporal gyrus displayed an overshot recovery of MEG responses. During the postoperative period there was no obvious recovery in MEG signals and no compensatory activity in the contralateral fusiform gyrus. This case demonstrates that lexicosemantic centers involved in the reading process can be noninvasively localized using MEG and that the results obtained are highly reliable for surgical planning. The results of the repeated MEG reflected sequentially the patient's recovery from dyslexia. This is the first report in which MEG studies have been shown to predict preoperatively the risk of dyslexia and demonstrate its serial physiological recovery.     

MR磁化传递对比成像观察新生儿轻度窒息

目的观察MR磁化传递对比(MTC)成像在轻度窒息新生儿中的应用价值。方法对15例轻度窒息新生儿(Apgar评分10分,病例组)及25名正常新生儿(对照组)采集脑常规T1WI、3D-T1WI和T1WI-MTC,计算脑磁化率(MTR),配准于标准新生儿脑模板后行统计分析。采用3dRegAna对病例组MTR与Apgar评分进行回归分析。结果相比对照组,病例组右颞极、左颞下回、左额上回、右缘上回、右眶额皮质、左额中叶、右额中回及左上额叶MTR显著降低;右梭状回、右顶叶下回、右枕中回、右颞中回、右颞下回、右颞上极、右楔叶、右角回、右舌回及右颞上回MTR显著增加。回归分析显示,病例组左中央后回、右颞下叶(前)、右额中回、右颞上极、左眶额皮质及右颞下叶(后)MTR与Apgar评分呈正相关,右壳核、右眶额皮质、左杏仁核、右颞下回、左舌回、右舌回、左颞中回、左枕中回、延髓及右梭状回呈负相关。组间MTR差异有统计学意义、且病例组MTR与Apgar评分呈正相关脑区为右额中叶、右颞极,呈负相关脑区则为右舌叶及右梭状回。结论 MR MTC成像能检出轻度窒息新生儿缺血缺氧脑区;缺血缺氧主要导致新生儿右侧脑损害。     

Regional specificity of format-specific priming effects in mirror word reading using functional magnetic resonance imaging

The speed and accuracy with which subjects can read words is enhanced or "primed" by a prior presentation of the same words. Moreover, priming effects are generally larger when the physical form of the words is maintained from the first to the second presentation. We investigated the neural basis of format-specific priming in a mirror word-reading task using event-related functional magnetic resonance imaging (fMRI). Participants read words that were presented either in mirror-image (M) orientation or in normal (N) orientation and were repeated either in the same or the alternate orientation, creating 4 study-test conditions, N-N, M-N, N-M, and M-M. Priming of N words resulted in reductions in fMRI signal in multiple brain regions, even though reading times (RTs) were unchanged. Priming of M words showed a pattern of RTs consistent with format-specific priming, with greater reductions when the prime matched the form of the test word. Priming-related reductions in fMRI activity were evident in all regions involved in mirror-image reading, regardless of the orientation of the prime. Importantly, reductions in several posterior regions, including fusiform, superior parietal, and superior temporal regions were also format specific. That is, signal reductions in these regions were greatest when the visual form of the prime and target matched (M-M compared with N-M). The results indicate that, although there are global neural priming effects due to stimulus repetition, it is also possible to identify regional brain changes that are sensitive to the specific perceptual overlap of primes and targets.     

Dissociable cortical processing of recognizable and non-recognizable biological movement: analysing gamma MEG activity

Recognition of biological motion is one of the essential ingredients of human evolutionary survival. When biological motion is revealed solely by a set of light dots on the joints of an invisible human figure, the perceptual system reliably distinguishes it from similar configurations. Here, we assessed the changes in neuromagnetic cortical responses during visual perception of biological motion. Healthy humans saw a randomized set of stimuli consisting of a point-light canonical walker and a scrambled configuration in which the spatial positions of dots were randomly rearranged on the screen. In separate runs, configurations were presented either within an upright or inverted (180 degrees ) orientation in the image plane. Participants performed a one-back repetition task lifting a forefinger in response to the second of two consecutive identical stimuli of each type. Both recognizable upright and non-recognizable inverted walkers evoke enhancements in oscillatory gamma brain activity (25-30 Hz) over the left occipital cortices as early as 100 ms from stimulus onset. Only a recognizable upright walker, however, yields further consecutive peaks over the parietal (130 ms) and right temporal (170 ms) lobes. Scrambled displays do not elicit any increases in the gamma response. The stimulus-specific time course and topographic dynamics of cortical oscillatory activity indicate that the brain rapidly dissociates spatial coherence and meaning revealed through biological movement.     

Time course of neural activity correlated with colored-hearing synesthesia

Synesthesia is defined as the involuntary and automatic perception of a stimulus in 2 or more sensory modalities (i.e., cross-modal linkage). Colored-hearing synesthetes experience colors when hearing tones or spoken utterances. Based on event-related potentials we employed electric brain tomography with high temporal resolution in colored-hearing synesthetes and nonsynesthetic controls during auditory verbal stimulation. The auditory-evoked potentials to words and letters were different between synesthetes and controls at the N1 and P2 components, showing longer latencies and lower amplitudes in synesthetes. The intracerebral sources of these components were estimated with low-resolution brain electromagnetic tomography and revealed stronger activation in synesthetes in left posterior inferior temporal regions, within the color area in the fusiform gyrus (V4), and in orbitofrontal brain regions (ventromedial and lateral). The differences occurred as early as 122 ms after stimulus onset. Our findings replicate and extend earlier reports with functional magnetic resonance imaging and positron emission tomography in colored-hearing synesthesia and contribute new information on the time course in synesthesia demonstrating the fast and possibly automatic processing of this unusual and remarkable phenomenon.     

Domain-general and domain-sensitive prefrontal mechanisms for recollecting events and detecting novelty

Recollecting the past and discriminating novel from familiar memoranda depend on poorly understood prefrontal cortical (PFC) mechanisms hypothesized to vary according to memory task (e.g. recollection versus novelty detection) and domain of targeted memories (e.g. perceptual versus conceptual). Using event-related fMRI, we demonstrate that recollecting conceptual or perceptual details surrounding object encounters similarly recruits left frontopolar and posterior PFC compared with detecting novel stimuli, suggesting that a domain-general control network is engaged during contextual remembering. In contrast, left anterior ventrolateral PFC coactivated with a left middle temporal region associated with semantic representation, and right ventrolateral PFC with bilateral occipito-temporal cortices associated with representing object form, depending on whether recollections were conceptual or perceptual. These PFC/posterior cortical dissociations suggest that during recollection, lateralized ventrolateral PFC mechanisms bias posterior conceptual or perceptual feature representations as a function of memory relevance, potentially improving the gain of bottom-up memory signals. Supporting this domain-sensitive biasing hypothesis, novelty detection also recruited right ventrolateral PFC and bilateral occipito-temporal cortices compared with conceptual recollection, suggesting that searching for novel objects heavily relies upon perceptual feature processing. Collectively, these data isolate task- from domain-sensitive PFC control processes strategically recruited in the service of episodic memory.