Post-traumatic stress disorder: a right temporal lobe syndrome?

In a recent paper (Georgopoulos et al 2010 J. Neural Eng. 7 016011) we reported on the power of the magnetoencephalography (MEG)-based synchronous neural interactions (SNI) test to differentiate post-traumatic stress disorder (PTSD) subjects from healthy control subjects and to classify them with a high degree of accuracy. Here we show that the main differences in cortical communication circuitry between these two groups lie in the miscommunication of temporal and parietal and/or parieto-occipital right hemispheric areas with other brain areas. This lateralized temporal-posterior pattern of miscommunication was very similar but was attenuated in patients with PTSD in remission. These findings are consistent with observations (Penfield 1958 Proc. Natl Acad. Sci. USA 44 51-66, Penfield and Perot 1963 Brain 86 595-696, Gloor 1990 Brain 113 1673-94, Banceaud et al 1994 Brain 117 71-90, Fried 1997 J. Neuropsychiatry Clin. Neurosci. 9 420-8) that electrical stimulation of the temporal cortex in awake human subjects, mostly in the right hemisphere, can elicit the re-enactment and re-living of past experiences. Based on these facts, we attribute our findings to the re-experiencing component of PTSD and hypothesize that it reflects an involuntarily persistent activation of interacting neural networks involved in experiential consolidation.

Understanding the parietal lobe syndrome from a neurophysiological and evolutionary perspective

In human and nonhuman primates parietal cortex is formed by a multiplicity of areas. For those of the superior parietal lobule (SPL) there exists a certain homology between man and macaques. As a consequence, optic ataxia, a disturbed visual control of hand reaching, has similar features in man and monkeys. Establishing such correspondence has proven difficult for the areas of the inferior parietal lobule (IPL). This difficulty depends on many factors. First, no physiological information is available in man on the dynamic properties of cells in the IPL. Second, the number of IPL areas identified in the monkey is paradoxically higher than that so far described in man, although this issue will probably be reconsidered in future years, thanks to comparative imaging studies. Third, the consequences of parietal lesions in monkeys do not always match those observed in humans. This is another paradox if one considers that, in certain cases, the functional properties of neurons in the monkey's IPL would predict the presence of behavioral skills, such as construction capacity, that however do not seem to emerge in the wild. Therefore, constructional apraxia, which is well characterized in man, has never been described in monkeys and apes. Finally, only certain aspects, i.e. hand directional hypokinesia and gaze apraxia (Balint's psychic paralysis of gaze), of the multifaceted syndrome hemispatial neglect have been described in monkeys. These similarities, differences and paradoxes, among many others, make the study of the evolution and function of parietal cortex a challenging case.

The synchronous neural interactions test as a functional neuromarker for post-traumatic stress disorder (PTSD): a robust classification method based on the bootstrap

Traumatic experiences can produce post-traumatic stress disorder (PTSD) which is a debilitating condition and for which no biomarker currently exists (Institute of Medicine (US) 2006 Posttraumatic Stress Disorder: Diagnosis and Assessment (Washington, DC: National Academies)). Here we show that the synchronous neural interactions (SNI) test which assesses the functional interactions among neural populations derived from magnetoencephalographic (MEG) recordings (Georgopoulos A P et al 2007 J. Neural Eng. 4 349-55) can successfully differentiate PTSD patients from healthy control subjects. Externally cross-validated, bootstrap-based analyses yielded >90% overall accuracy of classification. In addition, all but one of 18 patients who were not receiving medications for their disease were correctly classified. Altogether, these findings document robust differences in brain function between the PTSD and control groups that can be used for differential diagnosis and which possess the potential for assessing and monitoring disease progression and effects of therapy.

A magnetoencephalography study of choice bias

Many factors can influence, or bias, human decision making. A considerable amount of research has investigated the neural correlates of such biases, mostly correlating hemodynamic responses in brain areas with some aspect of the decision. These studies, typically done using functional magnetic resonance imaging or positron emission tomography, have provided useful information about the location of processing in the brain. However, comparatively little research has examined when these processes occur. The present experiment addressed this question by using magnetoencephalography (MEG) to record brain activity while subjects chose preferred options from decision sets. We found that MEG signal deviations for biased decisions occurred as early as 250-750 ms following stimulus onset. Such deviations occurred earliest in sensors over the right anterior cortex. These findings improve our understanding of temporal dynamics of decision biases and suggest ways that existing explanations for this bias could be refined.

Cerebral cortical mechanisms of copying geometrical shapes: a multidimensional scaling analysis of fMRI patterns of activation

We used multidimensional scaling (MDS) to characterize the integrative neural mechanisms during viewing and subsequently copying nine geometrical shapes. Human subjects initially looked at a central fixation point ("rest" period), then looked at a geometrical shape ("visual" period) which they copied without visual feedback ("copying" period). BOLD signal was recorded from voxels in 28 cortical areas (14 from each hemisphere) using a 4 Tesla magnet. For each voxel, signal ratios of "Visual versus Rest" (VR), and "Copy versus Visual" (CV) were calculated and used to construct two sets of Euclidean distance dissimilarity matrices for the nine shapes, with separate matrices defined for each region of interest (ROI) across subjects. The relations of perceptual and motor aspects of the shapes to MDS dimensions and specific ROIs were assessed using stepwise multiple regressions. The optimal individually scaled (INDSCAL) solutions were 2-dimensional. For the VR condition, MDS dimensions were significantly associated with the presence of crossing in a shape (Dimension 1), and with perimeter, height, cycles, peak segment speed, and horizontal symmetry (Dimension 2). ROIs most prominently associated with these dimensions essentially comprised the medial frontal lobe bilaterally, the inferior frontal gyrus bilaterally, and the left intraparietal sulcus (Dimension 1), and visual areas, including the calcarine sulcus and cuneus bilaterally (Dimension 2). These results document the expected involvement of visual areas and support the hypothesis advanced on the basis of previous findings (Lewis et al. 2003a) that a motor rehearsal of the upcoming shape copying is occurring during this visual presentation period. For the CV condition, practically one motor feature (number of segments drawn) dominated both dimensions, with a secondary engagement of horizontal symmetry in Dimension 1. The right postcentral gyrus, right intraparietal sulcus, right superior parietal lobule and right inferior parietal lobule contributed mostly to Dimension 1; the superior frontal gyrus bilaterally, right middle frontal gyrus, left postcentral gyrus, left inferior parietal lobule contributed mostly to Dimension 2; and the left superior parietal lobule and left intraparietal sulcus contributed to both dimensions approximately equally. CV BOLD activation of ROIs contributing to Dimension 1 (or to both dimensions) was significantly associated with the number of shape segments drawn. Since the direction of movement differs in successively drawn shape segments, the number of segments (minus one) equals the number of changes in the direction of movement. We conclude that this fundamental spatial motor aspect of drawing geometrical shapes is the critical variable, independent of the particular shape drawn, that dominates cortical activation during copying.

Differential contribution of superior parietal and dorsal-lateral prefrontal cortices in copying

In this study we examined the differential contribution of superior parietal cortex (SPC) and caudal dorsal-lateral prefrontal cortex (dlPFC) to drawing geometrical shapes. Monkeys were trained to draw triangles, squares, trapezoids and inverted triangles while we recorded the activity of small ensembles of neurons in caudal area 46 and areas 5 and 2 of parietal cortex. We analyzed the drawing factors encoded by individual neurons by fitting a step-wise general-linear model using as our dependent variable the firing rate averaged over segments of the produced trajectories. This analysis demonstrated that both cognitive (shape and segment serial position) and motor (maximum speed, position and direction of segment) factors modulated the activity of individual neurons. Furthermore, SPC had an enriched representation of both shape and motor factors, with the motor enrichment being stronger than the shape enrichment. Following this we used the activity in the simultaneously recorded neural ensembles to predict the hand velocity. In these analyses we found that the prediction of the hand velocity was better when we estimated different linear decoding functions for each shape than when we estimated a single function across shapes, although it was a subtle effect. Furthermore, we also found that ensembles of caudal dlPFC neurons carried considerable information about hand velocity, a purely motor factor. However, the SPC ensembles carried more information at the ensemble level as a function of the ensemble size than the caudal dlPFC ensembles, although the differences were not dramatic. Finally, an analysis of the response latencies of individual neurons showed that the caudal dlPFC representation was more sensory than the SPC representation, which was equally sensory and motor. Thus, this neurophysiological evidence suggests that both SPC and caudal dlPFC have a role in drawing, but that SPC plays a larger role in both the cognitive and the motor components.

Functional organization of the basal ganglia: contributions of single-cell recording studies

Studies of single-cell discharge in the basal ganglia of behaving primates have revealed: characteristic patterns of spontaneous discharge in the striatum, external (GPe) and internal (GPi) globus pallidus, pars reticulata and pars compacta of the substantia nigra, and the subthalamic nucleus (STN); phasic changes in neural discharge in relation to movements of specific body parts (e.g. leg, arm, neck, face); short-latency (sensory) neural responses to passive joint rotation; a somatotopic organization of movement-related neurons in GPe, GPi, and STN; a clustering of functionally similar neurons in the putamen and globus pallidus; greater representation of the proximal than of the distal portion of the limb; changes in neural activity in reaction-time tasks, suggesting a greater role of the basal ganglia in the execution than in the initiation of movement in this paradigm; a clear relation of neuronal activity to direction, amplitude (?velocity) of movement, and force; a preferential relation of neural activity to the direction of movement, rather than to the pattern of muscular activity. Some of these findings suggest that the basal ganglia may play a role in the control of movement parameters rather than (or independent of) the pattern of muscular activity. Loss of basal ganglia output related to amplitude may account for the bradykinesia in Parkinson's disease. The presence of somatotopic organization in the putamen and globus pallidus, together with known topographic striopallidal connections, suggests that segregated, parallel cortico-subcortical loops subserve 'motor' and 'complex' functions.

Neurostatistics: applications, challenges and expectations

Brain function and its relations to cognition and behavior can be elucidated only by the use of various complementary methods. Over the past 20 years, we have been studying the brain mechanisms underlying spatial processes using different methods, including the recording of single cell activity in behaving monkeys, functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) in human subjects, all performing the same tasks. These methods provide partially overlapping perspectives, resulting in a gain in knowledge beyond the province of the individual method. A common aspect in this endeavor is the statistical analysis of the data acquired by different methods, especially regarding the encoding of information in unitary elements (single cell activity in neurophysiology, blood oxygenation level-dependent (BOLD) activation of voxels in fMRI, magnetic field strength in MEG) and the decoding of information from ensembles. In this paper we illustrate the various approaches, their data analysis and possible applications to medicine in the context of operations in space.

Ultra-high field parallel imaging of the superior parietal lobule during mental maze solving

We used ultra-high field (7 T) fMRI and parallel imaging to scan the superior parietal lobule (SPL) of human subjects as they mentally traversed a maze path in one of four directions (up, down, left, right). A counterbalanced design for maze presentation and a quasi-isotropic voxel (1.46 x 1.46 x 2 mm thick) collection were implemented. Fifty-one percent of single voxels in the SPL were tuned to the direction of the maze path. Tuned voxels were distributed throughout the SPL, bilaterally. A nearest neighbor analysis revealed a "honeycomb" arrangement such that voxels tuned to a particular direction tended to occur in clusters. Three-dimensional (3D) directional clusters were identified in SPL as oriented centroids traversing the cortical depth. There were 13 same-direction clusters per hemisphere containing 22 voxels per cluster, on the average; the mean nearest-neighbor, same-direction intercluster distance was 9.4 mm. These results provide a much finer detail of the directional tuning in SPL, as compared to those obtained previously at 4 T (Gourtzelidis et al. Exp Brain Res 165:273-282, 2005). The more accurate estimates of quantitative clustering parameters in 3D brain space in this study were made possible by the higher signal-to-noise and contrast-to-noise ratios afforded by the higher magnetic field of 7 T as well as the quasi-isotropic design of voxel data collection.

Local shaping of function in the motor cortex: motor contrast, directional tuning

In this review we bring together three different lines of evidence to bear on the issue of local shaping of function in the motor cortex. The first line of evidence comes from the description by Cajal (1904) of the recurrent collaterals of pyramidal cell axons in the precentral gyrus. The second line of evidence comes from the electrophysiological study of the functional effects of these collaterals [Stefanis, C., Jasper, H. 1964a. Intracellular microelectrode studies of antidromic responses in cortical pyramidal tract neurons. J. Neurophysiol. 27, 828-854.; Stefanis, C., Jasper, H. 1964b. Recurrent collateral inhibition in pyramidal tract neurons. J. Neurophysiol. 27, 855-877.] and associated interneurons [Stefanis, C. 1969. Interneuronal mechanisms in the cortex. In: The Interneuron, Brazier, M.A.B. (ed.), Berkeley, CA: University of California Press, pp. 497-526.] using intracellular recordings. And third came the discovery of directional tuning in the motor cortex [Georgopoulos, A.P., Kalaska, J.F., Caminiti, R., Massey, J.T. 1982. On the relations between the direction of two-dimensional arm movements and cell discharge in primate motor cortex. J. Neurosci. 2, 1527-1537.] in the behaving monkey. We hazard the hypothesis that the bell-shaped directional tuning curve is the outcome of orderly, local neuronal interactions in the motor cortex in which the recurrent pyramidal cell collaterals play a crucial role. Specifically, we propose that these collaterals and the intercalated interneurons they impinge upon serve to spatially sharpen the motor cortical activation to a locus corresponding to the direction of the intended movement. Thus, the originally proposed role of the pyramidal cell collaterals in enhancing "motor contrast" [Stefanis, C. 1969. Interneuronal mechanisms in the cortex. In: The Interneuron, Brazier, M.A.B. (ed.), Berkeley, CA: University of California Press, pp. 497-526.] would translate to creating a "directional tuning field" on the motor cortical surface, where the enhanced motor contrast would correspond to high activity at the center of directional field, and the suppression of the fringe would correspond to lower activity at the periphery of the field, resulting, together in spatial tuning.

Cortical mechanisms subserving reaching

The generation and control of reaching in space is a function of several structures, cortical and subcortical. This paper summarizes some principles of the cortical mechanisms subserving this function, as revealed by recording the impulse activity of neurons in motor cortex and area 5 of the posterior parietal cortex in behaving monkeys. Large populations of neurons in these cortical areas are engaged in reaching. This engagement is early in time; for example, cell activity in the motor cortex begins to change 60-80 ms after target onset, and slightly later in area 5. The time course of cell recruitment in the active population is very similar for reaching movements of equal amplitude directed to different targets. In contrast, the intensity of cell discharge in both motor and parietal cortex is clearly modulated with respect to the direction of reaching. Typically, the firing rate is a cosine function of the direction of the movement in space. An unambiguous distributed code for the direction of reaching exists in neuronal populations in the cortical areas studied. The outcome of this population code can be visualized as a vector in neural space that points in the direction of the upcoming movement.

Synchronous neural interactions assessed by magnetoencephalography: a functional biomarker for brain disorders

We report on a test to assess the dynamic brain function at high temporal resolution using magnetoencephalography (MEG). The essence of the test is the measurement of the dynamic synchronous neural interactions, an essential aspect of the brain function. MEG signals were recorded from 248 axial gradiometers while 142 human subjects fixated a spot of light for 45-60 s. After fitting an autoregressive integrative moving average (ARIMA) model and taking the stationary residuals, all pairwise, zero-lag, partial cross-correlations (PCC(ij)(0)) and their z-transforms (z(ij)(0)) between i and j sensors were calculated, providing estimates of the strength and sign (positive, negative) of direct synchronous coupling at 1 ms temporal resolution. We found that subsets of z(ij)(0) successfully classified individual subjects to their respective groups (multiple sclerosis, Alzheimer's disease, schizophrenia, Sjögren's syndrome, chronic alcoholism, facial pain, healthy controls) and gave excellent external cross-validation results.

Mapping of the preferred direction in the motor cortex

Directional tuning is a basic functional property of cell activity in the motor cortex. Previous work has indicated that cells with similar preferred directions are organized in columns perpendicular to the cortical surface. Here we show that these columns are organized in an orderly fashion in the tangential dimension on the cortical surface. Based on a large number of microelectrode penetrations and systematic exploration of the proximal arm area of the motor cortex while monkeys made free reaching 3D movements, it was estimated that (i) directional minicolumns are approximately 30 mum in width, (ii) minicolumns with similar preferred directions tend to occur in doublets or triplets, and (iii) such minicolumns tend to repeat every approximately 240 mum (estimated width of a column), with intermediate preferred directions represented in a gradient. These findings provide evidence for an orderly mapping of the preferred direction in the motor cortex.

Reading in a deep orthography: neuromagnetic evidence for dual-mechanisms

Despite substantial efforts to connect cognitive-linguistic models with appropriate anatomical correlates, the question of which cognitive model best accounts for the neuropsychological and functional neuroimaging evidence remains open. The two most popular models are grounded in conceptually different bases and thus make quasi-distinct predictions in regard to the patterns of activation that should be observed in imaging investigations of linguistic processing. Dual-mechanism models propose that high-frequency regular and irregular words are processed through a lexicon-based word code, which facilitates their processing and pronunciation latencies relative to pseudowords. In contrast, single-mechanism models suggest the same behavioral effects can be explained through semantic mediation without the existence of a lexicon. In most previous studies, words and pronounceable pseudowords were presented in lexical-decision or word reading paradigms, and hemodynamic techniques were utilized to distinguish involved anatomical areas. The results typically indicated that both word classes activated largely congruent tissues, with a magnitude advantage for pseudowords in most or all activated regions. However, since the dual-mechanism model predicts both word types utilize the entire linguistic network, but that certain operations are merely obligatorily involved, these results do not sharply refute nor clearly support the model's main tenets. In the current study, we approach the dual- versus single-mechanism question differently by focusing on the temporal dynamics of MEG imaged neuronal activity, during performance of an oddball version of continuous lexical-decision, to determine whether the onset latency of any cortical language region shows effects of word class that are indicative of preferential versus obligatory processing pathways. The most remarkable aspect of our results indicated that both words and pseudowords initially activate the left posterior fusiform region, but that the spatiotemporal dynamics clearly distinguish the two word classes thereafter. For words, this left fusiform activation was followed by engagement of the left posterior inferior temporal, and subsequently activation reached the left posterior superior temporal region. For pseudowords, this sequential order of left temporal area activations was reversed, as activity proceeded from the left fusiform to the left superior temporal and then the left inferior temporal region. For both classes, this dynamic sequential spread manifested within the first 300 ms of stimulus processing. We contend these results provide strong support for the existence of dual-mechanisms underlying reading in a deep orthographic language (i.e., English).

Large-Scale Organization of Preferred Directions in the Motor Cortex. II. Analysis of Local Distributions

The spatial arrangement of preferred directions (PDs) in the primary motor cortex has revealed evidence for columnar organization and short-range order. We investigated the large-scale properties of this arrangement. We recorded neural activity at sites on a grid covering a large region of the arm area of the motor cortex while monkeys performed a 3D reaching task. Sites were projected to the cortical surface along anatomically defined cortical columns and a PD was extracted from each site with directionally tuned activity. We analyzed the resulting 2D surface map of PDs. Consistent with previous studies, we found that any particular reaching direction was rerepresented at many points across the recorded area. In particular, we determined that the median radius of a cortical region required to represent the full complement of reaching directions is at most 1 mm. We also found that for the majority of regions of this size, the distribution of PDs within them exhibits an enrichment for the representation of forward and backward reaching directions (see companion paper). Finally, we found that the error of a population vector estimate of reaching direction constructed from neural activity within these regions is small on average, but varies significantly across different sections of the motor cortex, with the highest levels of error sustained near the fundus of the central sulcus and lowest levels achieved near the crown. We interpret these findings in the context of two well-known features of motor cortex, that is, its highly distributed anatomical organization and its behaviorally dependent plasticity.

Classification of adolescent psychotic disorders using linear discriminant analysis

BACKGROUND

The differential diagnosis between schizophrenia and bipolar disorder during adolescence presents a major clinical problem. Can these two diagnoses be differentiated objectively early in the courses of illness?

METHODS

We used linear discrimination analysis (LDA) to classify 28 adolescent subjects into one of three diagnostic categories (healthy, N=8; schizophrenia, N=10; bipolar, N=10) using subsets from a pool of 45 variables as potential predictors (22 neuropsychological test scores and 23 quantitative structural brain measurements). The predictor variables were adjusted for age, gender, race, and psychotropic medication. All possible subsets composed of k=2-12 variables, from the set of 45 variables available, were evaluated using the robust leaving-one-subject-out method.

RESULTS

The highest correct classification (96%) of the 3 diagnostic categories was yielded by 9 sets of k=12 predictors, comprising both neuropsychological and brain structural measures. Although each one of these sets misclassified one case, each set correctly classified (100%) at least one group, such that a fully correct diagnosis could be reached by a tree-type decision procedure.

CONCLUSIONS

We conclude that LDA with 12 predictor variables can provide correct and robust classification of subjects into the three diagnostic categories above. This robust classification relies upon both neuropsychological and brain structural information. Our results demonstrate that, despite overlapping clinical symptoms, schizophrenia and bipolar disorder can be differentiated early in the course of disease. This finding has two important implications. Firstly, schizophrenia and bipolar disorder are different illnesses. If schizophrenia and bipolar are dissimilar clinical manifestations of the same disease, we would not be able to use non-clinical information to classify ('diagnose') schizophrenia and bipolar disorder. Secondly, if this study's findings are replicated, brain structure (MRI) and brain function (neuropsychological) used together may be useful in the diagnosis of new patients.

Large-scale organization of preferred directions in the motor cortex. I. Motor cortical hyperacuity for forward reaching

We used statistical methods for spherical density estimation to evaluate the distribution of preferred directions of motor cortical cells recorded from monkeys making reaching movements in 3D space. We found that this distribution, although broad enough to represent the entire 3D continuum of reaching directions, exhibited an enrichment for reaching forward from the body and, to a lesser degree, for reaching backward toward the body. The distribution of preferred directions of cells in the motor cortex may have important implications for motor cortical function and for the decoding of arm trajectories from population activity.

Neurophysiology of perceptual and motor aspects of interception

The interception of moving targets is a complex activity that involves a dynamic interplay of several perceptual and motor processes and therefore involves a rich interaction among several brain areas. Although the behavioral aspects of interception have been studied for the past three decades, it is only during the past decade that neural studies have been focused on this problem. In addition to the interception itself, several neural studies have explored, within that context, the underlying mechanisms concerning perceptual aspects of moving stimuli, such as optic flow and apparent motion. In this review, we discuss the wealth of knowledge that has accumulated on this topic with an emphasis on the results of neural studies in behaving monkeys.