Human cortical responses during one-bit short-term memory. A high-resolution EEG study on delayed choice reaction time tasks.

OBJECTIVE: We investigated whether a very simple short-term memory (STM) demand induces a visible change of EEG rhythms over the two hemispheres. METHODS: High-resolution EEG was obtained in young adults during two delayed choice reaction time tasks. In the STM condition, a simple cue stimulus (one bit) was memorized along a brief delay period (3.5-5.5 s). The task was visuo-spatial in nature. RESULTS: In the control (NSTM) condition, the cue stimulus remained available along the delay period. Compared to the control condition, the theta power (4-6 Hz) decreased in left frontal and bilateral parietal areas (delay period). Furthermore, low alpha power (6-8 Hz) decreased in bilateral frontal and left parietal areas, while high alpha power (10-12 Hz) decreased in the left fronto-parietal areas. CONCLUSIONS: The decrease of the alpha power is as an expression of the efficient information transfer within thalamo-cortical pathways. The significance of the study stands in the fact that even a very simple STM task (only one bit to be memorized) revealed changes in fronto-parietal theta and alpha rhythms.     

Voluntary activation and cortical activity during a sustained maximal contraction: an fMRI study

Motor fatigue is an exercise-induced reduction in the force-generating capacity. The underlying mechanisms can be separated into factors residing in the periphery or in the central nervous system. We designed an experiment in which we investigated central processes underlying motor fatigue by means of magnetic resonance imaging in combination with the twitch interpolation technique. Subjects performed a sustained maximal abduction (2 min) with the right index finger. Brain activation was recorded with an MR scanner, together with index finger abduction force, EMG of several hand muscles and interpolated twitches. Mean activity per volume was calculated for the primary motor cortex and the secondary motor areas (supplementary motor, premotor, and cingulate areas) as well as mean force and mean rectified EMG amplitude. Results showed a progressive decline in maximal index finger abduction force and EMG of the target muscles combined with an increase in brain activity in the contralateral primary motor cortex and secondary motor areas. Analysis of the twitches superimposed on the sustained contraction revealed that during the contraction the voluntary drive decreased significantly. In conclusion, our data showed that despite an increase in brain activity the voluntary activation decreased. This suggests that, although the CNS increased its input to the relevant motor areas, this increase was insufficient to overcome fatigue-related changes in the voluntary drive.     

Human prefrontal and sensory cortical activity during divided attention tasks

In our natural environment, the ability to divide attention is essential since we attend simultaneously to a number of sensory modalities, e.g., to visual and auditory stimuli. In this study, functional magnetic resonance imaging (fMRI) was used to study brain activation while a divided attention task was performed. Brain activation was also assessed under selective attention. Fourteen healthy male subjects aged between 19 and 28 years underwent fMRI studies using gradient EPI sequences. Cingulate activation was evident in all attention tasks. Focusing attention on one modality (visual or auditory) increased the activity in the corresponding primary and secondary sensory area. When attention is divided between both modalities, the activation in the sensory areas is decreased, possibly due to a limited capacity of the system for controlled processing. Left prefrontal activation, however, was evident selectively during the divided attention task. The present results suggest that this area may be important in the execution of controlled processing when attention is divided between two sources of information. These results support the view that the prefrontal cortex is involved in the central executive system and controls attention and information flow.     

Different cortical activation patterns during voluntary eccentric and concentric muscle contractions: an fMRI study

Concentric and eccentric muscle contractions have distinct differences in their neuromuscular and neurophysiologic characteristics. However, although many evidences regarding the features of these types of muscle contraction have emerged, there have been few neuroimaging studies to compare the two types of contractions. Therefore, we investigated whether cortical activity associated with eccentric contraction of the wrist extensors differed from that of concentric contraction, using functional MRI (fMRI). Fifteen right-handed healthy subjects were enrolled in this study. During 4 repeating blocks of eccentric and concentric muscle contraction paradigms, the brain was scanned with fMRI. The differences in the BOLD signal intensities during the performance of eccentric and concentric exercise were compared in the predetermined regions of interest. Our findings revealed that many cortical areas associated with motor performance were activated, including the primary motor area, the inferior parietal lobe, the pre-supplementary area (pre-SMA), the anterior cingulate cortex, the prefrontal area, and the cerebellum. In addition, lower signal intensities were seen in the right primary motor cortex and right cerebellum during eccentric contractions compared with concentric contractions, whereas higher signal intensities were detected in other cortical areas during eccentric contractions. In the study, we demonstrated that eccentric and concentric muscle contractions induced quite different patterns of cortical activity respectively. These findings might be attributed to different strategy of neuro-motor processing and a higher level of cognitive demand for the performance of motor task with a higher degree of difficulty such as that required during eccentric contractions in comparison of concentric contractions.     

Functional neuroanatomy of interference in overlapping dual tasks: an fMRI study

A basic characteristic of the human action and cognition system is the occurrence of interference when participants attempt to perform two tasks at the same time. Such interference has been studied for a long time with so-called overlapping dual tasks, where two stimuli presented in rapid succession require separate responses. As an indicator of interference, reaction times on the second stimulus increase the smaller the interval between both tasks. While most behavioral studies investigated the temporal dynamics of the interference, we focused on the functional neuroanatomy of overlapping dual-task performance by using functional magnetic resonance imaging (fMRI). Participants were asked to perform two choice reaction tasks concurrently [Pashler, Psychol. Bull., 116 (1994) 220-244]. When activation in this overlapping dual-task situation was compared with the summed activation of the single component tasks, activation in the prefrontal, temporal, parietal, and occipital cortices was detected. These data suggest that the processing of the overlapping dual tasks requires an extensive and distributed network of processing centers. However, the main focus of the dual-task-related activation was located in regions surrounding the left inferior frontal sulcus. Based on our findings and on findings of other recent neuroimaging studies, we argue that activation of the left inferior frontal sulcus reflects increased synaptic activity related to the need to manage interfering information in order to determine the appropriate action.     

Differential amygdala activation during emotional decision and recognition memory tasks using unpleasant words: an fMRI study

This study used fMRI to examine the response of the amygdala in the evaluation and short-term recognition memory of unpleasant vs. neutral words in nine right-handed healthy adult women. To establish specificity of the amygdala response, we examined the fMRI BOLD signal in one control region (visual cortex). Alternating blocks of unpleasant and neutral trials were presented. During the emotional decision task, subjects viewed sets of three unpleasant or three neutral words while selecting the most unpleasant or neutral word, respectively. During the memory task, subjects identified words that were presented during the emotional decision task (0.50 probability). Images were detrended, filtered, and coregistered to standard brain coordinates. The Talairach coordinates for the center of the amygdala were chosen before analysis. The BOLD signal at this location in the right hemisphere revealed a greater amplitude signal for the unpleasant relative to the neutral words during the emotional decision but not the memory task, confirmed by Time Course x Word Condition ANOVAs. These results are consistent with the memory modulatory view of amygdala function, which suggests that the amygdala facilitates long-term, but not short-term, memory consolidation of emotionally significant material. The control area showed only an effect for Time Course for both the emotional decision and memory tasks, indicating the specificity of the amygdala response to the evaluation of unpleasant words. Moreover, the right-sided amygdala activation during the unpleasant word condition was strongly correlated with the BOLD response in the occipital cortex. These findings corroborate those by other researchers that the amygdala can modulate early processing of visual information in the occipital cortex. Finally, an increase in subject's state anxiety (evaluated by questionnaire) while in the scanner correlated with amygdala activation under some conditions.     

Abnormal cortical sensory activation in dystonia: an fMRI study.

Despite the obvious motor manifestations of focal dystonia, it is recognised that the sensory system plays an important role in this condition. This functional magnetic resonance imaging study examines the sensory representations of individual digits both within the subregions of the primary sensory cortex (SI) and in other nonprimary sensory areas. Patients with focal dystonia and controls were scanned during vibrotactile stimulation of both the index (digit 2) and little (digit 5) fingers of their dominant hand (which was the affected hand in all the dystonic subjects). The activation maps obtained were analysed for location, size, and magnitude of activation and three-dimensional (3-D) orientation of digit representations. Data from both groups were compared. There were significant differences in the average 3-D separation between the two digit representations in area 1 of SI between subject groups (9.6 +/- 1.2 mm for controls and 4.1 +/- 0.2 mm for dystonic subjects). There were also strong trends for reversed ordering of the representation of the two digits in both the secondary sensory cortex and posterior parietal area between the two groups. In addition, in dystonic subjects, there was significant under activation in the secondary somatosensory cortex (SII/area 40) for both digits and in the posterior parietal area for digit 5. These results indicate the presence of widespread activation abnormalities in the cortical sensory system in dystonia.     

An fMRI study of musicians with focal dystonia during tapping tasks

Musician’s dystonia is a type of task specific dystonia for which the pathophysiology is not clear. In this study, we performed functional magnetic resonance imaging to investigate the motor-related brain activity associated with musician’s dystonia. We compared brain activities measured from subjects with focal hand dystonia and normal (control) musicians during right-hand, left-hand, and both-hands tapping tasks. We found activations in the thalamus and the basal ganglia during the tapping tasks in the control group but not in the dystonia group. For both groups, we detected significant activations in the contralateral sensorimotor areas, including the premotor area and cerebellum, during each tapping task. Moreover, direct comparison between the dystonia and control groups showed that the dystonia group had greater activity in the ipsilateral premotor area during the right-hand tapping task and less activity in the left cerebellum during the both-hands tapping task. Thus, the dystonic musicians showed irregular activation patterns in the motor-association system. We suggest that irregular neural activity patterns in dystonic subjects reflect dystonic neural malfunction and consequent compensatory activity to maintain appropriate voluntary movements.     

Common deactivation patterns during working memory and visual attention tasks: an intra-subject fMRI study at 4 Tesla

This parametric functional magnetic resonance imaging (fMRI) study investigates the balance of negative and positive fMRI signals in the brain. A set of visual attention (VA) and working memory (WM) tasks with graded levels of difficulty was used to deactivate separate but overlapping networks that include the frontal, temporal, occipital, and limbic lobes; regions commonly associated with auditory and emotional processing. Brain activation (% signal change and volume) was larger for VA tasks than for WM tasks, but deactivation was larger for WM tasks. Load-related increases of blood oxygenation level-dependent (BOLD) responses for different levels of task difficulty cross-correlated strongly in the deactivated network during VA but less so during WM. The variability of the deactivated network across different cognitive tasks supports the hypothesis that global cerebral blood flow vary across different tasks, but not between different levels of task difficulty of the same task. The task-dependent balance of activation and deactivation might allow maximization of resources for the activated network.     

Making sense during conversation: an fMRI study.

Although language is thought of as a left hemisphere function, there is increasing evidence that the right hemisphere contributes to language processing by identifying the theme of spoken and written language. Using fMRI, we examined the role played by the right and left hemispheres in making sense of a conversation. When this process involves implicit appraisal of changes in the conversation's topic, the neural network has a right hemisphere bias and includes Broca's and Wernicke's areas, their right hemisphere homologues, right dorsolateral prefrontal cortex, and the cerebellum. When making sense of conversation involves appraisal of the conversation's reasoning, however, the network includes Broca's and Wernicke's areas. Thus, right and left hemisphere systems contribute uniquely to the linguistic skills involved in making sense of a conversation.     

Cerebellar activity evoked by common tool-use execution and imagery tasks: an fMRI study

The purpose of this study is to identify the functional brain networks activated in relation to actual tool-use in humans. Although previous studies have identified brain activity related to tool-use gestures (Moll et al., 2000), they did not investigate the brain activity involved in such tool-use. We investigated brain activity using functional magnetic resonance imaging (fMRI) while human subjects mentally imagined using sixteen common tools and while they actually used them. Brain activity for both actual and imagined tool-use was found in the posterior part of the parietal cortex, in the supplementary motor area, and in the cerebellum. Under imagined tool-use conditions, we found brain activity in the premotor and right pars opercularis. Under actual tool-use conditions, we found it in the primary motor area, in the thalamus, and in the left pars opercularis. Our precise analysis in the cerebellum indicated that activity evoked by imagery was located significantly more lateral to that evoked by actual use. We found a relationship between activity in the tool imagery and execution conditions by comparing their t-value-weighted centroid of activation coordinates. Moreover, for half of the subjects the spatial distribution pattern for each tool was similar, suggesting that neural mechanisms contributing to skillful tool-use are modularly organized in the cerebellum.     

Interictal cortical reorganization in episodic migraine without aura: an event-related fMRI study during parametric trigeminal nociceptive stimulation

Reversible posterior leukoencephalopathy syndrome (RPLS) is theoretically associated with hypertensive encephalopathy because the most patients demonstrate abrupt increasing of blood pressure (BP). A 59-year-old woman, who had undergone cholecystectomy 4 days before, complained of a headache and rapidly progressing visual disturbance. Her BP was postoperatively controlled at around 150/80, but her BP was 89/46 when she noticed her symptoms. Magnetic resonance imaging showed vasogenic edema in bilateral occipital and right parietal lobes, and intracranial magnetic resonance angiography revealed bilateral diffuse peripheral vasoconstriction. After discontinuing ropivacaine administration via epidural catheter, her BP rose to 114/62 and her symptoms completely disappeared within a few days. Except for hypotension, the clinical course and the radiological evidences in our case were consistent with RPLE. This case supports another hypothesis of RPLS mechanism that arterial endothelial injury by toxic drug effect results in transudation of fluid from blood vessels causing vasogenic brain edema.     

Activation of cortical and cerebellar motor areas during executed and imagined hand movements: an fMRI study.

Brain activation during executed (EM) and imagined movements (IM) of the right and left hand was studied in 10 healthy right-handed subjects using functional magnetic resonance imagining (fMRI). Low electromyographic (EMG) activity of the musculi flexor digitorum superficialis and high vividness of the imagined movements were trained prior to image acquisition. Regional cerebral activation was measured by fMRI during EM and IM and compared to resting conditions. Anatomically selected regions of interest (ROIs) were marked interactively over the entire brain. In each ROI activated pixels above a t value of 2.45 (p<0.01) were counted and analyzed. In all subjects the supplementary motor area (SMA), the premotor cortex (PMC), and the primary motor cortex (M1) showed significant activation during both EM and IM; the somatosensory cortex (S1) was significantly activated only during EM. Ipsilateral cerebellar activation was decreased during IM compared to EM. In the cerebellum, IM and EM differed in their foci of maximal activation: Highest ipsilateral activation of the cerebellum was observed in the anterior lobe (Larsell lobule H IV) during EM, whereas a lower maximum was found about 2-cm dorsolateral (Larsell lobule H VII) during IM. The prefrontal and parietal regions revealed no significant changes during both conditions. The results of cortical activity support the hypothesis that motor imagery and motor performance possess similar neural substrates. The differential activation in the cerebellum during EM and IM is in accordance with the assumption that the posterior cerebellum is involved in the inhibition of movement execution during imagination.     

Activation of somatosensory cortical areas varies with attentional state: an fMRI study

The differing roles of SI and SII areas in the somatosensory system have received relatively little interest in previous research. In the present study fMRI was applied to determine possible changes in activations of these areas as a function of attentional modulation (attending vs. not attending to the stimulation of a finger). The results showed that attention induced larger regional changes, mostly enlargements of activated areas, at SII than at SI. The number of instances where new, emerging activations, not present in the non-attend condition, were observed was larger at SII than at SI. These differential attentional effects indicate that SII areas may have a role in more complex tactile functions such as tactile working memory mechanisms.     

Hyperfrontality in patients with schizophrenia during saccade and antisaccade tasks: A study with fMRI

Aims:  Antisaccadic eye movements, requiring inhibition of a saccade toward a briefly appearing peripheral target, are known to be impaired in schizophrenia. Previous neuroimaging studies have indicated that patients with schizophrenia show diminished activations in the frontal cortex and basal ganglia. These studies used target fixation as a baseline condition. However, if the levels of brain activities at baseline are not compatible between patients and healthy subjects, between-group comparison on antisaccade-related activations is consequently invalidated. One possibility is that patients with schizophrenia may present with greater activation during fixation than healthy subjects. In order to examine this possibility, here we investigated brain activities associated with antisaccade in the two groups without using target fixation at baseline.
Methods:  Functional brain images were acquired during prosaccades and antisaccades in 18 healthy subjects and 18 schizophrenia patients using a box-car functional magnetic resonance imaging design. Eye movements were measured during scanning.
Results:  In the patient group, the elevated activities in the dorsolateral prefrontal cortex (DLPFC) and thalamus, normally seen in antisaccade tasks relative to saccade tasks, were no longer observed. Moreover, in normal subjects, activities in the DLPFC and thalamus were greater during the antisaccade task than during the saccade task. In patients, no such difference was observed between the two tasks, suggesting that these brain regions are likely to be highly activated even by a simple task such as fixation. In particular, the DLPFC and thalamus in patients were not activated at a level commensurate with the difficulty of the tasks presented.
Conclusions:  From these results, it is suggested that schizophrenia entails dysfunctions in the fronto-striato-thalamo-cortical network associated with motor function control.     

High-frequency cortical responses reflect lexical processing: an MEG study

Meaningful words and matched pseudowords, such as moon vs. noom, are of equal perceptual complexity, but invoke different cognitive processes. To investigate high-frequency cortical responses to these stimuli, biomagnetic signals were recorded simultaneously over both hemispheres of right-handed individuals listening to words and pseudowords. Consistent with earlier EEG studies, evoked spectral responses recorded from the left hemisphere revealed depression of spectral power in the low gamma band (around 30 Hz) after pseudowords but not after words. Similar differences between stimulus categories were present in the beta range. These results indicate that distinct patterns of high-frequency cortical responses correspond to the different cognitive processes invoked by words and pseudowords. It is hypothesized that differential high-frequency cortical responses signal the activation or activation failure of distributed Hebbian cell assemblies representing words and other elements of cognitive processing.