Lisa Koski

Regional differences in the effects of task difficulty and motor output on blood flow response in the human anterior cingulate cortex : a review of 107 PET activation studies

WE reviewed 107 blood flow activation studies carried out with positron emission tomography and published between January 1993 and November 1996. These studies had reported their findings as peaks of significant difference in cerebral blood-flow (CBF) between two scans/tasks and had located the peaks in standardized stereotaxic space. We coded each task along several dimensions, including the type and rate of input and output, the types of cognitive processes, and the relative difficulty of tasks within a study. Based on this coding, a difference score (A–B) was calculated for each subtraction. Subsequently, the frequency distributions of the difference scores for subtractions yielding a peak in the anterior cingulate region (cingulate peak) were compared with those distributions obtained from subtractions without a cingulate peak (no cingulate-peak). The cingulate peak subtractions (n = 158) differed from the no cingulate peak subtractions (n = 229) in terms of difficulty level (p = 0.001) and the presence of a remote memory component (p = 0.01). Regional differences in the frequency distribution of certain task parameters, such as difficulty level, recent memory and the use of the hand for responding, were also observed when peaks found in the anterior cingulate cortex (ACC) were further classified as located in the rostral vs caudal ACC, supracallosal vs subcallosal ACC, and limbic vs paralimbic parts of the supracallosal ACC. We conclude that task difficulty plays a major role in modulating blood-flow response in the ACC, possibly interacting with other parameters such as the nature of the response and memory demands.

Reafferent copies of imitated actions in the right superior temporal cortex

Imitation is a complex phenomenon, the neural mechanisms of which are still largely unknown. When individuals imitate an action that already is present in their motor repertoire, a mechanism matching the observed action onto an internal motor representation of that action should suffice for the purpose. When one has to copy a new action, however, or to adjust an action present in ones motor repertoire to a different observed action, an additional mechanism is needed that allows the observer to compare the action made by another individual with the sensory consequences of the same action made by himself. Previous experiments have shown that a mechanism that directly matches observed actions on their motor counterparts exists in the premotor cortex of monkeys and humans. Here we report the results of functional magnetic resonance experiments, suggesting that in the superior temporal sulcus, a higher order visual region, there is a sector that becomes active both during hand action observation and during imitation even in the absence of direct vision of the imitators hand. The motor-related activity is greater during imitation than during control motor tasks. This newly identified region has all the requisites for being the region at which the observed actions, and the reafferent motor-related copies of actions made by the imitator, interact.

Functional connectivity of the anterior cingulate cortex within the human frontal lobe: a brain-mapping meta-analysis

Abstract. A database of positron-emission-tomography studies published between January 1993 and November 1996 was created to address several questions regarding the function and connectivity of the human anterior cingulate cortex (ACC). Using this database, we have previously reported on the relationship between behavioural variables and the probability of blood-flow response in distinct subdivisions of the ACC. The goal of the current analysis was to discover which areas of the frontal cortex show increased blood-flow co-occurring consistently with increased blood-flow in the ACC. Analyses of the frequency distributions of peaks in the ACC and the remaining frontal cortex (FC) yielded several important findings. First, FC peaks in the precentral gyrus, superior frontal gyrus, middle frontal gyrus, inferior frontal gyrus, medial frontal gyrus and orbitomedial frontal gyri were more frequent in subtractions that also yielded a peak in the ACC than in those that did not yield an ACC peak. Second, regional differences in the frequency distribution of these FC peaks were observed when the ACC peaks were subdivided into the rostral versus caudal ACC and supracallosal versus subcallosal ACC. Peaks in the precentral gyrus and in the vicinity of the supplementary motor area were more prevalent in subtractions with co-occurring peaks in the caudal than with the rostral ACC. Peaks in the middle frontal gyrus were more frequent in subtractions with co-occurring peaks in the paralimbic part of the supracallosal ACC, relative to the subcallosal or limbic supracallosal ACC. These observations are consistent with known differences in the anatomic connectivity in these cortical regions, as defined in non-human primates. Further analyses of the influence of behavioural variables on the relationships between the ACC and other regions of the frontal cortex suggested that this type of meta-analysis may provide testable hypotheses about functional and effective connectivity within the human frontal lobe.

Lateralization of the Human Mirror Neuron System

A cortical network consisting of the inferior frontal, rostral inferior parietal, and posterior superior temporal cortices has been implicated in representing actions in the primate brain and is critical to imitation in humans. This neural circuitry may be an evolutionary precursor of neural systems associated with language. However, language is predominantly lateralized to the left hemisphere, whereas the degree of lateralization of the imitation circuitry in humans is unclear. We conducted a functional magnetic resonance imaging study of imitation of finger movements with lateralized stimuli and responses. During imitation, activity in the inferior frontal and rostral inferior parietal cortex, although fairly bilateral, was stronger in the hemisphere ipsilateral to the visual stimulus and response hand. This ipsilateral pattern is at variance with the typical contralateral activity of primary visual and motor areas. Reliably increased signal in the right superior temporal sulcus (STS) was observed for both left-sided and right-sided imitation tasks, although subthreshold activity was also observed in the left STS. Overall, the data indicate that visual and motor components of the human mirror system are not left-lateralized. The left hemisphere superiority for language, then, must be have been favored by other types of language precursors, perhaps auditory or multimodal action representations.

Immediate and Long-Term Changes in Corticomotor Output in Response to Rehabilitation: Correlation with Functional Improvements in Chronic Stroke:

The goal of this study was to determine whether transcranial magnetic stimulation (TMS) measures of the corticomotor pathways might reflect and predict functional improvements during rehabilitation of a hemiparetic upper extremity. Ten patients with variable levels of functional impairment in upper extremity use were enrolled at least 3 months after a stroke. TMS was used to obtain serial measures of motor thresholds and motor-evoked potential (MEP) size for a muscle from both hands before and after each session of an intervention aimed at improving functional motor control. Functional ability and cortical map area and volume were measured before each therapy session. At intake, all TMS measures from the affected side were impaired compared with the unaffected side but they did not reliably predict the level of functional improvement. Motor thresholds decreased, whereas MEP amplitude and map size increased with treatment. The amount of change in affected side measures was correlated with the amount of improvement in hand/arm function. Normalization of MEP amplitude asymmetry in response to the 1st 2 therapy sessions predicted long-term improvement in Fugl-Meyer motor score. Within limits, TMS appears to be useful in both moderate and more impaired patients as a physiological assay of treatment-induced plasticity and behavioral gains. Methodological differences in the literature, however, currently obscure a full understanding of the potential contributions of TMS to rehabilitation research.

Brain Plasticity in the Adult: Modulation of Function in Amblyopia with rTMS

Amblyopia is a cortically based visual disorder caused by disruption of vision during a critical early developmental period. It is often thought to be a largely intractable problem in adult patients because of a lack of neuronal plasticity after this critical period [1]; however, recent advances have suggested that plasticity is still present in the adult amblyopic visual cortex [2-6]. Here, we present data showing that repetitive transcranial magnetic stimulation (rTMS) of the visual cortex can temporarily improve contrast sensitivity in the amblyopic visual cortex. The results indicate continued plasticity of the amblyopic visual system in adulthood and open the way for a potential new therapeutic approach to the treatment of amblyopia.

Deconstructing apraxia: understanding disorders of intentional movement after stroke

Impairments in praxic functioning are common after stroke, most frequently when the left hemisphere is affected. Several recent studies of apraxia after stroke have made advances in understanding the right hemisphere contribution to praxis, particularly for the performance of novel actions. Moreover, quantitative lesion analysis in stroke patients indicates the importance of cortical regions such as the intraparietal sulcus and the middle frontal gyrus for subserving praxic function. Complex neuropsychological models have been developed to account for the many dissociations observed in the types of errors observed in stroke patients. Relatively lacking, however, are models that attempt to relate the neurological data to what is known about praxis from functional neuroimaging in normal subjects and from physiological studies in the monkey. Moreover, a coherent interpretation of the results of apraxia studies remains hampered by the lack of a standard testing instrument to assess the nature and severity of apraxic impairments in the groups tested.

Computerized testing augments pencil‐and‐paper tasks in measuring HIV‐associated mild cognitive impairment*

Existing tools for rapid cognitive assessment in HIV‐positive individuals with mild cognitive deficits lack sensitivity or do not meet psychometric requirements for tracking changes in cognitive ability over time.

Directed attention after unilateral frontal excisions in humans

We investigated the ability of patients with frontal-lobe lesions to benefit from advance information in a simple reaction-time task. The task involved pressing a button in response to the appearance of a peripheral target (visual angle of 11.5 degrees). A cue, presented in the centre of the screen, preceded the target onset by either a short (average 500 ms) or a long (average 3000 ms) interval. In half of the trials, the cue was an arrow indicating the location, in the left or right hemifield, of the upcoming target; in the other half, the cue was an uninformative plus sign. In addition to patients with unilateral excisions of frontal cortex, we tested patients with anterior temporal-lobe excisions and normal controls. The frontal-lobe group was mildly impaired with respect to the temporal-lobe group in using advance spatial cues to speed response to the visual target. While the size of the cueing effect changed across the range of cue-target intervals tested, there was no variation across intervals in the size of the impairment exhibited by the frontal-lobe group. The site and the volume of the lesions were determined on the basis of magnetic resonance image (MRI) scans in 10 of the 17 patients in the frontal group. There was no correlation between lesion volume and benefit score in these patients.

Measuring Cognition in a Geriatric Outpatient Clinic: Rasch Analysis of the Montreal Cognitive Assessment

Objective: To evaluate the psychometric properties of the Montreal Cognitive Assessment as a quantitative measure of cognitive ability. Data analyzed: A total of 222 cases extracted from a clinical database (57-91 years) of patients screened for cognitive impairment in outpatient geriatric assessment clinics. Data collected: Demographic information and individual item responses to Montreal Cognitive Assessment. Results: Comparison of the data with a unidimensional Rasch model indicated that the total score obtained by summing across all items yields a reliable (0.75) quantitative estimate of global cognitive ability. All items fit the model and together spanned a range of difficulty from -3.75 to +2.88 logits. Items were assessed for differential item functioning across such patient characteristics as age, education, and language spoken. We provide a table for converting Montreal Cognitive Assessment total scores onto a linearly scaled score, with guidelines for interpreting changes in Montreal Cognitive Assessment score in terms of their statistical significance. Conclusions: The Montreal Cognitive Assessment can provide a reliable and valid quantitative estimate of cognitive ability in a geriatric cognitive disorders clinic setting.