Kathleen Pirog Revill

Neural correlates of partial lexical activation

As a spoken word unfolds over time, it is temporarily consistent with the acoustic forms of multiple words. Previous behavioral research has shown that, in the face of temporary ambiguity about how a word will end, multiple candidate words are briefly activated. Here, we provide neural imaging evidence that lexical candidates only temporarily consistent with the input activate perceptually based semantic representations. An artificial lexicon and novel visual environment were used to target human MT/V5 and an area anterior to it which have been shown to be recruited during the reading of motion words. Participants learned words that referred to novel objects and to motion or color/texture changes that the objects underwent. The lexical items corresponding to the change events were organized into phonologically similar pairs differing only in the final syllable. Upon hearing spoken scene descriptions in a posttraining verification task, participants showed greater activation in the left hemisphere anterior extent of MT/V5 when motion words were heard than when nonmotion words were heard. Importantly, when a nonmotion word was heard, the level of activation in the anterior extent of MT/V5 was modulated by whether there was a phonologically related competitor that was a motion word rather than another nonmotion word. These results provide evidence of activation of a perceptual brain region in response to the semantics of a word while lexical competition is in process and before the word is fully recognized.

Context and Spoken Word Recognition in a Novel Lexicon.

Three eye movement studies with novel lexicons investigated the role of semantic context in spoken word recognition, contrasting 3 models: restrictive access, access-selection, and continuous integration. Actions directed at novel shapes caused changes in motion (e.g., looming, spinning) or state (e.g., color, texture). Across the experiments, novel names for the actions and the shapes varied in frequency, cohort density, and whether the cohorts referred to actions (Experiment 1) or shapes with action-congruent or action-incongruent affordances (Experiments 2 and 3). Experiment 1 demonstrated effects of frequency and cohort competition from both displayed and non-displayed competitors. In Experiment 2, a biasing context induced an increase in anticipatory eye movements to congruent referents and reduced the probability of looks to incongruent cohorts, without the delay predicted by access-selection models. In Experiment 3, context did not reduce competition from non-displayed incompatible neighbors as predicted by restrictive access models. The authors conclude that the results are most consistent with continuous integration models.

Motor demand-dependent activation of ipsilateral motor cortex

The role of ipsilateral primary motor cortex (M1) in hand motor control during complex task performance remains controversial. Bilateral M1 activation is inconsistently observed in functional (f)MRI studies of unilateral hand performance. Two factors limit the interpretation of these data. As the motor tasks differ qualitatively in these studies, it is conceivable that M1 contributions differ with the demand on skillfulness. Second, most studies lack the verification of a strictly unilateral execution of the motor task during the acquisition of imaging data. Here, we use fMRI to determine whether ipsilateral M1 activity depends on the demand for precision in a pointing task where precision varied quantitatively while movement trajectories remained equal. Thirteen healthy participants used an MRI-compatible joystick to point to targets of four different sizes in a block design. A clustered acquisition technique allowed simultaneous fMRI/EMG data collection and confirmed that movements were strictly unilateral. Accuracy of performance increased with target size. Overall, the pointing task revealed activation in contralateral and ipsilateral M1, extending into contralateral somatosensory and parietal areas. Target size-dependent activation differences were found in ipsilateral M1 extending into the temporal/parietal junction, where activation increased with increasing demand on accuracy. The results suggest that ipsilateral M1 is active during the execution of a unilateral motor task and that its activity is modulated by the demand on precision.

Neural Correlates of Inter-Trial Priming and Role-Reversal in Visual Search

Studies of priming of visual perception demonstrate that observers respond more quickly to targets in a field of distractors when relevant features are repeated versus novel or role-reversed. In a recent brain imaging study by Kristjánsson et al. (2007), participants were presented with two items of one color and a single item in a different color with the task of reporting the orientation of the uniquely colored item. Consistent with previous behavioral reports, they found that observers were faster to respond when the target and distractor colors were identical to the previous trial than when they were reversed. They found reduced BOLD activity in brain areas linked with attentional control on trials where the target and distractor colors were repeated relative to reversed, which they interpreted as reflecting response suppression (decreased BOLD signal for repeated stimuli). However, since their design only compared repeated versus reversed task demands, it is logically possible that this pattern reflects increased BOLD signal for role-reversed stimuli: activity required to inhibit previously facilitated information and select previously inhibited information. We explored this possibility with a task where we contrasted the signal generated by repeated, reversed, and novel features. Our data suggest that the majority of the change in neural signal elicited by priming of pop-out reflects increased activation when selection criteria are reversed.

Distinct anatomy for visual search and bisection: a neuroimaging study.

Individuals with spatial neglect following brain injury often show biased performance on landmark bisection tasks (judging if a single item is transected at its midpoint) and search tasks (where they seek target(s) from an array of items). Interestingly, it appears that bisection deficits dissociate from other measures of neglect (including search tasks), and neglect patients with bisection deficits typically have more posterior injury than those without these symptoms. While previous studies in healthy adults have examined each of these tasks independently, our aim was to directly contrast brain activity between these two tasks. Our design used displays that were interpreted as landmark bisection stimuli in some blocks of trials and as search arrays on other trials. Therefore, we used a design where low-level perceptual and motor responses were identical across tasks. Both tasks generated significant activity in bilateral midfusiform gyrus, largely right lateralized activity in the posterior parietal cortex, left lateralized activity in the left motor cortex (consistent with right handed response) and generally right lateralized insular activation. Several brain areas showed task-selective activations when the two tasks were directly compared. Specifically, the superior parietal cortex was selectively activated during the landmark task. On the other hand, the search task caused stronger bilateral activation in the anterior insula, along with midfusiform gyrus, medial superior frontal areas, thalamus and right putamen. This work demonstrates that healthy adults show an anatomical dissociation for visual search and bisection behavior similar to that reported in neurological patients, and provides coordinates for future brain stimulation studies.

Neuroimaging somatosensory perception and masking

ABSTRACT The specific cortical and subcortical regions involved in conscious perception and masking are uncertain. This study sought to identify brain areas involved in conscious perception of somatosensory stimuli during a masking task using functional magnetic resonance (fMRI) to contrast perceived vs. non‐perceived targets. Electrical trains were delivered to the right index finger for targets and to the left index finger for masks. Target intensities were adjusted to compensate for threshold drift. Sham target trials were given in ˜10% of the trials, and target stimuli without masks were delivered in one of the five runs (68 trials/run). When healthy dextral adult volunteers (n=15) perceived right hand targets, greater left‐ than right‐cerebral activations were seen with similar patterns across the parietal cortex, thalamus, insula, claustrum, and midbrain. When targets were not perceived, left/right cerebral activations were similar overall. Directly comparing perceived vs. non‐perceived stimuli with similar intensities in the masking task revealed predominate activations contralateral to masks. In contrast, activations were greater contralateral to perceived targets if no masks were given or if masks were given but target stimulus intensities were greater for perceived than non‐perceived targets. The novel aspects of this study include: 1) imaging of cortical and subcortical activations in healthy humans related to somatosensory perception during a masking task, 2) activations in the human thalamus and midbrain related to perception of stimuli compared to matched non‐perceived stimuli, and 3) similar left/right cerebral activation patterns across cortical, thalamic and midbrain structures suggesting interactions across all three levels during conscious perception in humans. HighlightsfMRI study of conscious perception of somatosensory stimuli during a masking task.Activations in the human thalamus and midbrain related to perception.Similar left/right activation patterns across cortex, thalamus and midbrain.Patterns suggest interactions across all three levels during conscious perception.

Abnormally reduced primary motor cortex output is related to impaired hand function in chronic stroke

Stroke often involves primary motor cortex (M1) and its corticospinal projections (CST). As hand function is critically dependent on these structures, its recovery is often incomplete. The neuronal substrate supporting affected hand function is not well understood but likely involves reorganized M1 and CST of the lesioned hemisphere (M1IL and CSTIL). We hypothesized that affected hand function in chronic stroke is related to structural and functional reorganization of M1IL and CSTIL. We tested 18 patients with chronic ischemic stroke involving M1 or CST. Their hand function was compared with 18 age-matched healthy subjects. M1IL thickness and CSTIL fractional anisotropy (FA) were determined with MRI and compared with measures of the other hemisphere. Transcranial magnetic stimulation (TMS) was applied to M1IL to determine its input-output function [stimulus response curve (SRC)]. The plateau of the SRC (MEPmax), inflection point, and slope parameters of the curve were extracted. Results were compared with measures in 12 age-matched healthy controls. MEPmax of M1IL was significantly smaller ( P = 0.02) in the patients, indicating reduced CSTIL motor output, and was correlated with impaired hand function ( P = 0.02). M1IL thickness ( P < 0.01) and CSTIL-FA ( P < 0.01) were reduced but did not correlate with hand function. The results indicate that employed M1IL or CSTIL structural measures do not explain the extent of impairment in hand function once M1 and CST are sufficiently functional for TMS to evoke a motor potential. Instead, impairment of hand function is best explained by the abnormally low output from M1IL. NEW & NOTEWORTHY Hand function often remains impaired after stroke. While the critical role of the primary motor cortex (M1) and its corticospinal output (CST) for hand function has been described in the nonhuman primate stroke model, their structure and function have not been systematically evaluated for patients after stroke. We report that in chronic stroke patients with injury to M1 and/or CST an abnormally reduced M1 output is related to impaired hand function.