Antony D. Passaro

Functional neuroimaging with MEG: Normative language profiles

The reliability of language-specific brain activation profiles was assessed using Magnetoencephalography (MEG) in five experiments involving ninety-seven normal volunteers of both genders ranging in age from seven to eighty-four years. MEG data were analyzed with a fully automated method to eliminate subjective judgments in the process of deriving the activation profiles. Across all experiments, profiles were characterized by significant bilateral activity centered in the superior temporal gyrus, and in activity lateralized to the left middle temporal gyrus. These features were invariant across age, gender, variation in task characteristics, and mode of stimulus presentation. The absolute amount of activation, however, did decline with age in the auditory tasks. Moreover, contrary to the commonly held belief that left hemisphere dominance for language is greater in men than in women, our data revealed an opposite albeit a not consistently significant trend.

Functional Disruption of the Brain Mechanism for Reading: Effects of Comorbidity and Task Difficulty Among Children With Developmental Learning Problems

OBJECTIVE The study investigated the relative degree and timing of cortical activation associated with phonological decoding in poor readers. METHOD Regional brain activity was assessed during performance of a pseudoword reading task and a less demanding, letter-sound naming task by three groups of students: children who experienced reading difficulties without attention problems (N = 50, RD) and nonreading impaired (NI) readers either with (N = 20) or without attention-deficit/hyperactivity disorder (ADHD; N = 50). Recordings were obtained with a whole-head neuromagnetometer, and activation profiles were computed through a minimum norm algorithm. RESULTS Children with RD showed decreased amplitude of neurophysiological activity in the superior temporal gyrus, bilaterally, and in the left supramarginal and angular gyri during late stages of decoding, compared to typical readers. These effects were restricted to the more demanding pseudoword reading task. No differences were found in degree of activity between NI and ADHD students. Regression analyses provided further support for the crucial role of left hemisphere temporoparietal cortices and the fusiform gyrus for basic reading skills. CONCLUSIONS Results were in agreement with fMRI findings and replicate previous MEG findings with a larger sample, a higher density neuromagnetometer, an overt pseudoword reading task, and a distributed current source-modeling method.

Explorations of object and location memory using fMRI

Content-specific sub-systems of visual working memory (VWM) have been explored in many neuroimaging studies with inconsistent findings and procedures across experiments. The present study employed functional magnetic resonance imaging (fMRI) and a change detection task using a high number of trials and matched stimulus displays across object and location change (what vs. where) conditions. Furthermore, individual task periods were studied independently across conditions to identify differences corresponding to each task period. Importantly, this combination of task controls has not previously been described in the fMRI literature. Composite results revealed differential frontoparietal activation during each task period. A separation of object and location conditions yielded a distributed system of dorsal and ventral streams during the encoding of information corresponding to bilateral inferior parietal lobule (IPL) and lingual gyrus activation, respectively. Differential activity was also shown during the maintenance of information in middle frontal structures bilaterally for objects and the right IPL and left insula for locations. Together, these results reflect a domain-specific dissociation spanning several cortices and task periods. Furthermore, differential activations suggest a general caudal-rostral separation corresponding to object and location memory, respectively.

Optimizing estimation of hemispheric dominance for language using magnetic source imaging

The efficacy of magnetoencephalography (MEG) as an alternative to invasive methods for investigating the cortical representation of language has been explored in several studies. Recently, studies comparing MEG to the gold standard Wada procedure have found inconsistent and often less-than accurate estimates of laterality across various MEG studies. Here we attempted to address this issue among normal right-handed adults (N=12) by supplementing a well-established MEG protocol involving word recognition and the single dipole method with a sentence comprehension task and a beamformer approach localizing neural oscillations. Beamformer analysis of word recognition and sentence comprehension tasks revealed a desynchronization in the 10-18Hz range, localized to the temporo-parietal cortices. Inspection of individual profiles of localized desynchronization (10-18Hz) revealed left hemispheric dominance in 91.7% and 83.3% of individuals during the word recognition and sentence comprehension tasks, respectively. In contrast, single dipole analysis yielded lower estimates, such that activity in temporal language regions was left-lateralized in 66.7% and 58.3% of individuals during word recognition and sentence comprehension, respectively. The results obtained from the word recognition task and localization of oscillatory activity using a beamformer appear to be in line with general estimates of left hemispheric dominance for language in normal right-handed individuals. Furthermore, the current findings support the growing notion that changes in neural oscillations underlie critical components of linguistic processing.

Aberrant spatiotemporal activation profiles associated with math difficulties in children: A magnetic source imaging study

The study investigates the relative degree and timing of cortical activation in parietal, temporal, and frontal regions during simple arithmetic tasks in children who experience math difficulties. Real-time brain activity was measured with magnetoencephalography during simple addition and numerosity judgments in students with math difficulties and average or above average reading skills (MD group, N = 14), students with below average scores on both math and basic reading tests (MD/RD group, N = 16) and students with above average scores on standardized math tests (control group, N = 25). Children with MD showed increased degree of neurophysiological activity in inferior and superior parietal regions in the right hemisphere compared to both controls and MD/RD students. Left hemisphere inferior parietal regions did not show the expected task-related changes and showed activity at a significant temporal delay. MD students also showed increased early engagement of prefrontal cortices. Taken together, these findings may indicate increased reliance on a network of right hemisphere parietal (and possibly frontal areas as well) for simple math calculations in students who experience math difficulties but perform within normal range in reading.

When Epilepsy Interferes With Word Comprehension: Findings in Landau-Kleffner Syndrome

Landau-Kleffner syndrome is characterized by a regression in receptive language. The factors that affect the clinical expression of this syndrome remain unclear. This study presents neuroimaging findings in 2 patients showing different clinical evolutions. Linguistic regression persisted in 1 patient and evolved positively in the other. In patient A (with severe linguistic regression) there was an overlap between areas engaged during word recognition and those involved in generating the epileptiform activity; in patient B (with better linguistic evolution), receptive language was predominantly represented in the right hemisphere (unaffected). Patient A underwent multiple subpial transections. The 2-year follow-up indicated linguistic improvement, absence of epileptiform activity, and activation of the left temporal cortex during word comprehension. These results suggest that the resolution of the linguistic deficit in Landau-Kleffner syndrome may be modulated by the language-specific cortex freed from interfering epileptiform activity or by reorganization of the receptive language cortex triggered by the epileptic activity.

The Timing and Strength of Regional Brain Activation Associated with Word Recognition in Children with Reading Difficulties

The study investigates the relative degree and timing of cortical activation across parietal, temporal, and frontal regions during performance of a continuous visual-word recognition task in children who experience reading difficulties (N = 44, RD) and typical readers (N = 40, NI). Minimum norm estimates of regional neurophysiological activity were obtained from magnetoencephalographic recordings. Children with RD showed bilaterally reduced neurophysiological activity in the superior and middle temporal gyri, and increased activity in rostral middle frontal and ventral occipitotemporal cortices, bilaterally. The temporal profile of activity in the RD group, featured near-simultaneous activity peaks in temporal, inferior parietal, and prefrontal regions, in contrast to a clear temporal progression of activity among these areas in the NI group. These results replicate and extend previous MEG and fMRI results demonstrating atypical, latency-dependent attributes of the brain circuit involved in word reading in children with reading difficulties.

MEG correlates of bimodal encoding of faces and persons' names.

Learning associations between peoples faces and names is a universal cognitive function with important social implications. The goal of the present study was to examine brain activity patterns associated with cross-modal encoding of names and faces. Learning face-name pairs was compared to unimodal learning tasks using the same visual and auditory stimuli. Spatiotemporal brain activation profiles were obtained with magnetoencephalography using an automated source estimation method. Results showed activation foci in left (for names) and right (for faces) temporal lobe perisylvian cortices, predominantly right-hemisphere occipital and occipitotemporal regions (for faces), and right hemisphere dorsolateral prefrontal regions during the encoding phase for both types of stimuli presented in isolation. Paired (face-name) stimulus presentation elicited bilateral prefrontal and temporal lobe perisylvian activity for faces and enhanced visual cortex activation in response to names (compared to names in the unpaired condition). These findings indicate distinct patterns of brain activation during the formation of associations between meaningful visual and auditory stimuli.

Change detection for the study of object and location memory.

Seven adult human participants were tested in change detection tasks for object and location memory with large and small sets of four different stimulus types. Blocked tests demonstrated that participants performed similarly in separate object and location tests with matched parameters and displays. In mixed tests, participants were informed that they would be tested with either object changes or location changes; surprisingly, they were nearly as accurate remembering both objects and locations as when either was tested alone. By contrast, in the large-set condition, performance was lower than baseline on surprise probe test trials in which participants were tested (on 13% of trials) with the change type opposite to the present block (e.g., location probe trials during the object change block). These probe-test results were further supported by the reduction in probe-baseline differences when tested with small sets (6) of these item types. Small sets required remembering locations and objects to resolve object-location confounds. Together these results show that humans can remember both objects and locations with little loss of accuracy when instructed to do so, but do not learn these contextual associations without instruction.

Spectral Power of Brain Activity Associated with Emotion — A Pilot MEG Study

In this study we analyzed broadband, whole-head MEG activity to investigate affective processing in the human brain and assess whether spectral power in nonoverlapping frequency bands correlates preferentially with specific anatomical locations and distinct emotions elicited by affective stimuli that differed in valence from unpleasant to pleasant and arousal from low to high. We analyzed recordings from 20 normal young adults using linear mixed effects models computed separately in each frequency band on a reduced set of recording sites represented by 8 principal components. Our results showed statistically significant differences between the pleasant and neutral conditions in all frequency bands and between the pleasant and unpleasant condition in the theta band.