Biomedical subjects

J M Maisog

Publications and source records attributed to J M Maisog.

At least 19 recordsLinked to original sources

PubMed1999

A functional lesion in developmental dyslexia: left angular gyral blood flow predicts severity.

Functional imaging studies have shown reduced regional cerebral blood flow (rCBF) in temporal and inferior parietal regions in dyslexia. To relate such abnormalities to the severity of dyslexia, correlations between reading skill and rCBF during a series of reading tasks and visual fixation were mapped for 17 right-handed dyslexic men, ages 18-40, and 14 matched controls. These correlations uniquely identified the left angular gyrus as the most probable site of a functional lesion in dyslexia: Here, higher rCBF was associated with better reading skill in controls (p <.01), but with worse reading skill in dyslexia (p <.01). This suggests that greater reliance on this region normally facilitates reading, but impairs reading in dyslexia. Thus, developmental dyslexia may share a common localization with alexia.

Adolescent↗

PubMed1999

Pain intensity processing within the human brain: a bilateral, distributed mechanism.

Functional imaging studies of human subjects have identified a diverse assortment of brain areas that are engaged in the processing of pain. Although many of these brain areas are highly interconnected and are engaged in multiple processing roles, each area has been typically considered in isolation. Accordingly, little attention has been given to the global functional organization of brain mechanisms mediating pain processing. In the present investigation, we have combined positron emission tomography with psychophysical assessment of graded painful stimuli to better characterize the multiregional organization of supraspinal pain processing mechanisms and to identify a brain mechanism subserving the processing of pain intensity. Multiple regression analysis revealed statistically reliable relationships between perceived pain intensity and activation of a functionally diverse group of brain regions, including those important in sensation, motor control, affect, and attention. Pain intensity-related activation occurred bilaterally in the cerebellum, putamen, thalamus, insula, anterior cingulate cortex, and secondary somatosensory cortex, contralaterally in the primary somatosensory cortex and supplementary motor area, and ipsilaterally in the ventral premotor area. These results confirm the existence of a highly distributed, bilateral supraspinal mechanism engaged in the processing of pain intensity. The conservation of pain intensity information across multiple, functionally distinct brain areas contrasts sharply with traditional views that sensory-discriminative processing of pain is confined within the somatosensory cortex and can account for the preservation of conscious awareness of pain intensity after extensive cerebral cortical lesions.

Adult↗

PubMed1998

An area specialized for spatial working memory in human frontal cortex.

Working memory is the process of maintaining an active representation of information so that it is available for use. In monkeys, a prefrontal cortical region important for spatial working memory lies in and around the principal sulcus, but in humans the location, and even the existence, of a region for spatial working memory is in dispute. By using functional magnetic resonance imaging in humans, an area in the superior frontal sulcus was identified that is specialized for spatial working memory. This area is located more superiorly and posteriorly in the human than in the monkey brain, which may explain why it was not recognized previously.

Animals↗

PubMed1998

Modeling hemodynamic response for analysis of functional MRI time-series.

The standard Gaussian function is proposed for the hemodynamic modulation function (HDMF) of functional magnetic resonance imaging (fMRI) time-series. Unlike previously proposed parametric models, the Gaussian model accounts independently for the delay and dispersion of the hemodynamic responses and provides a more flexible and mathematically convenient model. A suboptimal noniterative scheme to estimate the hemodynamic parameters is presented. The ability of the Gaussian function to represent the HDMF of brain activation is compared with Poisson and Gamma models. The proposed model seems valid because the lag and dispersion values of hemodynamic responses rendered by the Gaussian model are in the ranges of their previously reported values in recent optical and fMR imaging studies. An extension of multiple regression analysis to incorporate the HDMF is presented. The detected activity patterns exhibit improvements with hemodynamic correction. The proposed model and efficient parameter estimation scheme facilitated the investigation of variability of hemodynamic parameters of human brain activation. The hemodynamic parameters estimated over different brain regions and across different stimuli showed significant differences. Measurement of hemodynamic parameters over the brain during sensory or cognitive stimulation may reveal vital information on physiological events accompanying neuronal activation and functional variability of the human brain, and should lead to the investigation of more accurate and complex models.

Brain↗

PubMed1998

Cortical regions involved in visual texture perception: a fMRI study.

To determine visual areas of the human brain involved in elementary form processing, functional magnetic resonance imaging (fMRI) was used to measure regional responses to two types of achromatic textures. Healthy young adults were presented with 'random' textures which lacked spatial organization of the black and white pixels that make up the image, and 'correlated' textures in which the pixels were ordered to produce extended contours and rectangular blocks at multiple spatial scales. Relative to a fixation condition, random texture stimulation resulted in increased signal intensity primarily in the striate cortex, with slight involvement of the cuneus and middle occipital, lingual and fusiform gyri. Correlated texture stimulation also resulted in activation of these areas, yet the regional extent of this activation was significantly greater than that produced by random textures. Unlike random stimulation, correlated stimulation additionally resulted in middle temporal activation. Direct comparison of the two stimulation conditions revealed significant differences most consistently in the anterior fusiform gyrus, but also in striate, middle occipital, lingual and posterior temporal regions in subjects with robust activation patterns. While both random and correlated stimulation produced activation in similar areas of the occipital lobe, the increase in regional activation during the correlated condition suggests increased recruitment of neuronal populations occurs in response to textures containing visually salient features. This increased recruitment occurs within striate, extrastriate and temporal regions of the brain, also suggesting the presence of receptive field mechanisms in the ventral visual pathway that are sensitive to features produced by higher-order spatial correlations.

Adult↗

PubMed1998

Positron emission tomography [15O]water studies with short interscan interval for single-subject and group analysis: influence of background subtraction.

Use of short interscan interval [15O]water positron emission tomography (PET) studies reduces the overall study duration and may allow an increased number of scans for single-subject analysis of unique cases (e.g., stroke). The purpose of this study was to examine how subtraction of residual radioactivity from the previous injection (corrected scan) compared to nonsubtraction (uncorrected scan) in a PET short interscan interval (6 minutes) study affects single-subject and group data analysis using a motor activation task. Two currently widely used analytic strategies, Worsley's method and the SPM technique, were applied. Excellent agreement between activation maps obtained from corrected and uncorrected data sets was obtained both in single-subject analyses performed on data sets from the six normal subjects and three stroke (subcortical infarct) patients, and in group analysis (six normal subjects) within a particular statistical method. The corrected and uncorrected data were very similar in the (1) number of activated brain regions; (2) size of clusters of activated brain voxels; (3) Talairach coordinates of the activated region; and (4) t or Z value of the peak intensity for every significantly activated motor brain structure (both for large activations such as in motor cortex and small activations such as in putamen and thalamus). [15O]Water PET data obtained with a short interscan interval (6 minutes) produce similar results whether or not the background is subtracted. Thus, if injection dose and timing are constant, one can achieve the advantage of a short interscan interval without the added complexity of correcting for background radioactivity.

Adult↗

PubMed1998

fMRI study of face perception and memory using random stimulus sequences.

A new functional magnetic resonance imaging (fMRI) method was used to investigate the functional neuroanatomy of face perception and memory. Whole-brain fMRI data were acquired while four types of stimuli were presented sequentially in an unpredictable pseudorandom order at a rate of 0.5 Hz. Stimulus types were a single repeated memorized target face, unrepeated novel faces, nonsense scrambled faces, and a blank screen. Random stimulus sequences were designed to generate a functional response to each stimulus type that was uncorrelated with responses to other stimuli. This allowed fMRI responses to each stimulus type to be examined separately using multiple regression. Signal increases were found for all stimuli in ventral posterior cortex. Responses to intact faces extended to more anterior locations of occipitotemporal cortex than did responses to scrambled faces, consistent with previous studies of face perception. Responses evoked by novel faces were in regions of ventral occipitotemporal cortex medial to regions in which significant responses were evoked by the target face. The repeated target face stimulus also evoked activity in widely distributed regions of frontal and parietal cortex. These results demonstrate that cortical hemodynamic responses to interleaved novel and repeated stimuli can be distinguished and measured using fMRI with appropriate stimulus sequences and data analysis methods. This method can now be used to examine the neural systems involved in cognitive tasks that were previously impossible to study using positron emission tomography or fMRI.

Adult↗

PubMed1998

An efficient method for correcting the edge artifact due to smoothing.

Spatial smoothing is a common pre-processing step in the analysis of functional brain imaging data. It can increase sensitivity to signals of specific shapes and sizes (Rosenfeld and Kak [1982]: Digital Picture Processing, vol. 2. Orlando, Fla.: Academic; Worsley et al. [1996]: Hum Brain Mapping 4:74-90). Also, some amount of spatial smoothness is required if methods from the theory of Gaussian random fields are to be used (Holmes [1994]: Statistical Issues in Functional Brain Mapping. PhD thesis, University of Glasgow). Smoothing is most often implemented as a convolution of the imaging data with a smoothing kernel, and convolution is most efficiently performed using the Convolution Theorem and the Fast Fourier Transform (Cooley and Tukey [1965]: Math Comput 19:297-301; Priestly [1981]: Spectral Analysis and Time Series. San Diego: Academic; Press et al. [1992]: Numerical Recipes in C: The Art of Scientific Computing, 2nd ed. Cambridge: Cambridge University Press). An undesirable side effect of smoothing is an artifact along the edges of the brain, where brain voxels become smoothed with non-brain voxels. This results in a dark rim which might be mistaken for hypoactivity. In this short methodological paper, we present a method for correcting functional brain images for the edge artifact due to smoothing, while retaining the use of the Convolution Theorem and the Fast Fourier Transform for efficient calculation of convolutions.

Artifacts↗

PubMed1997

Attention-related modulation of activity in primary and secondary auditory cortex.

We investigated the effects of auditory attention on brain activity using functional magnetic resonance imaging. Subjects listened to three word lists, three times each, and were instructed to count the number of times they heard a target word during two of these presentations. For the third, they listened to the words without counting. All subjects showed significant areas of activation in auditory cortex during the listening conditions compared to rest. There was significantly more activation and a larger area of activation, particularly in association cortex, in the left temporal lobe during counting of targets compared to the no-target conditions, with a similar trend in the right hemisphere. These results provide evidence of an attention-related enhancement of both activation magnitude and extent in auditory cortex.

Adult↗

PubMed1997

A positron emission tomographic study of impaired word recognition and phonological processing in dyslexic men.

BACKGROUND: Developmental dyslexia is characterized by impaired word recognition, which is thought to result from deficits in phonological processing. Improvements during the course of development are thought to disproportionately involve orthographic components of reading; phonological deficits persist into adulthood. OBJECTIVE: To localize the neural correlates of impaired word recognition and phonological processing in men with developmental dyslexia. METHODS: Regional cerebral blood flow was measured with oxygen 15 positron emission tomography in 17 men with dyslexia and in 14 matched controls during the performance of phonological and orthographic tasks--pronunciation (reading aloud) and lexical decision making--designed to activate posterior and anterior perisylvian cortices, respectively. RESULTS: Altered patterns of activation (reduced activation, unusual deactivation) were seen in dyslexic men in mid- to posterior temporal cortex bilaterally and in inferior parietal cortex, predominantly on the left, during both pronunciation and decision making. In contrast, dyslexic men demonstrated essentially normal activation of left inferior frontal cortex during both phonological and orthographic decision making. CONCLUSION: These, along with prior findings, are compatible with a hypothesis of bilateral involvement of posterior temporal and parietal cortices in dyslexia.

Adolescent↗

PubMed1997

Phonological and orthographic components of word recognition. A PET-rCBF study.

Pronunciation (of irregular/inconsistent words and of pseudowords) and lexical decision-making tasks were used with 15O PET to examine the neural correlates of phonological and orthographic processing in 14 healthy right-handed men (aged 18-40 years). Relative to a visual-fixation control task, all four experimental tasks elicited a left-lateralized stream of activation involving the lingual and fusiform gyri, perirolandic cortex, thalamus and anterior cingulate. Both pronunciation tasks activated the left superior temporal gyrus, with significantly greater activation seen there during phonological (pseudoword) than during orthographic (real word) pronunciation. The left inferior frontal cortex was activated by both decision-making tasks; more intense and widespread activation was seen there during phonological, than during orthographic, decision making, with the activation during phonological decision-making extending into the left insula. Correlations of reference voxels in the left superior temporal gyrus and left inferior frontal region with the rest of the brain were highly similar for the phonological and orthographic versions of each task type. These results are consistent with connectionist models of reading, which hypothesize that both real words and pseudowords are processed within a common neural network.

Adolescent↗

PubMed1997

Altered patterns of cerebral activity during speech and language production in developmental stuttering. An H2(15)O positron emission tomography study.

To assess dynamic brain function in adults who had stuttered since childhood, regional cerebral blood flow (rCBF) was measured with H2O and PET during a series of speech and language tasks designed to evoke or attenuate stuttering. Speech samples were acquired simultaneously and quantitatively compared with the PET images. Both hierarchical task contrasts and correlational analyses (rCBF versus weighted measures of dysfluency) were performed. rCBF patterns in stuttering subjects differed markedly during the formulation and expression of language, failing to demonstrate left hemispheric lateralization typically observed in controls; instead, regional responses were either absent, bilateral or lateralized to the right hemisphere. Significant differences were detected between groups when all subjects were fluent-during both language formulation and non-linguistic oral motor tasks-demonstrating that cerebral function may be fundamentally different in persons who stutter, even in the absence of stuttering. Comparison of scans acquired during fluency versus dysfluency-evoking tasks suggested that during the production of stuttered speech, anterior forebrain regions-which play an a role in the regulation of motor function-are disproportionately active in stuttering subjects, while post-rolandic regions-which play a role in perception and decoding of sensory information-are relatively silent. Comparison of scans acquired during these conditions in control subjects, which provide information about the sensorimotor or cognitive features of the language tasks themselves, suggest a mechanism by which fluency-evoking maneuvers might differentially affect activity in these anterior and posterior brain regions and may thus facilitate fluent speech production in individuals who stutter. Both correlational and contrast analyses suggest that right and left hemispheres play distinct and opposing roles in the generation of stuttering symptoms: activation of left hemispheric regions appears to be related to the production of stuttered speech, while activation of right hemispheric regions may represent compensatory processes associated with attenuation of stuttering symptoms.

Adult↗

PubMed1996

Abnormal processing of visual motion in dyslexia revealed by functional brain imaging.

It is widely accepted that dyslexics have deficits in reading and phonological awareness, but there is increasing evidence that they also exhibit visual processing abnormalities that may be confined to particular portions of the visual system. In primate visual pathways, inputs from parvocellular or magnocellular layers of the lateral geniculate nucleus remain partly segregated in projections to extrastriate cortical areas specialized for processing colour and form versus motion. In studies of dyslexia, psychophysical and anatomical evidence indicate an anomaly in the magnocellular visual subsystem. To investigate the pathophysiology of dyslexia, we used functional magnetic resonance imaging (fMRI) to study visual motion processing in normal and dyslexic men. In all dyslexics, presentation of moving stimuli failed to produce the same task-related functional activation in area V5/MT (part of the magnocellular visual subsystem) observed in controls. In contrast, presentation of stationary patterns resulted in equivalent activations in V1/V2 and extrastriate cortex in both groups. Although previous studies have emphasized language deficits, our data reveal differences in the regional functional organization of the cortical visual system in dyslexia.

Adult↗

PubMed1996

Face encoding and recognition in the human brain.

A dissociation between human neural systems that participate in the encoding and later recognition of new memories for faces was demonstrated by measuring memory task-related changes in regional cerebral blood flow with positron emission tomography. There was almost no overlap between the brain structures associated with these memory functions. A region in the right hippocampus and adjacent cortex was activated during memory encoding but not during recognition. The most striking finding in neocortex was the lateralization of prefrontal participation. Encoding activated left prefrontal cortex, whereas recognition activated right prefrontal cortex. These results indicate that the hippocampus and adjacent cortex participate in memory function primarily at the time of new memory encoding. Moreover, face recognition is not mediated simply by recapitulation of operations performed at the time of encoding but, rather, involves anatomically dissociable operations.

Adult↗

PubMed1996

Functional magnetic resonance imaging of human visual cortex during face matching: a comparison with positron emission tomography.

Cortical areas associated with the perception of faces were identified using functional magnetic resonance imaging (fMRI). T2*-weighted gradient echo, echo-planar MR images were obtained using a modified 1.5-T GE Signa MRI. In all nine subjects studied, performance of a face-matching task was associated with a region of significantly increased MR signal in the ventral occipitotemporal cortex, extending from the inferior occipital sulcus to the lateral occipitotemporal sulcus and fusiform gyrus. Smaller and more variable signal increases were found in dorsolateral occipitoparietal cortex near the intraparietal sulcus. Signal decreases were found in the angular gyrus and posterior cingulate cortex. Single-subject fMRI analyses revealed discrete areas of activation with well-defined borders. Group analyses of spatially smoothed fMRI data produced results that replicated most aspects of previous studies of face processing using positron emission tomography (PET). These results show that PET and fMRI identify functional areas with similar anatomical locations. In addition, fMRI reveals interindividual variation in the anatomical location of higher-level processing areas with greater anatomical precision.

Adult↗

PubMed1996

Visual cortical dysfunction in Alzheimer's disease evaluated with a temporally graded "stress test" during PET.

OBJECTIVE: Visual-processing abnormalities commonly contribute to typical Alzheimer's disease symptoms, but their detailed pathophysiology remains unknown. To investigate why patients with Alzheimer's disease have greater difficulty performing visuoconstructive (magnocellular-dominated) tasks than face- or color-perception (parvocellular-dominated) tasks, the authors measured brain activation in response to a temporally graded visual stimulus (neural stress test) during positron emission tomography. METHOD: The stress test measured regional cerebral blood flow (CBF) in response to a patterned flash stimulus in the resting state (0 Hz in the dark) and at frequencies of 1, 2, 4, 7, and 14 Hz. Ten patients with Alzheimer's disease and 12 age- and sex-matched comparison subjects were studied. RESULTS: The striate response at 7 Hz and 14 Hz (the degree of regional CBF increase from that at 0 Hz) was significantly less in the patients than in the comparison subjects, whereas the change in regional CBF at the lower frequencies did not differ between groups. In bilateral middle temporal association areas activated by motion and dominated by magnocellular input, regional CBF at 1 Hz (the frequency with maximal apparent motion) was significantly greater than at 0 Hz in the comparison subjects but not in the patients. CONCLUSIONS: The magnocellular visual system normally responds to high-frequency input and motion; the failure of response in the striate cortex at high but not low frequencies in the Alzheimer's patients suggests greater magnocellular than parvocellular dysfunction at these levels. Activation failure in the middle temporal areas in the patients supports magnocellular dysfunction. The finding that the Alzheimer's disease group had abnormal visual cortical function emphasizes the importance of clinical visuospatial evaluation of patients with Alzheimer's disease to fully understand symptom production and to plan interventions.

Aged↗

PubMed1996

Cerebral glucose metabolism during pharmacologic studies: test-retest under placebo conditions.

UNLABELLED: The reliability of serial [18F]fluorodeoxyglucose (FDG) PET scans for psychopharmacologic studies was tested by using placebo infusions. METHODS: FDG scans were obtained before and after a 30 min placebo infusion (n = 10; Group 1) or after each of two bolus infusions with placebo (n = 8; Group 2). Subjects performed a continuous performance task (CPT) during each scan. Cardiovascular measures and ratings of anxiety were obtained in all subjects. Samples for determination of plasma norepinephrine (NE) were taken at multiple time points in Group 1. RESULTS: A slight increase in apparent global metabolism occurred between scans in both Groups 1 and 2. A few regions significantly increased in both groups. While an apparent increase in sympathetic activity occurred during the placebo infusion, neither NE levels, anxiety ratings nor cardiovascular measures correlated with global or regional FD6 uptake. CONCLUSION: Test-retest differences of global and regional glucose metabolism were highly consistent across two experimental designs. While increases in cerebral glucose metabolism appeared to occur during the second scan, differences between scans were small. This method may offer advantages for selected psychopharmacologic studies.

Adult↗

PubMed1995

Age-related reductions in human recognition memory due to impaired encoding.

The participation of the medial temporal cortex and other cerebral structures in the memory impairment that accompanies aging was examined by means of positron emission tomography. Cerebral blood flow (rCBF) was measured during encoding and recognition of faces. Young people showed increased rCBF in the right hippocampus and the left prefrontal and temporal cortices during encoding and in the right prefrontal and parietal cortex during recognition. Old people showed no significant activation in areas activated during encoding in young people but did show right prefrontal activation during recognition. Age-related impairments of memory may be due to a failure to encode the stimuli adequately, which is reflected in the lack of cortical and hippocampal activation during encoding.

Adult↗