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G McCarthy

Publications and source records attributed to G McCarthy.

174 records · Page 10Linked to original sources
PubMedDate not supplied

When words collide: orthographic and phonological interference during word processing.

The interaction between orthographic and phonological information was studied in two experiments by requiring subjects to match visually presented word pairs on the basis of their visual or rhyming similarity. Word pairs either rhymed and looked alike, rhymed but did not look alike, looked alike but did not rhyme, or did not rhyme and did not look alike. In Experiment 1 under rhyme matching, reaction time (RT) was markedly increased whenever there was a conflict between orthographic and phonological cues. Under visual matching, overall RT was shorter than rhyme matching, with visually similar rhyming and non-rhyming pairs producing equally rapid and short responses compared to the non-rhyming but visually different word pairs. Most subjects also responded slower to rhyming and visually different stimuli compared to word pairs that did no look alike or rhyme. Experiment 2 sought to specify the processing locus of these effects by recording event-related brain potentials (ERPs) under task conditions similar to the first experiment. The RT data essentially replicated the effects found in Experiment 1 for both matching tasks. The ERP data viewed in the context of these results suggested that the interaction of the orthographic and phonological codes begins at least at the stimulus comparison processing stage, but that the conflict may also contribute to delays in response selection. The results are discussed in terms of several current models of word processing.

Discrimination Learning↗
PubMedDate not supplied

Human extrastriate visual cortex and the perception of faces, words, numbers, and colors.

Electrophysiological correlates of the processing of visual information were studied in epileptic patients with electrodes chronically implanted on the surface of striate and extrastriate cortex. In separate experiments patients viewed faces, letter strings (words and non-words), numbers, and control stimuli. A negative potential, N200, was evoked by faces, letter strings, and numbers, but not by the control stimuli. N200 was recorded bilaterally from discrete regions of the fusiform and inferior temporal gyri. These category-specific face, letter-string, and number "modules" vary in location. In most cases there was no overlap in the location of face and letter-string modules, suggesting a mosaic of functionally discrete regions. In some cases letter-string and number N200s were recorded from the same location, suggesting that these modules may be less spatially and functionally discrete. Face N200-like potentials can be recorded from temporal scalp, allowing the possibility of studying early face processing in normal subjects. Longer-latency face-specific potentials were recorded from the inferior surface of the anterior temporal lobe. Potentials evoked by colored checkerboards were recorded from a region of the fusiform gyrus posterior to the fusiform region from which category-specific N200s were recorded. These results suggest that there are several processing streams in inferior extrastriate cortex. In addition to object recognition systems previously proposed for faces and words, our preliminary results suggest a separate system dealing with numbers. Postulated systems dealing with larger manipulable objects and animals have not been detected.

Color Perception↗
PubMedDate not supplied

Activation of human prefrontal cortex during spatial and nonspatial working memory tasks measured by functional MRI.

Separate working memory domains for spatial location, and for objects, faces, and patterns, have been identified in the prefrontal cortex (PFC) of nonhuman primates. We have used functional magnetic resonance imaging to examine whether spatial and nonspatial visual working memory processes are similarly dissociable in human PFC. Subjects performed tasks which required them to remember either the location or shape of successive visual stimuli. We found that the mnemonic component of the working memory tasks affected the hemispheric pattern of PFC activation. The spatial (LOCATION) working memory task preferentially activated the middle frontal gyrus (MFG) in the right hemisphere, while the nonspatial (SHAPE) working memory task activated the MFG in both hemispheres. Furthermore, the area of activation in the left hemisphere extended into the inferior frontal gyrus for nonspatial SHAPE task. A perceptual target (DOT) detection task also activated the MFG bilaterally, but at a level approximately half that of the working memory tasks. The activation in the MFG occurred within 3-6 s of task onset and declined following task offset. Time-course analysis revealed a different pattern for cingulate gyrus, in which activation occurred upon task completion. Cingulate gyrus activation was greatest following the SHAPE task and was greater in the left hemisphere. The present results support the prominent role of the PFC and, specifically, the MFG in working memory, and indicate that the mnemonic content of the task affects the relative weighting of hemispheric activation.

Adult↗
PubMedDate not supplied

Electrophysiological studies of human face perception. I: Potentials generated in occipitotemporal cortex by face and non-face stimuli.

This and the following two papers describe event-related potentials (ERPs) evoked by visual stimuli in 98 patients in whom electrodes were placed directly upon the cortical surface to monitor medically intractable seizures. Patients viewed pictures of faces, scrambled faces, letter-strings, number-strings, and animate and inanimate objects. This paper describes ERPs generated in striate and peristriate cortex, evoked by faces, and evoked by sinusoidal gratings, objects and letter-strings. Short-latency ERPs generated in striate and peristriate cortex were sensitive to elementary stimulus features such as luminance. Three types of face-specific ERPs were found: (i) a surface-negative potential with a peak latency of approximately 200 ms (N200) recorded from ventral occipitotemporal cortex, (ii) a lateral surface N200 recorded primarily from the middle temporal gyrus, and (iii) a late positive potential (P350) recorded from posterior ventral occipitotemporal, posterior lateral temporal and anterior ventral temporal cortex. Face-specific N200s were preceded by P150 and followed by P290 and N700 ERPs. N200 reflects initial face-specific processing, while P290, N700 and P350 reflect later face processing at or near N200 sites and in anterior ventral temporal cortex. Face-specific N200 amplitude was not significantly different in males and females, in the normal and abnormal hemisphere, or in the right and left hemisphere. However, cortical patches generating ventral face-specific N200s were larger in the right hemisphere. Other cortical patches in the same region of extrastriate cortex generated grating-sensitive N180s and object-specific or letter-string-specific N200s, suggesting that the human ventral object recognition system is segregated into functionally discrete regions.

Adolescent↗
PubMedDate not supplied

Electrophysiological studies of human face perception. II: Response properties of face-specific potentials generated in occipitotemporal cortex.

In the previous paper the locations and basic response properties of N200 and other face-specific event-related potentials (ERPs) were described. In this paper responsiveness of N200 and related ERPs to the perceptual features of faces and other images was assessed. N200 amplitude did not vary substantially, whether evoked by colored or grayscale faces; normal, blurred or line-drawing faces; or by faces of different sizes. Human hands evoked small N200s at face-specific sites, but evoked hand-specific ERPs at other sites. Cat and dog faces evoked N200s that were 73% as large as to human faces. Hemifield stimulation demonstrated that the right hemisphere is better at processing information about upright faces and transferring it to the left hemisphere, whereas the left hemisphere is better at processing information about inverted faces and transferring it to the right hemisphere. N200 amplitude was largest to full faces and decreased progressively to eyes, face contours, lips and noses viewed in isolation. A region just lateral to face-specific N200 sites was more responsive to internal face parts than to faces, and some sites in ventral occipitotemporal cortex were face-part-specific. Faces with eyes averted or closed evoked larger N200s than those evoked by faces with eyes forward. N200 amplitude and latency were affected by the joint effects of eye and head position in the right but not in the left hemisphere. Full and three-quarter views of faces evoked larger N200s than did profile views. The results are discussed in relation to behavioral studies in humans and single-cell recordings in monkeys.

Animals↗
PubMedDate not supplied

Electrophysiological studies of human face perception. III: Effects of top-down processing on face-specific potentials.

This is the last in a series of papers dealing with intracranial event-related potential (ERP) correlates of face perception. Here we describe the results of manipulations that may exert top-down influences on face recognition and face-specific ERPs, and the effects of cortical stimulation at face-specific sites. Ventral face-specific N200 was not evoked by affective stimuli; showed little or no habituation; was not affected by the familiarity or unfamiliarity of faces; showed no semantic priming; and was not affected by face-name learning or identification. P290 and N700 were affected by semantic priming and by face-name learning and identification. The early fraction of N700 and face-specific P350 exhibited significant habituation. About half of the AP350 sites exhibited semantic priming, whereas the VP350 and LP350 sites did not. Cortical stimulation evoked a transient inability to name familiar faces or evoked face-related hallucinations at two-thirds of face-specific N200 sites. These results are discussed in relation to human behavioral studies and monkey single-cell recordings. Discussion of results of all three papers concludes that: face-specific N200 reflects the operation of a module specialized for the perception of human faces; ventral and lateral occipitotemporal cortex are composed of a complex mosaic of functionally discrete patches of cortex of variable number, size and location; in ventral cortex there is a posterior-to-anterior trend in the location of patches in the order letter-strings, form, hands, objects, faces and face parts; P290 and N700 at face-specific N200 sites, and face-specific P350, are subject to top-down influences.

Arousal↗
PubMedDate not supplied

Computer-assisted coregistration of multislice SPECT and MR brain images by fixed external fiducials.

We have developed and validated in a phantom a method of computer-assisted coregistration using multislice SPECT and MR images. Reusable fiducial markers were fabricated from nylon-based plastic and consist of two parts: a base that remains fixed to the skin with adhesive between scans and a removable, spherical cavity insert that can be filled with contrast agents appropriate for multiple imaging modalities. Markers external and internal to a three-dimensional brain phantom provided a means of quantifying the method's accuracy. A computer algorithm was used to derive transformation matrices for image sets by minimizing the root mean squared deviations obtained for multiple permutations (n = 10) of increasing numbers (range 3-11) of external SPECT/MRI point pairs. As defined by the average +/- SEM mean residual deviations of noncoregistered internal fiducials, the minimal accuracy was 2.4 +/- 0.3 mm (no marker > 3.6 mm) for three coregistration points and did not improve beyond seven to eight fiducials (2.2 +/- 0.1 mm; no marker > 3.2 mm). The method's true accuracy is likely to be better than estimates of minimal accuracy, however, since such measures reflect surmountable random errors in fiducial location. With use of identical MRI (or SPECT) data sets, measures of intraoperator (0.5 +/- 0.1 and 0.5 +/- 0.1 mm, respectively) and interoperator (0.5 +/- 0.3 and 0.8 +/- 0.1 mm) reliability were also obtained, establishing the method as highly reproducible and objective. Preliminary results in a human subject suggest its feasibility for clinical studies.

Brain↗
PubMedDate not supplied

A dental splint for use during peroral endoscopy.

Damage to teeth is a well known complication of peroral endoscopy. A case is described in which potentially fatal complications of such damage occurred. This spurred on the development of a dental splint as an aid in the prevention of such damage. This splint is described, and evidence for the beneficial results of its use is presented.

Aged↗
PubMedDate not supplied

Imaging findings in hippocampal sclerosis: correlation with pathology.

We evaluated the ability of preoperative radiologic imaging to detect hippocampal sclerosis in 31 patients who underwent surgery for intractable epilepsy. Hippocampal sclerosis is commonly associated with surgically treatable temporal lobe epilepsy. It is pathologically described as neuronal cell loss with associated gliosis in the hippocampus. While previous reports have correlated imaging results with clinical or qualitative histologic findings, this study used quantitative pathologic criteria (neuronal cell density) to diagnosis hippocampal sclerosis. We focused our study on the 11 patients with cryptogenic temporal lobe epilepsy. Of these, nine had hippocampal sclerosis by pathologic criteria. MR findings included unilateral hippocampal atrophy, an increased signal in the hippocampus on long TR scans, and atrophy in the adjacent white matter and temporal lobe. Hippocampal atrophy was most frequently seen in the red nucleus plane on coronal scans, corresponding to the body of the hippocampus. We also compared hippocampal size on MR with neuronal density in surgical specimens of the 11 patients with cryptogenic temporal lobe epilepsy. A statistically significant correlation was found between MR size and neuronal density in CA3 and CA4 of the cornu ammonis and the granular cell layer of the hippocampus. Since temporal lobectomy eliminated seizures in seven of nine patients with hippocampal sclerosis, preoperative diagnosis by MR has important therapeutic consequences.

Adolescent↗
PubMedDate not supplied

A comparison of attendance for drug misuse to Dublin accident & emergency departments 1985-1993.

We compared attendance by Drug Misusers to the Accident & Emergency Departments (A&E) in the greater Dublin area during September 1985 and April 1993. The study took the form of a questionnaire administered by the previously trained A&E staff to attenders who were currently (within the past six months): experimenting with drugs, abusing drugs or drug dependent. Seventy three drug misusers were identified in September 1985 and 68 in April 1993. Poly substance misuse was a major feature of the 1993 survey and Benzodiazepines replaced Heroin as the major drug of misuse. There was a significant increase in the misuse of prescribed opiates, in particular morphine sulphate, methadone and DF118. Physical problems associated with drug misuse were more common in 1993 because of the new trend of crushing and injecting the morphine sulphate tablet. Patients continue to inject drugs and are poor attenders at needle exchange programmes. We feel that all doctors should exercise caution in their prescribing habits in view of the alarming increase in the misuse of prescribed medication as indicated by our figures.

Adolescent↗
PubMedDate not supplied

MR findings after depth electrode implantation for medically refractory epilepsy.

PURPOSE: To evaluate using MR imaging chronic changes in the brain parenchyma after electroencephalography depth electrode placement in patients with medically refractory epilepsy. METHODS: A retrospective review of MR scans in 57 patients who underwent stereotactic placement of 210 depth electrodes was performed. Scans were evaluated for evidence of gliosis, hemorrhage, or infection along the probe tracts. RESULTS: Signal abnormalities along the probe tracts were seen in 38 patients (67%). Of the 210 probe tracts evaluated on long-repetition-time images, 85 (41%) were associated with punctate hyperintensity and four (2%) with punctate hypointensity; 120 (57%) showed no MR changes to suggest prior electrode placement. One probe placement was complicated by a significant intraparenchymal hemorrhage. CONCLUSIONS: Depth electrode implantation for electroencephalography monitoring results in imperceptible or minimal chronic changes as detected by MR in almost all patients. The punctate hypersensitivity on long-repetition-time images is though to be caused by gliosis along the probe tracts. These signal changes should not be confused with the seizure focus. Significant hemorrhage or infection is a rare complication of probe placement. Incorporation of MR angiographic data with conventional spin-echo images at the time of stereotactic probe placement may further reduce the low incidence of probe-related hemorrhage.

Adolescent↗