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Biomedical subjects

M R Harter

Publications and source records attributed to M R Harter.

At least 19 recordsLinked to original sources
PubMed1995

Event related potentials in adults diagnosed as reading disabled in childhood.

The purpose of this study was to identify electrophysiological correlates of reading disability (RD) in adults with psychometrically documented childhood reading histories. Specific a-priori hypotheses for these correlates were generated from the findings of Harter, Anllo-Vento, Wood & Schroeder (1988a); Harter, Diering & Wood (1988b). The subjects were 32 males with normal intelligence and no history of attention deficit disorder or current major psychopathology. Event related potentials were recorded over O1, O2, C3', C4', F3, and F4 to letter stimuli using an intralocation selective attention paradigm. Subjects with RD showed a general reduction in positivity starting at 150 ms and continuing up to 500 ms. The reduced positivity at left central P240 replicated the findings of Harter et al. (1988b) with children. However, adult RD was associated with more diffuse, bilateral reduction in electrophysiological response to all stimuli. A possible relationship between the bilaterality of the neural deficit and the severity of the dyslexia was proposed: that a bilateral deficit may be involved in severe cases.

Adult↗
PubMed1989

Event-related potentials, spatial orienting, and reading disabilities.

Event-related potentials (ERPs) were used to investigate the effects of visual-spatial orienting on selective neural processing in boys with learning disabilities. Twenty-seven 8-12 year old boys were classified into four groups depending on whether or not they had a reading disability or attention deficit disorder. Event-related potentials were recorded over the left and right occipital, central, and frontal cortical regions. The behavioral task required the subjects 1) to rapidly switch their attention from the center to the periphery of the visual field (spatial component), and 2) to selectively respond to a target versus a nontarget flash when the target was presented in the relevant visual field (nonspatial component). The amplitude of two ERP components was enhanced in response to relevant as compared to irrelevant stimuli. The enhancement of an early negative occipital-central component, which peaked 180-200 ms following targets (N1), indicated that selective neural processing associated with spatial attention could be switched in 600 ms. This enhancement of N1 was greater in boys with than without a reading disability, which implies that reading disability is associated with enhanced spatial attention. The enhancement of a later positive component, which peaked 300-340 ms following targets (P3), suggested that nonspatial target selection was reduced in boys with a reading disability, particularly over the left occipital hemisphere. Target relevance and reading disability also influenced trial-to-trial variability in the ERP waveform. The effects of reading disability on event-related potentials did not vary as a function of attention deficit disorder, indicating that these two disorders are distinct.

Arousal↗
PubMed1988

Separate brain potential characteristics in children with reading disability and attention deficit disorder: relevance-independent effects.

The effects of reading disability (RD) and attention deficit disorder (ADD) on event-related potentials (ERPs) were investigated in 52 boys between 8 and 12 years of age. There were four groups: 25 children without RD divided into two groups (17 without ADD and 8 with ADD) and 27 children with RD divided into two groups (11 without ADD and 16 with ADD). The children participated in three tasks ("games") designed to assess selective attention to the color black (vs white), to any of two letters (vs. two patterns), and to any of 12 letters (vs. 12 patterns). ERPs were recorded over the left and right occipital, central, and frontal hemispheres. Children with RD as compared to those without RD were characterized by the following ERPs: (1) a smaller amplitude positive wave at about 240 msec (P240) over the left central hemisphere: (2) a smaller amplitude positive wave at about 500 msec (P500) over the right central hemisphere: (3) a larger P500 over the right central hemisphere: (3) a larger P500 over both left and right occipital hemispheres: and (4) smaller within-subject-and-condition variability of ERP waveform. These effects did not differ significantly for the three games and, therefore, were not specific to letter processing per se. The effects of RD did not interact with the effects of ADD for the most part. The ERPs clearly indicated these two disorders, in part, involve different underlying brain processes.

Arousal↗
PubMed1988

Separate brain potential characteristics in children with reading disability and attention deficit disorder: color and letter relevance effects.

This experiment was on event-related potential (ERP) indicants of the selective neural processing of black vs. white and letter vs. nonletter stimuli in boys (8-12 years of age) with and without a reading disability (RD) and/or attentional deficit disorder (ADD). Selective neural processing was measured by the increase or difference in ERP amplitude in response to stimuli that were relevant as compared to irrelevant to the color or letter attention task. The 52 children that participated in the study constituted four groups: 25 normal reading children (17 without ADD and 8 with ADD), and 27 RD children (11 without ADD and 16 with ADD). ERPs were recorded over the left and right occipital, central, and frontal regions. Selective neural processing due to stimulus relevance was reduced in boys with RD as compared to normal readers. This reduced selectivity was indicated by a predominantly symmetrical reduction in the magnitude of a positive difference potential over the central regions, between 300 and 360 msec, and then by a left greater than right hemisphere reduction in the magnitude of a positive difference potential over the occipital regions, at about 400 msec. Task relevance increased the within-subject and condition variability of this occipital positive component and this effect was greater for boys without than with RD, particularly over the left hemisphere. Selective neural processing due to stimulus relevance was greater in boys with ADD as compared to those without ADD. This was indicated by an increase in the magnitude of a positive difference potential between 320 and 400 msec over the central and frontal regions and a slow, late, negative difference potential between 600 and 800 msec over the central and occipital regions. These ADD effects tended to be greater over the right than left hemisphere. The unique polarity, scalp distribution, and time course of the effects of RD as compared to ADD on ERPs to relevant stimuli clearly indicated these two disorders, in part, involve different underlying brain deficits.

Arousal↗
PubMed1987

Separate verbal memory and naming deficits in attention deficit disorder and reading disability.

In this study, verbal memory and naming abilities were investigated in reading disabled (RD) and control children who were characterized according to the presence or absence of attention deficit disorder (ADD). Results indicate that deficits in learning and memory for recently acquired information occur as a function of ADD rather than RD while deficits in naming are specific to RD rather than ADD. We conclude that ADD is a major source of additional and separate cognitive morbidity in RD children.

Attention Deficit Disorder with Hyperactivity↗
PubMed1986

Visual event-related potentials to colored patterns and color names: attention to features and dimension.

Four right-handed males and 4 right-handed females were instructed to match pairs of stimuli (colored flashes with either colored patterns or color names) presented sequentially to the central retina. Subjects were to respond to the second stimulus of a pair when it matched the first stimulus in terms of sensory color or word meaning. ERPs recorded from the second stimulus of a pair over occipital and frontal cortical regions indicate the following: Interdimension effects reflect an early and more global discrimination process between colored patterns and word patterns per se. The source of this effect appears to be localized in occipital cortical regions. Intradimension effects were evident later in time and reflect a more refined discrimination process between particular features within a dimension rather than between dimensions. The intradimension color effect began earlier in time than the word effect (229 msec versus 318 msec in the occipital data) and appears to be localized in posterior temporal regions. The onset of the word effect appears to have two neural generators: an early effect localized in frontal regions (274 msec) and a later effect localized in occipital regions (318 msec). The hierarchical model of language processing seems to hold true predominantly in posterior cortical regions. Effects associated with linguistic processing were evident in frontal regions before effects were noted in the occipital regions. This result suggests that either: word information is processed simultaneously and independently in the different regions, or anterior regions feedback onto posterior regions and, therefore, influence the processing in this region.

Adult↗
PubMed1986

Discussion of neural-specificity model of selective attention: a response to Hillyard and Mangun and to Näätänen.

Hillyard and Mangun (this issue) and Näätänen (this issue) have made a number of reflective, important observations in regard to the model we described in our earlier paper (Harter & Aine, 1984). The attention they have given to our paper and model is greatly appreciated and has helped us more clearly conceptualize some of the assertions and observations set forth in our original paper. The scholarly contributions of Hillyard, Näätänen, and their colleagues during the last 20 years, along with those by others related to the neurophysiology and neuroanatomy of the visual system, have led us to propose the somewhat different perspective represented by the neural specificity model of selective attention. Hillyard's and Mangun's and then Näätänen comments will be abstracted and discussed in order.

Attention↗
PubMed1983

Interocular suppression in adults and infants using anaglyphic stimuli: visually evoked potential measures.

Interocular suppression, assessed by monocular VEP amplitude reduction, was investigated in 3 infants, average age of 3 months, and 8 adults. A dichoptic simultaneous masking paradigm was employed, the two eyes being disassociated by red and green filters which transmitted non-overlapping wave lengths of light (anaglyphic method). One eye was continuously presented with either darkness, a diffuse green light, or two green dot patterns, containing either 20' or 80' dots. The other eye was flashed with either red diffuse light, a red 20' dot pattern, or a red 80' dot pattern. In both adults and infants, monocular VEP amplitude was reduced as the size of the continuously presented dots increased (pattern suppression) and a smaller amplitude response occurred when dots of equal size were presented than when data of different size were presented (size-specific suppression). The relative and absolute magnitudes of interocular pattern suppression were similar for adults and infants. However, the relative and absolute magnitudes of size-specific suppression were larger for adults than for infants. Pattern interocular suppression was inferred to be relatively more mature in 3 month olds than size-specific suppression. The relative lack of size-specific suppression suggests a relative immaturity of binocular spatial frequency or size channels.

Adult↗
PubMed1982

Hemispheric differences in the neural processing of stimulus location and type: effects of selective attention on visual evoked potentials.

Hemispheric differences in a negative brain potential associated with selectively attending the location and type of stimulation were investigated. The earlier portion of this negativity (between 125 and 222 msec after stimulation) was associated with attending the location of the stimulus. It was symmetrical in the central scalp regions but was greater in the hemisphere contralateral to the attended visual field in the posterior scalp region. The latter portion of this negativity (from 222 to 272 msec after stimulation) primarily was associated wih attending one of the different types of stimuli presented at a given location and was greater over the left posterior regions of the scalp. These results were interpreted in relationship to the time-course of different types of information processing in the left and right hemisphere.

Adult↗