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Biomedical subjects

W J Lovegrove

Publications and source records attributed to W J Lovegrove.

At least 19 recordsLinked to original sources
PubMed2000

Transient deficit hypothesis and dyslexia: examination of whole-parts relationship, retinal sensitivity, and spatial and temporal frequencies.

A defect affecting the transient visual sub-system is believed to be one of the prime factors affecting reading disability. In this study, the transient deficit hypothesis was tested using the global precedence paradigm, examining retinal sensitivity, and comparing of patterns of responses to large versus small stimuli. Participants were three groups of dyslexic, chronologically age-matched, and reading age-matched children. The results revealed that although dyslexic individuals did not show any deficit in processing (a) wholes and parts (Experiment 1); (b) information in peripheral locations of the retina (Experiment 2); and (c) various sizes of the stimulus (Experiment 3); they showed a deficit in temporal processing of visual information. These findings challenge the transient deficit hypothesis in the sense that the transient sub-system has been suggested to be associated with processing of the global level, low spatial frequencies, and peripheral vision; however, they confirm that hypothesis in the sense that this visual sub-system is suggested to be associated with processing of high temporal frequencies. Transient deficit hypothesis and dyslexia: examination of whole-parts relationship, retinal sensitivity, and spatial and temporal frequencies.

Case-Control Studies↗
PubMed1999

Do the global advantage and interference effects covary?

The global precedence hypothesis has been operationally defined as a faster or earlier processing of the global than of the local properties of an image (global advantage) and as interference by processing at the global level with processing at the local level (global interference). Navon (1977) proposed an association between the global advantage and interference effects. Other studies have shown a dissociation between the two effects (e.g., Lamb & Robertson, 1988). It seems that the controversy in previous research resulted from not equalizing the eccentricities of global and local properties. In the present study, the eccentricities of the two levels were equalized by using stimuli with all their elements located along their perimeters. The results of the first experiment demonstrated that although the global level was identified faster than the local level in both the central and the peripheral locations of the visual field (global advantage), the pattern of global interference varied across the visual field. Consistency of global and local levels increased the speed of processing of the local level displayed at the center of the visual field but slowed down the processing of that level at peripheral locations. The results of Experiment 2 demonstrated that it was most likely that the variation in the pattern of global interference was determined by the variable of eccentricity, rather than by the sizes of the global and local levels.

Fixation, Ocular↗
PubMed1993

Visual and language processing deficits are concurrent in dyslexia.

Research has demonstrated that dyslexic subjects have language processing problems. More recent evidence indicates that dyslexic subjects also suffer a low-level visual information processing deficit. Little evidence is available to indicate the extent to which dyslexic subjects simultaneously show a visual and language processing dysfunction. In this study, 35 normal and 35 dyslexic subjects aged from 7.9 to 14 years of age were compared on three reading process variables, a visual processing score, a test of phonological coding and a test of language comprehension, each of which were shown to be related to reading performance. The visual processing score was the slope of the regression line predicting the duration of visible persistence as a function of spatial frequency. The language processing measures were a test of phonological coding of orthographically legal Non-Words and a test of language comprehension, the Token Test. The results showed that the visual processing score was significantly predictive of group membership with 91% of the dyslexic group and only 20% of the normal readers having low scores on this measure. The Non-Word test was found to be a perfect discriminator for dyslexia by indicating that every subject in this group had a major phonological coding deficit. An unexpected finding of the present results was that the Token Test did not discriminate between the groups. The results are interpreted as providing evidence for the concurrence of visual and language deficits in dyslexia.

Adolescent↗
PubMed1992

Visual processing deficits in dyslexia: receptors or neural mechanisms?

In 1989 and 1990 Grosser and Spafford attributed abnormalities in the peripheral vision of dyslexic subjects to an anomalous distribution of rods and cones. We argue that at the light levels used in these experiments cone responses should dominate both central and peripheral vision. A more likely explanation of their findings is that there is a postreceptoral deficit in the transient visual channel. This hypothesis is supported by independent anatomical, physiological, and psychophysical evidence.

Attention↗
PubMed1991

Metacontrast with masks varying in spatial frequency and wavelength.

In a metacontrast masking paradigm, adult subjects were required to detect briefly presented target lines followed at various delays by a flanking mask varying in spatial frequency and wavelength. Detection accuracy, the dependent measure, was recorded as a function of the delay of the mask. The results showed that long wavelength masks produced maximum masking at a relatively short delay, while short wavelength masks produced maximum masking at a relatively long delay. Results are discussed within the framework of transient/sustained (magnocellular/parvocellular) theory of visual processing, and suggest that low spatial frequency channels respond with shorter latency and/or faster rise time to short wavelength stimuli, and high spatial frequency channels respond with greater sensitivity to long wavelength stimuli.

Adult↗
PubMed1990

Low-frequency filtering and the processing of local-global stimuli.

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.

Adult↗
PubMed1990

Experimental evidence for a transient system deficit in specific reading disability.

This paper summarizes the experimental evidence for a transient system deficit in specific reading disability. Differences have been found between controlled readers and specifically disabled readers in measurements of visible persistence, pattern contrast sensitivity and temporal or flicker contrast sensitivity. This transient system deficit was found in approximately 75% of the specifically reading disabled subjects tested. A further series of experiments failed to demonstrate differences in sustained system function between the two groups. Evidence is also presented which suggests that the visual system deficit precedes the reading disability. Possible mechanisms of the impact of a transient system deficit on reading are discussed.

Afterimage↗
PubMed1988

Uniform-field flicker masking in control and specifically-disabled readers.

Possible transient-system deficiencies in subjects with specific reading disabilities (SRDs) were investigated in groups of 13-year-old SRDs and control normal readers. In experiment 1, in which a 6 Hz uniform-field flicker (UFF) mask and a stationary test stimulus were used, it was found that the overall effect of UFF masking was to reduce differences in contrast sensitivity between SRDs and normal readers. In experiments 2a and 2b, with UFF masks of 6 and 20 Hz and a 6 Hz moving (experiment 2a) or flickering (experiment 2b) test stimulus, contrast sensitivity in both groups was decreased in the presence of the 6 Hz UFF mask. Only the control group, however, showed a further decrease in sensitivity with the 20 Hz UFF mask. This indicates that the groups differ in terms of a mechanism sensitive to high temporal frequencies. A 20 Hz counterphase flickering test stimulus was used in experiment 3 in the presence of 6 Hz UFF, and it was found that SRDs are less sensitive than controls to 20 Hz flicker across all spatial frequencies used. The 6 Hz mask, however, did not differentially affect the two groups. These findings provide further evidence for a transient-system deficit in the visual systems of SRDs, but also suggest a more complex situation by showing that the two groups differ in a high-temporal-frequency mechanism.

Child↗
PubMed1988

The effects of spatial frequency and temporal waveform on three measures of temporal processing.

The effects of spatial frequency and temporal transition of sine-wave grating onset and offset were assessed using measures of reaction time, visual persistence, and temporal order judgements. The stimuli were lateralized fields, separated by 1 degree of visual angle. Slow temporal transition resulted in significantly poorer performance than did abrupt onset and offset, but spatial frequency had a minimal effect. Thus, the latency, temporal resolution, and temporal ordering of events are mediated by a mechanism that is sensitive to abrupt temporal transients. The stimulus conditions employed did not result in a shift in the point of subjective simultaneity.

Attention↗
PubMed1987

The effects of grating complexity on transient evoked potentials.

In the present investigation, the effects of spatial frequency (1.0, 3.0 and 9.0 c/deg), grating complexity (1 + 3 or 3 + 9), and relative phase relationships (peaks-add or peaks-subtract) on the amplitude and latency of VEP components were examined with a pattern appearance (50 msec, 1.9 Hz) technique. The effects of temporally modulating one spatial component of a complex grating while the other component was continuously viewed were also investigated. Two components of the response obtained with pattern appearance were differentially affected by these stimulus manipulations. In general, most conditions that involved spatial complexity resulted in a loss of information that could be obtained when simple sine-waves were used.

Adult↗
PubMed1985

Spatial-frequency-dependent visible persistence and specific reading disability.

Visible persistence duration for sine-wave gratings was measured in 9-year-old normal and specific-reading-disabled children. Experiment 1 investigated the influence of stimulus duration on visible persistence. The results demonstrated a Reading Group X Spatial Frequency interaction with disabled readers showing a smaller increase in persistence duration with increasing spatial frequency than controls. This interaction was greatest with stimulus durations longer than 80 msec. In Experiments 2a and 2b persistence was measured across a range of contrasts from .1 to .5. The stimulus durations employed were 100 msec in Experiment 2a and 300 msec in Experiment 2b. In both experiments, increasing contrast decreased persistence duration at 2 and 12 cycles per degree (c/deg) for the control group. In the specific-reading-disabled group, however, contrast had little effect on the persistence of 2-c/deg gratings in either experiment. In Experiment 2a the persistence of the 12-c/deg grating decreased with increasing contrast for both groups. In Experiment 2b contrast had significantly less effect on persistence duration in the specific-reading-disabled group, however, contrast had little effect on the persistence of 2-c/deg ratings in either experiment. In Experiment 2b contrast had significantly less effect on persistence duration in the specific-reading-disabled group than in the control group at 12 c/deg. Consequently, contrast had less effect on persistence in specific-reading-disabled children than in normal readers, especially at durations longer than the "critical duration" for each spatial frequency. Experiment 3 extended this finding to gratings with spatial frequencies of 4 and 8 c/deg. These results indicate a difference between normal and specific-reading-disabled children in cortical visible persistence. Two scores of visual processing were derived from the above experiments. On these scores the reading-disabled children were divided into Visual Disabled Readers (approximately 70%--eight subjects) and Nonvisual Disabled Readers (approximately 30%--four subjects). The percentages of disabled readers in each category remained constant when the sample size was increased to 61 normal and disabled readers.

Attention↗
PubMed1984

Visible persistence as a function of spatial frequency, number of cycles and retinal area.

Using a variety of measures it has been shown that processing time increases with increasing spatial frequency. Long and Sakitt (1981) [Vision Res. 21, 1387-1393] investigated duration of visible persistence as a function of both spatial frequency and number of cycles present. They concluded that number of cycles and not spatial frequency is the crucial variable in determining duration of visible persistence. The present paper investigates this issue in three experiments. Experiments 1 and 2 determined duration of visible persistence with spatial frequencies of 2, 4, 8 and 10 c/deg while holding both number of cycles and grating area constant in a dark surround and in a light surround. Stimulus durations of 50 and 300 msec were used in Experiments 1 and 2 respectively. The results at each stimulus duration showed an increase in duration of visible persistence with increasing spatial frequency similar to that found in most previous reports. This increase was less with a 300 than with a 50 msec stimulus duration. Whether the gratings were presented in a light or a dark surround had no significant effect. Experiment 3 showed that at 2 c/deg duration of visible persistence increased with increasing size of the grating stimuli when all stimulus sizes fell within the area of spatial summation. This effect was greater in a dark than in light surround. It is concluded that visible persistence does increase with spatial frequency. Previous results inconsistent with this conclusion are explained in terms of spatial summation.

Afterimage↗
PubMed1984

Visible persistence as a function of viewing condition and eye-handedness relationship.

Duration of visible persistence was investigated as a function of spatial frequency, orientation and viewing condition (binocular, dominant eye and non-dominant eye). Persistence increased with increasing spatial frequency and was longer for oblique than vertical gratings. Viewing condition also influenced visible persistence such that binocular persistence was significantly shorter than for either monocular condition. A post-hoc analysis indicated that this was only true with subjects who were right-handed and right-eyed. Subjects mixed on this relationship showed no differences in persistence across viewing conditions. A second experiment which selected subjects on the basis of this relationship confirmed the results of Experiment 1. The results for the right-right group broaden the generality of the rule showing an inverse relationship between response strength and persistence duration. The results for the Mixed group indicate a lack of binocular summation.

Dominance, Cerebral↗
PubMed1980

Specific reading disability: differences in contrast sensitivity as a function of spatial frequency.

Contrast thresholds for sine-wave gratings of spatial frequencies of 2, 4, 12, and 16 cycles per degree were determined for normal and disabled readers at a range of stimulus durations. Normal readers demonstrated monotonically decreasing sensitivity with increasing spatial frequency at exposure durations between 40 and 100 milliseconds. At exposure durations of 150 to 1000 milliseconds, they showed peak sensitivity at 4 cycles per degree. In comparison, disabled readers showed monotonically decreasing sensitivity with increasing spatial frequency at all stimulus durations. The difference in sensitivity pattern across spatial frequencies was greatest at stimulus durations approximately equal to fixation durations during reading.

Afterimage↗