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Biomedical subjects

Sheila G Crewther

Publications and source records attributed to Sheila G Crewther.

7 recordsLinked to original sources
PubMed2003

Change detection is impaired in children with dyslexia.

The severe deficits in rapid automatized naming demonstrated by children with developmental dyslexia has usually been interpreted in terms of a deficit in speed of access to the lexicon rather than as a possible deficit in speed of visual object recognition. Yet fluent reading requires rapid visual recognition and semantic interpretation of new letters and words appearing in successive fixations of the eyes. Thus we wondered whether change detection performance was related to reading ability. We investigated whether children with developmental dyslexia (DD) were less able to detect change in a simple display--gap--display paradigm than normal reading (NR) children of the same age and children with impaired reading and mentation (LD). In a first experimental phase, the DDs required a longer initial exposure of four letter items in order to detect change of a single letter at a level of 71% correct, compared with NRs performing at the same level. Thus the deficit in reading in DD is associated with a deficit in early processes associated with visual recognition. In a second experimental phase (using the individual target display exposures measured in the first phase), cues appeared during the 250 ms gap for a period of either 0 (no cue), 50 or 200 ms immediately prior to the presentation of the second (comparison) display. Children of all groups showed dependence on the presence of the cue to help make a judgement of change (versus no change), with the NRs least affected. When change was detected in the presence of a cue, the NRs were better able to identify the new letter than either of the other groups. However, only about 50% of the correct detections were accompanied by a correct identification. Despite published reports of a mini-neglect for left visual field in dyslexic adults, none of our groups showed such an effect. However, a significant upper visual field (UpVF) advantage in change detection performance was found across groups, which we interpret in terms of the interactions of the ventral and dorsal streams.

Adolescent↗
PubMed2002

Temporal processing of global and local information varies with global precedence.

The concept of global precedence, which suggests that the global aspect of a scene is processed more rapidly than local details, was examined using the attentional blink paradigm. Eighteen adult subjects observed multiple sequences of complex global-local letter figures to see whether the attentional blink duration would be affected by the visual angle size of the stimulus. Within each sequence, the subject was directed to identify either a global or local red target letter and to detect whether a global or local probe letter (X) was presented in the sequence following the target letter. Stimuli were presented at three different sizes. Results showed significantly higher probe detection rates for global probes than for local at small stimulus sizes. However, using large stimulus sizes, mean correct probe detection was significantly higher in conditions requiring local attention compared to global. No significant difference in probe detection performance was observed between global and local conditions at medium stimulus sizes. The results suggest that the rate of visual information processing varies according to the visual angle of the particular information. The results support the suggestion that the precedence of information is an important factor in the temporal processing of global-local information.

Adolescent↗
PubMed2002

Normal readers have an upper visual field advantage in change detection.

The ability to detect change has not been well studied in children.A research paradigm was used involving search for a change made to one of four letter targets, presented twice, with a gap of 250 ms.Sixty-one schoolchildren, aged 7.3 - 12.9 years,participated in the study. The duration of the first presentation was such that each child was detecting change at a rate of 71% correct. The mean duration was 0.91 s (SD = 0.58 s),with older children not significantly faster in detecting change. In the second phase, using the duration established in the first phase, the effect of cue condition on detection was studied. An upper visual field advantage for change detection was clearly evident for un-cued trials. The most likely explanation derives from the masking produced by the reappearance of the four targets, which interferes with the process of change detection. The greater dorsal pathway representation of the lower visual field implies greater masking there, yielding better change detection in the upper visual field.

Child↗
PubMed2002

Effects of optical defocus and spatial contrast on anterior chamber depth in chicks.

The goal of this study was to investigate the effect of optical defocus and spatial contrast on refractive development and, in particular,on anterior chamber growth. Ninety chicks were raised from day 4-10 post-hatching wearing monocular lenses (+/-10 Dor 0 D), in an environment with either high, low or no spatial contrast patterns: 30%, 6% or 0% contrast, respectively. At day 10, the chicks' refractive state and ocular components were assessed using retinoscopy and A-scan ultrasonography. Ocular defocus resulted in sign-dependent significant differences in refractive error, axial length and vitreous chamber depth. Lens wear also led to significant spatial contrast dependent changes in anterior chamber depth. Varying ambient spatial contrast in the chick's environment did not inhibit emmetropization processes; however, anterior chamber growth was particularly susceptible to changes in spatial contrast.

Animals↗
PubMed2002

Separation of contour and area dependent components in the first and second order kernels of the multifocal pattern appearance evoked potential.

The non-linearities of the diffuse flash visual evoked potential (VEP) have been shown to generate separable waves corresponding to Magnocellular and Parvocellular contributions. The authors wished to investigate whether, under conditions of isoluminance, the chromatic pattern appearance VEP shows waveforms that are related to both the processing of contours (the pattern) and the processing of the diffuse stimulation (from the area lying between the contours). The VERIS multifocal system was used to record monocular VEP. Standard 2-ring, 3-ring, 5-ring and 7-ring VERIS pattern stimuli were employed with a cardboard circular aperture used to blank the stimulus monitor to a 9-cm diameter field. The results for four subjects were recorded. The first order kernel responses showed a significant increase in amplitude with total contour length. In comparison, the amplitudes of the first and second slices of the second order kernel did not increase with contour length. It appears that the contour-dependent component has negligible non-linear components. It is proposed that the constant, non-zero responses of the higher order kernel components reflects contributions from the diffuse chromatic VEP.

Adult↗
PubMed2001

Is there an association between functional vision and learning to read?

Background: Controversy exists about the role of visual parameters and vision in learning to read. This study aims to determine whether ocular parameters or performance on a dynamic test of visual function differs for children of differing reading ability. Methods: Two hundred and eighty-four children (mean age 9.9 +/- 1.8 years) received a vision screening emphasising binocular anomalies associated with discomfort at near (distance and near visual acuity, distance vision challenged with binocular +1 D lenses, near heterophoria, near point of convergence, stereopsis and accommodative facility). Non-verbal mentation age and reading accuracy were assessed. One hundred and six children performed a computerised task of motion coherence detection. Children were classified as normal readers (n = 195), children with dyslexia (n = 49) or learning disabled children (n = 40) based on their mentation age and their reading age. Results: There were no statistically significant differences or correlations between visual parameters and reading performance. Over thirty per cent of the children had accommodative facilities below or equal to six cycles per minute. Children with learning disabilities performed worst on the motion coherence task but this was statistically significant only when compared to the performance of dyslexics. Discussion: The lack of association between ophthalmic parameters and poor reading ability supports the view of the Committee on Children with Disabilities. However, 39 per cent of the children might be expected to experience difficulty 'reading to learn', as suggested by the American Academy of Optometry, as they showed anomalies associated with visual discomfort with prolonged reading. The motion coherence test did not differentiate dyslexics from normal readers and was worst in children with learning disability. Accommodative facility testing remained the most useful predictor of potential visual discomfort.

Journal Article↗