An exploration of transient deficit hypothesis in Specific Reading Disability

"The focus of this study is to investigate the transient deficit hypothesis in relation to children who experience Specific Reading Disability (SRD)." -- abstract. The focus of this study is to investigate the transient deficit hypothesis in relation to children who experience Specific Reading Disability (SRD). Specific Reading Disability is defined as a child of normal intelligence (or above) who has no behavioural or emotional problems but their reading age is two or more years behind their age group. The transient deficit hypothesis is a major approach in vision reading research used to account for the differences found between normal readers and children with SRD. This approach proposes that two pathways are involved in visual processing: the transient pathway is suggested to be sensitive to global features, movement, peripheral information and low spatial and high temporal frequencies; and the sustained pathway is proposed to process central features, stationary images, colour and high spatial and low temporal frequencies (Lovegrove, 1993). Transient deficit hypothesis suggests that a weak transient channel can adversely affect the two systems combining properly during reading. A sluggish transient channel may cause a superimposition of letters, causing the SRD child to see letters that appear to overlap (Lovegrove, 1993). This study investigated differences in visual processing between three groups (Chronological age-matched, SRD, and Reading age-matched) of 18 children. The transient deficit hypothesis was examined in the first experiment by using the global precedence paradigm. In Experiments 2 and 3, the sensitivity of the retina and the effect of variation of the size of stimulus were explored.. Secondary to these experiments is the fourth experiment where the influence of incongruent processing on the visual processing of SRD children was explored. The aim of Experiment 1 was to compare the performance of SRD to the performance of normal readers in processing whole and parts of a compound stimulus. Following the transient deficit hypothesis, SRD children should have shown difficulty in processing the global stimulus in comparison to the local aspect of a stimulus. The results of Experiment 1 showed that the global level was detected faster than the local level by all three experimental groups. The reaction times (RT) of SRDwere significantly slower (77 milliseconds, p <0.05) than the Chronological age-matched group, and the Reading age-matched group's times were significantly slower (96 milliseconds, p <0.05) than those of the SRD. Under the transient deficit hypothesis it could be expected that if SRD children have a weak transient channel this may lead to a deficit in processing peripheral information. Experiment 2 found that for all three groups, as stimuli were presented further from the fovea, the RT patterns best fitted with an increasing linear function. The Chronological age-matched group RT was faster than the SRD group (187 milliseconds, p <0.01), and SRD group RT was faster than the Reading age-matched group (31 milliseconds, p <0.10). The purpose of Experiment 3 was to ascertain whether SRD children would have greater difficulty in processing larger stimuli as compared to smaller stimuli. By following the spatial frequency theory it is suggested that low spatial frequency could be associated with larger stimuli, and this may lead to a slower performance by SRD. The results of Experiment 3, indicate that all three groups processed larger stimuli more slowly than they did smaller stimuli. Results for all three groups formed decreasing logarithmetic functions. SRD were significantly behind the Chronological age-matched (70 milliseconds, p <0.05), and significantly in front of the Reading age-matched group (140 milliseconds, p <0.01). From Experiment 4, it appears that conflicting information between the local and global levels, results in the global having an inhibitory influence on responding to the local level. Similar to Experiment 1, the pattern of results to global and local levels formed quadratic functions. The consistent stimuli were detected faster than the inconsistent stimuli in all three groups. In other words, SRD were not significantly different from the other two reading groups in response to inconsistent and consistent stimuli. The inconsistency of stimuli did not have a detrimental effect on their performance. Results from the four experiments show that SRD children do not have any difficulties in processing wholes in comparison to parts, or problems in processing peripheral visual information in comparison to central, and no deficits in processing low spatial frequencies in comparison to high. Additionally in regard to incongruent information, SRD children did not show any significant differences from normal readers. However, the SRD children were significantly slower in the processing of any type of visual information in comparison to the Chronological age-matched children. In conclusion, the results show that a low-level transient deficit did not lead to difficulties for SRD children in processing global information, stimuli at peripheral locations, and large and inconsistent stimuli. The findings do not support the transient deficit hypothesis in the sense that the transient sub-system is suggested to be associated with global level processing, low spatial frequencies, peripheral vision and incongruent information. However, the results of this study confirmed the transient deficit hypothesis in a sense that the transient sub-system is suggested to be associated with high temporal frequency. In this study, the slower pattern of RT for the SRD group could be related a deficit in visual processing of SRD individuals, or the difference in average IQ between the SRD group and the Chronological age-matched group.