***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: A pilot study suggests that playing video games may enhance a range of spatial vision functions in adults with amblyopia We describe several experiments on  contour interactions and crowding effects at the resolution limit of the visual system . As test stimuli we used characters that are often employed in optometric practice for testing visual acuity : L and olt C 's , Snellen E 's , and rectangular gratings . We tested several hypotheses that have been put forward to explain  contour interaction  and  crowding effects . In Experiment 1 and Experiment 2 , L and olt C 's were the test stimuli , and bars , or L and olt C 's , or gratings served as distractors . In Experiment 1 , we showed that neither scale invariance nor spatial frequency selectivity is a characteristic of foveal crowding effects . These results allowed us to conclude that mechanisms other than lateral masking contribute to observers ' performance in ' crowded ' tasks . R. F. Hess , S. C. Dakin , and N. Kappor ( 2000 ) suggested that the spatial frequency b and most appropriate for target recognition is shifted by the surrounding bars to higher spatial frequencies that can not be resolved by observers . Our Experiment 2 rejects this hypothesis as the experimental data do not follow theoretical predictions . In Experiment 3 , we employed Snellen E 's , both as test stimuli and as distractors . The masking functions were similar to those measured in Experiment 1 when the test L and olt C was surrounded by L and olt C 's . In Experiment 4 , we extended the range of test stimuli to rectangular gratings ; same-frequency or high-frequency gratings were distractors . In this case , if the distracting gratings had r and om orientation from trial to trial , the critical spacing was twice larger than in the first three experiments . If the orientation of the distractors was fixed during the whole experiment , the critical spacing was similar to that measured in the first three experiments . We suggest that the visual system can use different mechanisms for the discrimination of different test stimuli in the presence of particular surround . Different receptive fields with different spatial characteristics can be employed . To explain why crowding effects at the resolution limit of the visual system are not scale invariant , we suggest that a range of stimuli , slightly varying in size , may all be processed by the same neural channel -- the channel with the smallest receptive fields of the visual system Purpose . The aim of this study was to investigate the effect of test design ( crowding ) and age on visual acuity in a sample of young children . Methods . Vision was measured in 103 children aged between 4 and 9 years using five different visual acuity tests . The tests included three crowded tests : logMAR Crowded test , the Sonksen logMAR test , and the Crowded Kay Picture test , and two single optotype tests : the Single Kay Picture and Sheridan Gardiner tests . Tests were presented in a r and om order using st and ardized instructions and a defined end-point . Results were analyzed in two age groups , younger ( 4 to 6 years ) and older ( 7 to 9 years ) . Results . In both groups , there was a significant main effect of test on acuity ( younger : F = 63.92 , dF = 4 , p < 0.001 ; older : F = 63.59 , dF = 4 , p < 0.001 ) . In the younger group , an effect of crowding was seen in all three crowded tests , but in the older group , an effect of crowding was seen only in the crowded letter tests . In both groups , mean acuity was lowest with the logMAR Crowded test , which has the closest interoptotype spacing ( 0.5 ) , slightly higher with the Sonksen test ( with interoptotype spacing of 1.0 ) and highest with the single optotype tests ( no crowding ) . More crowding was seen in the younger children . Conclusions . Our results show that maturation of line acuity is still taking place between the ages of 4 and 9 years . Measured acuity is affected by the amount of contour interaction induced by the type of optotype ( letter or picture ) and by the interoptotype separation . Another factor , probably a maturation of gaze control or selective attention is responsible for the reduction in crowding with age The visual function of 42 children with haemorrhagic and /or ischaemic cerebral lesions acquired before a gestational age of 35 weeks was examined and related to cranial ultrasound in the neonatal period and to MRI and neurodevelopmental status at follow-up . All 37 children with abnormal ultrasound scans and one of the five with normal ultrasound scans showed impairment of one or more aspects of visual function . While impaired acuity was more frequent among infants with MRI evidence of visual pathway damage , this was not an invariable finding . Normal or near-normal visual acuity did not preclude the presence of other functional visual deficits . The authors conclude that preterm cerebral insults may produce a variety of visual difficulties , the pattern and severity of which can not be predicted on imaging . Each child therefore requires individual assessment of multiple aspects of visual function Contour interaction was investigated in 12 preschool children 3 to 4 years of age , and compared to the results obtained from 5 normal adults tested under an identical paradigm .  Observers viewed the display from a distance at which they could identify the location of a gap ( up or down ) in an isolated C correctly on 90 to 95 % of the trials . The isolated C and C 's with bars tangentially located at various positions above and below the test optotype were intermixed r and omly . Percent correct was plotted as a function of the angular subtense of the gap width of the test optotype . A significant decrease in performance was found when the bars were positioned at 0.71 to 1.42 times the angular subtense of the gap for both the preschool children and the adults . The results suggest that preschool children demonstrate contour interaction that is quantitatively similar to adults . Because the spacing of letters on st and ard acuity charts is typically larger than the range over which contour interaction occurs , the poorer acuity often measured with charts compared to isolated letter presentation in preschool children suggests that factors other than contour interaction ( perhaps attentional factors ) are involved 
    Output:
    
Results Children with normal vision show an extent of contour interaction over an area 1.5–3 × as large as that seen in adults NV .
The magnitude of contour interaction normally ranges between 1–2 lines on an acuity chart and this magnitude is even larger when stimuli are arranged in a circular configuration .
Adults with congenital nystagmus ( CN ) show interaction areas that are 2 × larger than those seen adults with NV .
The magnitude of the crowding effect is also 2 × as large in individuals with CN as in individuals with NV .
This is the first systematic review to compare crowding ratios and it shows that charts with 50 % interoptotype spacing were most sensitive to capture crowding effects .
The groups that showed the largest crowding effects were individuals with CN , VI adults with central scotomas and children with CVI .
Perceptual Learning seems to be a promising technique to reduce excessive foveal crowding effects