Clinical vision

Two colour blind simulation tools evaluated using simulcheck

Julio Lillo, Leticia Alvaro, Humberto Moreira and Delia Majarín

Three colour vision alteration simulation tools, belonging to two different types (optical simulation, Variantor goggles. Software simulation, vischeck and coblis) were evaluated using simulcheck. Real and simulated dichromats participated in the evaluation. On the one hand, real dichromats responded to original stimuli and, on the other, normal observers responded to stimuli transformed by the evaluated simulator. It was assumed that the less difference between the results provided, the better a simulator was. The first part of simulcheck (pseudo-achromatic selection task) required real (5 protanopes and 5 deuteranopes) and simulated (10 normal trichromats) dichromats to select the least chromatic stimulus from a colour set where the chromatic angle (H*) was systematically changed (9? between adjacent stimuli) to cover the full colour circle. The second part of simulcheck (lightness matching task) was the application of a spatial version of the AMLA method used to measure the luminance factor (Y/Yn) of the stimuli selected in the first part. There were important differences between the simulation tools accuracy. Vischeck provided the best results (only a minor significant difference appeared when simulated dichromates were compared with real ones). On the other hand Coblis produced important errors for both simulcheck tasks and dichromat types. The most important ones appeared for protanopes and the simulated pseudoachromatic red (luminance-lightness very much over the one perceived by real protanopes). Variantor goggles shows and intermediate accuracy level. The main limitation of Variantor derives of not doing different simulations for protanopes and deuteranopes. The practical implications of these results are discussed within the universal design framework

Number sense in autism

Elizabeth Pellicano, David Aagten-Murphy, Niki Daniel and David Burr

Number skills are often reported anecdotally and in the mass media as a relative strength for individuals with autism. The handful of studies on mathematical achievement nevertheless demonstrates that while some children with autism show talents in this area, a substantial proportion of children struggle with this aspect of the school curriculum. This study therefore sought to examine whether children with autism, like typical children, possess an intuitive feel for numbers – a “number sense” – and whether variation in their mathematical skills can be explained by strengths and weaknesses in this sense. 24 cognitively able children with autism (range = 8 – 12 years) and 24 typical children of similar age and ability were administered a standardized test of mathematical achievement and two experimental estimation tasks, one psychophysical non-symbolic estimation task and one symbolic estimation (number-line) task. Children with autism showed difficulties only on the number-line task, which required matching spatial quantities with a numerical value. Furthermore, individual differences in performance on the number-line task were significantly correlated with autistic children’s academic achievement, such that better number sense skills went hand-in-hand with better arithmetic skills. These findings question the widespread belief that mathematical skills are generally enhanced in autism.

Perception of numerosity, time and attention in preterm children of low birth-weight

Francesca Tinelli, Giovanni Anobile, Monica Gori, David Aagten-Murphy, Giovanni Cioni, David Charles Burr and Maria Concetta Morrone

Attention, perception of time and numerosity are high cognitive functions important for everyday life. Children born long before term show a high incidence of learning disorders, especially in mathematics. In this study we measured performance of school-aged children born pre-term (gestational age less than 32 weeks, N=20, no apparent brain lesions) and age-matched controls on several tasks: 1) numerosity discrimination; 2) visual, auditory and audio-visual time bisection; 3) mapping number onto space; 4) sustained visual attention (multiple object tracking); 5) contrast sensitivity for stationary and moving gratings. The preterm group showed significant impairments in all tasks except contrast sensitivity, and the degree of deficit correlated strongly with gestational age and with birth weight. The results suggest that the deficits may be specific to high-level visual functions probably mediated by parietal cortex.

Functional modulation of object-selective areas in stoke patients with ventral cortical lesions

Maren Prass, Cathleen Grimsen, Freimuth Brunner, Andreas Kastrup and Manfred Fahle

Specific cortical areas, such as lateral occipital complex (LOC), fusiform face area (FFA) and parahippocampal place area (PPA), are engaged in processing of complex visual scenes. We used functional magnetic resonance imaging (fMRI) to investigate how these areas are functionally modulated in stroke patients with unilateral ventral cortical lesions. Patients performed an object categorization task, with images presented to the left or right of a fixation point. LOC, FFA and PPA were defined in a separate mapping session. BOLD responses were normal in LOC, while activity in FFA was down-regulated bilaterally in patients compared to healthy controls, suggesting impairments of both, the damaged and the intact hemisphere. Usually, activity is higher for contralaterally than for ipsilaterally presented stimuli (contralateral bias). Both, FFA and PPA of the lesioned hemisphere lacked the contralateral bias, indicating disturbed object processing in the lesioned hemisphere. These results correspond to our behavioral data, since patients made more errors and reacted slower when stimuli were presented in the contralesional visual hemifield. In summary, the functional modulation of the ventral object network is disturbed after focal brain lesions. This is reflected by impaired performance and altered activity pattern in cortical object areas, suggesting incomplete reorganization processes.

Retinotopic maps in human visual cortex following hemispherectomy at age 3

Frans Cornelissen and Koen Haak

Human visual cortex contains maps of the visual field. Much research has been dedicated to answering whether and when these visual field maps change when critical components of the visual circuitry are damaged. Here, we present the case of a sixteen-year-old patient who lost the entire left cerebral hemisphere at age of three. Using a functional magnetic resonance imaging (fMRI) retinotopic mapping procedure and population receptive field modeling, we found that 1) despite the long period since the hemispherectomy, the retinotopic organization of early visual cortex remained unaffected by the removal of an entire cerebral hemisphere, and 2) the intact lateral occipital cortex contained an exceptionally large representation of the center of the visual field. The same method also indicates that the neuronal receptive fields in these lateral occipital regions are extraordinarily small. While these features are clearly abnormal, we argue that they do not necessarily stem from cortical reorganization: they can also be explained by the notion that the hemispherectomy took place during a critical period and arrested its normal development. Studying visual field maps under atypical conditions such as presented here, may therefore be a very fruitful way to understanding normal development.

Consequences of chiasmatic abnormalities on the organisation of the ventral processing stream

Falko R. Kaule, Barbara Wolynski, Anil Kumar, Irene Gottlob, Jörg Stadler, Oliver Speck, Martin Kanowski, Synke Meltendorf and Michael B. Hoffmann

Purpose: Due to an abnormal projection of the temporal retina to the contralateral and ipsilateral hemisphere in albinism and achiasma, respectively, the visual cortex receives additional input from the ipsilateral visual hemifield. Consequences of these misprojections on visual function are surprisingly small. Methods: We scanned 5 albinotic, 1 achiasmic, and 5 human control subjects with optimised retinotopic mapping procedures at 7 or 3 Tesla to detail the organization of early and higher visual cortex (voxel size: 2.53 mm3; TR: 2.4 s; volumes per scan: 105). Results: The ipsilateral visual field was represented as a retinotopic map, in albinism contralateral and in achiasma ipsilateral to the stimulated eye. This abnormal representation was mirror-symmetrically superimposed onto the normal representation of the contralateral visual field, not only in early, but also higher visual areas of the ventral processing stream (V4, VO1/2, PHC1/2). Conclusion: The observed organisation of large parts of the visual cortex highlights conservative geniculo-striate and cortico-cortical projection patterns, which result in extensive abnormal cortical visual field representations. We conclude that, in both albinism and achiasmia, these are made available for visual perception by subtle changes of the intracortical architecture. Supported by DFG (HO 2002/10-1), and the federal state of Saxony-Anhalt (CBBS)