Perceptual learning improves visual functions in patients with albinistic bilateral amblyopia: A pilot study.

BACKGROUND: Several visual functions are impaired in patients with oculocutaneous albinism (OCA) associated to albinistic bilateral amblyopia (ABA). OBJECTIVE: In this study, we aimed at exploring whether perceptual learning (PL) can improve visual functions in albinism. METHOD: Six patients and six normal sighted controls, were trained in a contrast detection task with lateral masking. Participants were asked to choose which of the two intervals contained a foveally presented low-contrast Gabor patch. Targets were presented between higher contrast collinear flankers with equal spatial frequency. When increasing target-to-flanker distance, lateral interactions effect normally switches from inhibition to facilitation, up to no effect. RESULTS: Our findings showed that before PL, only controls showed facilitation. After PL, results suggest that facilitatory lateral interactions are found both in controls as well as in albino patients. These results suggest that PL could induce higher processing efficiency at early cortical level. Moreover, PL positive effect seems to transfer to higher-level visual functions, but results were not very consistent among tasks (visual acuity, contrast sensitivity function, hyperacuity and foveal crowding). CONCLUSIONS: Although a small sample size was tested, our findings suggest a rehabilitative potential of PL in improving visual functions in albinism.

Dyslexia and the magnocellular-parvocellular coactivaton hypothesis.

Previous studies showed that the lateral masking of a fast-moving low spatial frequency (SF) target was strong when exerted by static flankers of lower or equal to the target SF and absent when flankers' SF was higher than the target's one. These masking and unmasking effects have been interpreted as due to Magnocellular-Magnocellular (M-M) inhibition and Parvocellular-on-Magnocellular (P-M) disinhibitory coactivation, respectively. Based on the hypothesis that the balance between the two systems is perturbed in Developmental Dyslexia (DD), we asked whether dyslexic children (DDs) behaved differently than Typically Developing children (TDs) in conditions of lateral masking. DDs and TDs performed a motion discrimination task, of a .5c/deg Gabor target moving at 16 deg/sec, either isolated or flanked by static Gabors with a SF of .125, .5 or 2 c/deg (Experiment 1). As a control, they also performed a contrast detection task of a static target, either isolated or flanked (Experiment 2). DDs did not perform any different from TDs with either a static target or an isolated moving target of low spatial frequency, thus suggesting efficient feedforward Magnocellular (M) and Parvocellular (P) processing. Also, DDs showed similar contrast thresholds to TDs in the M-M inhibition condition. Conversely, DDs did not recover from lateral masking in the M-P coactivation condition. In addition, their performance in this condition negatively correlated with non-words accuracy, supporting the suggestion that an inefficient Magno-Parvo coactivation may possibly be associated to both higher visual suppression and reduced perceptual stability during reading.

Testing the visual field of children and adults with Rarebit: The role of task repetition on sensitivity.

Rarebit is a simple and user-friendly perimetry that tests the visual field by using tiny supra-threshold dot stimuli. It appears to be especially useful for examining the visual field of children who are under 12 years of age. However, previous data showed that the number of errors was higher in children than adults. We ask whether the different number of errors in these two groups depended on task learning and whether it may be accounted for by sensitivity differences or a response bias. Thirty-one children between 9 and 12 years of age and thirty-nine adults were tested three times with Rarebit perimetry. A bias-free sensitivity index, d', rather than the simple hit rate, revealed a group difference that remained after extensive task repetition. Indeed, d' increased with task learning in a similar way in the two groups so that group difference remained after practice. The response bias differed in the two groups, being conservative in the older group (criterion C >0) and liberal in the younger (criterion C < 0). Both biases disappeared with task learning in the third session, suggesting that response bias cannot account for the group difference in sensitivity after practice. When bias-free measures of sensitivity are used and task learning effects are minimized, Rarebit perimetry may be a more valuable method than simple mean hit rate (MHR) to enlighten sensitivity differences in the visual field assessment within the pediatric population.

Developmental dyslexia: A deficit in magnocellular-parvocellular co-activation, not simply in pure magnocellular activation.

The magnocellular deficit theory of dyslexia suggests a selective impairment in contrast detection of stimuli involving pure magnocellular response (e.g. Gabor patches of 0.5 c/deg, 30 Hz, low contrast). An alternative hypothesis is that, dyslexia may be associated with a reduction of typical facilitation that normal readers present for stimuli relying on low-level magno-parvo co-activation, relative to stimuli eliciting pure magno activation. According to this hypothesis, any advantage in contrast sensitivity, produced by either decreasing stimuli temporal frequency (from 30 to 10 Hz, Experiment 1) or using static stimuli of increasing spatial frequency (from 0.5 to 4 c/deg, Experiment 2), would be ascribed to the coexisting responses of the magnocellular and parvocellular systems. In the control group, this advantage in contrast sensitivity was found for a 0.5 c/deg Gabor (either static or flickering at 10 Hz) and for a static Gabor of 4 c/deg. In contrast to magnocellular deficit theory predictions, dyslexic individuals showed no deficit in the unmixed magnocellular response. However, they showed no advantage when the relative weight between magnocellular and parvocellular inputs was thrown off balance in favor of the latter. These results suggest that in order to interpret low-level visual deficits in dyslexia, it is worth considering that fast, feedforward low-frequency representations of spatial structures may result from the coexisting responses of two systems. Our results suggest that in dyslexia, the relative contribution of these two systems in visual processing is perturbed, and that this may have detrimental consequences in word processing, both within the parafovea and the fovea during fixation.