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.

Neural Restoration Training improves visual functions and expands visual field of patients with homonymous visual field defects.

BACKGROUND: In recent years, the introduction of visual rehabilitation for patients with homonymous visual field defects has been met with both enthusiasm and caution. Despite the evidence that restitutive training results in expansion of the visual field, several concerns have been raised. OBJECTIVE: We tested the effectiveness of a new rehabilitative protocol called "Neuro Restoration Training" (NRT) in reducing visual field defects and in restituting visual functions in the restored hemianopic area. METHODS: Ten patients with homonymous visual field defects (lesion age >6 months) where trained in detecting low contrast Gabor patches randomly presented in the blind field, which refers to regions of 0 dB sensitivity, and along the hemianopic boundary between absolute (0 dB) and partial blindness (>0 dB). Training included static, drifting, and flickering Gabors in different blocks. Positions along the hemianopic boundary were systematically shifted toward the blind field according to the threshold reduction during the training. Before and after the training, we assessed visual field expansion and improvement in different high-level transfer tasks (i.e., letter identification and shape recognition) performed in the hemianopic boundary and in the blind field. RESULTS: NRT led to significant visual field enlargement (≈5 deg), as indicated by the conventional Humphrey perimetry, and two custom made evaluations of visual field expansion with eye movement control (one static and one dynamic). The restored area acquired new visual functions such as small letter recognition and perception of moving shapes. Finally, for some patients, NRT also improved detection, either aware or not, of high contrast flickering grating and recognition of geometrical shapes entirely presented within the blind field. CONCLUSION: These results suggest that NRT may lead to visual field enlargement and translate into untrained visual functions.

Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia.

BACKGROUND: Amblyopic observers present abnormal spatial interactions between a low-contrast sinusoidal target and high-contrast collinear flankers. It has been demonstrated that perceptual learning (PL) can modulate these low-level lateral interactions, resulting in improved visual acuity and contrast sensitivity. OBJECTIVE: We measured the extent and duration of generalization effects to various spatial tasks (i.e., visual acuity, Vernier acuity, and foveal crowding) through PL on the target's contrast detection. METHODS: Amblyopic observers were trained on a contrast-detection task for a central target (i.e., a Gabor patch) flanked above and below by two high-contrast Gabor patches. The pre- and post-learning tasks included lateral interactions at different target-to-flankers separations (i.e., 2, 3, 4, 8λ) and included a range of spatial frequencies and stimulus durations as well as visual acuity, Vernier acuity, contrast-sensitivity function, and foveal crowding. RESULTS: The results showed that perceptual training reduced the target's contrast-detection thresholds more for the longest target-to-flanker separation (i.e., 8λ). We also found generalization of PL to different stimuli and tasks: contrast sensitivity for both trained and untrained spatial frequencies, visual acuity for Sloan letters, and foveal crowding, and partially for Vernier acuity. Follow-ups after 5-7 months showed not only complete maintenance of PL effects on visual acuity and contrast sensitivity function but also further improvement in these tasks. CONCLUSION: These results suggest that PL improves facilitatory lateral interactions in amblyopic observers, which usually extend over larger separations than in typical foveal vision. The improvement in these basic visual spatial operations leads to a more efficient capability of performing spatial tasks involving high levels of visual processing, possibly due to the refinement of bottom-up and top-down networks of visual areas.

The Effects of Aging on Orientation Discrimination.

Visual perception relies on low-level encoding of local orientation. Recent studies show an age-dependent impairment in orientation discrimination of stimuli embedded in external noise, suggesting that encoding of orientation is inefficient in older adults. In the present study we ask whether aging also reduces decoding, i.e., selecting the neural representations of target orientation while discarding those conflicting with it. We compared younger and older participants capability (mean age 24 and 68 years respectively) in discriminating whether the orientation of a Gabor target was left or right from the vertical. We measured (d'), an index of discrimination sensitivity, for orientation offset ranging from 1° to 12°. In the isolated target condition, d' was reduced by aging and, in the older group, did not increase with orientation offset, thus resulting in a larger group difference at large than small orientation offsets from the vertical. Moreover, oriented elements in the background impaired more discrimination in the older group. However, distractors reduced more d' when target-background orientation offset was large than when target and flanker had similar orientation, indicating that the effect of the background was not local, i.e., due to target inhibition by similarly oriented flankers. Altogether, these results indicate that aging reduces the efficiency in discarding the response to orientations differing from the target. Our results suggest that neural decision-making mechanisms, involving not only signal enhancement but also non-signal inhibition, become inefficient with age. This suggestion is consistent with the neurophysiological evidence of inefficient visual cortical inhibition in aging.

Effects of aging on interference control in selective attention and working memory.

Working memory decay in advanced age has been attributed to a concurrent decrease in the ability to control interference. The present study contrasted a form of interference control in selective attention that acts upon the perception of external stimuli (access) with another form that operates on internal representations in working memory (deletion), in order to determine both of their effects on working memory efficiency in younger and older adults. Additionally, we compared memory performance under these access and deletion functions to performance in their respective control conditions. The results indicated that memory accuracy improved in both age groups from the access functions, but that only young adults benefited from the deletion functions. In addition, intrusion effects in the deletion condition were larger in older than in younger adults. The ability to control the irrelevant perception- and memory-elicited interference did not decline in general with advancing age; rather, the control mechanisms that operate on internal memory representations declined specifically.

Effects of aging on visual contour integration and segmentation.

PURPOSE: Perception of circular disconnected contours requires the integration of relevant local orientation information across space and the suppression of irrelevant orientations. Using a detection of deviation from circularity (DFC) task, the present study examined whether the efficiency of either integrative or suppressive visual mechanisms, or both, declines with age. METHODS: Younger and older observers' sensitivities in detecting the DFC of a contour formed by Gabors were compared in three conditions: when all elements were oriented tangentially to the contour, with and without the presence of randomly oriented background noise; and when they had alternated tangential and orthogonal orientations, without background noise. RESULTS: In agreement with previous studies, the authors found that younger observers were not impaired in the mixed condition with respect to the tangential condition, suggesting the involvement of a high-level mechanism responding to the global closure information provided by tangential local orientations, even if they are interspersed with orthogonal ones. Instead, older observers were specifically impaired in the mixed condition, suggesting a reduced capability of suppressing nontangential information along the contour, and were also less efficient in suppressing irrelevant orientations in the background. CONCLUSIONS: These results support the suggestion that, whereas integrative mechanisms are not affected by age, suppressive mechanisms are.