Zahra Hussain

Perceptual learning reduces crowding in amblyopia and in the normal periphery

Amblyopia is a developmental visual disorder of cortical origin, characterized by crowding and poor acuity in central vision of the affected eye. Crowding refers to the adverse effects of surrounding items on object identification, common only in normal peripheral but not central vision. We trained a group of adult human amblyopes on a crowded letter identification task to assess whether the crowding problem can be ameliorated. Letter size was fixed well above the acuity limit, and letter spacing was varied to obtain spacing thresholds for central target identification. Normally sighted observers practiced the same task in their lower peripheral visual field. Independent measures of acuity were taken in flanked and unflanked conditions before and after training to measure crowding ratios at three fixed letter separations. Practice improved the letter spacing thresholds of both groups on the training task, and crowding ratios were reduced after posttest. The reductions in crowding in amblyopes were associated with improvements in standard measures of visual acuity. Thus, perceptual learning reduced the deleterious effects of crowding in amblyopia and in the normal periphery. The results support the effectiveness of plasticity-based approaches for improving vision in adult amblyopes and suggest experience-dependent effects on the cortical substrates of crowding.

Perceptual learning modifies inversion effects for faces and textures.

We trained subjects to identify either upright or inverted faces in a 10AFC task and measured performance subsequently in four conditions: same- and different-upright faces, and same- and different-inverted faces. Performance improved for both the upright-trained and the inverted-trained groups. The improvements were highly specific to the trained face exemplars, and largely specific to the trained face orientations. This pattern of results yielded an increase in the face-inversion effect after upright-training, and a decrease in the inversion effect after inverted-training, but only for the trained set of faces in both groups. A similar pattern of results was found for phase-scrambled faces in which the configural structure of faces had been removed: although there was no baseline inversion effect for the scrambled stimuli, inversion effects emerged after training. We consider the implications of this pattern of learning for current views on the face-inversion effect, and face-encoding more generally.

Superior Identification of Familiar Visual Patterns a Year After Learning

Practice improves visual performance on simple tasks in which stimuli vary along one dimension. Such learning frequently is stimulus-specific and enduring, and has been associated with plasticity in striate cortex. It is unclear if similar lasting effects occur for naturalistic patterns that vary on multiple dimensions. We measured perceptual learning in identification tasks that used faces and textures, stimuli that engage multiple stages in visual processing. Performance improved significantly across 2 consecutive days of practice. More important, the effects of practice were remarkably stable across time: Improvements were maintained approximately 1 year later, and both the relative difficulty of identifying individual stimuli and individual differences in performance were essentially constant across sessions. Finally, the effects of practice were largely stimulus-specific. Our results suggest that the characteristics of perceptual learning are similar across a spectrum of stimulus complexities.

Versatile perceptual learning of textures after variable exposures.

Perceptual learning of 10-AFC texture identification is stimulus specific: after practice, identification accuracy drops substantially when textures are rotated 180°, reversed in contrast polarity, or when a novel set of textures is presented. Here we asked if perceptual learning occurs without any repetition of items during training, and whether exposure to greater stimulus variation during training influences transfer of learning. We trained three groups of subjects in a 10-AFC texture identification task on 2 days. The Standard group viewed a fixed set of 10 textures throughout training. The Variable group viewed 840 novel sets of textures. The Switch group viewed different fixed sets of 10 textures on Days 1 and 2. In all groups, transfer of learning was tested by using fixed sets of textures on Days 3 and 4 and having half of the subjects from each group switch to a novel set on Day 4. During training, the most learning was obtained by the Standard group, and gradual but significant learning was obtained by the other two groups. On Day 4, performance of the Standard group was adversely affected by a switch to novel textures, whereas performance of the Variable and Switch groups remained intact. Hence, slight but significant learning occurred without repetition of items during training, and stimulus specificity was influenced significantly by the type of training. Increasing stimulus variability by reducing the number of times stimuli are repeated during practice may cause subjects to adopt strategies that increase generalization of learning to new stimuli. Alternatively, presenting new stimuli on each trial may prevent subjects from adopting strategies that result in stimulus specific learning.

The challenges of developing a contrast-based video game for treatment of amblyopia.

Perceptual learning of visual tasks is emerging as a promising treatment for amblyopia, a developmental disorder of vision characterized by poor monocular visual acuity. The tasks tested thus far span the gamut from basic psychophysical discriminations to visually complex video games. One end of the spectrum offers precise control over stimulus parameters, whilst the other delivers the benefits of motivation and reward that sustain practice over long periods. Here, we combined the advantages of both approaches by developing a video game that trains contrast sensitivity, which in psychophysical experiments, is associated with significant improvements in visual acuity in amblyopia. Target contrast was varied adaptively in the game to derive a contrast threshold for each session. We tested the game on 20 amblyopic subjects (10 children and 10 adults), who played at home using their amblyopic eye for an average of 37 sessions (approximately 11 h). Contrast thresholds from the game improved reliably for adults but not for children. However, logMAR acuity improved for both groups (mean = 1.3 lines; range = 0–3.6 lines). We present the rationale leading to the development of the game and describe the challenges of incorporating psychophysical methods into game-like settings.

Robust perceptual learning of faces in the absence of sleep

This study examines the effects of sleep on learning in a face identification task. Five groups of subjects performed a 1-of-10 face identification task in two sessions separated by 3, 12, and 24h. Session 1 consisted of four blocks of 105 trials each; Session 2 consisted of eight blocks of trials. All groups exhibited significant improvement in response accuracy within each session. Furthermore, between-session learning - defined as the difference in proportion correct between sessions 1 and 2 - was significant for all groups. Between-session learning was greater in groups that slept between sessions, but the effect was small and affected performance only in the first block of trials in Session 2. Overall, we find that sleeps contribution is a small proportion of the total amount learned in face identification, with improvements continuing to accrue in its absence.

The Rapid Emergence of Stimulus Specific Perceptual Learning

Is stimulus specific perceptual learning the result of extended practice or does it emerge early in the time course of learning? We examined this issue by manipulating the amount of practice given on a face identification task on Day 1, and altering the familiarity of stimuli on Day 2. We found that a small number of trials was sufficient to produce stimulus specific perceptual learning of faces: on Day 2, response accuracy decreased by the same amount for novel stimuli regardless of whether observers practiced 105 or 840 trials on Day 1. Current models of learning assume early procedural improvements followed by late stimulus specific gains. Our results show that stimulus specific and procedural improvements are distributed throughout the time course of learning.

The representation of audiovisual regularities in the human brain

Neural representation of auditory regularities can be probed using the MMN, a component of ERPs generated in the auditory cortex by any violation of that regularity. Although several studies have shown that visual information can influence or even trigger an MMN by altering an acoustic regularity, it is not known whether audiovisual regularities are encoded in the auditory representation supporting MMN generation. We compared the MMNs elicited by the auditory violation of (a) an auditory regularity (a succession of identical standard sounds), (b) an audiovisual regularity (a succession of identical audiovisual stimuli), and (c) an auditory regularity accompanied by variable visual stimuli. In all three conditions, the physical difference between the standard and the deviant sound was identical. We found that the MMN triggered by the same auditory deviance was larger for audiovisual regularities than for auditory-only regularities or for auditory regularities paired with variable visual stimuli, suggesting that the visual regularity influenced the representation of the auditory regularity. This result provides evidence for the encoding of audiovisual regularities in the human brain.

Contrast-reversal abolishes perceptual learning.

We tested the effects of contrast reversal on perceptual learning in a 10AFC texture identification task. Four groups of subjects performed the task on two consecutive days. One group saw the same textures on both days, whereas three other groups saw novel, rotated (180 deg), or contrast-reversed textures on the second day. Response accuracy improved during the first day in all groups. Accuracy decreased significantly at the start of Day 2 in the groups who saw novel, rotated or contrast-reversed textures, but not in the group who saw the same textures. Moreover, the drop in performance was the same in the groups who saw novel, rotated, and contrast-reversed textures. Control experiments showed that making subjects aware of the stimulus transformations at the start of either the first or second day did not alter the results. Hence, the effects of contrast-reversal and 180 deg rotation on the generalization of learning were the same as the effect of using novel stimuli, and knowledge of the stimulus transformation did not reduce their effects. We consider the implications of this pattern of results for the neural mechanisms recruited during the identification and learning of two-dimensional visual patterns.

Estimation of cortical magnification from positional error in normally sighted and amblyopic subjects

We describe a method for deriving the linear cortical magnification factor from positional error across the visual field. We compared magnification obtained from this method between normally sighted individuals and amblyopic individuals, who receive atypical visual input during development. The cortical magnification factor was derived for each subject from positional error at 32 locations in the visual field, using an established model of conformal mapping between retinal and cortical coordinates. Magnification of the normally sighted group matched estimates from previous physiological and neuroimaging studies in humans, confirming the validity of the approach. The estimate of magnification for the amblyopic group was significantly lower than the normal group: by 4.4 mm deg(-1) at 1° eccentricity, assuming a constant scaling factor for both groups. These estimates, if correct, suggest a role for early visual experience in establishing retinotopic mapping in cortex. We discuss the implications of altered cortical magnification for cortical size, and consider other neural changes that may account for the amblyopic results.