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Jaffe-Dax, S, Lieder I, Biron T, Ahissar M.  2016.  Dyslexics' usage of visual priors is impaired.. Journal of vision. 16(9):10. Abstract
Human perception benefits substantially from familiarity, via the formation of effective predictions of the environment's pattern of stimulation. Basic stimulation characteristics are automatically retrieved and integrated into our perception. A quantitatively measurable manifestation of the integration of priors is known as "contraction to the mean"; i.e., perception is biased toward the experienced mean. We previously showed that in the context of auditory discrimination, the magnitude of this bias is smaller among dyslexic individuals than among good readers matched for age and general reasoning skills. Here we examined whether a similarly reduced contraction characterizes dyslexics' behavior on serial visual tasks. Using serial spatial frequency discrimination tasks, we found that dyslexics' bias toward the experiment's mean spatial frequency was smaller than that observed for the controls. Thus, dyslexics' difficulties in automatic detection and integration of stimulus statistics are domain-general. These difficulties are likely to impede the acquisition of reading expertise.
Jaffe-Dax, S, Raviv O, Jacoby N, Loewenstein Y, Ahissar M.  2015.  A Computational Model of Implicit Memory Captures Dyslexics' Perceptual Deficits.. The Journal of neuroscience : the official journal of the Society for Neuroscience. 35(35):12116-26. Abstractjaffe-dax_et_al._-_2015_-_a_computational_model_of_implicit_memory_captures_dyslexics_perceptual_deficits_-_journal_of_neuroscience.pdf
Dyslexics are diagnosed for their poor reading skills, yet they characteristically also suffer from poor verbal memory and often from poor auditory skills. To date, this combined profile has been accounted for in broad cognitive terms. Here we hypothesize that the perceptual deficits associated with dyslexia can be understood computationally as a deficit in integrating prior information with noisy observations. To test this hypothesis we analyzed the performance of human participants in an auditory discrimination task using a two-parameter computational model. One parameter captures the internal noise in representing the current event, and the other captures the impact of recently acquired prior information. Our findings show that dyslexics' perceptual deficit can be accounted for by inadequate adjustment of these components; namely, low weighting of their implicit memory of past trials relative to their internal noise. Underweighting the stimulus statistics decreased dyslexics' ability to compensate for noisy observations. ERP measurements (P2 component) while participants watched a silent movie indicated that dyslexics' perceptual deficiency may stem from poor automatic integration of stimulus statistics. This study provides the first description of a specific computational deficit associated with dyslexia.
Doron, A, Manassi M, Herzog MH, Ahissar M.  2015.  Intact crowding and temporal masking in dyslexia.. Journal of vision. 15(14):13. Abstractintact_crowding_and_temporal_masking_in_dyslexia.pdf
Phonological deficits in dyslexia are well documented. However, there is an ongoing discussion about whether visual deficits limit the reading skills of people with dyslexia. Here, we investigated visual crowding and backward masking. We presented a Vernier (i.e., two vertical bars slightly offset to the left or right) and asked observers to indicate the offset direction. Vernier stimuli are visually similar to letters and are strongly affected by crowding, even in the fovea. To increase task difficulty, Verniers are often followed by a mask (i.e., backward masking). We measured Vernier offset discrimination thresholds for the basic Vernier task, under crowding, and under backward masking, in students with dyslexia (n = 19) and age and intelligence matched students (n = 27). We found no group differences in any of these conditions. Controls with fast visual processing (good backward masking performance), were faster readers. By contrast, no such correlation was found among the students with dyslexia, suggesting that backward masking does not limit their reading efficiency. These findings indicate that neither elevated crowding nor elevated backward masking pose a bottleneck to reading skills of people with dyslexia.
Jacoby, N, Ahissar M.  2015.  Assessing the applied benefits of perceptual training: Lessons from studies of training working-memory.. Journal of vision. 15(10):6. Abstracti1534-7362-15-10-6.pdf
In the 1980s to 1990s, studies of perceptual learning focused on the specificity of training to basic visual attributes such as retinal position and orientation. These studies were considered scientifically innovative since they suggested the existence of plasticity in the early stimulus-specific sensory cortex. Twenty years later, perceptual training has gradually shifted to potential applications, and research tends to be devoted to showing transfer. In this paper we analyze two key methodological issues related to the interpretation of transfer. The first has to do with the absence of a control group or the sole use of a test-retest group in traditional perceptual training studies. The second deals with claims of transfer based on the correlation between improvement on the trained and transfer tasks. We analyze examples from the general intelligence literature dealing with the impact on general intelligence of training on a working memory task. The re-analyses show that the reports of a significantly larger transfer of the trained group over the test-retest group fail to replicate when transfer is compared to an actively trained group. Furthermore, the correlations reported in this literature between gains on the trained and transfer tasks can be replicated even when no transfer is assumed.
Daikhin, L, Ahissar M.  2015.  Fast Learning of Simple Perceptual Discriminations Reduces Brain Activation in Working Memory and in High-level Auditory Regions.. Journal of cognitive neuroscience. :1-14. Abstractjocn_a_00786.pdf
Introducing simple stimulus regularities facilitates learning of both simple and complex tasks. This facilitation may reflect an implicit change in the strategies used to solve the task when successful predictions regarding incoming stimuli can be formed. We studied the modifications in brain activity associated with fast perceptual learning based on regularity detection. We administered a two-tone frequency discrimination task and measured brain activation (fMRI) under two conditions: with and without a repeated reference tone. Although participants could not explicitly tell the difference between these two conditions, the introduced regularity affected both performance and the pattern of brain activation. The "No-Reference" condition induced a larger activation in frontoparietal areas known to be part of the working memory network. However, only the condition with a reference showed fast learning, which was accompanied by a reduction of activity in two regions: the left intraparietal area, involved in stimulus retention, and in posterior superior-temporal area, involved in representing auditory regularities. We propose that this joint reduction reflects a reduction in the need for online storage of the compared tones. We further suggest that this change reflects an implicit strategic shift "backwards" from reliance mainly on working memory networks in the "No-Reference" condition to increased reliance on detected regularities stored in high-level auditory networks.
Weiss, AH, Granot RY, Ahissar M.  2014.  The enigma of dyslexic musicians.. Neuropsychologia. 54:28-40. Abstractweiss_granot_ahissar_-_2014_-_the_enigma_of_dyslexic_musicians._-_neuropsychologia.pdf
Musicians are known to have exceptional sensitivity to sounds, whereas poor phonological representations (or access to these representations) are considered a main characteristic of dyslexic individuals. Though these two characteristics refer to different abilities that are related to non-verbal and verbal skills respectively, the recent literature suggests that they are tightly related. However, there are informal reports of dyslexic musicians. To better understand this enigma, two groups of musicians were recruited, with and without a history of reading difficulties. The pattern of reading difficulties found among musicians was similar to that reported for non-musician dyslexics, though its magnitude was less severe. In contrast to non-musician dyslexics, their performance in pitch and interval discrimination, synchronous tapping and speech perception tasks, did not differ from the performance of their musician peers, and was superior to that of the general population. However, the auditory working memory scores of dyslexic musicians were consistently poor, including memory for rhythm, melody and speech sounds. Moreover, these abilities were inter-correlated, and highly correlated with their reading accuracy. These results point to a discrepancy between their perceptual and working memory skills rather than between sensitivity to speech and non-speech sounds. The results further suggest that in spite of intensive musical training, auditory working memory remains a bottleneck to the reading accuracy of dyslexic musicians.
Raviv, O, Lieder I, Loewenstein Y, Ahissar M.  2014.  Contradictory behavioral biases result from the influence of past stimuli on perception.. PLoS computational biology. 10(12):e1003948. Abstractraviv_et_al._-_2014_-_contradictory_behavioral_biases_result_from_the_influence_of_past_stimuli_on_perception._-_plos_computational_biol.pdfjournal.pcbi_.1003948.pdf
Biases such as the preference of a particular response for no obvious reason, are an integral part of psychophysics. Such biases have been reported in the common two-alternative forced choice (2AFC) experiments, where participants are instructed to compare two consecutively presented stimuli. However, the principles underlying these biases are largely unknown and previous studies have typically used ad-hoc explanations to account for them. Here we consider human performance in the 2AFC tone frequency discrimination task, utilizing two standard protocols. In both protocols, each trial contains a reference stimulus. In one (Reference-Lower protocol), the frequency of the reference stimulus is always lower than that of the comparison stimulus, whereas in the other (Reference protocol), the frequency of the reference stimulus is either lower or higher than that of the comparison stimulus. We find substantial interval biases. Namely, participants perform better when the reference is in a specific interval. Surprisingly, the biases in the two experiments are opposite: performance is better when the reference is in the first interval in the Reference protocol, but is better when the reference is second in the Reference-Lower protocol. This inconsistency refutes previous accounts of the interval bias, and is resolved when experiments statistics is considered. Viewing perception as incorporation of sensory input with prior knowledge accumulated during the experiment accounts for the seemingly contradictory biases both qualitatively and quantitatively. The success of this account implies that even simple discriminations reflect a combination of sensory limitations, memory limitations, and the ability to utilize stimuli statistics.
Weiss, AH, Biron T, Lieder I, Granot RY, Ahissar M.  2014.  Spatial vision is superior in musicians when memory plays a role.. Journal of vision. 14(9) Abstracti1534-7362-14-9-18.pdf
Musicians' perceptual advantage in the acoustic domain is well established. Recent studies show that musicians' verbal working memory is also superior. Additionally, some studies report that musicians' visuospatial skills are enhanced although others failed to find this enhancement. We now examined whether musicians' spatial vision is superior, and if so, whether this superiority reflects refined visual skills or a general superiority of working memory. We examined spatial frequency discrimination among musicians and nonmusician university students using two presentation conditions: simultaneous (spatial forced choice) and sequential (temporal forced choice). Musicians' performance was similar to that of nonmusicians in the simultaneous condition. However, their performance in the sequential condition was superior, suggesting an advantage only when stimuli need to be retained, i.e., working memory. Moreover, the two groups showed a different pattern of correlations: Musicians' visual thresholds were correlated, and neither was correlated with their verbal memory. By contrast, among nonmusicians, the visual thresholds were not correlated, but sequential thresholds were correlated with verbal memory scores, suggesting that a general working memory component limits their performance in this condition. We propose that musicians' superiority in spatial frequency discrimination reflects an advantage in a domain-general aspect of working memory rather than a general enhancement in spatial-visual skills.
Cohen, Y, Daikhin L, Ahissar M.  2013.  Perceptual learning is specific to the trained structure of information.. Journal of cognitive neuroscience. 25(12):2047-60. Abstractcohen_daikhin_ahissar_-_2013_-_perceptual_learning_is_specific_to_the_trained_structure_of_information_-_journal_of_cognitive_neurosci.pdf
What do we learn when we practice a simple perceptual task? Many studies have suggested that we learn to refine or better select the sensory representations of the task-relevant dimension. Here we show that learning is specific to the trained structural regularities. Specifically, when this structure is modified after training with a fixed temporal structure, performance regresses to pretraining levels, even when the trained stimuli and task are retained. This specificity raises key questions as to the importance of low-level sensory modifications in the learning process. We trained two groups of participants on a two-tone frequency discrimination task for several days. In one group, a fixed reference tone was consistently presented in the first interval (the second tone was higher or lower), and in the other group the same reference tone was consistently presented in the second interval. When following training, these temporal protocols were switched between groups, performance of both groups regressed to pretraining levels, and further training was needed to attain postlearning performance. ERP measures, taken before and after training, indicated that participants implicitly learned the temporal regularity of the protocol and formed an attentional template that matched the trained structure of information. These results are consistent with Reverse Hierarchy Theory, which posits that even the learning of simple perceptual tasks progresses in a top-down manner, hence can benefit from temporal regularities at the trial level, albeit at the potential cost that learning may be specific to these regularities.
Banai, K, Ahissar M.  2013.  Musical experience, auditory perception and reading-related skills in children.. PloS one. 8(9):e75876. Abstractbanai_ahissar_-_2013_-_musical_experience_auditory_perception_and_reading-related_skills_in_children._-_plos_one.pdf
The relationships between auditory processing and reading-related skills remain poorly understood despite intensive research. Here we focus on the potential role of musical experience as a confounding factor. Specifically we ask whether the pattern of correlations between auditory and reading related skills differ between children with different amounts of musical experience.
Jacoby, N, Ahissar M.  2013.  What does it take to show that a cognitive training procedure is useful? A critical evaluation. Progress in brain research. 207:121-40. Abstractjacoby_ahissar_-_2013_-_what_does_it_take_to_show_that_a_cognitive_training_procedure_is_useful_a_critical_evaluation._-_progress_in_br.pdf
Individuals substantially improve with training, indicating that a large degree of plasticity is retained across ages. In the past 20 years, many studies explored the ability to boost cognitive skills (reasoning, linguistic abilities, working memory, and attention) by training with other tasks that exploit limited cognitive resources. Indeed, individuals with long-term training on challenging skills (musicians and action video gamers) show impressive behavior on related tasks (linguistic and visual attention, respectively). However, a critical evaluation of training studies that last weeks to months shows typically mild effects, mainly with respect to control groups that either did not practice or practiced with less challenging, rewarding, or exciting conditions. These findings suggest that future training studies should evaluate these factors carefully and assess whether they mainly impact the testing sessions or actual longer-term skills, and whether their impact can be further strengthened. The lack of a comprehensive theory of learning that integrates cognitive, motivational, and alertness aspects poses a bottleneck to improving current training procedures.
Ramus, F, Ahissar M.  2012.  Developmental dyslexia: The difficulties of interpreting poor performance, and the importance of normal performance.. Cognitive neuropsychology. Abstractramus_ahissar_-_2012_-_developmental_dyslexia_the_difficulties_of_interpreting_poor_performance_and_the_importance_of_normal_performance._-_cognitive_neuropsychology.pdf
This paper provides a selective review of data on phonology, audition, vision, and learning abilities in developmental dyslexia, with a specific focus on patterns of normal alongside poor performance. Indeed we highlight the difficulties of interpreting poor performance, and we criticize theories of dyslexia that are exclusively suited to explaining poor performance, at the risk of overgeneralizing and predicting deficits in many more situations than are observed. We highlight a number of tasks and conditions where individuals with dyslexia seem to show perfectly normal performance, and we discuss the value of taking such data seriously into account and the difficulties of current theories to explain them. Finally, we discuss the experimental challenges for tasks investigating the nature of cognitive deficits in dyslexia and in other developmental disorders and the challenges for any proper theory of dyslexia aiming to explain cases of normal as well as poor performance.
Ahissar, M.  2012.  Perceptual learning 2012.. Vision research. 61:1-3.ahissar_-_2012_-_perceptual_learning_2012._-_vision_research.pdf
Daikhin, L, Ahissar M.  2012.  Responses to deviants are modulated by subthreshold variability of the standard.. Psychophysiology. 49(1):31-42. Abstractdaikhin_ahissar_-_2012_-_responses_to_deviants_are_modulated_by_subthreshold_variability_of_the_standard_-_unknown.pdf
Auditory mechanisms automatically detect both basic features of sounds and the rules governing their presentation. In the oddball paradigm, the auditory system detects the sameness (or no-variability) rule when the same reference tone is consistently repeated. We used two oddball protocols, the classical one with a fixed reference and a modified one with a jittered reference, to determine whether the auditory system can detect subthreshold violations of sameness. We found that the response to the repeated standard was not modified by the small jitter. However, the response to the frequency oddball was smaller under the jittered protocol, indicating hypersensitivity to sameness. The sensitivity to jitter was largest when the oddball deviated by 8%, was smaller for 40%, and disappeared at 100% deviation, indicating that sensitivity to sameness is context dependent; namely, it is scaled with respect to the overall range of stimuli.
Oganian, Y, Ahissar M.  2012.  Poor anchoring limits dyslexics' perceptual, memory, and reading skills.. Neuropsychologia. Abstractoganian_ahissar_-_2012_-_poor_anchoring_limits_dyslexics_perceptual_memory_and_reading_skills_-_neuropsychologia.pdf
The basic deficits underlying the severe and persistent reading difficulties in dyslexia are still highly debated. One of the major topics of debate is whether these deficits are language specific, or affect both verbal and non-verbal stimuli. Recently, Ahissar and colleagues proposed the "anchoring-deficit hypothesis" (Ahissar, Lubin, Putter-Katz, & Banai, 2006), which suggests that dyslexics have a general difficulty in automatic extraction of stimulus regularities from auditory inputs. This hypothesis explained a broad range of dyslexics' verbal and non-verbal difficulties. However, it was not directly tested in the context of reading and verbal memory, which poses the main stumbling blocks to dyslexics. Here we assessed the abilities of adult dyslexics to efficiently benefit from ("anchor to") regularities embedded in repeated tones, orally presented syllables, and written words. We also compared dyslexics' performance to that of individuals with attention disorder (ADHD), but no reading disability. We found an anchoring effect in all groups: all gained from stimulus repetition. However, in line with the anchoring-deficit hypothesis, controls and ADHD participants showed a significantly larger anchoring effect in all tasks. This study is the first that directly shows that the same domain-general deficit, poor anchoring, characterizes dyslexics' performance in perceptual, working memory and reading tasks.
Raviv, O, Ahissar M, Loewenstein Y.  2012.  How Recent History Affects Perception: The Normative Approach and Its Heuristic Approximation. PLoS Comput Biol. 8:e1002731. Abstractjournal.pcbi_.1002731.pdf
Author Summary

In this paper we study how history affects perception using an auditory delayed comparison task, in which human participants repeatedly compare the frequencies of two, temporally-separated pure tones. We demonstrate that the history of the experiment has a substantial effect on participants' performance: when both tones are high relative to past stimuli, people tend to report that the 2nd tone was higher, and when they are relatively low, they tend to report that the 1st tone was higher. Interestingly, only the most recent trials bias performance, which can be interpreted as if the participants assume that the statistics of stimuli in the experiment is highly volatile. Moreover, this bias persists even in settings, in which it is detrimental to performance. These results demonstrate the abilities, as well as limitations, of the cognitive system when incorporating expectations in perception.

Nahum, M, Daikhin L, Lubin Y, Cohen Y, Ahissar M.  2010.  From Comparison to Classification: A Cortical Tool for Boosting Perception. J. Neurosci.. 30:1128–1136. Abstractnahum_et_al._-_2010_-_from_comparison_to_classification_a_cortical_tool_for_boosting_perception._-_the_journal_of_neuroscience_the_official_journal_of_the_society_for_neuroscience.pdf
Humans are much better in relative than in absolute judgments. This common assertion is based on findings that discrimination thresholds are much lower when measured with methods that allow interstimuli comparisons than when measured with methods that require classification of one stimulus at a time and are hence sensitive to memory load. We now challenged this notion by measuring discrimination thresholds and evoked potentials while listeners performed a two-tone frequency discrimination task. We tested various protocols that differed in the pattern of cross-trial tone repetition. We found that best performance was achieved only when listeners effectively used cross-trial repetition to avoid interstimulus comparisons with the repeated reference tone. Instead, they classified one tone, the nonreference tone, as either high or low by comparing it with a recently formed internal reference. Listeners were not aware of the switch from interstimulus comparison to classification. Its successful use was revealed by the conjunction of improved behavioral performance and an event-related potential component {(P3),} indicating an implicit perceptual decision, which followed the nonreference tone in each trial. Interestingly, tone repetition itself did not suffice for the switch, implying that the bottleneck to discrimination does not reside at the lower, sensory stage. Rather, the temporal consistency of repetition was important, suggesting the involvement of higher-level mechanisms with longer time constants. These findings suggest that classification is based on more automatic and accurate mechanisms than interstimulus comparisons and that the ability to effectively use them depends on a dynamic interplay between higher- and lower-level cortical mechanisms.
Banai, K, Ahissar M.  2010.  On the importance of anchoring and the consequences of its impairment in dyslexia. Dyslexia. 16:240–257.
Nahum, M, Nelken I, Ahissar M.  2010.  Stimulus uncertainty and perceptual learning: Similar principles govern auditory and visual learning. Vision research. 50:391–401.
Nahum, M, Renvall H, Ahissar M.  2009.  Dynamics of cortical responses to tone pairs in relation to task difficulty: A MEG study. Human brain mapping. 30:1592–1604.
Rokem, A, Ahissar M.  2009.  Interactions of cognitive and auditory abilities in congenitally blind individuals. Neuropsychologia. 47:843–848.
Banai, K, Ahissar M.  2009.  Perceptual learning as a tool for boosting working memory among individuals with reading and learning disability. Learning & Perception. 1:115–134.
Ahissar, M, Nahum M, Nelken I, Hochstein S.  2009.  Reverse hierarchies and sensory learning. Philosophical Transactions of the Royal Society B: Biological Sciences. 364:285–299. Abstractahissar_et_al._-_2009_-_reverse_hierarchies_and_sensory_learning._-_philosophical_transactions_of_the_royal_society_of_london._series_b_biological_sciences.pdf
Revealing the relationships between perceptual representations in the brain and mechanisms of adult perceptual learning is of great importance, potentially leading to significantly improved training techniques both for improving skills in the general population and for ameliorating deficits in special populations. In this review, we summarize the essentials of reverse hierarchy theory for perceptual learning in the visual and auditory modalities and describe the theory's implications for designing improved training procedures, for a variety of goals and populations.