Lateral masking, levels of processing and stimulus category: a comparative study between normal and dyslexic readers

The lateral masking effect results in lower performance on letter recognition when items are flanked by other stimuli. Using a new paradigm based on discrimination (feature analysis) and categorization (memory access) tasks, we investigated the influence of level of processing (as addressed, respectively, by these two tasks) and stimulus type (Latin letters, Korean letters and geometrical figures) on lateral masking. In addition, performance of dyslexic and non-dyslexic adult readers was compared. The non-dyslexic participants demonstrated a classical lateral masking effect with lower performance for flanked items than isolated ones. In addition, lateral masking was stronger in the categorization than in the discrimination task and was restricted to familiar items, i.e., Latin letters and geometrical figures. Dyslexic participants showed poorer performance than non-dyslexics on processing isolated items, and the pattern of decrease in performance for lateral masking was similar to non-dyslexics. However, they also showed a stronger decrease in performance in categorization and a stronger decrease related to the lateral masking for this categorization task. Our results in normal readers suggest that lateral masking relies on the interference between the target and the flankers during feature integration that may result in marked impairment of memory access (categorization task). Poorer performance in dyslexic readers may reflect impaired parafoveal/peripheral low-level processing during feature integration that may have worsened during the flanked condition due to a target selection/spatial-attentional disorder. Moreover, dyslexic subjects presented an additional categorization deficit that may relate to a specific left-hemispheric disorder.     

Visual backward masking: deficits in locating targets are specific to schizophrenia and not related to intellectual decline

Visual backward masking deficits have been postulated as potential vulnerability markers for schizophrenia. This study investigated the diagnostic specificity of a location and an identification variant of the backward masking task for schizophrenia and analyzed masking performance during the course of the tasks. The influence of schizophrenia patients' intellectual decline on masking performance was also examined. Twenty-eight schizophrenia patients were compared to 28 patients with unipolar depression and 28 healthy controls on a letter location task and a letter identification task applying a low spatial frequency mask. Schizophrenia patients made significantly more detection errors on the location task than depressives at an interstimulus interval (ISI) of 50 ms and healthy controls at ISIs of 16.7, 33.3, 50, and 66.7 ms. Thus, the location masking dysfunction of schizophrenia patients was distinctive at a rather long interstimulus interval (50 ms). On the identification task the performance of schizophrenia patients did not differ from that of the two control groups. Identification but not location masking performance improved during the course of the task for all groups. Intellectual deterioration of schizophrenia patients was not correlated with location or identification masking performance. Schizophrenia patients are characterized by specific impairments in spatial visual processing which appear to be independent of intellectual decline. Potential explanations of the location masking deficit found in schizophrenia are discussed.     

Mental retardation, inspection time, and central attentional impairment

Performance of mentally retarded and nonretarded adults on simple visual discrimination tasks was compared. Experiment 1 involved a lines-discrimination task used in previous research to measure inspection time. A dichoptic stimulus presentation procedure was employed, with target and masking stimuli presented to different eyes so that only central masking could occur. Experiment 2 involved a discrimination between two lights in different positions, demonstrating that masking was characterized by constant stimulus-onset-asynchrony, because the relationship between target duration and the interval between target offset and mask onset was additive. Results from both experiments confirmed that for both retarded and nonretarded subjects, the backward masking procedures used interfered with visual processing at a central level. In Experiment 3 markedly deviant smooth pursuit eye movements in a pendulum tracking task occurred among retarded subjects but not among nonretarded subjects. This deviancy was additional to other indications of poor attention. In the retarded group the correlation between estimates of inspection time and deviant eye tracking was .64, but other instances of inattentiveness were not related to either of these two measures. This finding was interpreted in terms of an involuntary dysfunction within some central attentional system that involves at least three qualitatively different levels.     

Cross-diagnostic comparison of visual processing in bipolar disorder and schizophrenia

Patients with Schizophrenia (SZ) show deficits across various stages of visual information processing. Whether patients with Bipolar Disorder (BD) exhibit these deficits is unclear. In this study, we conducted a detailed comparison of specific stages of early visual perception in BD and SZ. Forty-three BD patients, 43 SZ patients, and 51 matched healthy control subjects (HC) were administered three visual processing paradigms emphasizing: 1) an early stage of object formation (location backward masking), 2) a middle stage of object substitution (four-dot backward masking), and 3) a later stage at the perception–attention interface (rapid serial visual processing (RSVP) task eliciting the attentional blink). SZ performed significantly worse than BD and HC on location and four-dot masking. BD did not significantly differ from HC on either masking task. Both patient groups performed significantly worse than HC on the RSVP task; unlike SZ, BD did not show a significant attentional blink effect compared to HC. Our results indicate that BD patients were intact at the early and middle stages of visual processing (object formation and substitution) but intermediate between the SZ and HC groups at a later processing stage involving perceptual and attentional processes (RSVP task). These findings suggest that SZ is characterized by a diffuse pathophysiology affecting all stages of visual processing whereas in BD disruption is only at the latest stage involving higher order attentional functions.     

A pilot study of visual backward masking performance among affected versus unaffected offspring of parents with bipolar disorder

BACKGROUND: Cognitive dysfunction is evident in some euthymic patients with established bipolar disorder (BD), including deficits in visual backward masking (VBM) tasks which map to a specific neural pathway. A high-risk paradigm would clarify the temporal relation of cognitive dysfunction to clinical course. METHOD: We compared euthymic offspring (age range: 18-32 years) of lithium-responsive bipolar parents with and without a previous lifetime history of psychiatric illness to healthy comparison subjects with a negative family history, on a VBM task that requires target location. RESULTS: High-risk offspring with no lifetime psychiatric history performed the VBM task at levels of healthy controls. High-risk offspring with a previous history of a mood disorder, in complete remission, made significantly more errors at short target-mask intervals than control or never ill offspring. These higher error rates were not a consequence of faster response times. CONCLUSIONS: There is preliminary evidence of specific cognitive dysfunction early in the course of illness in affected offspring of parents with lithium responsive BD. VBM is ideal for future longitudinal studies addressing whether cognitive dysfunction in BD is a trait marker or a consequence of illness manifestation.     

Neurophysiological,linguistic, and cognitive predictors of children’s ability to perceive speech in noise

Hearing in noisy environments is a complicated task that engages attention, memory, linguistic knowledge, and precise auditory-neurophysiological processing of sound. Accumulating evidence in school-aged children and adults suggests these mechanisms vary with the task’s demands. For instance, co-located speech and noise demands a large cognitive load and recruits working memory, while spatially separating speech and noise diminishes this load and draws on alternative skills. Past research has focused on one or two mechanisms underlying speech-in-noise perception in isolation; few studies have considered multiple factors in tandem, or how they interact during critical developmental years. This project sought to test complementary hypotheses involving neurophysiological, cognitive, and linguistic processes supporting speech-in-noise perception in young children under different masking conditions (co-located, spatially separated). Structural equation modeling was used to identify latent constructs and examine their contributions as predictors. Results reveal cognitive and language skills operate as a single factor supporting speech-in-noise perception under different masking conditions. While neural coding of the F0 supports perception in both co-located and spatially separated conditions, neural timing predicts perception of spatially separated listening exclusively. Together, these results suggest co-located and spatially separated speech-in-noise perception draw on similar cognitive/linguistic skills, but distinct neural factors, in early childhood.     

Regional brain activity associated with visual backward masking

In visual backward masking, the visibility of a briefly presented visual target is disrupted by a mask that is presented shortly thereafter. The goal of the current study was to identify regions in the human cortex that may provide the neural basis of visual masking. We searched for areas whose activity correlated with perception as we systematically varied the strength of masking. A total of 13 subjects performed a backward masking task during functional magnetic resonance imaging. Target and mask were presented at three delay intervals (34, 68, and 102 msec) and behavioral measures confirmed that the targets were more visible at longer masking intervals. Two sets of regions of interest were identified: Distinct regions in the visual cortex (V1/V2, LO, hMT+) were segregated using scans to localize visual processing drawn from the existing literature. Additional cortical regions were selected in a data-driven approach based on their activity during the backward masking task. For each set, we determined the regions whose magnitude of activation increased at longer masking intervals. Nine of the subjects provided valid behavioral performance data on the visual masking task and imaging data from these subjects were used for subsequent analysis. The scans of visual processing areas identified four regions, including: early visual areas (V1 and V2), the motion-sensitive regions in the lateral occipital (LO) lobe (hMT+), and two components (dorsal and ventral) of the object-sensitive region, LO. Of these, the ventral and dorsal LO regions were sensitive to the strength of the mask. For the data-driven approach, six regions were identified on the basis of a difference map in which all masking intervals were contrasted with rest. These included the inferior parietal, anterior cingulate, precentral, insula, thalamic, and occipital areas. The predicted effects of more activity with weaker masking were seen in the thalamus, inferior parietal, and anterior cingulate. This study isolated three types of visual processing areas. The first included regions that subserve key stages of vision (including object and motion processing). The second type responded to the presentation of brief ly presented visual stimuli, regardless of masking interval. The third type (selected from the first two) included regions sensitive to the interval between the target and mask. These latter regions (including ventral LO, inferior parietal, anterior cingulate, and thalamus) may form the neural substrate of backward masking.     

Are early stages of visual information processing in schizophrenic patients disturbed? Results of an experimental study

In a study of early stages of visual information processing in schizophrenic patients, twenty acute schizophrenics, twenty remitted schizophrenics and twenty chronic schizophrenics were compared with control groups consisting of twenty healthy subjects and twenty depressive patients (psychiatric controls). The groups tested were matched for age, sex, and educational level. The psychiatric groups were selected according to the research diagnostic criteria (RDC) of Spitzer et al. Performance in a picture integration task and a backward masking task were measured. Chronic and acute schizophrenics showed similarly poor performance in the masking task compared to both control groups. The remitted schizophrenics, however, did not differ from the healthy controls or from the psychiatric controls. This finding contradicts the claim recently made by Merritt et al., that poor performance in backward masking is a trait marker for schizophrenia.     

Visual information processing after severe closed head injury: effects of forward and backward masking.

Three tachistoscopic tasks were employed to assess whether survivors of severe closed head injury (CHI) exhibit a disturbance of information processing within peripheral and/or central visual pathways. Twelve survivors of severe CHI and 12 individually matched control subjects completed a recognition threshold (no mask) task, a monoptic, forward masking by visual noise task (to assess processing within relatively peripheral pathways), and a dichoptic, backward masking by pattern task (to assess processing within central pathways). For each experimental procedure, the minimum exposure durations required by subjects to identify correctly single consonants and triple consonants were determined. Survivors of severe CHI showed deficits on all three visual tasks. Both groups also had higher threshold durations for the more complex stimuli (triple v single consonants), but differences in threshold were greater in the patients with CHI. The degree of perceptual impairment exhibited by patients with CHI was highly variable and not consistently related to injury characteristics or residual motor or speech and language impairment.     

Perceptual and memory components of the superior recall of letters from the right visual half-fields

Right-hander's show consistently better recall from the right visual half-field (VHF) when digit sequences are presented simultaneously at fixation and in one of the visual half-fields. This right VHF superiority has been attributed to the more direct connection between the right VHF and the speech areas in the left hemisphere of the brain.The effect of amount of material to be remembered, interstimulas interval, and presentation time was studied in order to assess the importance of short-term memory, backward masking, and perceptual factors on this task. Sequences of five letter pairs were presented at two different interstimulas intervals (165 msec and 330 msec) and three presentation times (165 msec, 280 msec and 330 msec). Increasing the amount of material to be remembered and increasing the interstimulas interval appeared to increase the asymmetry between the VHF's. Changing presentation time (from 330 msec to 165 msec) had little effect on either the VHF asymmetry or recall efficiency. This suggests that the controlled fixation paradigm is a memory and backward masking task with the right VHF superiority resulting primarily from the memory aspect of the task.     

Attention-related modulation of auditory brainstem responses during contralateral noise exposure

As determinants facilitating attention-related modulation of the auditory brainstem response (ABR), two experimental factors were examined: (i) auditory discrimination; and (ii) contralateral masking intensity. Tone pips at 80 dB sound pressure level were presented to the left ear via either single-tone exposures or oddball exposures, whereas white noise was delivered continuously to the right ear at variable intensities (none--80 dB sound pressure level). Participants each conducted two tasks during stimulation, either reading a book (ignoring task) or detecting target tones (attentive task). Task-related modulation within the ABR range was found only during oddball exposures at contralateral masking intensities greater than or equal to 60 dB. Attention-related modulation of ABR can thus be detected reliably during auditory discrimination under contralateral masking of sufficient intensity.     

Modulation of attention during visual masking in schizophrenia

OBJECTIVE: Schizophrenia patients consistently demonstrate performance deficits on visual masking procedures. The present study examined whether attentional manipulation would improve subjects' performance on visual masking. METHOD: A metacontrast task was administered to 105 schizophrenia patients and 52 healthy comparison subjects. Attention was manipulated by associating selected trials of the task with monetary reward. RESULTS: Schizophrenia patients exhibited poorer performance than the comparison subjects across conditions. Patients demonstrated modest, but statistically significant, improvement in performance with the attentional manipulation. This improvement was not significant for the comparison subjects. CONCLUSIONS: These findings suggest that early visual processes in schizophrenia are responsive to attentional manipulation but that the degree of improvement is relatively small, suggesting that these processes are not easily altered.     

Event-related potentials reflect individual differences in age-invariant auditory skills

Previous work has found that auditory event-related potentials show maturational changes, with latency and amplitude of late components (N1 and P2) decreasing and increasing with age respectively. Our goal was to test the hypothesis that these changes reflect increased speed of neural processing in the auditory system. Thirty-three listeners, aged 10-50 years, were tested on a frequency discrimination task and an auditory backward recognition masking task. P1 and N1b event-related potential components were measured to tones. The N1b became larger and earlier with age, and the latency of P1 decreased with age. However, thresholds on the behavioural tasks did not change with age. Nevertheless, individual differences in the peak amplitude of N1b were independently related to frequency discrimination and degree of masking. Thus, the relationship that does exist between individual differences in psychoacoustic performance and the auditory N1b reflect a stable characteristic of the individual rather than a maturational change.     

Early visual information processing in schizophrenia compared to recurrent depression

Patients with schizophrenia have repeatedly shown deficits in early visual processing using backward masking (VBM) tasks. Whether this represents a specific dysfunction in schizophrenia is an unsolved question. Patients with recurrent unipolar depression represent an interesting comparison group to examine the question of specificity, but have never previously been assessed on VBM. In addition to comparing VBM performance in patients with schizophrenia and patients with depression, we wanted to examine the relations between VBM and clinical symptoms. Fifty-one patients with schizophrenia were compared to 49 patients with recurrent unipolar depression and 47 healthy controls. All subjects were administered a two-digit identification task in a no-masking and four masking conditions. Patients with schizophrenia performed significantly worse than normal controls on four of the five conditions. No significant difference was found between depression patients and normal controls. The effect of masking stimuli had no differential effects on the three groups. VBM correlated strongly with positive symptoms in the schizophrenia group.     

Visual masking as a probe for abnormal gamma range activity in schizophrenia.

BACKGROUND: Visual masking procedures assess very early stages of visual perception. Patients with schizophrenia consistently show deficits on visual masking tasks, and these deficits likely reflect vulnerability to schizophrenia. We conducted two experiments to determine whether visual masking procedures can reveal underlying abnormalities in gamma range oscillations in schizophrenia. METHODS: In the first experiment, we conducted nonlinear modeling of visual masking performance data from 89 male schizophrenic patients and 20 male comparison subjects. In the second experiment, electrophysiological recordings of event-related gamma activity were taken during a visual masking task in a subset of eight patients and seven control subjects. RESULTS: In the first experiment, nonlinear modeling of the performance data revealed evidence of oscillations in the gamma range (30 and 35 Hz) for the comparison group but not patients. In the second experiment, the comparison group, but not the patients, showed a burst of gamma range activity 200-400 msec following target presentation. The difference between patients and comparison subjects in this time period was significant (p <.05). CONCLUSIONS: Visual masking procedures can serve as a probe for underlying gamma range activity, which appears to be aberrant in schizophrenia. Perceptual problems in schizophrenia may, at least in part, be due to a failure to establish and/or maintain gamma range oscillations.     

Auditory perceptual consolidation in early-onset blindness

Early-onset blindness (EB) produces measurable advantages in auditory perception, attention, memory and language. Neville and Bavelier [Neville, H. J., & Bavelier, D. (2001) Variability of developmental plasticity. In J. L. McClelland, R. S. Siegler (Eds.) Mechanisms of cognitive development: Behavioral andellon symposia on cognition (pp. 271-301)] hypothesized that faster temporal processing underlies many auditory compensatory effects in the blind. We tested this hypothesis by comparing early-onset blind individuals and sighted counterparts (SC) by assessing their rates of perceptual consolidation, the accurate perceptual representation of auditory stimuli. Firstly, we first tested both groups on a temporal-order judgment task (TOJ). EB subjects had significantly lower TOJ thresholds than the SC subjects. Secondly, we assessed perceptual consolidation speed using auditory backward masking tasks, taking into account individual TOJ thresholds. Discrimination performance was unaffected at all mask delays in the EB group while the SC subjects needed a mask delay of 160 ms to perform comparably. A backward masking task using single tone stimuli found no differences between the EB and SC groups any mask delay. A simultaneous masking task demonstrated that the mask effectively impaired discrimination in EB subjects at sensory stages. These results suggest that advantages in perceptual consolidation may reflect a mechanism responsible for the short response times and better performance reported in early blind individuals across a number of complex auditory tasks.     

The magnocellular visual system and schizophrenia: what can the color red tell us?

Previous research has suggested that genetic loading for schizophrenia is related to a dysfunctional magnocellular (M) subcortical visual pathway-responsible for processing movement and location. However, data substantiating this mechanism remains inconclusive. The present study used a novel technique to selectively suppress the M system in order to investigate the impact of genetic loading for schizophrenia on its functioning. A visual backward masking task was administered to 28 healthy first-degree relatives of persons with schizophrenia and 31 healthy controls. The task was administered on both a red and neutral background, as diffuse red light has been shown to selectively suppress the M system in basic vision research. On a location condition of backward masking, controls demonstrated reduced accuracy on the red compared to the neutral background. In contrast, relatives did not display differential performance between the two backgrounds. The differential effect on the two groups appears to be attributable to a difference in activity of the M pathway. Performance in the relatives was consistent with the notion of a hyperactive M pathway.     

GABAA agonist reduces visual awareness: a masking-EEG experiment

Consciousness can be manipulated in many ways. Here, we seek to understand whether two such ways, visual masking and pharmacological intervention, share a common pathway in manipulating visual consciousness. We recorded EEG from human participants who performed a backward-masking task in which they had to detect a masked figure form its background (masking strength was varied across trials). In a within-subject design, participants received dextromethorphan (a N-methyl-d-aspartate receptor antagonist), lorazepam (LZP; a GABA(A) receptor agonist), scopolamine (a muscarine receptor antagonist), or placebo. The behavioral results show that detection rate decreased with increasing masking strength and that of all the drugs, only LZP induced a further decrease in detection rate. Figure-related ERP signals showed three neural events of interest: (1) an early posterior occipital and temporal generator (94-121 msec) that was not influenced by any pharmacological manipulation nor by masking, (2) a later bilateral perioccipital generator (156-211 msec) that was reduced by masking as well as LZP (but not by any other drugs), and (3) a late bilateral occipital temporal generator (293-387 msec) that was mainly affected by masking. Crucially, only the intermediate neural event correlated with detection performance. In combination with previous findings, these results suggest that LZP and masking both reduce visual awareness by means of modulating late activity in the visual cortex but leave early activation intact. These findings provide the first evidence for a common mechanism for these two distinct ways of manipulating consciousness.     

Functional magnetic resonance imaging of a human auditory cortex area involved in foreground–background decomposition

Auditory foreground–background decomposition is a pattern recognition process which combines simultaneous and sequential grouping in complex sound sequences. Using functional magnetic resonance imaging with reduced scanner noise and stimulation through a new type of earphones, we investigated the possibility that this process activates topographically distinct areas of human auditory cortex. A basic matching-to-sample task with variable tones (sequential grouping) caused significant activity in three separate landmark-related territories on the supratemporal plane. A similar task in the presence of a strongly masking acoustic background pattern to challenge simultaneous grouping led to the distinction of the subterritory in which foreground signal-related or task-related signal properties were exclusively seen. In contrast to the remainder of territories the level of activity and the periodicity of the signal time-course was resistant to the masking influence of the background. This suggests that auditory foreground–background decomposition involves a specialized non-primary auditory cortex field. Generally, the findings demonstrate functional parcellation of auditory cortex for which the evidence in humans, in contrast to other primates, is only indirect to date.     

Masking interrupts figure-ground signals in V1

In a backward masking paradigm, a target stimulus is rapidly (<100 msec) followed by a second stimulus. This typically results in a dramatic decrease in the visibility of the target stimulus. It has been shown that masking reduces responses in V1. It is not known, however, which process in V1 is affected by the mask. In the past, we have shown that in V1, modulations of neural activity that are specifically related to figure-ground segregation can be recorded. Here, we recorded from awake macaque monkeys, engaged in a task where they had to detect figures from background in a pattern backward masking paradigm. We show that the V1 figure-ground signals are selectively and fully suppressed at target-mask intervals that psychophysically result in the target being invisible. Initial response transients, signalling the features that make up the scene, are not affected. As figure-ground modulations depend on feedback from extrastriate areas, these results suggest that masking selectively interrupts the recurrent interactions between V1 and higher visual areas.