Hemispheric asymmetries in feature integration during visual word recognition

Although the definitive source of the left hemisphere's superiority for visual word recognition remains illusive, some argue that the left (LH) and right (RH) hemispheres engage different strategies during early perceptual processes involved in stimulus encoding. In particular, it is proposed that the LH treats a word as a unitary perceptual group whereas the RH processes the letters comprising a word as a series of individual perceptual units. The present study investigated support for this processing distinction by examining hemispheric strategies for temporal integration using Prinzmetal and Millis-Wright's (1984 Prinzmetal, W. and Millis-Wright, M. 1984. Cognitive and linguistic factors affect visual feature integration. Cognitive Psychology, 16: 305–340. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar]) feature-binding paradigm. A total of 20 participants identified the colour and identity of a target letter, presented within a three-letter word (e.g., ART) or nonword (e.g., HRF), directed to their left or right visual field. Errors were classified on the basis of whether they involved substitution of a colour present within the stimulus but at a different location (ON error), or the substitution of a colour not present within the stimulus (OFF error). As anticipated, for word stimuli there was a higher proportion of OFF errors associated with trials directed to the RH, consistent with the notion that the LH treats words as single perceptual units and is hence biased toward miscombination of perceptual information present within the stimulus. The pattern of ON errors across stimulus type provided clear evidence of RH sequential encoding effects, with the number of errors increasing markedly across the ordinal position of the letters comprising the stimulus string. As such, these data provide new evidence that the LH's advantage for visual word recognition arises, at least in part, from the ability to encode verbal stimuli as single perceptual units.     

Hemispheric asymmetries in feature integration during visual word recognition

Although the definitive source of the left hemisphere's superiority for visual word recognition remains illusive, some argue that the left (LH) and right (RH) hemispheres engage different strategies during early perceptual processes involved in stimulus encoding. In particular, it is proposed that the LH treats a word as a unitary perceptual group whereas the RH processes the letters comprising a word as a series of individual perceptual units. The present study investigated support for this processing distinction by examining hemispheric strategies for temporal integration using Prinzmetal and Millis-Wright's (1984) feature-binding paradigm. A total of 20 participants identified the colour and identity of a target letter, presented within a three-letter word (e.g., ART) or nonword (e.g., HRF), directed to their left or right visual field. Errors were classified on the basis of whether they involved substitution of a colour present within the stimulus but at a different location (ON error), or the substitution of a colour not present within the stimulus (OFF error). As anticipated, for word stimuli there was a higher proportion of OFF errors associated with trials directed to the RH, consistent with the notion that the LH treats words as single perceptual units and is hence biased toward miscombination of perceptual information present within the stimulus. The pattern of ON errors across stimulus type provided clear evidence of RH sequential encoding effects, with the number of errors increasing markedly across the ordinal position of the letters comprising the stimulus string. As such, these data provide new evidence that the LH's advantage for visual word recognition arises, at least in part, from the ability to encode verbal stimuli as single perceptual units.     

Differential modulation of word recognition by semantic and spatial orienting of attention

In the present study, we investigated the ability to orient attention to abstract associative features of complex stimuli, more specifically, to the semantic categories of visual word stimuli. We compared the behavioral and electrophysiological effects of semantic orienting with those elicited by spatial orienting to word stimuli. Two parallel, cued lexical-decision tasks, with semantic- or spatial-orienting cues, were used. Results showed that both semantic and spatial orienting facilitated behavioral performance. The event-related potential analysis revealed different and non-overlapping patterns of modulation of word processing by semantic and spatial orienting. Modulation by semantic orienting started later, affecting only the potentials linked to conceptual or semantic processing (N300 and N400). The pattern of N300/N400 modulation in the semantic-orienting condition was similar to that observed in semantic-priming tasks, and was compatible with the operation of controlled semantic processes. Spatial orienting significantly enhanced the amplitude of the early visual potential P1 as well as the language-related N400 potential. These findings showed that the similar end-result of behavioral facilitation by semantic and spatial orienting is achieved through largely distinct mechanisms acting upon separate levels of stimulus analysis.     

Unilateral attention deficits and hemispheric asymmetries in the control of visual attention

The present study sought to determine the roles of the two hemispheres in arousal and the selective components of attention. Ten patients with left and right parietal lesions and ten with left and right temporal lesions participated in the experiment. The hypothesis that posterior parietal lesions, whether left or right, cause two selective attentional deficits, namely, a reduced reactivity to stimuli in the visual field contralateral to the lesion, and a reduced reactivity to any stimulus which occupies a relative contralateral spatial position, was tested by asking the patient to tilt their head either to the left or to the right by 90 degrees and to respond to two stimuli displayed above and on either side of fixation mark. The arousal component of attention was studied by analysing the overall RT to visual stimuli independent of their spatial positions. The results showed that (1) patients with either left or right parietal damage are impaired in shifting attention from the ipsilateral to the contralateral visual field, and, within each visual field, in a direction contraversive to the lesion and (2) these two attentional deficits are more severe after right than after left parietal damage. Furthermore, the results show that the difficulty in maintaining a high level of alertness is a specific deficit of patients with right hemispheric lesion and not of patients with an extinction syndrome, insofar as there is no significant difference in overall RT between patients with parietal and temporal lesions.     

Neuroleptics reverse attention asymmetries in schizophrenic patients

Discussions of hemispheric asymmetry in psychopathology are often confounded by the effects of medication. We examined the effect of neuroleptic drugs on attention asymmetries in acutely psychotic patients admitted for the first time to a psychiatric hospital before the initiation of drug treatment and again after a period of treatment with neuroleptics. Overall performance did not change significantly; however, attention asymmetry was clearly related to the medication status of the patient: unmedicated patients showed inattention to the right hemispace, which changed to more prominent left-sided inattention when medicated. A longer time on medication or a higher daily dose were associated with a shift of inattention from the right to left hemispace. This suggests that neuroleptics may normalize left hemisphere performance, at the expense of deteriorated right hemisphere performance.     

Cerebral blood flow asymmetries in headache-free migraineurs

Regional cerebral blood flow (rCBF) asymmetries were studied in controls and patients with common and classic/complicated migraine using 133Xe inhalation with 8 homologously situated external collimators over each cerebral hemisphere. Migraine patients as a group more frequently had posterior rCBF asymmetries than controls (p less than 0.03). Although there were no differences in the number of anterior rCBF asymmetries, migraine patients had 2 or more asymmetric probe pairs more often than controls (p less than 0.02). The posterior rCBF asymmetries, consistent with the site of activation of many migraine attacks, may be related to more labile control of the cerebral circulation.     

Lateralised covert attention in word identification

The right visual field superiority in word recognition has been attributed to an attentional advantage by the left brain hemisphere. We investigated whether such advantage involves lateralised covert attention, in the absence of overt fixations on prime words. In a lexical decision task target words were preceded by an identical or an unrelated prime word. Eye movements were monitored. In Experiment 1 lateralised (to the left or right of fixation) prime words were parafoveally visible but foveally masked, thus allowing for covert attention but preventing overt attention. In Experiment 2 prime words were presented at fixation, thus allowing for both overt and covert attention. Results revealed positive priming in the absence of fixations on the primes when these were presented in the right visual field. The effects of covertly attended primes were nevertheless significantly reduced in comparison with those of overtly attended primes. It is concluded that word identification can be accomplished to a significant extent by lateralised covert attention alone, with right visual field advantage.     

Lateralised covert attention in word identification

The right visual field superiority in word recognition has been attributed to an attentional advantage by the left brain hemisphere. We investigated whether such advantage involves lateralised covert attention, in the absence of overt fixations on prime words. In a lexical decision task target words were preceded by an identical or an unrelated prime word. Eye movements were monitored. In Experiment 1 lateralised (to the left or right of fixation) prime words were parafoveally visible but foveally masked, thus allowing for covert attention but preventing overt attention. In Experiment 2 prime words were presented at fixation, thus allowing for both overt and covert attention. Results revealed positive priming in the absence of fixations on the primes when these were presented in the right visual field. The effects of covertly attended primes were nevertheless significantly reduced in comparison with those of overtly attended primes. It is concluded that word identification can be accomplished to a significant extent by lateralised covert attention alone, with right visual field advantage.     

Finding the right word: hemispheric asymmetries in the use of sentence context information

The cerebral hemispheres have been shown to be differentially sensitive to sentence-level information; in particular, it has been suggested that only the left hemisphere (LH) makes predictions about upcoming items, whereas the right (RH) processes words in a more integrative fashion. The current study used event-related potentials to jointly examine the effects of expectancy and sentential constraint on word processing. Expected and unexpected but plausible words matched for contextual fit were inserted into strongly and weakly constraining sentence frames and presented to the left and right visual fields (LVF and RVF). Consistent with the prediction/integration view, the P2 was sensitive to constraint: words in strongly constraining contexts elicited larger P2s than those in less predictive contexts, for RVF/LH presentation only. N400 responses for both VFs departed from the typical pattern of amplitudes graded by cloze probability. Expected endings in strongly and weakly constraining contexts were facilitated to a similar degree with RVF/LH presentation, and expected endings in weakly constraining contexts were not facilitated compared to unexpected endings in those contexts for LVF/RH presentation. These data suggest that responses seen for central presentation reflect contributions from both hemispheres. Finally, a late positivity, larger for unexpected endings in strongly constraining contexts, observed for these stimuli with central presentation was not seen here for either VF. Thus, some phenomena observed with central presentation may be an emergent property of mechanisms that require interhemispheric cooperation. These data highlight the importance of understanding hemispheric asymmetries and their implications for normal language processing.     

An fMRI investigation of strategies for word recognition

A common procedure used to study visual word recognition is the lexical decision task (LDT). Behavioral studies have demonstrated that overall performance in this task is modulated by the type of foils presented. There are divergent claims about the impact of different types of foils on overall processing strategies in the LDT: some researchers claim that pseudohomophone foils (e.g., BRANE) encourage participants to selectively de-emphasize phonological processing, while other researchers claim that pseudohomophone foils encourage participants to engage in more extensive processing of all types (orthographic, phonological, and semantic). To evaluate these claims, we conducted a functional magnetic resonance imaging study. Participants (N = 18) completed three lexical decision tasks, each with a different foil type: consonant strings (e.g., BVRNT), pseudowords (e.g., BLINT), and pseudohomophones (e.g., BRANE). We presented homophones (e.g., MAID) and nonhomophones (e.g., MESS) on word trials in order to be able to calculate the homophone effect as a marker of phonological processing for word stimuli in each foil condition. Comparison of behavioral results in the different foil conditions showed that reaction times were longest, error rates were highest, and homophone effects were largest in the LDT with pseudohomophones. Imaging results showed the greatest magnitude of activity in several regions, including the inferior frontal cortex, during the LDT with pseudohomophone foils. A comparison to inferior frontal activity produced during an additional task (rhyme judgment) supported the conclusion that LDT with pseudohomophone foils is a difficult task in which readers engage in an overall response strategy involving extensive processing of phonological information.     

Cortical dynamics of word recognition

While functional neuroimaging studies have helped elucidate major regions implicated in word recognition, much less is known about the dynamics of the associated activations or the actual neural processes of their functional network. We used intracerebral electroencephalography recordings in 10 patients with epilepsy to directly measure neural activity in the temporal and frontal lobes during written words' recognition, predominantly in the left hemisphere. The patients were presented visually with consonant strings, pseudo-words, and words and performed a hierarchical paradigm contrasting semantic processes (living vs. nonliving word categorization task), phonological processes (rhyme decision task on pseudo-words), and visual processes (visual analysis of consonant strings). Stimuli triggered a cascade of modulations in the gamma-band (>40 Hz) with reproducible timing and task-sensitivity throughout the functional reading network: the earliest gamma-band activations were observed for all stimuli in the mesial basal temporal lobe at 150 ms, reaching the word form area in the mid fusiform gyrus at 200 ms, evidencing a superiority effect for word-like stimuli. Peaks of gamma-band activations were then observed for word-like stimuli after 400 ms in the anterior and middle portion of the superior temporal gyrus (BA 38 and BA 22 respectively), in the pars triangularis of Broca's area for the semantic task (BAs 45 and 47), and in the pars opercularis for the phonological task (BA 44). Concurrently, we observed a two-pronged effect in the prefrontal cortex (BAs 9 and 46), with nonspecific sustained dorsal activation related to sustained attention and, more ventrally, a strong reflex deactivation around 500 ms, possibly due to semantic working memory reset.     

Re-evaluating split-fovea processing in word recognition: Effects of word length

Several studies have claimed that, when fixating a word, letters to the left and right of fixation project to different hemispheres and are consequently subjected to different processes. In support of this claim, Lavidor M, et al. (2001; hereafter LES&B) report that lexical decisions were affected by increasing the number of letters to the left of fixation but not to the right, and that this indicates divided hemispheric access at the point of fixation to length-sensitive processes in the right hemisphere (RH). We re-evaluated these claims in Experiment 1 using Lavidor et al.'s original stimuli and procedure of merely instructing participants where to fixate. In contrast to the earlier study, increases in the number of letters to the left and right of the designated fixation location produced near-identical effects on reaction time, and increases to the left actually improved response accuracy and increases to the right impaired it. When larger stimuli were used to improve stimulus perceptibility and an eye-tracker monitored fixation accuracy (Experiment 2), left and right increases in the number of letters again produced near-identical effects on reaction time (and accuracy), but frequent and substantial fixation errors were revealed. When an eye-tracker ensured accurate fixations (Experiment 3), left and right increases in the number of letters again produced near-identical effects on reaction time and accuracy. Thus, the findings of all three experiments provide no support for the findings of LES&B (2001) and no evidence of split-fovea processing. The findings also indicate the dangers of assuming fixation of precisely-specified locations within words, both in experiments designed to reveal split-foveal processing and hemispheric asymmetry and in more normal circumstances of word perception.     

Neuromagnetic evidence for early semantic access in word recognition

Magnetic brain responses recorded in the human magnetoencephalogram (MEG) distinguished between words with different semantics but carefully matched for frequency and length. Multiple recordings from a single subject showed that 100 ms following stimulus onset, significantly stronger neuromagnetic responses were elicited by words with strong multimodal semantic associations than by other word material. At this early processing step, there was a highly significant correlation (0.80) between the magnitude of brain responses to individual words recorded over parieto-occipital areas and their semantic association strengths. Subsequent to this early difference related to word meaning, additional differences in MEG responses emerged for words from different grammatical categories. Together, these results suggest that word meaning can be reflected by early neuromagnetic brain responses and before the grammatical information about the word is encoded.     

Attenuation of spatial attentional asymmetries with poor sustained attention

The co-existence of deficits in sustained and spatial attention in patients with acquired damage to the right cerebral hemisphere has led to the proposition that sustained attention could be a marker for left spatial inattention, or neglect. We investigated the possibility that reductions in leftward spatial attentional asymmetries could arise from individual differences in the capacity for sustained attention even within healthy adult populations. We observed that healthy participants who performed poorly on a test of sustained attention had a significantly attenuated left spatial bias, relative to those with good sustained attention capacity, on a free-viewing spatial attention test. Our results provide further support for the notion that sustained attention may exert a modulatory influence on spatial attention.     

Hemispheric asymmetries in mediating intention, but not selective attention

Heilman and Van Den Abell [Neuropsychologia 17, 315-321, 1979] reported a right hemisphere dominance for cerebral activation. In the present study, we further examined the nature of this finding in an experimental paradigm in which selective attention and intention (response readiness) were manipulated independently. Normal subjects were tested on a choice reaction time task in which they were given preliminary information about where a target stimulus would occur (selective attention) and which hand to use for responding (intention). Our findings indicate that at short foreperiod durations, the right hemisphere is superior for mediating intention. There was no evidence for a right hemisphere superiority for selective attention.     

Inter-limb coupling in coordinated bimanual movement: attention and asymmetries

Inter-limb coupling, a phenomenon whereby each of the upper limbs tends to take on characteristics of the intended movement of the other, represents a limitation on the ability to perform asymmetrical bimanual movements. Two experiments each employing 16 dextral and 16 sinistral normal subjects are reported. In the first experiment evidence of inter-limb coupling was observed during a continuous bimanual rotary task. This coupling appeared to be asymmetrical, with the nonpreferred hand contributing more to coupling than the preferred hand, especially in dextrals. In the second experiment asymmetries in inter-limb coupling were found to be modified by the conscious direction of attention to one or other hand. This suggests that the often reported strong inter-limb asymmetry associated with dextrality, and the weaker assymetry associated with sinistrality, may be partly due to an underlying inter-limb attentional asymmetry in the former, and a relative lack of attentional asymmetry in the latter.     

Cerebral asymmetries in monozygotic twins: An fMRI study

Lateralization for language, spatial judgment and face processing was assessed in 42 pairs of identical twins, 21 discordant and 21 concordant pairs for handedness, using fMRI. Individual laterality indices were calculated based on the observed activation patterns. All tasks showed expected asymmetry, favoring the left-hemisphere for language and the right-hemisphere for spatial judgment and face processing. The intra-class correlations on the laterality indices were significant only on the language task and only for the concordant group, but not the discordant group, suggesting a stronger genetic influence for language asymmetry in concordant twins. The expected asymmetry was greater for the concordant group only on the language task. The difference was not significant, but conformed quite well to Annett's genetic model, which assumes a right-shift (RS) gene with one allele (RS+) biasing toward right-handedness and left-cerebral language dominance, and the other (RS−) leaving both asymmetries to chance. The model also assumes that the genetic influence is additive for handedness but dominant-recessive for left-cerebral language dominance, which explains the high concordance for language dominance in twins discordant for handedness. Our data suggest that the same gene has no influence on right-hemisphere dominance for spatial judgment or face processing, and offer little support for mirror-imaging in MZ twins other than that due to chance.     

Neurophysiological correlates of word recognition in dyslexia

Summary. The neurobiological basis of learning word spellings and recognition of recently learned words was assessed in a learning experiment in 9 dyslexics and 9 controls male adolescents. In a recognition paradigm previously learned pseudowords and graphic symbols were presented 50 times each interspersed pseudo-randomly between 3 unlearned items which were repeated 50 times and 150 filler pseudowords. The electrophysiological correlate of recognition of learned pseudowords and graphic symbols was a positivity around 600ms. For pseudowords the amplitude of this ERP component was significantly attenuated in the dyslexic group, no differences between the groups were found for recognition of graphic material. These data suggest that dyslexic children are able to learn the spelling of simple words, however, the neurophysiological correlate of recognition of these learned words is significantly attenuated. This result strengthens the view that dyslexic children are not generally impaired in recognition memory but specific for linguistic material like words.