Interhemispheric connectivity during lateralized lexical decision

The well‐established right visual field (RVF‐lh) advantage in word recognition is commonly attributed to the typical left hemisphere dominance in language; words presented to the LVF‐rh are processed less efficiently due to the need for transcallosal transfer from the right to left hemisphere. The exact stage for this hemispheric transfer is currently unsettled. Some studies suggest that transfer occurs at very early stages between primary visual regions, whereas other studies suggest that transfer occurs between the left visual word form area and its right hemisphere homolog. This study explores these conflicting accounts and finds evidence for both. Participants conducted a lateralized lexical decision task with both unilateral and bilateral display conditions. Connectivity analyses were conducted from magnetoencephalography signals that were localized to the left middle occipital gyrus (LMOG), right middle occipital gyrus (RMOG), left visual word form area (LVWFA), and right visual word form area (RVWA). Results from unilateral trials showed asymmetrical interhemispheric connectivity from the RMOG to LMOG and symmetrical interhemispheric connectivity between the LVWFA and RVWFA. Furthermore, bilateral presentations led to reduced interhemispheric connectivity between both homologous region of interest pairs. Together, these results suggest that lateralized word recognition involves multiple stages of interhemispheric interactions and that these interactions are reduced with bilateral displays.     

Dissociation of ear preference in monaural word and voice recognition

The ambiguity in the literature concerning cerebral dominance for voice recognition, has been investigated with a monaural go-no-go task, using male and female voices. A left ear advantage for the male, and a right ear advantage for the female voice was found, suggesting that both hemispheres can perform voice recognition. A monaural word recognition test showed the expected left hemisphere dominance.     

Visual Hemifield Asymmetry for Naming Concrete Nouns and Verbs in Children Between Seven and Eleven Years of Age

Three-letter concrete nouns (Experiment 1) or verbs (Experiment 2) were exposed tachistoscopically in the right and left visual hemifields using a technique of bilateral stimulation with cued order of report. For subjects aged seven to eleven years, both types of word gave a small RVF advantage on words reported first and a larger RVF advantage on words reported second. This interaction between order of report and visual hemifield has been found by Young, Bion and Ellis (1980) to characterize stimuli which the right hemisphere is able to recognize. The absence of age × visual hemifield interactions argues against any progressive lateralisation through childhood, but may be indicative of the way the right hemisphere acquires its reading vocabulary.     

Hemispheric differences in odour recognition

Two experiments were designed in order to study hemispheric differences in odour recognition in normal subject. In the first experiment subjects first smelled an odour and then a visual stimulus (a picture or a word related or unrelated to the odour) was flashed either to the left or to the right hemisphere for 150 milliseconds. Subjects had to press a key if the two stimuli (olfactory and visual) matched and another key if they did not. Reaction time analysis showed that the responses were faster when the second stimulus (either picture or word) was presented to the right hemisphere. In a second experiment the first stimulus was auditory rather than olfactory. Subjects were instructed to listen to names of odorants and then to respond to pictures or written names as in the first experiment. In this case a left hemisphere advantage emerged, presumably related to the priming of the left hemisphere by the auditorially presented verbal stimulus.     

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.     

ERP correlates of the bilateral redundancy gain for words

Neurophysiological correlates of hemispheric asymmetry and interhemispheric interaction in lexical processing were investigated in a lexical decision task with tachistoscopic stimulus presentation either unilaterally, to the right or left visual field, or bilaterally, with identical stimulus copies to each visual hemi-field. Behavioral data confirmed both right visual field advantage and bilateral redundancy gain for words but not for pronounceable orthographically regular pseudowords. ERPs showed a significant amplitude increase 160-200 after stimulus presentation specifically for words after bilateral redundant stimulation, which was present in the recordings from both hemispheres. Localization of cortical sources using minimum norm estimation indicated stronger cortical activity for words in temporal regions of both hemispheres after bilateral presentation compared with each of the unilateral stimulation conditions individually. Pseudoword presentation did not lead to a general increase of cortical activation in the bilateral condition compared with unilateral presentation. The specific activation increase for words in the bilateral redundant condition relative to unilateral stimulation and the absence of this effect for pseudowords, which became manifest in a significant interaction of the factors lexicality and presentation mode, is best explained by summation of neuronal activation from both hemispheres within distributed lexical circuits. Source estimation indicates that temporal areas, particularly in the left hemisphere, are the primary cortical loci where such stimulus-specific activity increases occurred.     

Functional magnetic resonance imaging study of word recognition in normal elders

Four healthy, cognitively intact elders participated. Subjects underwent fMRI scanning while performing a word recognition task with an easy condition (low demand) and a difficult condition titrated to each subject's ability (titrated demand). Relative to low, titrated demand was associated with increased activation of the left medial frontal (cluster level P <.002), right superior temporal (P <.007) and right superior parietal cortices (P<.001). Increased activation of bilateral cortical areas by elders during the more challenging titrated demand compared with low demand may indicate recruitment of additional brain regions, enabling subjects to maintain performance despite increasing difficulty. Alternatively, the bilateral activation on this word recognition task may reflect compensatory use of right hemisphere networks.     

Attentional mechanisms and perceptual asymmetries in tachistoscopic recognition of words and faces.

In simultaneous bilateral tachistoscopic recognition tasks, normal right-handers named more words in the right visual field and recognized more faces in the left visual field. Priming the left hemisphere with a verbal task diminished the left field superiority for faces, and priming the right hemisphere with a face-recognition task, reduced the right visual field superiority for words. These priming effects disappeared when a recognition procedure was used. When words and faces were presented simultaneously, subjects required to attend to the faces and report them first showed a left visual field superiority for faces and a right visual field superiority for words. Subjects told to attend to the words and report them first showed only the right visual field superiority for words.     

Hemi-field asymmetries in memory for incongruous scenes

The purpose of the present study was to investigate hemispheric asymmetries of long-term semantic memory in normal subjects. A hemi-field reaction time (RT) and accuracy paradigm was used and the test stimuli consisted of pictures, with different semantic organization. Two separate experiments were conducted. In the first, memory for pictures depicting common scenes or incongruous scenes was measured. In the second, we compared memory for incoherent scenes and the same incongruous scenes. The results showed no difference between the visual half-fields in accuracy scores. However, in both experiments the RT results showed that incongruous scenes were recognized consistently faster in the right visual half-field than in the left half-field. In addition, in the first experiment there was no visual field difference for common scenes. In the second experiment, there was a left visual field advantage for unorganized scenes. The results were interpreted to reflect hemispheric asymmetry in long-term semantic memory, in general, and showing a left hemisphere specialization for uncommon, unusual aspects of otherwise regular percepts, in particular. Applying schemata theory to the results, we discussed the possibility that the left hemisphere uses/stores schemata conducive to "flexible" thinking strategies while the right hemisphere uses/stores schemata that involve "rigid" thinking strategies.     

Determinants of the enhancement of the right visual field advantage by bilateral vs. unilateral stimuli

Bilateral presentation of two different verbal inputs usually produces a stronger left hemispheric/right visual field (RVF) processing advantage than unilateral presentation. Previous reports of this effect confounded stimulus load with display configuration. Two experiments are presented in which the number of items during unilateral and bilateral trials was equated. The RVF advantage was enhanced during a free recall paradigm (Experiment I) but not during a partial report paradigm in which subjects were cued in advance to report only the top or the bottom of the display (Experiment II). This demonstrates that the effect does not depend upon the number of inputs per display and can not invariably be produced by bilateral/bihemispheric stimulation. It is suggested that subjects may be strongly biased to give priority to items delivered to the RVF/left hemisphere during bilateral trials. However, when this bias is constrained by rendering half of the RVF inputs irrelevant, subjects may be forced to pay equal attention to LVF inputs. Thus, eliminating the attentional bias eliminates the enhancement of the RVF advantage during bilateral trials.     

Foveal word reading requires interhemispheric communication

The left cerebral hemisphere is dominant for language processing in most individuals. It has been suggested that this asymmetric language representation can influence behavioral performance in foveal word-naming tasks. We carried out two experiments in which we obtained laterality indices by means of functional imaging during a mental word-generation task, using functional transcranial Doppler sonography and functional magnetic resonance imaging, respectively. Subsequently, we administered a behavioral word-naming task, where participants had to name foveally presented words of different lengths shown in different fixation locations shifted horizontally across the screen. The optimal viewing position for left language dominant individuals is located between the beginning and the center of a word. It is shifted toward the end of a word for right language dominant individuals and, to a lesser extent, for individuals with bilateral language representation. These results demonstrate that interhemispheric communication is required for foveal word recognition. Consequently, asymmetric representations of language and processes of interhemispheric transfer should be taken into account in theoretical models of visual word recognition to ensure neurological plausibility.     

Reduced hippocampal activation during encoding and recognition of words in schizophrenia patients

OBJECTIVE: In patients with schizophrenia, impaired hippocampal activation either during encoding or recognition tasks has been observed in a few functional imaging experiments. In this fMRI study, the authors report results of word encoding and recognition in schizophrenia patients and healthy comparison subjects, with a special focus on correcting for behavioral recognition success in order to prevent a bias related to lower task performance in the schizophrenia patients. METHOD: The verbal encoding and recognition tasks were both first analyzed irrespective of recognition success. In a second analysis, recognition success was included in the block-designed encoding task as a covariate of no interest, and incorrectly classified items were rejected from the analysis of the event-related recognition task. RESULTS: Patients performed poorer on the recognition task than the comparison subjects. Bilateral hippocampal activation during encoding and recognition was observed in both groups. Right hippocampal activation in patients during recognition became significant only after exclusion of wrongly classified items. Group comparison revealed greater activation in the healthy comparison subjects in the left anterior hippocampus during encoding and bilaterally during recognition. Greater bilateral hippocampal activation in the healthy subjects and greater activation in the right anterior hippocampus in the schizophrenic patients were revealed after presentation of novel words, which were intermixed with previously encoded words in the recognition task. After exclusion of incorrectly classified items, the differences in the right hippocampus remained significant. CONCLUSIONS: This study provides evidence for disturbed hippocampal function during verbal encoding and recognition in patients with schizophrenia. It extends previous studies by correcting for the possible confound of differences in behavioral task performance. This approach further supports the concept of hippocampal dysfunction in schizophrenia.     

Attentional and sensory effects of lowered levels of intrinsic alertness

Low levels of intrinsic alertness are associated with lateralized performance in visual tasks, similar to neglect of the left (ipsilesional) visual hemi-field. However, it is unclear whether reduced alertness produces a specific lateralization of spatial-attentional processes in terms of the prioritization of right- over left-side stimuli, or whether it affects more basic functions of visuo-sensory coding, and/or higher function of the top-down control of selection, of stimuli on the left side. To decide between these alternatives, the present study examined the effects of lowered alertness, induced by a 50-min vigilance task, in a partial-report paradigm of briefly presented letter displays. With only one (unilateral) stimulus in display, no specific hemi-field effects were found under low-alertness conditions, indicating that reduced alertness impairs neither sensory effectiveness nor the top-town control of selection. However, with dual, bilateral stimuli, report accuracy was specifically affected for left-side targets (in subjects who showed comparable performance for both sides under normal-alertness conditions). This pattern can be interpreted in terms of a specific bias in spatial-attentional weighting, where prioritization of stimuli on the right leads to (mild) extinction of targets on the left. Moreover, participants who had a lower general level of alertness also showed a more pronounced re-distribution of weights, evidenced by a more severe imbalance in report accuracy, in a low compared to a normal state of alertness. This suggests that a low general level of intrinsic alertness engenders a specific vulnerability to neglect-like performance with a (mild) left-side extinction.     

Brain activation for lexical decision and reading aloud: two sides of the same coin?

This functional magnetic resonance imaging study compared the neuronal implementation of word and pseudoword processing during two commonly used word recognition tasks: lexical decision and reading aloud. In the lexical decision task, participants made a finger-press response to indicate whether a visually presented letter string is a word or a pseudoword (e.g., "paple"). In the reading-aloud task, participants read aloud visually presented words and pseudowords. The same sets of words and pseudowords were used for both tasks. This enabled us to look for the effects of task (lexical decision vs. reading aloud), lexicality (words vs. nonwords), and the interaction of lexicality with task. We found very similar patterns of activation for lexical decision and reading aloud in areas associated with word recognition and lexical retrieval (e.g., left fusiform gyrus, posterior temporal cortex, pars opercularis, and bilateral insulae), but task differences were observed bilaterally in sensorimotor areas. Lexical decision increased activation in areas associated with decision making and finger tapping (bilateral postcentral gyri, supplementary motor area, and right cerebellum), whereas reading aloud increased activation in areas associated with articulation and hearing the sound of the spoken response (bilateral precentral gyri, superior temporal gyri, and posterior cerebellum). The effect of lexicality (pseudoword vs. words) was also remarkably consistent across tasks. Nevertheless, increased activation for pseudowords relative to words was greater in the left precentral cortex for reading than lexical decision, and greater in the right inferior frontal cortex for lexical decision than reading. We attribute these effects to differences in the demands on speech production and decision-making processes, respectively.     

Independent functioning of the two cerebral hemispheres for recognizing bilaterally presented tachistoscopic visual-half-field stimuli.

Bilateral tachistoscopic presentation of verbal stimuli produces a significantly larger right visual half-field (VHF) superiority than does unilateral presentation, when fixation is controlled by a center digit. This experiment tested whether the increased asymmetry was due to (a) subjects attending the right VHF and ignoring the left VHF or (b) interference between the hemispheres due to competition for the left hemisphere language areas. Words, shaptes, and pictures of faces were presented bilaterally to each VHF, with fixation controlled by a center digit. In three conditions, the same type of stimuli was presented in each VHF (e.g., a word in both the left and right VHF). In two conditions words were presented to one VHF and nonverbal stimuli to the opposite VHF (words paired with words and words paired with faces). It was found that the stimulus pairings did not affect VHF asymmetry for any stimulus. Words showed a large right VHF superiority in all conditions. Shapes showed a significantly smaller right VHF superiority in all conditions. Faces showed no VHF asymmetry in any condition. It was concluded that attentional factors were not important since shapes or faces could be recognized accurately from the left VHF without lowering verbal recognition from the right VHF. Thus the low recognition accuracy from the left VHF is specific for verbal stimuli rather than attentional. The interference hypothesis was also not supported since all the right VHF stimuli (words, shapes, or faces) were associated with low recognition of words from the left VHF. It was suggested instead that VHF asymmetry under unilateral and bilateral presentation reflect two different mechanisms. Under conditions of unilateral presentation, VHF asymmetries are caused by loss of information when any given stimulus must cross the callosum to reach the hemisphere specialized for its processing. However, with bilateral VHF presentation and fixation control, the two hemispheres act as independent channels for information processing. Under this condition, each hemisphere recognized the stimulus from its contralateral VHF. Thus the large right VHF superiority for words with bilateral presentation reflects the superior ability of the left hemisphere for verbal recognition.     

Assessing effects of stimulus orientation on perception of lateralized words and nonwords

Numerous studies of the processes of visual word recognition in the left and right cerebral hemispheres have attempted to control for confounding differences in the retinal placement (and hence visual acuity) of the beginnings of words by re-orientating normally-horizontal words vertically. However, despite the popularity of this approach, little is known about the precise effects that vertically orientating normally-horizontal words exert on hemispheric processes of word recognition. In this study, we investigated perception of horizontal and vertical English words and nonwords in the left visual field (LVF) and right visual field (RVF). An eye-tracking device ensured central fixation and a 2AFC paradigm (Reicher-Wheeler task) suppressed influences of non-perceptual bias. Horizontal stimuli produced a strong right visual field advantage for words but not for nonwords, whereas, vertical stimuli produced no hemifield differences at all. Moreover, vertical stimuli produced an advantage for words over nonwords in both visual fields whereas horizontal stimuli produced this effect only in the right visual field. Implications of these findings for the sensitivity of processes of word perception to stimulus orientation in the two cerebral hemispheres are discussed.     

Visual half-field experiments are a good measure of cerebral language dominance if used properly: evidence from fMRI

Traditional neuropsychology employs visual half-field (VHF) experiments to assess cerebral language dominance. This approach is based on the assumption that left cerebral dominance for language leads to faster and more accurate recognition of words in the right visual half-field (RVF) than in the left visual half-field (LVF) during tachistoscopic presentation. Information in the RVF is directly projected to the left hemisphere, whereas information presented in the LVF needs interhemispheric transfer to reach the left half of the brain. This interpretation of the RVF superiority for word recognition lacks direct evidence however, and a multitude of studies have lead to contradictory findings. To investigate this matter further we try to establish the ideal parameters for VHF experiments to measure language dominance, and subsequently compare laterality indices (LIs) obtained from RT patterns in bilateral VHF tasks to those LIs acquired in the same individuals during a mental word generation task in the fMRI scanner. Our results reveal a direct link between VHF advantages and individual language lateralization. Differences in behavioral performance between left-hemisphere dominant and right-hemisphere dominant individuals suggest that carefully designed VHF tests can be used as a reliable predictor of cerebral language dominance.     

Symmetrical hemispheric priming in spatial neglect: A hyperactive left-hemisphere phenomenon?

Hemispheric rivalry models of spatial neglect suggest that the left hemisphere becomes hyperactive following right-hemisphere lesions since the two hemispheres normally exert an inhibitory influence on each other via callosal connections. Using a masked hemifield priming paradigm, we investigated whether the putative change in hemispheric balance involves other, higher-order abstract representational systems in spatial neglect. Participants consisted of 12 neglect patients with right-hemisphere damage and three groups of control participants, i.e., 12 young healthy controls, 10 age-matched healthy controls and 10 right-hemisphere patients without spatial neglect. In each trial, participants made semantic categorization about a centrally presented target word which was preceded by a masked prime flashed either to the left or right visual field. All three control groups exhibited strong left-hemisphere advantage in inhibitory syllabic priming, consistent with the known left-hemisphere dominance in lexical inhibition during reading. By contrast, neglect patients exhibited a symmetrical pattern of priming between the left and right visual fields. These results suggest that (1) the neglected hemifield can rapidly extract abstract information even from weak and normally non-perceptible visual stimuli, but that (2) the normal left hemispheric dominance in reading is absent in neglect patients probably because of the generalized hyperactivity of the left hemisphere. Our results demonstrate a covert behavioral change in spatial neglect which may reflect the altered inter-hemispheric balance in the bilateral word recognition system encompassing lexico-semantic memory.     

An FMRI study of episodic encoding and recognition of words in patients with schizophrenia in remission

OBJECTIVE: Verbal memory deficits are among the most severe cognitive deficits observed in patients with schizophrenia. This study examined patterns of brain activity during episodic encoding and recognition of words in patients with schizophrenia. METHOD: Functional magnetic resonance imaging (fMRI) was used to study regional brain activation in 10 healthy male comparison subjects and 10 male outpatients with schizophrenia during performance of a modified version of the words subtest of Warrington's Recognition Memory Test. RESULTS: Despite having intact performance in word recognition, the patients with schizophrenia had less activation of the right dorsolateral and anterior prefrontal cortex, right anterior cingulate, and left lateral temporal cortex during word encoding, compared with the healthy comparison subjects. During word recognition, the patients had impairments in activation of the bilateral dorsolateral prefrontal and lateral temporal cortices. CONCLUSIONS: Schizophrenia was associated with attenuated frontotemporal activation during episodic encoding and recognition of words. These results from an fMRI study replicate earlier findings derived from a positron emission tomography study.