Hemispheric facilitation as a result of threatening and nonthreatening words: blocked vs mixed presentation effects and order effects.

Sixteen right-handed participants (eight male and eight female students) were administered a tachistoscopic unilateral letter-naming task. The design contained three valance conditions and two types of presentation. In baseline conditions no concurrent task was given. In threat and nonthreat conditions, each unilateral stimulus was preceded by a threatening or nonthreatening word in central vision. Participants were instructed to recall this word after reporting the unilateral letter string. With blocked presentations, a series of trials had the same emotional valence (threatening or nonthreatening), whereas with mixed presentations the valences were alternating within a series. Analysis of order effects for the blocked presentations revealed sustained effects of the initial block valence on visual field asymmetries. The sustained effect of the initial threatening block was a reduction of the right visual-field advantage. Mixed presentations resulted in a enhanced right visual-field advantage following the presentation of threatening information and an enhanced left visual-field advantage following the presentation of nonthreatening information. The research suggests that tonic and phasic reactions to emotional stimuli may affect visual field asymmetry in different ways.     

Interhemispheric cooperation and activation in integration of verbal information

Subjects had to judge the semantic relation between two tachistoscopically presented German adjectives. In Exp. I, the two words were projected simultaneously, either both to the left visual field (i.e. to the right hemisphere, LVF-RH), or both to the right visual field (i.e. to the left hemisphere, RVF-LH), or one word to each visual field (i.e. to both hemispheres, EVF-BH). The results yielded a significantly higher performance in the condition EVF-BH than in the two unilateral conditions, without a significant difference between the latter two conditions. In Exp. II, the two words were shown successively. With bilateral presentation, a distinction was made as to whether the left visual field (EVF-BH-L) or the right visual field (EVF-BH-R) was stimulated first. The significantly highest performance was shown in conditions RVF-LH and EVF-BH-R compared with the condition EVF-BH-L, and the significantly poorest performance in the condition LVF-RH compared with the condition EVF-BH-L. The results provide evidence for (1) an intrahemispheric interference with simultaneous unilateral presentation, (2) a cooperative interhemispheric interaction with simultaneous bilateral presentation, and (3) an interhemispheric priming effect with successive bilateral presentation when the left hemisphere was stimulated first.     

Word repetition within- and across-visual fields: an event-related potential study

A divided visual field (DVF) procedure was used to investigate the scalp distribution of the event-related potential (ERP) repetition effect. ERPs were recorded from 27 scalp sites whilst subjects (n = 20) discriminated between words and non-words presented to either the left (LVF) or the right (RVF) visual field. A proportion of the words were repeated on the trial immediately following their first presentation. In two within-field repetition conditions the two encounters with a word occurred in the same visual field (LVF or RVF). In two across-field repetition conditions, the two encounters with a word occurred in different visual fields. For both words and non-words, task performance was better for RVF presentations than for LVF presentations. In each repetition condition there was a positive-going shift in the ERP elicited by repeated words compared to that elicited by words on their first presentation. This ERP repetition effect was equivalent in magnitude and lateralised to the right hemisphere to an equivalent degree in all four repetition conditions. It is suggested that the ERP effects largely reflect the processing of visual form thought to occur predominately in the right hemisphere.     

Effects of lateralized presentation on levels of processing in memory

The present study utilized lateralized presentation of words in a level-of-processing paradigm. Eighty words were presented to subjects in four level-of-processing conditions: orthographic, phonetic, syntactic and semantic. Words were presented either to the left or right visual fields (Experiments 1 and 2), or centrally (Experiment 3). Both free recall and recognition retrieval conditions were utilized. There were no visual field differences in the recognition condition. In the free recall condition, there was an interaction between visual field of presentation and level of processing. Recall was greatest for phonetically and semantically processed words which had been presented to the right visual field. A left visual field advantage was found for syntactically processed words. Results are interpreted in the context of retrieval strategies and hemisphere-linked processing capacities.     

Verbal visual extinction in right/left hemisphere lesion patients and the problem of lexical access

In order to explore perceptual processes preceding visual lexical access, we compared single words reading in both visual hemifields in right or left hemisphere damaged patients. Words were tachistoscopically presented in three conditions: (1) one word in one visual hemifield, (2) two words, one in each visual hemifield aside the fovea, (3) one longer word in both visual hemifields, centred on the fovea. In many patients, suffering from right as well as left hemisphere lesions, reading remains normal for unilateral presentation whereas bilateral presentation induced extinction in the hemifield contralateral to the lesion. Nevertheless, extinction was not frequently observed for the right or the left part of long centred words. Extinction proved to be most frequent in lesions centred on the occipito-temporo-parietal area. In addition, we found visual extinction not to be related to auditory extinction in verbal dichotic listening.     

Evidence for hemispheric specialization of lexical distinctions in bilingual Chinese-Mandarin speakers

Lexical decision vocal reaction times of a group of English-proficient, Chinese-Mandarin speakers and group of monolingual, English speakers were measured to unilaterally presented concrete and abstract English words. Results of an ANOVA showed a significant group x visual field x stimulus type interaction. Post-hoc analysis showed a significant right visual field advantage for the Chinese subjects. For the English speaking subjects, right visual field stimulations yielded significantly faster vocal reaction times to the abstract words than to the concrete ones, while the opposite occurred for left visual-field inputs. Also, a correlation between the two lateral conditions was significant for the Chinese subjects but not for the English speakers. These findings suggest that the English speakers evidenced dissociated left and right hemispheric linguistic processing while the Chinese subjects left hemisphere was responsible for the final phonological stages of linguistic analysis. Such findings support a phonological "monitor-user" hypothesis for cerebral dominance characteristics in bilingual Chinese speakers.     

Attentional Bias and Visual Field Asymmetry

In mixed list tachistoscopic presentation of abstarct words and random shapes, the probability of word and shape stimuli was varied to generate attentional bias and selective activation of the left or right hemispheres. While a right visual field advantage in accuracy was observed for words, and an effect of stimulus probability was observed across both visual fields in reaction time, no interactions were found between attentional bias and visual field asymmetry. The results can therefore be explained without reference to the attentional model and do not provide evidence of hemispheric activation or priming.     

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.     

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.     

Cerebral hemispheric differences in memory of emotional and non-emotional words in normal individuals

The present study was designed to examine the cerebral hemispheric differences in memory of positive, negative and non-emotional words using a new method of successive presentation to each visual half-field in which perception of each item was nearly perfect thereby allowing laterality differences for effects of emotion on memory to emerge unconfounded by perception (Experiment 1). This procedure was compared with traditional perceptual identification (Experiment 2). In Experiment 1, 12 words were presented successively in each half field in each trial followed by free recall at the end of the trial. The results showed that recall of positive and negative emotional words was better than that of non-emotional words in both visual fields. Recall of positive and negative emotional words was not different in left and right visual fields (RVFs) although the recall of non-emotional words was better in the RVF than in the left visual field (LVF). The differences in recall between emotional and non-emotional words was greater in the LVF than in the RVF. Experiment 2 used the more traditional method of perceptual identification following each visual half-field presentation of a single item. Perceptual identification was better in the RVF than the LVF in each word condition. There were no visual field differences in perceptual identification between emotional and non-emotional words, as there was for memory in Experiment 1. The results supported the hypothesis that explicit memory for emotional words was dependent more on the right hemisphere, whereas perception of both emotional and non-emotional words was more dependent on the left hemisphere.     

Effects of spelling-to-sound regularity on word identification following brief presentation in the right or left visual field

In this experiment subjects were briefly presented with a vertically written word to either the left or right visual field. On half the critical trials the target word was irregular in its spelling-to-sound correspondence (e.g. PINT) and in the other half it was regular (e.g. LIME). With right visual field presentation substantially more regular than irregular words were reported correctly but with left visual field presentation no such effect was found. The data also showed the presence of far more "regularization" errors following right visual field presentation. The data are seen as consistent with the view that the right hemisphere is not capable of generating phonology.     

Laterality differences in recognition of Japanese and Hangul words by monolinguals and bilinguals

Two-character nonsense Kana words, individual Kanji words and individual Hangul words are presented tachistoscopically in the left or right visual field to 20 normal, right-handed Japanese students and 13 normal, right-handed Korean subjects. The former did not know Hangul letters. The latter were born and raised in Japan, in exclusively Japanese-speaking families, but they could read Hangul letters and write them a little because they have learned the Hangul language for 6 months. In each of three conditions (Kana, Kanji and Hangul work recognition), each subject was required to move the index finger of his right or left hand leftwards as fast as possible after the presentation of two of four stimuli and rightwards after the presentation of the other two. The reaction time was measured. A significant right field superiority for the recognition of Kana words and no lateral asymmetry for Kanji words were shown in both Japanese and Korean groups. However, for the Hangul recognition, a significant left field superiority for Japanese subjects and a significant right field superiority for Korean subjects were obtained. These findings are interpreted as follows. Kana and Kanji are processed somewhat differently in the cerebral hemispheres. Japanese subjects do not recognize Hangul stimuli as orthographic characters but as shapes or figures. Korean subjects can identify Hangul stimuli as letters. Both the first language (Kana) and second language (Hangul) are processed in the dominant left hemisphere by right-handed Korean subjects.     

Left visual-field advantage in the dual-stream RSVP task and reading-direction: A study in three nations

In the dual-stream Rapid Serial Visual Presentation task, a stream of stimuli containing two target stimuli is rapidly presented left and right. In previous studies, the second target was better identified in the left than in the right hemifield. In all those studies, alphanumeric stimuli were used both as targets and distracters. We examined to what extent this left visual-field advantage is dependent on reading-direction. The task was performed by Germans (with Latin characters), Israelis (with Latin and Hebrew characters) and Taiwanese (with Latin and Chinese characters). If caused by overlearnt associative links between Latin characters and left-to-right reading, the prominent left visual-field bias should be reversed in Hebrew and disappear in Chinese. Furthermore, if caused by direction of reading in the participant's native language, the left visual-field advantage in Latin conditions should be larger in Germans than in Israelis and Taiwanese. A left visual-field advantage was always observed, though slightly smaller in Hebrew and in Chinese, and there was no difference in the Latin conditions between the three nations. Therefore, it seems that the left visual-field advantage in speeded target identification is not primarily caused by the left-to-right reading-direction, but may be a combined effect resulting from the asymmetric organization of general mechanisms of visual processing and from stimulus-induced preferences.     

Visual-field superiority as a function of stimulus type and content

The objective of this study was to examine whether hemispheric superiority is determined more by stimulus type (facial, lexical) or content (neutral, emotional). A split, visual-field experiment was designed using a computer-based program with bilateral presentation (left visual-field, right visual-field) for four sets of stimuli: face (neutral, emotional) x word (neutral, emotional), N = 40. The dependent measures were the frequency of correct response and response latency of correct responses. The visual-field effect was nonsignificant for correct responses; however, the interaction of stimulus type x content was found significant. The interaction of visual-field x stimulus type was significant with response time as the dependent measure. Facial stimuli were processed faster in the left visual-field (a right hemispheric function) and lexical stimuli were processed faster in the right visual-field (a left hemispheric function). No hemispheric effect was observed for stimulus content.     

Laterality and sex differences for visual recognition of emotional and non-emotional words

Clinical and experimental evidence indicate that the right hemisphere possesses a special capacity for emotional non-verbal material. The possibility that this ability would extend to language was tested with a lexical decision paradigm using tachistoscopic double simultaneous presentation of emotional and non-emotional words. Male subjects showed an overall right visual field superiority with the predicted effect of emotional words in the left visual field. In contrast, the field advantages for females were more variable and showed evidence of a larger effect of emotional words in the right visual field. The imageability potential of words also appeared to influence the female performance.     

The right hemisphere fails to orient to the negative valence of visually presented words

To assess cerebral functional asymmetry for accessing the meaning of emotional words, native English speakers were asked to make lexical decisions to emotional words, neutral words and nonwords briefly flashed in their left or right visual field, that is, perceived by their right or left visual cortex, respectively. Emotionally negative words had an overall probability of occurrence of 12.5% in each visual field and were expected to elicit a P3b event-related potential modulation indexing shifts of attention. Emotionally negative words elicited a P3b when perceived by the left visual cortex but failed to trigger such attention-related response when perceived by the right hemisphere. The results suggest relative insensitivity on the part of the right hemisphere in registering the negative emotional valence of written words.     

Differential processing of Kanji and Kana stimuli in Japanese people: Some implications from stroop-test results

RTs of Japanese subjects responding to Kanji and Kana Stroop type colour words presented in the left or the right visual field were measured.When subjects tried to respond to Kanji Stroop stimuli, they showed markedly greater interference effect in the case of left visual field presentation, while they showed no visual field differences when they responded to Kana Stroop stimuli.These results support the notion that Kanji has a different property from Kana and suggest a specialization of the right hemisphere for Kanji processing.     

Evaluating a split fovea model of visual word recognition: effects of case alternation in the two visual fields and in the left and right halves of words presented at the fovea

Two experiments are reported exploring the effect of cAsE aLtErNaTiOn on lexical decisions to words and nonwords presented laterally or centrally. In line with previous research, Experiment 1 found that case alternation slowed lexical decision responses to words more in the right visual field (RVF) than in the left visual field (LVF). In Experiment 2, the words and nonwords were all presented centrally. There were three conditions, a condition in which the word and nonwords were presented in lower case letters, a condition in which the letters to the left of the central fixation were case alternated (e.g., aMbItion, mOdLants) and a condition in which the letters to the right of fixation were case alternated (e.g., collApSe, pireNtOl). Alternating the case of letters to the right of fixation slowed lexical decision responses more than alternating letter case to the left of fixation. The results provide further support for a split fovea account of visual word recognition according to which those letters of a centrally-fixated word that fall to the left of fixation are processed initially by the right cerebral hemisphere while those letters that fall to the right of fixation are processed initially by the left cerebral hemisphere, with the characteristics of the left and right hemispheres being revealed in the processing of initial and final letters in centrally presented words.     

VISUAL-FIELD SUPERIORITY AS A FUNCTION OF STIMULUS TYPE AND CONTENT

The objective of this study was to examine whether hemispheric superiority is determined more by stimulus type (facial, lexical) or content (neutral, emotional). A split, visual-field experiment was designed using a computer-based program with bilateral presentation (left visual-field, right visual-field) for four sets of stimuli: face (neutral, emotional) &#50 word (neutral, emotional), N = 40. The dependent measures were the frequency of correct response and response latency of correct responses. The visual-field effect was nonsignificant for correct responses; however, the interaction of stimulus type &#50 content was found significant. The interaction of visual-field &#50 stimulus type was significant with response time as the dependent measure. Facial stimuli were processed faster in the left visual-field (a right hemispheric function) and lexical stimuli were processed faster in the right visual-field (a left hemispheric function). No hemispheric effect was observed for stimulus content.     

The importance of being first: an analysis of tachistoscopic presentations of words

The importance of the first letter of a word as a cue for the rest of the word was explored in a tachistoscopic presentation of four letter English words to the two hemispheres. The positioning of words presented bilaterally and the frequency of the words were manipulated so that the role of the first letter could be examined. The results indicate that the first letter does not play the critical role in word recognition that has been ascribed to it. In the standard bilateral presentation, subjects' recall patterns indicated that first letters were recalled equally well whether the word was presented in the right or left visual field. However, subjects did not recall as many words correctly when they went to the left visual field. The results were interpreted as supporting an information processing view of hemispheric specialization. Such a view proposes several levels of functioning in which the two hemispheres overlap, cooperate, or specialize, depending on the task to be accomplished.