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.     

Opposite hemispheric activations as a result of emotionally threatening and non-threatening words.

Twenty right-handed males participated in a tachistoscopic unilateral letter recognition task with three conditions. In the control condition, each trial consisted of three consonants that were flashed horizontally to the left or to the right visual field. In the threat and non-threat conditions, each lateral three-letter presentation was preceded by an emotionally threatening or non-threatening word presented in central fixation. Across conditions, subjects identified more letters correctly in the right visual field than in the left visual field. The concurrent presentation of threatening words resulted in a selective enhancement of left visual-field performances. The concurrent presentation of non-threatening words resulted in a selective right visual-field enhancement. Our conclusion is that threatening stimuli prime the right hemisphere and can alter predicted laterality patterns.     

Hemisphere Division and its Effect on Selective Attention: A Generality Examination of Lavie's Load Theory

The present study examined the role of visual presentation mode (unilateral vs. bilateral visual fields) on attentional modulation. We examined whether or not the presentation mode affects the compatibility effect, using a paradigm involving two task-relevant letter arrays. Sixteen participants identified a target letter among task-relevant letters while ignoring either a compatible or incompatible distracter letter that was presented to both hemispheres. Two letters arrays were presented to visual fields, either unilaterally or bilaterally. Results indicated that the compatibility effect was greater in bilateral than in unilateral visual field conditions. Findings support the assumption that the two hemispheres have separate attentional resources.     

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.     

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.     

Hemispheric asymmetry for word naming: effects of frequency and regularity of pronunciation

Observers named visually presented words as quickly as possible when the words were presented to the centre of a viewing screen (Experiments 1, 5, and 6) or were flashed to either the left visual field/right hemisphere (LVF/RH) or right visual field/left hemisphere (RVF/LH) on each trial (Experiments, 2, 3, and 4). Words varied in frequency of occurrence in the language and in regularity of pronunciation. On lateralised trials, there was a RVF/LH advantage for both error rate and reaction time, with this hemispheric asymmetry for word naming being independent of both frequency and regularity. The RVF/LH advantage was also independent of whether the letters within a word were arranged horizontally or vertically. Error rates and reaction times were lower for high-than for low-frequency words and lower for words with a regular pronunciation than for exception words. In addition, on centre trials the effects of regularity were larger for low-frequency words than for high-frequency words. However, when words were presented in the visual periphery, the effects of regularity were of the same magnitude for high-and low-frequency words. In view of the theoretical importance of the Frequency by Regularity interaction, this latter result suggests that word processing is qualitatively different in the visual periphery than in the centre of vision; indicating that perceptual asymmetries in a typical visual half-field experiment may be limited in what they can tell us about the relative contributions of the two hemispheres to processing words in the centre of vision.     

Hemispheric Asymmetry for Word Naming: Effects of Frequency and Regularity of Pronunciation

Observers named visually presented words as quickly as possible when the words were presented to the centre of a viewing screen (Experiments 1, 5, and 6) or were flashed to either the left visual field/right hemisphere (LVF/RH) or right visual field/left hemisphere (RVF/LH) on each trial (Experiments, 2, 3, and 4). Words varied in frequency of occurrence in the language and in regularity of pronunciation. On lateralised trials, there was a RVF/LH advantage for both error rate and reaction time, with this hemispheric asymmetry for word naming being independent of both frequency and regularity. The RVF/LH advantage was also independent of whether the letters within a word were arranged horizontally or vertically. Error rates and reaction times were lower for high-than for lowfrequency words and lower for words with a regular pronunciation than for exception words. In addition, on centre trials the effects of regularity were larger for low-frequency words than for high-frequency words. However, when words were presented in the visual periphery, the effects of regularity were of the same magnitude for high-and low-frequency words. In view of the theoretical importance of the Frequency by Regularity interaction, this latter result suggests that word processing is qualitatively different in the visual periphery than in the centre of vision; indicating that perceptual asymmetries in a typical visual halffield experiment may be limited in what they can tell us about the relative contributions of the two hemispheres to processing words in the centre of vision.     

Magnetic stimulation of the left visual cortex impairs expert word recognition

One of the hallmarks of expert reading is the ability to identify arrays of several letters quickly and in parallel. Such length-independent reading has only been found for word stimuli appearing in the right visual hemifield (RVF). With left hemifield presentation (LVF), response times increase as a function of word length. Here we investigated the comparative efficiency with which the two hemispheres are able to recognize visually presented words, as measured by word length effects. Repetitive transcranial magnetic stimulation (rTMS) of the left occipital cortex disrupted expert processing of the RVF such that a length effect was created (Experiment 1). Right occipital rTMS, on the other hand, had no such effect on RVF words and nor did it modulate the length effect already present in the LVF. Experiment 2 explored the time course of these TMS-induced effects by applying single pulses of TMS at various stimulus-onset asynchronies for the same task. We replicated the TMS-induced length effect for RVF words, but only when a single pulse was applied to the left visual cortex 80 msec after target presentation. This is the first demonstration of TMS-induced impairment producing a word length effect, and as such confirms the specialization of the left hemisphere in word recognition. It is likely that anatomical differences in the pathway linking retinal input to higher level cortical processing drive this effect.     

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.     

Right words and left words: electrophysiological evidence for hemispheric differences in meaning processing.

Both cerebral hemispheres are involved in language processing, each playing a unique role that may derive from differences in knowledge organization and on-line meaning integration. Here, we examine lateralized differences in knowledge representation and retrieval using event-related potentials (ERPs) elicited by words in sentences. Volunteers read pairs of sentences ending with three target types: (1) expected words, (2) unexpected words from the expected semantic category, and (3) unexpected words from an unexpected category. Context was presented word by word at fixation while targets were presented two degrees to the right or left of fixation. ERPs to unexpected endings were more negative than those to expected endings in both visual fields. However, when presented to the right visual field (left hemisphere), unexpected items from the expected category elicited smaller N400s than those from an unexpected category. In contrast, when presented to the left visual field (right hemisphere) all unexpected endings elicited N400s of similar amplitude. Thus, while both hemispheres are sensitive to context, only the left hemisphere is sensitive to semantic similarity between an unexpected ending and the expected completion. The results suggest lateralized differences in how new information is integrated into sentences. We propose that right hemisphere processing is best characterized as 'integrative'; new information is compared directly with context information. In contrast, left hemisphere processing is better characterized as 'predictive'; the processing of context leads to an expectation about the semantic features of upcoming items and new information is compared with that expectation rather than directly with the context.     

Phonological information in short-term memory of sentences

Potter and Lombardi (1990) showed that words that are semantically similar to nouns of auditorily or visually presented sentences intrude into sentence recall if presented in an unrelated word list (given either before or after the sentence). The authors interpreted this finding in terms of the conceptual regeneration hypothesis. This hypothesis states that sentence recall is based on conceptual information whereas the role of phonological information is negligible. However, this general interpretation is questionable. In order to demonstrate the involvement of phonological information in sentence recall, we used Potter and Lombardi's intrusion paradigm. Participants were either presented with auditory or visual material. With respect to visual presentations we used two conditions: a rapid presentation rate (rapid serial visual presentation, RSVP) and a slow presentation rate. Under all conditions the word list was presented before the sentence. The intrusion effect was only found for RSVP but not for the slow conditions. This finding provides evidence for the assumption that phonological information generally plays an important role in short-term sentence recall.     

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.     

An investigation of hemispheric asymmetry in size and semantic discriminations and related visual ERPs

The present study examined possible hemispheric differences in discriminations of different sizes of geometric shapes (rectangles) and different meanings of words, and determined whether left and right hemisphere derived visual event related potentials (ERPs) were related to performance. Eighteen right-handed subjects (10 male and 8 female) participated in two separate sessions conducted on different days. The visual ERPs were recorded from over left parietal (P3) and right parietal (P4) scalp locations. Subjects were required to make discriminations of three words (PARE, PAIR, PEAR) and three sizes of rectangles (small, medium, large). Each word and rectangle was singly presented for 40 msec at 1 degree 24 minutes of arc to the left and right of central fixation (LVF and RVF respectively). The major findings were as follows: 1) there were no performance and ERP differences between hemispheres in the verbal task; 2) the left hemisphere excelled in the spatial task; however, ERPs derived from the two hemispheres were similar; 3) subjects experienced greater difficulty in their discriminations of geometric size, as compared to words, regardless of field of presentation. Discrimination of size seemed to have influenced P3 (P300) latency, i.e., it was longer (both hemispheres) when subjects made size discriminations as compared to words. It was proposed that the more difficult discriminations involved in size discrimination required a greater time for stimulus evaluation and that this was reflected in the delayed P3 response.     

Hemispheric Differences in Matching Stroop-Type Letter Stimuli

Two experiments are reported in which normal right-handed males and females had to match two Stroop-type letter stimuli, according to either their global level (Experiment 1) or their local level (Experiment 2).Stimulus presentation was sequential with the first letter presented foveally and the second letter presented to either the right visual field (RVF) or the left visual field (LVF). A manual “Same-Different” response was required.In Experiment 1, the left hemisphere (RVF) showed interference from the irrelevant local level features, supporting an analytic processing mechanism, while the right hemisphere (LVF) showed no such effect, supporting a holistic matching mechanism. In Experiment 2, both hemispheres showed interference from the irrelevant global features, with the right showing a non-significant tendency towards a larger Stroop effect.     

Visual field effects and short-term memory for verbal material

The role of memory in the usual right visual field superiority for verbal material reported with normal subjects was investigated. Nonsense words were presented vertically in either the left or right visual field. The exposure duration, which was individually determined for each subject to produce about 75% correct responses, ranged from 5 to 25 msec. After a delay of 0, 5, or 10 sec the subject decided whether or not a nonsense word exposed in central vision was the same as the stimulus word.With males, the results suggested that the right visual field superiority represents left hemisphere specialization for retention of verbal material and hemispheric equality in reception. With females no visual field superiority was found perhaps indicating that both hemispheres receive and retain verbal information equally well. The findings support the concept of less-complete lateralization of linguistic and spatial functions in females.     

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.     

Emotional words: what's so different from just words?

Aphasic patients, in particular global aphasics, may still swear and produce emotional utterances with ease. Based on these clinical observations we investigated emotional word "reading" in a series of different experiments over 25 years, not only in aphasic patients, but also in the left (LVF) and right (RVF) visual fields of healthy subjects, and in a depth-recorded epileptic patient. Across these experiments we found: i) behaviorally a strong emotional word effect in the left visual field (right hemisphere - RH) of normals, correlating well with the emotional word performance of aphasic patients, pointing to a specific role of the right hemisphere; ii) electro-physiologically a specific early (100-140 msec) brain response to emotional words during scalp recordings in healthy subjects subsequent to right visual field (left hemisphere - LH) stimulation, that source localization procedures project to posterior areas of the right hemisphere; iii) preliminary data of a very early (60 msec) activation of the left amygdala in a depth-recorded epileptic patient when the same emotional words were presented to the right visual field (left hemisphere); and iv) a consistent gender difference showing that the above findings might be relevant for men only. Both hemispheres therefore appear to be implicated in emotional word "reading" but in different ways. We propose that the left amygdala via extrastriate connections acts as a detector of emotional word content at a very early stage of processing; that this amygdala response subsequently modulates the cortical response to emotional words asymmetrically, rendering the left visual cortex less sensitive to emotional words than that of the right hemisphere; and that this modulation is gender dependent.     

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.     

Interference and priming within and across visual fields in a lexical decision task

A lower-case target word or non-word was laterally presented by itself or simultaneously with an upper-case distractor word in the same or in the opposite visual field. The distractor words were either semantically associated or unrelated to the target. Subjects performed a lexical decision task on the target. Two kinds of selection within and across visual fields were investigated. Selection of an external target relevant for the response (filtering) and selection of an internal target by automatic priming. No lateral asymmetry was found for automatic priming. Filtering was equally efficient within hemispheres, but strongly asymmetrical across hemispheres. Unrelated distractor words presented to the right hemisphere interfered less with target processing in the left hemisphere than vice versa. Thus, all distractor words received equivalent semantic processing; those presented to the right hemisphere, however, were least costly to exclude from task-relevant processing in the left hemisphere.