Controlled semantic processes within and between the two cerebral hemispheres

To test the separate and combined abilities of the two cerebral hemispheres to perform controlled semantic selection and integration processes, Hebrew readers saw pairs of words and had to decide whether the two words were semantically related. The first word in each pair was presented centrally. The second word was presented in the left, right, or central visual field (LVF, RVF, and CVF). We compared response latencies for related pairs in two conditions: In the ambiguous condition, the first word was a homograph (either homophonic or heterophonic) and the second word was related to either its dominant or subordinate meaning. In the unambiguous condition, homographs were replaced with unambiguous control words. Irrespective of VF or homograph type, response times for ambiguous pairs were significantly longer than for unambiguous pairs only when targets were related to the subordinate meaning of the homograph. In the left hemisphere (RVF/LH), this ambiguity effect was larger for heterophones than for homophones, whereas in the right hemisphere (LVF/RH), similar patterns were observed for both types of homographs. Finally, performance patterns in the CVF revealed the same patterns as those in the RVF/LH, and were different from those in the LVF/RH. The implications of these results are discussed.     

The role of the right cerebral hemisphere in processing novel metaphoric expressions taken from poetry: a divided visual field study

Previous research suggests that the right hemisphere (RH) may contribute uniquely to the processing of metaphoric language. However, most studies have focused on familiar metaphoric expressions. The present study used the divided visual field paradigm to examine the role of the right cerebral hemisphere in processing novel metaphoric expressions taken from poetry. In two experiments, participants were presented with four types of word pairs, literal, conventional metaphoric and novel metaphoric expressions and unrelated word pairs, and asked to perform a semantic judgment task. Two different SOAs: 400 and 1100 ms were used. The results of both experiments showed that responses to LVF/RH presented target words were more accurate and faster than responses to RVF/LH target words for novel metaphoric expressions, but not for other types of word pairs. These results support previous research indicating that during word recognition, the RH activates a broader range of related meanings than the LH, including novel, nonsalient meanings. The findings thus suggest that the RH may be critically involved in at least one important component of novel metaphor comprehension, i.e., the integration of the individual meanings of two seemingly unrelated concepts into a meaningful metaphoric expression.     

Hemispheric asymmetries in discourse processing: evidence from false memories for lists and texts

Previous research suggests that the right hemisphere (RH) may contribute uniquely to discourse and text processing by activating and maintaining a wide range of meanings, including more distantly related meanings. The present study used the word-lists false memory paradigm [Roediger, H. L., III, & McDermott, K. B. (1995). Creating false memories: Remembering words not presented in lists. Journal of Experimental Psychology: Learning, Memory, and Cognition, 21, 803-814.] to examine the hypothesis that difference between the two cerebral hemispheres in discourse processing may be due, at least partly, to memory representations for implicit text-related semantic information. Specifically, we tested the susceptibility of the left hemisphere (LH) and RH to unpresented target words following the presentation of semantically related words appearing in either word lists or short texts. Findings showed that the RH produced more false alarms than the LH for unpresented target words following either word lists or texts. These findings reveal hemispheric differences in memory for semantically related information and suggest that RH advantage in long-term maintenance of a wide range of text-related word meanings may be one aspect of its unique contribution to the construction of a discourse model. The results support the RH coarse semantic coding theory [Beeman, M. (1998). Coarse semantic coding and discourse comprehension. In M. Beeman & C. Chiarello (Eds.), Right hemisphere language comprehension: Perspectives from cognitive neuroscience (pp. 255-284). Mahwah, NJ: Erlbaum.] and suggest that hemispheric differences in semantic processing during language comprehension extend also to verbal memory.     

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.     

Hemispheric semantic priming in the single word presentation task

Functional brain asymmetries in semantic activation were studied by presenting categorically related (e.g. TABLE-BED) or unrelated primes and targets to the left visual field (LVF)/right hemisphere or to the right visual field (RVF)/left hemisphere in the single word presentation lexical decision task. The results showed that the primes in the RVF/left hemisphere primed lexical decisions to the targets both in the RVF and in the LVF. However, the primes in the LVF/right hemisphere did not induce any priming in the LVF or RVF. These results suggest that the left hemisphere automatically activates categorically related meanings in both hemispheres. The role of the right hemisphere in automatic semantic processing may be very limited.     

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.     

A transfer appropriate processing approach to investigating implicit memory for emotional words in the cerebral hemispheres

Forty undergraduate students participated in two experiments designed to investigate the impact of perceptual and conceptual encoding manipulations on implicit memory for emotional words in each cerebral hemisphere. Adopting a transfer appropriate processing approach, the encoding manipulations were designed to promote processing of the surface features of stimuli in Experiment 1, and their semantic meaning in Experiment 2. In both experiments, participants completed the designated encoding task, followed by a lexical decision task where primed and unprimed words were presented to the left (LVF) and right visual fields (RVF). In Experiment 1, implicit memory was observed for RVF presentations of words primed according to their perceptual features. Word valence did not impact on visual field of presentation for primed or unprimed words. In Experiment 2, participation in the conceptual encoding task differentially impacted on processing and implicit memory for emotional words presented in the LVF, where priming the conceptual meaning of words facilitated the processing of positive, relative to negative and non-emotional words. In addition, implicit memory for conceptually primed negative words was reflected in inhibition of primed relative to unprimed negatively valenced words presented in the LVF. In contrast, for RVF presentations, there was evidence of implicit memory for conceptually primed non-emotional words, but not for emotional words. The results are generally consistent with the right hemisphere model of emotion, which posits greater right hemisphere involvement in both the processing and implicit memory of emotional stimuli. The results also support Nagae and Moscovitch's suggestion [Nagae, S., & Moscovitch, M. (2002). Cerebral hemispheric differences in memory of emotional and non-emotional words in normal individuals. Neuropsychologia, 40, 1601-1607] that level of processing be incorporated into studies examining the veracity of the right hemisphere and valence models of emotional processing. The study demonstrated the usefulness of adopting a transfer appropriate processing approach to investigating memory for word valence in each hemisphere.     

Cerebral asymmetries in the level of attention required for word recognition

Words presented to the right visual field (RVF) are recognised more readily than those presented to the left visual field (LVF). This RVF advantage could reflect: (a) the direct connection between the RVF and left hemisphere, (b) an attentional bias directed towards the RVF, or (c) an attentional advantage, where the left hemisphere is able to recognise words using less attention than the right hemisphere. The attentional bias and advantage models were tested in 20 dextral adults during a divided visual field word-naming task. Spatial attention was manipulated with valid, invalid, or neutral central cues. Error and reaction time measures revealed a RVF advantage for word recognition. If the attentional bias model is correct, the RVF advantage should have been attenuated for valid and invalid cues compared to neutral cues. Instead of this, an interaction emerged whereby the cueing effect was stronger for words in the LVF than the RVF. This interaction has been reported previously in studies using peripheral spatial cues. The interaction suggests that the RVF requires less attention to process words than the LVF. This left hemisphere attentional advantage may reflect asymmetries between the hemispheres in their word processing styles.     

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.     

Event related potentials (ERP) and behavioral measurements to verbal stimulation of visual fields

Event related potentials (ERP) to visually presented linguistic stimuli were examined using a lexical-decision task and an oddball paradigm. Stimuli were presented to the central, right or left visual fields (CVF, RVF and LVF) and generated ERP with very clear N100-P300 components. The question addressed was whether there is ERP evidence for left hemisphere (LH) superiority in linguistic discrimination as reported behaviorally. Nineteen young, right-handed male subjects participated. The main factor influencing the latency and amplitude of N100 was that of contralateral versus ipsilateral stimulation. Shorter N100 latency and larger amplitude were recorded over the hemisphere contralateral to the visual field stimulated. In contrast, the factors influencing the P300 parameters were the visual field stimulated and the hemisphere over which the ERP was recorded. P300 amplitude was significantly larger and P300 latency significantly shorter over the LH than over the RH. Significantly shorter P300 latency and larger peak amplitude were found for RVF than for LVF stimulation.     

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.     

The time course of semantic category processing in the cerebral hemispheres: an electrophysiological study

Using visual half-field presentations of words to the right (RVF) and to the left visual field (LVF), this study investigated the time course of the hemispheric involvement in the processing of semantic category information. Multi-channel event related brain potentials (ERPs) were recorded from 15 healthy subjects during a categorisation task of sequentially presented word pairs. Subjects had to judge mentally after the appearance of the second word whether the words of a pair were semantically related (SR) or not (SU). ERPs were computed, from 100 ms before the onset of the second word to 600 ms, for SR and SU conditions in the LVF and in the RVF separately. The temporal segmentation of ERP map series into sequences of quasi-stable map configurations revealed a total of seven segments in each visual field of which only the first five (S1-S5, appearing between 70 and 400 ms) showed different map configurations as a function of visual field but presented a similar temporal sequence in both visual fields. By contrast, of the last two segments (S6 and S7) which appeared between approximately 400 and approximately 600 ms, only S7 differentiated SR and SU conditions in terms of its duration. Source localisation analysis of the segments showed that following the initial activation of posterior brain regions as a function of the visual field of presentation, a common neural network was activated in the left hemisphere (LH) although the dynamics of activation varied as a function of visual field. Concerning the role of the right hemisphere (RH) in lexico-semantic processing, the results presented here appear to be compatible with a 'callosal relay model' and suggest that, in healthy subjects, information is transferred rapidly ( approximately 150 ms) from the RH to the language dominant-LH.     

Priming by natural category membership in the left and right cerebral hemispheres

The cerebral representation of category information was examined in a single word priming paradigm, during which the N400 component of the event-related potential (ERP) was recorded. The visual half-field paradigm was employed in order to selectively stimulate the two hemispheres. To investigate which aspects of category membership are relevant in producing priming, two types of related stimuli were presented. In one condition pairs of exemplars had a higher amount of feature overlap (e.g., MOSQUITO-FLEA) than in the other (e.g., SOFA–VASE). Significant priming was obtained only for stimuli in the high feature overlap condition and then only when these were presented to the left visual field (LVF)/right hemisphere (RH). This finding was interpreted within our recent model of semantic memory wherein the right hemisphere represents items on the basis of distributed individual features, whereas the left hemisphere (LH) represents semantic information locally, within a spreading activation system, where priming occurs exclusively through associative links. It was concluded that knowledge regarding category membership is maintained in the RH, on the basis of feature coding.     

The previous visual field: Effects of lateralization and response accuracy on current performance

The accuracy of performance in identifying syllables flashed to the RVF (right visual field) or LVF (left visual field) was strongly affected by the lateralization of and response accuracy for the previous stimulus, presented 10–20 sec earlier. For both the LVF and RVF, performance was better on Pre-LVF trials (previous stimulus in LVF) than on Pre-RVF trials (previous stimulus in RVF). The Pre-LVF advantage was the same in subjects with a strong RVF advantage and those with no asymmetry. Their performance was better when a previous stimulus had received adequate attention for correct encoding (Pre-Correct) than when it had not (Pre-Wrong), for both fields on Pre-LVF trials and the LVF, but not RVF, on Pre-RVF trials. Relations are discussed in terms of the capacities and properties of the RH (right hemisphere) and LH (left hemisphere) cerebral arousal systems.     

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.     

Semantic category effects modulate visual priming in neglect patients

Previous studies indicate that extinguished stimuli can still be unconsciously processed, leading to implicit priming effects. Here we investigated whether these implicit effects might be modulated by the semantic nature of the stimuli. Five neglect patients and ten controls performed an identification task of items belonging to living and non-living categories. In the study phase photographs of animals and artifacts were presented either to the left visual field (LVF) or to the right visual field (RVF). In the identification phase, each stimulus was displayed centrally and was revealed in a sequence of frames where the item was represented by an increasingly less and less filtered image up to a complete version. The results showed that lateralized stimuli differentially affected controls’ and neglect patients’ memory retrieval. In controls memory traces from the study phase served as efficient primes, thereby reducing the amount of information necessary for the identification of both stimulus categories. Moreover, hemispheric differences emerged with an advantage of the RVF/left hemisphere for artifact items, while no difference was found for living things. Neglect patients showed a priming effect for artifact items presented either to the RVF/left hemisphere or LVF/right hemisphere, as well as for living items presented to the RVF/left hemisphere, but not for living items presented to the LVF/right hemisphere. The priming effect observed for extinguished artifacts is consistent with the evidence of the existence of a specific mechanism destined to analyze, in an automatic and implicit fashion, motor-relevant information of manipulable objects and tools, which are important for identification process. Results are discussed in relation to current models of organization of conceptual knowledge within the framework of different processes performed by the two hemispheres.     

Interhemispheric Effects in Short-Term Recognition Memory for Single Words

Twenty undergraduate men participated in a short-term recognition memory experiment in which single words of four types, classified by high and low imagery value and high and low Thorndike-Lorge frequency, were each presented twice unilaterally to the right and left visual fields (RVF, LVF). Stimuli were projected either to the same or to the opposite visual field on successive presentations. Results showed that: (1) imagery value affected responses to initial presentations, but not to repetitions; and (2) the speed and accuracy of recognizing repetitions in the LVF were the same whether the stimuli had been presented initially to the LVF or the RVF, whereas speed and accuracy in the RVF were significantly poorer for words initially presented to the LVF than for words initially presented to the RVF. The latter findings are consistent with differential encoding or with asymmetrical storage of verbal information in the two hemispheres, but not with the hypothesis that the memory store for words is confined to the language-dominant hemisphere.     

Evidence for a new conceptualization of semantic representation in the left and right cerebral hemispheres

Two experiments are reported that examined qualitative differences in how semantic information is represented in the two hemispheres. In the first experiment, items that were associatively related but did not share semantic features or membership in semantic categories produced priming when delivered to the LH (RVF) but not to the RH (LVF). In the second experiment items that shared semantic features but were neither associates nor in the same category produced priming in the RH (LVF), but not in the LH (RVF). Together, the two experiments support the theory that, in the right hemisphere, semantic memories are represented within a distributed system, on the basis of semantic features, whereas, in the left hemisphere representations are, as in local models, relatively more holistic, and are connected via associative links.