Interhemispheric communication is via direct connections

Neural Coupling between Interhemispheric and Frontoparietal Functional Connectivity during Semantic Processing

Interhemispheric and frontoparietal functional connectivity have been reported to increase during explicit information processing. However, it is unclear how and when interhemispheric and frontoparietal functional connectivity interact during explicit semantic processing. Here, we tested the neural coupling hypothesis that explicit semantic processing promotes neural activity in the nondominant right hemispheric areas, owing to synchronization with enhanced frontoparietal functional connectivity at later processing stages. We analyzed electroencephalogram data obtained using a semantic priming paradigm, which comprised visual priming and target words successively presented under direct or indirect attention to semantic association. Scalp potential analysis demonstrated that the explicit processing of congruent targets reduced negative event-related potentials, as previously reported. Current source density analysis showed that explicit semantic processing activated the right temporal area during later temporal intervals. Subsequent dynamic functional connectivity and neural coupling analyses revealed that explicit semantic processing increased the correlation between right temporal source activities and frontoparietal functional connectivity in later temporal intervals. These findings indicate that explicit semantic processing increases neural coupling between the interhemispheric and frontoparietal functional connectivity during later processing stages.

Lateralized Repetition Priming for Familiar Faces: Evidence for Asymmetric Interhemispheric Cooperation

Repetition priming refers to facilitated recognition of stimuli that have been seen previously. Although a great deal of work has examined the properties of repetition priming for familiar faces, little has examined the neuroanatomical basis of the effect. Two experiments are presented in this paper that combine the repetition priming paradigm with a divided visual field methodology to examine lateralized recognition of familiar faces. In the first experiment participants were presented with prime faces unilaterally to each visual field and target faces foveally. A significant priming effect was found for prime faces presented to the right hemisphere, but not for prime faces presented to the left hemisphere. In Experiment 2, prime and target faces were presented unilaterally, either to the same visual field or to the opposite visual field (i.e., either within hemisphere or across hemispheres). A significant priming effect was found for the within right hemisphere condition, but not for the within left hemisphere condition, replicating the findings of the first experiment. Priming was also found in both of the across hemispheres conditions, suggesting that interhemispheric cooperation occurs to aid recognition. Taken in combination these experiments provide two main findings. First, an asymmetric repetition priming effect was found, possibly as a result of asymmetric levels of activation following recognition of a prime face, with greater priming occurring within the right hemisphere. Second, there is evidence for asymmetric interhemispheric cooperation with transfer of information from the right hemisphere to the left hemisphere to facilitate recognition.

How do our brain hemispheres cooperate to avoid false memories?

Memories are not always as reliable as they may appear. The occurrence of false memories can be reduced, however, by enhancing the cooperation between the two brain hemispheres. Yet is the communication from left to right hemisphere as helpful as the information transfer from right to left? To address this question, 72 participants were asked to learn 16 word lists. Applying the Deese-Roediger-McDermott paradigm, the words in each list were associated with an unpresented prototype word. In the test condition, learned words and corresponding prototypes were presented along with non-associated new words, and participants were asked to indicate which of the words they recognized. Crucially, both study and test words were projected to only one hemisphere in order to stimulate each hemisphere separately. It was found that false recognitions occurred significantly less often when the right hemisphere studied and the left hemisphere recognized the stimuli. Moreover, only the right-to-left direction of interhemispheric communication reduced false memories significantly, whereas left-to-right exchange did not. Further analyses revealed that the observed reduction of false memories was not due to an enhanced discrimination sensitivity, but to a stricter response bias. Hence, the data suggest that interhemispheric cooperation does not improve the ability to tell old and new apart, but rather evokes a conservative response tendency. Future studies may narrow down in which cognitive processing steps interhemispheric interaction can change the response criterion.

Do our brain hemispheres exchange some stimulus aspects better than others?

The communication between the two brain hemispheres involves considerable information losses. This study investigated whether these losses might be reduced for frequently processed stimulus attributes and currently attended stimulus properties. It was assumed that size should be more often processed than animateness, since estimating object size is essential for motor planning. Thus, if the transfer deficits were curbed for frequently processed stimulus aspects, then they should be smaller for size than for animateness. By contrast, if the transfer losses were reduced for currently attended stimulus features, then they should depend on the task: in a size judgement task they should be smaller for size, and in an animateness judgement task they should be smaller for animateness. To test the interhemispheric transmission of the two stimulus properties, a lateralized priming paradigm was implemented, in which one group of participants judged targets according to their size and another group according to their animateness. It was found that the transfer deficits were virtually absent for size, but very pronounced for animateness. Hence, the present data provide the first evidence that interhemispheric transmission losses vary for particular stimulus features and that they may be reduced with practice. Since there was no priming for unattended stimulus properties, it remains open whether attention to certain stimulus aspects can improve the interhemispheric exchange as well.

Automatic semantic priming in the left and right hemispheres

We investigated hemispheric differences and inter-hemispheric transfer of facilitation in automatic semantic priming, using prime-target pairs composed of words of the same category but not associated (e.g. skirt-glove), and a blank-target baseline condition. Reaction time and accuracy were measured at short (300 ms) intervals between prime and target onsets, using a go/no-go task to discriminate between word or non-word targets. Reaction times were facilitated more for target words presented in the right visual field (RVF) compared to the left visual field (LVF), and targets presented in RVF were primed in both visual fields, whereas targets presented in LVF were primed by primes in the LVF only. These results suggest that both hemispheres are capable of automatic priming at very short stimulus onset asymmetries (SOA), but cross-hemisphere priming occurs only in the left hemisphere.

Hemispheric asymmetry in the processing of Japanese script

The Japanese written language comprises both a logographic (kanji) and a phonetic (kana) script. Patterns of hemispheric asymmetry in the encoding of these scripts have been found to differ. The present study examined the way in which hemispheric differences at the level of orthographic encoding impact at the level of lexical access. A total of 32 participants performed a lexical decision task within an associative priming paradigm across both scripts. The results showed a right visual field advantage for the processing of kana, but no lateralised advantage for kanji stimuli. Patterns of facilitation also differed, with kanji stimuli eliciting a late-developing and eventually stronger priming effect in the left visual field. The results are discussed with reference to the recognition-with-phonology model in the processing of logographic script.

Hemispheric asymmetries in activation and integration of categorical information

Hemispheric asymmetries in activation and integration of semantic information were studied in the normal brain. In Experiment 1, categorically related primes and targets (HAND-NOSE) were presented to the left visual field (LVF) or to the right visual field (RVF) in a primed lexical decision task. The ratio of nonword targets in relation to all unrelated targets in the stimulus lists was manipulated. It was assumed that at the low nonword ratio the priming effects would primarily tap automatic semantic processing. At the higher ratio, priming would be generated predominantly by postlexical meaning integration or semantic matching. The results revealed automatic priming in the RVF/left hemisphere and postlexical priming in the LVF/right hemisphere. Experiment 2 measured automatic categorical activation with the lower nonword ratio and by presenting primes at the centre of the field. Bilateral priming was observed. It is suggested that the left hemisphere automatically activates categorically related word meanings in both hemispheres. The right hemisphere contributes by maintaining the meanings active and by retrospectively integrating them to the context.

Priming of strong semantic relations in the left and right visual fields: a time-course investigation

Prior time-course investigations of cerebral asymmetries in word processing have sometimes reported hemisphere differences in the onset and duration of semantic priming. In the current study, very strongly related word pairs (categorical associates such as arm-leg) were employed in a low relatedness proportion lexical decision priming paradigm. A range of prime-target stimulus onset asynchronies (SOAs: 150-800 ms) was included. Only very weak evidence was obtained for a LVF priming lag at the briefest SOA, while priming was bilateral at moderately long SOAs. We consider these data in the context of previous time-course studies and suggest that, when highly semantically similar word pairs are used, a right hemisphere priming lag is, at best, a very small effect.

Word imageability and N400 in an incidental memory paradigm

High imagery words are memorized better than low imagery words. To examine how these words are processed at encoding, event-related potentials (ERPs) were recorded in an incidental memory paradigm. Frequency-matched high and low imagery words (45 words each) were presented in a random order on a computer screen. Twelve university students were asked to rate the imageability of a word's referent on a five-point scale. High imagery words elicited a larger N400 than low imagery words did. The N400 was more left-lateralized for low imagery words than for high imagery words, suggesting that some neural generators (probably in the right hemisphere) were not involved in the processing of low imagery words. Difference waveforms showed that the N400 was followed by a second negativity (N800), which was also larger for high imagery words and had a scalp distribution similar to that of the N400. Subsequent free recall showed a classical imagery effect that high imagery words were recalled better than low imagery words. These results suggest that the superiority of high imagery words over low imagery words in incidental memory results from more extensive activation of a semantic network distributed across the left and right hemispheres, the latter of which probably deals with imagery-related information that is not activated by low imagery words.

Interhemispheric communication via direct connections for alternative meanings of ambiguous words

A priming experiment was used to investigate Burgess and Simpson's (1988) claim that interhemispheric cooperation plays an essential role in the interpretation of ambiguous text. In doing so, the merits of two models of interhemispheric cooperation, the homotopic inhibition theory (Cook, 1986) and the direct connections model (Collins & Coney, 1998), were examined. Priming of alternative meanings of ambiguous words was measured using homographs and their dominant (e.g., BARK-DOG) and subordinate meanings (e.g., BARK-TREE) as related pairs in a lexical decision task, with normal university students as subjects. Stimulus pairs were temporally separated by stimulus onset asynchronies (SOAs) of 180 and 350 ms and were independently projected to the left or right visual fields (LVF or RVF). At the shorter SOA, priming was restricted to LVF-RVF presentations, with homograph primes directed to the LVF equally facilitating responses to RVF targets which were associated with their dominant and subordinate meanings. This suggests that within 180 ms, a homograph projected to the right hemisphere activates a range of alternative meanings in the left hemisphere. At an SOA of 350 ms, LVF-RVF priming was obtained along with RVF-LVF and RVF-RVF priming. Evidently at this stage of processing, an ambiguous word directed to either hemisphere activates a range of alternative meanings in the contralateral hemisphere, while RVF primes also activate subordinate, but not dominant meanings in the left hemisphere. A homograph directed to the LVF did not activate dominant or subordinate meanings within the right hemisphere at either SOA. Generally, ambiguous words directed to either hemisphere activated a more extensive array of meanings in the contralateral hemisphere than in the hemisphere to which the prime was directed. This confirms the importance of interhemispheric cooperation in generating alternate meanings of ambiguous words. Strong support was found for the direct connections model (Collins & Coney, 1998), but no support for the homotopic inhibition theory (Cook, 1986).

Right hemisphere contributions to the comprehension of low-imagery words

A priming experiment, with normal university students as subjects, was used to investigate whether the right cerebral hemisphere contributes to the comprehension of low-imagery words. Each hemisphere's access to semantic representations of low-imagery words was gauged by comparing responses to low-imagery targets preceded by associated low-imagery primes (e.g., BELIEF-IDEAL) with responses to the same targets when they were preceded by unrelated primes (e.g., FATE-IDEAL). All primes and targets were independently projected to the left or right visual fields (LVF or RVF), and temporally separated by a stimulus onset asynchrony of 250 ms. There was a clear RVF advantage in response speed and accuracy measures, confirming the left hemisphere's advantage in processing low-imagery words. Nonetheless, the priming effects provided evidence that the right hemisphere contributes to the comprehension of low-imagery words, as primes projected to the RVF equally facilitated responses to associated targets subsequently appearing in either visual field. In contrast, primes directed to the LVF did not facilitate responses to associated targets projected to the LVF or RVF. The results suggest that low-imagery words projected to the left hemisphere activated low-imagery associates in both hemispheres to an equivalent degree, whereas low-imagery primes directed to the right hemisphere failed to activate low-imagery associates in either hemisphere. Like Kounios and Holcomb's (1994) study of event-related response potentials evoked by abstract and concrete words, the findings indicate that while the left hemisphere is the primary processor of low-imagery/abstract words, the right hemisphere plays a subsidiary role in the comprehension of these words.

Differences in semantic category priming in the left and right cerebral hemispheres under automatic and controlled processing conditions

The contribution of each cerebral hemisphere to the generation of semantic category meanings at automatic and strategic levels of processing was investigated in a priming experiment where prime and target words were independently projected to the left or right visual fields (LVF or RVF). Non-associated category exemplars were employed as related pairs in a lexical decision task and presented in two experimental conditions. The first condition was designed to elicit automatic processing, so related pairs comprised 20% of the positive set, stimulus pairs were temporally separated by a stimulus onset asynchrony (SOA) of 250 ms, and there was no allusion to the presence of related pairs in the instructions to subjects. The second condition, designed to invoke controlled processing, incorporated a relatedness proportion of 50%, stimulus pairs separated by an SOA of 750 ms, and instructions which informed subjects of the presence and use of category exemplar pairs in the stimulus set. In the first condition, a prime directed to either visual field facilitated responses to categorically related targets subsequently projected to the RVF, while in the second condition a prime directed to either visual field facilitated responses to related targets projected to the LVF. The facilitation effects obtained in both conditions appeared to reflect automatic processes, while strategic processes were invoked in the left, but not the right hemisphere in the second condition. The results suggest that both hemispheres have automatic access to semantic category meanings, although the timecourse of activation of semantic category meanings is slower in the right hemisphere than in the left.