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Several attempts have been made to understand when and how the two hemispheres of the brain work together to encode and retrieve information during memory tasks, but it remains unclear whether they are equally capable of encoding and retrieval, particularly when the stimuli do not evoke a leftward processing asymmetry. Using a divided visual field paradigm, we presented nonverbal visual stimuli to one visual field/hemisphere at encoding, and at retrieval presented the stimuli either to the same or opposite visual field/hemisphere. Recognition responses were faster and more accurate when the stimuli were initially presented at encoding and retrieval to the same hemisphere (Experiment 1), even when delay intervals between study and test were short (Experiment 2). Taken together, these findings suggest that recognition decisions for stimuli initially presented to a single hemisphere occur more quickly at shorter lags, perhaps due to a stronger memory representation in the original hemisphere of input compared to the indirectly activated hemisphere. Our results are significant because they demonstrate that each hemisphere of the brain can function to encode and retrieve memory representations equally well, as long as the stimuli contain no linguistic information.

The current study investigated behavioral correlates of structural asymmetry of the insula, and traditional perisylvian language regions, in a large sample of young adults (N=200). The findings indicated (1) reliable leftward surface area asymmetry of the anterior insula, (2) association of this asymmetry with divided visual field lateralization of visual word recognition, and (3) modulation of the correlation of structural and linguistic asymmetry by consistency of hand preference. Although leftward asymmetry of cortical surface area was observed for the anterior insula, pars opercularis and triangularis, and planum temporale, only the anterior insula asymmetry was associated with lateralized word recognition. We interpret these findings within the context of recent structural and functional findings about the human insula. We suggest that leftward structural lateralization of earlier developing insular cortex may bootstrap asymmetrical functional lateralization even if the insula is only a minor component of the adult language network., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Individual differences in reading and cerebral lateralisation were investigated in 200 college students who completed reading assessments and divided visual field word recognition tasks, and received a structural MRI scan. Prior studies on this data set indicated that little variance in brain-behaviour correlations could be attributed to the effects of sex and handedness variables (Chiarello, Welcome, Halderman, & Leonard, 2009; Chiarello, Welcome, Halderman, Towler, et al., 2009; Welcome et al., 2009). Here a more bottom-up approach to behavioural classification (cluster analysis) was used to explore individual differences that need not depend on a priori decisions about relevant subgroups. The cluster solution identified four subgroups of college age readers with differing reading skill and visual field lateralisation profiles. These findings generalised to measures that were not included in the cluster analysis. Poorer reading skill was associated with somewhat reduced VF asymmetry, while average readers demonstrated exaggerated RVF/left hemisphere advantages. Skilled readers had either reduced asymmetries, or asymmetries that varied by task. The clusters did not differ by sex or handedness, suggesting that there are identifiable sources of variance among individuals that are not captured by these standard participant variables. All clusters had typical leftward asymmetry of the planum temporale. However, the size of areas in the posterior corpus callosum distinguished the two subgroups with high reading skill. A total of 17 participants, identified as multivariate outliers, had unusual behavioural profiles and differed from the remainder of the sample in not having significant leftward asymmetry of the planum temporale. A less buffered type of neurodevelopment that is more open to the effects of random genetic and environmental influences may characterise such individuals.

In a recent critique Boles and Barth (2011) argue that their prior study investigating asymmetry/performance relationships (Boles, Barth, & Merrill, 2008) uncovered the "true" association (i.e., negative correlation) between lateralization of visual lexical processes and word recognition performance. They contend that our study reporting positive correlations of lexical asymmetry and reading performance (Chiarello et al., 2009) was flawed and hence inconclusive. In this response we address the two major objections raised by Boles and Barth (2011) regarding our selection of tasks and asymmetry measures. We conclude that the Boles and Barth principle of task purity is not relevant to the stated aims of our investigation, and that our linear regression method of measuring asymmetry is valid given the high level of accuracy for the tasks we reported. Because the aims of each investigation differed, we argue that it is unwise to attempt to fit each study into the framework favored by Boles and Barth (2011)., (Copyright © 2011 Elsevier Inc. All rights reserved.)

There are substantial individual differences in the size and shape of the corpus callosum and such differences are thought to relate to behavioral lateralization. We report findings from a large scale investigation of relationships between brain anatomy and behavioral asymmetry on a battery of visual word recognition tasks. A sample of 200 individuals was divided into groups on the basis of sex and consistency of handedness. We investigated differences between sex/handedness groups in callosal area and relationships between callosal area and behavioral predictors. Sex/handedness groups did not show systematic differences in callosal area or behavioral asymmetry. However, the groups differed in the relationships between area of the corpus callosum and behavioral asymmetry. Among consistent-handed males, callosal area was negatively related to behavioral laterality. Among mixed-handed males and consistent-handed females, behavioral laterality was not predictive of callosal area. The most robust relationship was observed in mixed-handed females, in whom behavioral asymmetry was positively related to callosal area. Our study demonstrates the importance of considering brain/behavior relationships within sub-populations, as relationships between behavioral asymmetry and callosal anatomy varied across subject groups.

Is it advantageous to be strongly lateralized? The current study investigated this question by examining the relationship between visual field asymmetries for lexical tasks and reading performance in a sample of 200 young adults. Larger visual field asymmetries were associated with better reading performance, but this relationship was obtained primarily in those with strong and consistent hand preferences. Among mixed handers, variation in visual field asymmetry accounted for little or no variance in reading skill. In addition, correlations between visual field asymmetry and reading performance were observed for word recognition tasks, but not for tasks requiring controlled semantic retrieval. The results are consistent with the idea that consistent and mixed handers may represent differing neurobehavioral populations. Because greater lateralization was associated with better reading skill only for consistent handers, reduced behavioral asymmetry cannot be assumed to be a risk factor for reading dysfunction in the population as a whole.

The authors report findings of a large-scale, multitask investigation of sex differences in both structural asymmetries and lateralization of word reading. Two hundred participants were tested in eight divided visual field lexical tasks, and each received a structural magnetic resonance imaging scan. The authors examined whether there was evidence for sex differences in overall measures of neuroanatomical and behavioral lateralization, in specific language tasks and brain regions, and in variation in asymmetry within and across tasks and brain regions. There was very little evidence for sex differences on any behavioral measure. The few indications of sex differences in the current report accounted for 2% or less of the individual variation in asymmetry and could not be replicated in independent subsamples. No sex differences were observed in the asymmetry of structures in Broca's and Wernicke's areas such as pars triangularis, pars opercularis, the planum temporale, planum parietale, or Heschl's gyrus. There were also no sex differences in the variability of neuroanatomical asymmetries within or between brain regions. However, a significant relationship between planum temporale and behavioral asymmetry was restricted to men.

The current experiment investigated how sentential form-class expectancies influenced lexical-semantic priming within each hemisphere. Sentences were presented that led readers to expect a noun or a verb and the sentence-final target word was presented to one visual field/hemisphere for a lexical decision response. Noun and verb targets in the semantically related condition were compared to an unrelated prime condition, which also predicted part of speech but did not contain any lexical-semantic associates of the target word. The semantic priming effect was strongly modulated by form-class expectancy for RVF/LH targets, for both nouns and verbs. In the LVF/RH, semantic priming was obtained in all conditions, regardless of whether the form-class expectancy was violated. However, the nouns that were preceded by a noun-predicting sentence showed an extremely high priming value in the LVF/RH, suggesting that the RH may have some sensitivity to grammatical predictions for nouns. Comparisons of LVF/RH priming to calculations derived from the LSA model of language representation, which does not utilize word order, suggested that the RH might derive message-level meaning primarily from lexical-semantic relatedness.

A lateralized backward masking paradigm was used to examine hemisphere differences in orthographic and phonological processes at an early time course of word recognition. Targets (e.g., bowl) were presented and backward masked by either pseudohomophones of the target word (orthographically and phonologically similar, e.g., BOAL), orthographically similar (little phonological similarity, e.g., BOOL), or unrelated (e.g., MANT) non-words. Stimuli were presented to the left visual field (LVF) or right visual field (RVF) using target/mask durations of 30 or 50 ms. Responses to RVF presentations were facilitated in the pseudohomophone condition relative to the orthographically similar condition. This provides evidence that the LH can access phonology in the earliest moments of word recognition. In contrast, responses to LVF stimuli showed a greater degree of facilitation for the orthographically similar condition relative to the unrelated condition as compared to RVF presentations. These results suggest the Strong Phonological Theory of Frost (1998) is potentially an accurate description of reading processes supported by the LH, but may not be applicable to the RH. The lexical route offered by the Dual Route Cascaded model may better describe reading processes at early moments in the RH (Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001).

It has been claimed that the typical RVF/LH advantage for word recognition is reduced or eliminated for imageable, as compared to nonimageable, nouns. To determine whether such word-class effects vary depending on the stimulus list context in which the words are presented, we varied the proportion of high- and low-image words presented in a lateralized lexical decision task (0, 25, 50, 75, or 100% high image). Although the RVF/LH advantage for high-image words was unaltered by word-class proportion, a significant linear trend was obtained for the low-image words such that the RVF/LH advantage increased as the proportion of low-image words increased. We discuss the implications of these findings for models of how lexical processing is distributed across hemispheres.

The RVF/LH advantage typically found for high image nouns may be reduced or eliminated when they are mixed with other types of words, such as low image nouns ([Chiarello et al., 2001]). Global stimulus context thus appears to affect the distribution of processing across the hemispheres ( [Chiarello et al., 2001]). This idea was further investigated in the present study by presenting moderately imageable words (e.g., HOBBY) intermixed with either low or high image nouns. It was predicted that these medium image words would show a RVF advantage when mixed with high image nouns. However, this RVF advantage was expected to disappear when they were mixed with low image nouns, because the medium image words would be relatively more imageable in that context. This hypothesis was not supported, as similar RVF advantages were found for each imageability condition. It is suggested that more heterogenous stimulus distributions may be necessary for context-dependent alterations of cerebral asymmetries to occur.

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.

In this article we examine whether the distribution of function across the right and left cerebral hemispheres for lexical processing is influenced by the global context within which words are presented. A review of previously published studies indicates that the ubiquitous right visual field (RVF)/left hemisphere advantage for word recognition may be reduced or eliminated for nouns, content words, or high image words, but only when such items are presented along with verbs, function words, or low image words. However, paradoxically, when the former items are presented in more homogeneous contexts, the RVF advantage is uniformly observed. We propose that the processing efficiency of a hemisphere for a given stimulus depends on that item's relation to the other stimuli provided, that is, the global context. This was examined in a visual half-field experiment that varied whether high and low image nouns were presented in homogeneous (blocked lists) or heterogeneous (mixed lists) contexts. An unvarying RVF advantage was observed for high image words in homogeneous contexts, but this advantage was eliminated when the same items were presented in heterogeneous contexts. We suggest that stimulus heterogeneity maximizes reliance on differing, but complementary, computational biases across hemispheres. Hence, the extent to which the left and right hemispheres are recruited for the recognition of individual word types can vary dynamically with variation in the stimulus environment., (Copyright 2001 Elsevier Science.)

Previous research has indicated that the right hemisphere (RH) exhibits priming for nonassociated category members, but that the left hemisphere (LH) does not (Chiarello et al., 1990; Chiarello & Richards, 1992). Subsequent research has shown that time course is an important factor, but what other variables might influence the priming of nonassociated semantic category members? We hypothesized that repeated stimulation of the same semantic category would produce priming within the LH. Previous studies have used few exemplars from a given semantic category and thus have not tested this idea. Our prediction was that the LH would show priming after an adequate number of category instances had been processed. Based on previous research, we predicted no change in the priming observed in the RH over trial block. Priming was obtained in the RH, but it diminished as category repetition increased. In contrast, priming was not significant in the LH, indicating that category repetition does not induce maintenance of category members within the LH.

This study explored the role of syntactic organization on semantic facilitation for target words presented to the right (R) and left (L) visual fields (VFs). Sentence and unstructured list primes were contrasted and, in each condition, the effect of intervening unrelated words on the durability of priming within each VF/hemisphere was investigated. Each prime contained a critical word which occurred Near (one intervening word) or Far (six intervening words) from the target word and was semantically related to it or a neutral control. It was hypothesized that, for word lists, facilitation for RVF target words would decrease with increasing distance between the critical and target words. For sentences no decrease in facilitation was expected for RVF targets. However, for LVF targets, facilitation was expected to decrease with distance both for sentences and lists with no greater priming for sentences than for lists. The results supported these hypotheses. Priming effects that spanned several intervening items were found only when the primes were well-structured, normal sentences and the target words were presented to the RVF. These results suggest that while word-level priming processes are available to both hemispheres, the left hemisphere makes greater use of message-level syntactic and semantic mechanisms for sentence comprehension.

Cerebral asymmetries in lexical ambiguity resolution were studied. In 2 experiments, targets related to the dominant and subordinate meanings of ambiguous word primes were presented for lexical decision after a 750-ms stimulus onset asynchrony. Experiment 1 compared presentation of target words to the left visual field/right-hemisphere (LVF/RH), to the right visual field/left-hemisphere (RVF/LH), or after redundant bilateral visual field (BVF) presentation. Experiment 2 examined unilateral priming in the absence of a BVF condition. On unilateral trials, priming was observed for dominant meanings in both the LVF/RH and RVF/LH, whereas subordinate priming was obtained only in the RVF/LH. These results suggest a possible role of hemispheric interaction in the availability of ambiguous word meanings. BVF performance evidenced a bilateral redundancy gain and priming that resembled that obtained on RVF/LH trials. Additional BVF analyses were not consistent with a strict race model interpretation and appear to implicate hemispheric cooperation in the bihemisperic processing of lexical information.

Recent views of cerebral laterality suggest that both hemispheres contribute to the performance of most cognitive tasks. This would appear to require regulatory mechanisms to coordinate, select, and integrate the processes subserved by each hemisphere. In this paper we consider whether interhemispheric inhibition is used to achieve unified performance from a bilateral system capable of producing simultaneous and potentially conflicting, outputs. We examine the theoretical bases and empirical support offered for three varieties of interhemispheric inhibition. Interhemispheric suppression refers to mechanisms that may permit one hemisphere to halt or prevent concurrent processing by the opposite hemisphere. Such inhibition may operate either in a reciprocal, or in a unidirectional, fashion. Interhemispheric isolation may require an inhibitory process that suspends information transfer to prevent potentially harmful interhemispheric intrusions. Finally, one hemisphere could impede processing within the other via interference, that is, by providing irrelevant or detrimental information. It is argued that, while such mechanisms are plausible, evidence supportive of each is not convincing. We consider why it has been difficult to obtain clear-cut support for interhemispheric inhibition and suggest some avenues for further research.

This paper critically examines the theoretical and empirical basis for two claims made by Schwartz and Kirsner (1986, Brain and Cognition, 5, 354-361): (1) that acuity gradients can account for most visual field effects/interactions reported for horizontally presented words, and (2) the acuity gradients should be considered the "default" explanation for such findings. It is argued here that the acuity gradient account of higher order laterality effects is based on a questionable theory of lexical access, has no empirical support, and cannot explain, even in principle, many lateralization results. It is concluded that acuity gradients make little or no contribution to laterality effects when the stimuli consist of short monomorphemic words, and that theoretical explanations for such effects ought not be accepted by default.

Exposure duration, retinal eccentricity and stimulus size were systematically varied to assess the contribution of viewing conditions to the lexical decision asymmetry. Contrary to Sergent's hypothesis (Psychol. Bull. 93, 481-512), the RVF advantage was unaltered over the range of conditions tested. This suggests that hemisphere differences in the ability to extract information from degraded signals cannot account for the asymmetry in this task. The results are compatible with the view that the lexical decision asymmetry arises from a relatively late stage of processing in which the information is no longer represented in sensory form.

This study investigated the processing of content and function words when input to the left vs right hemispheres. For both lexical decision and naming there was a larger RVF advantage for function as compared to content words: function words were processed more slowly than content in the LVF, but not in the RVF. These results do not replicate the previous report of Bradley and Garrett, Neuropsychologia 21, 155-159, 1983, and provide some support for the view that function words are less accessible to the right hemisphere. In a second experiment, there was no difference in VF asymmetry when acceptability judgments were required for function vs content word phrases. Grammaticality judgments, of any sort, may be predominantly processed in the left hemisphere.

Patients with unilateral left- or right-hemisphere lesions were asked to make similarity judgments to visually presented words on the basis of rhyme, meaning or visual similarity. The left-hemisphere-injured group was significantly impaired, relative to controls, for all types of lexical judgments, with the greatest impairment in the rhyme condition. Patients with right-hemisphere injury were also impaired, but only when lexical judgments were based on meaning. The results are generally consistent with normal and split-brain lateralization findings, and provide evidence supportive of a right-hemisphere contribution to some aspects of lexical semantic processing.