Effects of output order on hemispheric asymmetry for processing letter trigrams

Hemispheric asymmetries in hierarchical stimulus processing are modulated by stimulus categories and their predictability

Hemispheric dominance has been behaviourally documented for the local (left hemisphere, LH) or global (right hemisphere, RH) processing of hierarchical letters. However, Fink et al. (1997) indicated that stimulus category modulates this hemispheric asymmetry. The purpose of this study was to investigate the influence of the category (letters versus objects) on hemispheric specialisation for global and local processing using a visual half-field presentation in a task where participants ignored whether the target appeared at the global or local level. In Experiment 1 we replicated the classic hemispheric asymmetry for global/local processing of hierarchical letters. In Experiment 2, which consisted of hierarchical object processing, a RH dominance for the local level was observed. In Experiment 3 a within-participant design was used where anticipation about the stimulus category was precluded, resulting in the classic RH and LH specialisations for global and local processing for both letter-based and object-based stimuli. Taken together, these results suggest that the highly demanding local processing stage engages one hemisphere more than the other, according to the lateralisation of cerebral networks specialised for stimulus category. In addition, the direction of lateralisation for the local level was also modulated by the predictability of the stimulus category.

In your right mind: right hemisphere contributions to language processing and production

The verbal/nonverbal account of left and right hemisphere functionality is the prevailing dichotomy describing the cerebral lateralization of function. Yet the fact that the left hemisphere is the superior language processor does not necessarily imply that the right hemisphere is completely lacking linguistic ability. This paper reviews the growing body of research demonstrating that, far from being nonverbal, the right hemisphere has significant language processing strength. From prosodic and paralinguistic aspects of speech production, reception, and interpretation, to prelexical, lexical and postlexical components of visual word recognition; strong involvement of the right hemisphere is implicated. The evidence reviewed challenges the notion that language is solely a function of the "verbal" left hemisphere, indicating that the right cerebral hemisphere makes significant and meaningful contributions to normal language processing as well.

Impaired hemispheric communication in schizophrenia: a study using the consonant-vowel-consonant task

The ability of the left and right hemispheres to interact and cooperate in the processing of information is important for normal brain functioning. Some investigators have suggested that this cooperation in functioning occurs through a process called "metacontrol." Because studies have suggested that patients with schizophrenia may have problems with interhemispheric interaction, we considered that it could be informative to study such patients from the viewpoint of interhemispheric cooperation, and more specifically that of metacontrol. To do this we utilized an assessment procedure called the consonant-vowel-consonant (CVC) task, in which nonsense three-letter combinations (trigrams) are presented vertically to the left, right and both visual fields. The pattern of errors can be used to determine the type of processing the hemispheres use under the different conditions of presentation. Previous studies have shown that when trigram information is presented to both hemispheres, the left hemisphere appears to switch into a mode that more resembles right hemisphere processing, and we wished to determine if evidence for this switch in the mode of processing also occurred in a sample of patients with schizophrenia. To determine the specificity of any findings to schizophrenia, we also studied a group of patients with bipolar disorder. For the control and bipolar groups, our results showed the typical finding of superior processing in the right visual field (left hemisphere) over the left visual field, with presentation to both visual fields demonstrating a qualitative error pattern resembling that of the left visual field (right hemisphere). Patients with schizophrenia, however, did not demonstrate this switching into a right hemisphere mode upon bilateral presentation of the trigrams and instead seemed to show no switching, or actually switching into more of a left hemisphere mode. Our results suggest that patients with schizophrenia (but not bipolar disorder) may have abnormalities in the cooperative processing of information when it is presented to both hemispheres, and that this disturbance may reflect problems with metacontrol of hemispheric processing.

Hemispheric differences for identification of words and nonwords in urdu-English bilinguals

Hemispheric asymmetry was examined for Urdu-English bilinguals identifying printed Urdu words and nonwords, separated Urdu letter strings, digits, and English nonwords. In all cases, fewer errors occurred when stimuli were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH). Qualitative error patterns suggested that separated Urdu letter strings were processed more serially than Urdu letter strings joined to form words or pronounceable nonwords and more serially on RVF/LH than on LVF/RH trials. This qualitative laterality effect is similar to that found for Hebrew and Arabic but opposite that found for English and suggests that the qualitative manner of processing printed verbal material is influenced by language-specific factors such as scanning direction, orthographic-to-phonological mapping rules, and morphology.

Pre-frontal executive committee for perception, working memory, attention, long-term memory, motor control, and thinking: a tutorial review

As an explicit organizing metaphor, memory aid, and conceptual framework, the prefrontal cortex may be viewed as a five-member 'Executive Committee,' as the prefrontal-control extensions of five sub-and-posterior-cortical systems: (1) the 'Perceiver' (dominant-right-hemisphere ventral-lateral prefrontal cortex--VL/PERC-PFC) is the frontal extension of the ventral perceptual stream (the VL/PERC system) which represents the world and self in object coordinates; (2) the 'Verbalizer' (dominant-left-hemisphere ventral-lateral prefrontal cortex system--VL/VERB-PFC) is the frontal extension of the language stream (the VL/VERB system) which represents the world and self in language coordinates; (3) the 'Motivator' (ventral/medial-orbital pre-frontal cortex--VMO-PFC) is the frontal cortical extension of a subcortical extended-amygdala stream (the VMO system) which represents the world and self in motivational/emotional coordinates; (4) the 'Attender' (dorsal-medial/anterior cingulate--DM/AC-PFC) is the frontal cortical extension of a subcortical extended-hippocampal stream (the DM/AC system) which represents the world and self in spatiotemporal coordinates and directs attention to internal and external events; and (5) the 'Coordinator' (the dorsolateral prefrontal cortex--DL-PFC) is the frontal extension of the dorsal perceptual stream (the DL system) which represents the world and self in body- and eye-coordinates and controls willed action and working memory. This tutorial review examines the interacting roles of these five systems in perception, working memory, attention, long-term memory, motor control, and thinking.

Interhemispheric Interaction in Bilateral Redundancy Gain: Effects of Stimulus Format

Previous visual laterality experiments have shown that identification is better when 2 copies of the same stimulus are presented--1 to each hemisphere--than when only a single copy is presented to 1 hemisphere. New experiments were conducted to vary whether the 2 stimuli on a bilaterally redundant trial were physically identical or identical in format in addition to leading to identical responses. Substantial bihemispheric gain was obtained even when the 2 stimuli were perceptually distinctive (e.g., letter trigrams differing in case and font or the same numeric quantity represented by digits and dots). Thus, much of the bihemispheric advantage involves relatively abstract aspects of information processing. However, when the formats were sufficiently distinctive, there were small effects on bihemispheric performance, suggesting some role for less abstract processes that are sensitive to physical characteristics.

How the brain encodes the order of letters in a printed word: the SERIOL model and selective literature review

This paper describes a novel theoretical framework of how the position of a letter within a string is encoded, the SERIOL model (sequential encoding regulated by inputs to oscillations within letter units). Letter order is represented by a temporal activation pattern across letter units, as is consistent with current theories of information coding based on the precise timing of neural spikes. The framework specifies how this pattern is invoked via an activation gradient that interacts with subthreshold oscillations and how it is decoded via contextual units that activate word units. Using mathematical modeling, this theoretical framework is shown to account for the experimental data from a wide variety of string-processing studies, including hemispheric asymmetries, the optimal viewing position, and positional priming effects.

Hemispheric differences in object identification

Although the right hemisphere is thought to be preferentially involved in visuospatial processing, the specialization of the two hemispheres with respect to object identification is unclear. The present study investigated the effects of hemifield presentation on object and word identification by presenting objects (Experiment 1) and words (Experiment 2) in a rapid visual stream of distracters. In Experiment 1, object images presented in the left visual field (i.e., to the right hemisphere) were identified with shorter display times. In addition, the left visual field advantage was greater for inverted objects. In Experiment 2, words presented in the right visual field (i.e., to the left hemisphere) under similar conditions were identified with shorter display times. These results support the idea that the right hemisphere is specialized with regard to object identification.