Lateralization of auditory language functions: a dynamic dual pathway model

Spoken language comprehension requires the coordination of different subprocesses in time. After the initial acoustic analysis the system has to extract segmental information such as phonemes, syntactic elements and lexical-semantic elements as well as suprasegmental information such as accentuation and intonational phrases, i.e., prosody. According to the dynamic dual pathway model of auditory language comprehension syntactic and semantic information are primarily processed in a left hemispheric temporo-frontal pathway including separate circuits for syntactic and semantic information whereas sentence level prosody is processed in a right hemispheric temporo-frontal pathway. The relative lateralization of these functions occurs as a result of stimulus properties and processing demands. The observed interaction between syntactic and prosodic information during auditory sentence comprehension is attributed to dynamic interactions between the two hemispheres.     

The effect of spatial frequency on global precedence and hemispheric differences

There are many conditions in which identification proceeds faster for the global form of a hierarchical pattern than for its local parts. Since the global form usually contains more lower spatial frequencies than do the local forms, it has frequently been suggested that the higher transmission rate of low spatial frequencies is responsible for the global advantage. There are also functional hemispheric differences. While the right hemisphere is better at processing global information, the left hemisphere has an advantage with respect to local information. In accordance with the spatial-frequency hypothesis, it has been speculated that this difference is due to a differential capacity of the hemispheres for processing low and high spatial frequencies. To test whether low spatial frequencies were responsible for the global advantage and/or for the observed hemispheric differences, two experiments were carried out with unfiltered and highpass-filtered compound-letter stimuli presented at the left, right, or center visual field. The first experiment, in which the target level was randomized in each trial block, revealed that low spatial frequencies were not necessary for either global advantage or for hemispheric differences. Highpass filtering merely increased the response times. In the second experiment, the target level was held constant in each block. This generally increased the speed of responding and produced interactions between filtering and global-local processing. It was concluded that both sensory and attentional or control mechanisms were responsible for global precedence and that the hemispheres differed with respect to the latter.     

Functional hemispheric asymmetries of global/local processing mirrored by the steady-state visual evoked potential

While hemispheric differences in global/local processing have been reported by various studies, it is still under dispute at which processing stage they occur. Primarily, it was assumed that these asymmetries originate from an early perceptual stage. Instead, the content-level binding theory (Hübner & Volberg, 2005) suggests that the hemispheres differ at a later stage at which the stimulus information is bound to its respective level. The present study tested this assumption by means of steady-state evoked potentials (SSVEPs). In particular, we presented hierarchical letters flickering at 12 Hz while participants categorised the letters at a pre- cued level (global or local). The information at the two levels could be congruent or incongruent with respect to the required response. Since content-binding is only necessary if there is a response conflict, asymmetric hemispheric processing should be observed only for incongruent stimuli. Indeed, our results show that the cue and congruent stimuli elicited equal SSVEP global/local effects in both hemispheres. In contrast, incongruent stimuli elicited lower SSVEP amplitudes for a local than for a global target level at left posterior electrodes, whereas a reversed pattern was seen at right hemispheric electrodes. These findings provide further evidence for a level-specific hemispheric advantage with respect to content-level binding. Moreover, the fact that the SSVEP is sensitive to these processes offers the possibility to separately track global and local processing by presenting both level contents with different frequencies.     

Hemispheric differences in the perception of words and faces in deaf and hearing children

The purpose of this study was to investigate hemispheric functional asymmetry in 18 normal hearing children and 18 congenitally deaf children aged 13-14 years. The task was identification of a visual stimulus (3-letter word or photograph of a face) presented in either the left or right visual field. The children responded by pointing to the target stimulus on a response card which contained four different words or three different faces. The percentage of errors for presentations to the two visual fields were analysed to determine hemispheric dominance. The pattern of hemispheric differences for the hearing children was consistent with that from previous investigations. The results for the deaf children differed from those of the normals. In word perception we observed a right hemisphere advantage and in the face recognition a lack of hemispheric differences. These results point to a lack of auditory experiences which is affecting the functional organization of the two hemispheres. It is suggested that the necessity to make use of visuo-spatial information in the process of communication causes right hemisphere dominance in verbal tasks. This may influence the perception of other visuo-spatial stimuli which may yield a lack of hemispheric asymmetry in face recognition.     

Hemispheric asymmetries in the visual evoked potentials to temporal frequency: preliminary evidence

Steady-state evoked potentials were recorded in eight adult subjects from occipital and temporal leads of both hemispheres to investigate the effect of temporal frequency on the hemispheric specialization for basic visual information. A 3 cycles deg-1 grating was phase-reversed at different temporal frequencies (from 4 to 18 Hz), and the frequency spectrum of evoked potentials was computed by means of a fast Fourier transform program. Significant results were obtained for the component at twice the temporal frequency of stimulation. Occipital evoked potentials did not show hemispheric asymmetry, whereas temporal evoked potentials showed an interaction between hemisphere and temporal frequency: right and left hemispheres were respectively prominent for low (4 and 6 Hz) and for high (8-18 Hz) temporal frequencies. The results are discussed in the context of current research on hemispheric specialization for basic spatiotemporal parameters of visual information processing.     

Access to perceptual and conceptual information in the left and right hemispheres.

It is assumed that there are hemispheric differences in the type of information available for the processing of word meanings, e.g., categorical or associative information. In the present experiment, we used a semantic priming paradigm to examine whether perceptual or conceptual properties of word meanings would be associated with the left or right hemisphere. The present experiment also examined time-course activation of these properties across the hemispheres, using short and long stimulus onset asynchronies. The results indicated that perceptual information is available only in the right hemisphere at an early rather than a late stage of target processing, while conceptual information is available in both hemispheres at both early and later stages of target processing. It is suggested that the imagery system in the right hemisphere may contribute to the perceptual priming observed in this hemisphere.     

Repetition priming within and between the two cerebral hemispheres

Two experiments explored repetition priming benefits in the left and right cerebral hemispheres. In both experiments, a lateralized lexical decision task was employed using repeated target stimuli. In the first experiment, all targets were repeated in the same visual field, and in the second experiment the visual field of presentation was switched following repetition. Both experiments demonstrated hemispheric specialization for the task (a RVF advantage for word identification) and hemispheric interaction for word processing (lexicality priming from contralateral distracters). In the first experiment, words were identified more quickly and accurately following repetition, with repetition facilitating faster but fewer correct responses for non-words. Complex interactions between visual field of first and second presentation in the second experiment indicate asymmetric interhemispheric repetition priming effects. These results provide a broad picture of hemispheric asymmetries in word processing and of complex interaction between the hemispheres during word recognition.     

Perceptual categorization in the cerebral hemispheres

The suggestion of a functional hemispheric dissociation in performing perceptual categorization was examined in two tachistoscopic experiments with normal adults. Subjects were required to match physically identical or similar, but nonidentical, pictures of animals. The first experiment showed no hemispheric difference and indicated that both hemispheres could resort to two different types of categorization process as a function of the simultaneous or delayed mode of presentation of the stimuli. In the second experiment, the viewing conditions were manipulated so as to afford a reduced amount of stimulus energy, and a significant left-visual-field advantage was then obtained. The results are discussed with respect to models of information processing in the cerebral hemispheres, and in relation to impairments in object recognition following brain damage.     

Effects of response task and accessory stimuli on redundancy gain: tests of the hemispheric coactivation model

Two experiments tested predictions of the hemispheric coactivation model for redundancy gain (J. O. Miller, 2004). Simple reaction time was measured in divided attention tasks with visual stimuli presented to the left or right of fixation or redundantly to both sides. Experiment 1 tested the prediction that redundancy gain--the decrease in reaction time when 2 stimuli are presented rather than 1--should increase when the response requires a greater degree of bilateral control by both hemispheres. In different blocks, the response was made with the left hand, the right hand, or both hands simultaneously. In accordance with the model, redundancy gain was larger with bimanual than with unimanual responses. Experiment 2 tested and found support for the prediction that redundancy gain should decrease when both hemispheres are activated by an irrelevant auditory accessory stimulus. Thus, the results support the hemispheric coactivation model.     

The effect of congenital deafness on cerebral asymmetry in the perception of emotional and non-emotional faces.

Functional hemispheric specialization in recognizing faces expressing emotions was investigated in 18 normal hearing and 18 congenitally deaf children aged 13-14 years. Three kinds of faces were presented: happy, to express positive emotions, sad, to express negative emotions, and neutral. The subjects' task was to recognize the test face exposed for 20 msec in the left or right visual field. The subjects answered by pointing at the exposed stimulus on the response card that contained three different faces. The errors committed in expositions of faces in the left and right visual field were analyzed. In the control group the right hemisphere dominated in case of sad and neutral faces. There were no significant differences in recognition of happy faces. The differentiated hemispheric organization pattern in normal hearing persons supports the hypothesis of different processing of positive and negative emotions expressed by faces. The observed hemispheric asymmetry was a result of two factors: (1) processing of faces as complex patterns requiring visuo-spatial analysis, and (2) processing of emotions contained in them. Functional hemispheric asymmetry was not observed in the group of deaf children for any kind of emotion expressed in the presented faces. The results suggest that lack of auditory experience influences the organization of functional hemispheric specialization. It can be supposed that in deaf children, the analysis of information contained in emotional faces takes place in both hemispheres.     

Primary and secondard process in the context of cerebral hemispheric specialization

Increasing knowledge of human cerebral hemispheric specialization suggests a relationship between the different formal modes of thinking attributed to the two hemispheres and those psychoanalysis has traditionally assigned to primary (similar to recessive hemispheric) and secondary (similar to dominant hemispheric) processes. The fact of a lifelong capability at the neurophysiological level for coequal, simultaneous registration and organization of experience in these different cognitive modes allows primary process to be conceptualized in developmental terms. As the first organizing mode of infantile development, it shapes the primitive content of the dynamic unconscious. Thereafter, it can be viewed as coexistent and commingled with secondary process in dynamic tension, complementarity, and developing complexity. Observations from normal behavior, current research, and the functioning of the psychoanalyst support this thesis and reinforce an emerging viewpoint in contemporary psychoanalysis that primary processes are not confined to archaic levels but are open to growth and developmental integration into the complete range of ego functions.     

Hemispheric Asymmetry in Transient Visual Evoked Potentials Induced by the Spatial Factor of the Stimulation

In order to enhance the effect of spatial frequency on the hemispheric asymmetry of visual evoked potentials (VEP), the response amplitudes to ON-OFF modulated gratings were compared with the responses to pattern reversal stimulation. Sinusoidal gratings of different spatial frequencies were presented to six righthanders. VEPs were recorded from temporal leads on each hemisphere. In the left hemisphere, the amplitude was constant for the two modes of presentation and independent of spatial frequency. In the right hemisphere, the response amplitude was larger to the ONSET stage of ON-OFF stimulation than to reversal and presented the characteristic spatial frequency tuning curve. This asymmetry is assumed to reflect a difference in sensitivity of the two hemispheres to the spatiotemporal characteristics of the stimulus. The relevance of these findings is discussed in relation to the other hemispheric specialization models.     

Relative priming of temporal local-global levels in auditory hierarchical stimuli

Priming is a useful tool for ascertaining the circumstances under which previous experiences influence behavior. Previously, using hierarchical stimuli, we demonstrated (Justus & List, 2005) that selectively attending to one temporal scale of an auditory stimulus improved subsequent attention to a repeated (vs. changed) temporal scale; that is, we demonstrated intertrial auditory temporal level priming. Here, we have extended those results to address whether level priming relied on absolute or relative temporal information. Both relative and absolute temporal information are important in auditory perception: Speech and music can be recognized over various temporal scales but become uninterpretable to a listener when presented too quickly or slowly. We first confirmed that temporal level priming generalized over new temporal scales. Second, in the context of multiple temporal scales, we found that temporal level priming operates predominantly on the basis of relative, rather than absolute, temporal information. These findings are discussed in the context of expectancies and relational invariance in audition.     

Happiness takes you right: The effect of emotional stimuli on line bisection

Emotion recognition is mediated by a complex network of cortical and subcortical areas, with the two hemispheres likely being differently involved in processing positive and negative emotions. As results on valence-dependent hemispheric specialisation are quite inconsistent, we carried out three experiments with emotional stimuli with a task being sensitive to measure specific hemispheric processing. Participants were required to bisect visual lines that were delimited by emotional face flankers, or to haptically bisect rods while concurrently listening to emotional vocal expressions. We found that prolonged (but not transient) exposition to concurrent happy stimuli significantly shifted the bisection bias to the right compared to both sad and neutral stimuli, indexing a greater involvement of the left hemisphere in processing of positively connoted stimuli. No differences between sad and neutral stimuli were observed across the experiments. In sum, our data provide consistent evidence in favour of a greater involvement of the left hemisphere in processing positive emotions and suggest that (prolonged) exposure to stimuli expressing happiness significantly affects allocation of (spatial) attentional resources, regardless of the sensory (visual/auditory) modality in which the emotion is perceived and space is explored (visual/haptic).     

Effect of temporal constraints on hemispheric asymmetries during spatial frequency processing

Studies on functional hemispheric asymmetries have suggested that the right vs. left hemisphere should be predominantly involved in low vs. high spatial frequency (SF) analysis, respectively. By manipulating exposure duration of filtered natural scene images, we examined whether the temporal characteristics of SF analysis (i.e., the temporal precedence of low on high spatial frequencies) may interfere with hemispheric specialization. Results showed the classical hemispheric specialization pattern for brief exposure duration and a trend to a right hemisphere advantage irrespective of the SF content for longer exposure duration. The present study suggests that the hemispheric specialization pattern for visual information processing should be considered as a dynamic system, wherein the superiority of one hemisphere over the other could change according to the level of temporal constraints: the higher the temporal constraints of the task, the more the hemispheres are specialized in SF processing.     

On the nonrelation between spatial frequency and cerebral hemispheric competence

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies is discussed in reference to Christman's (1989) review. It is suggested that the relevant results from every study reviewed by Christman which support the hypothesis can also be explained in terms of the total amount of visible information or energy contained in a visual stimulus, and the right hemisphere's relative resistance to information degradation. A recent study provides evidence in support of this total visible information/energy hypothesis, and appropriate measures of information/energy are discussed. Furthermore, results from the studies reviewed may reflect response bias and not hemispheric specialization or competence. The few studies which examine response bias support such an interpretation.     

Individual differences in the hemispheric specialization of dual route variables

The dual route model suggests that reading of letter strings can occur through both a lexical and a nonlexical route. Hemispheric specialization of these routes has also been posited, suggesting that the left hemisphere has both lexical and nonlexical routes while the right hemisphere has only a lexical route. However, some recent data conflict with this hemispheric dual route model, suggesting that both hemispheres may have access to both routes. The purpose of the present study was to investigate individual differences in the hemispheric specialization of these routes and to determine whether these group differences in their specialization might explain conflicts in the literature. The effect of four individual difference factors was explored: handedness, biological sex, menstrual stage (i.e., fluctuations in estrogen), and self-rated degree of masculinity (i.e., sexual attribution). We looked at the interaction of these individual differences with the following dual route variables: (i) string length, (ii) word frequency, (iii) regularity of grapheme-phoneme correspondences of words, and (iv) the interaction of frequency and regularity using a bilateral lexical decision task. We observed that sex, menstrual stage, and masculinity each affected hemispheric specialization of the dual route variables, but did so in different ways. We posit that both hemispheres have orthographical (lexical) access as well as phonological (nonlexical) access to words. Further, we suggest that the presence of phonological processing in the right hemisphere depends on available resources and the strategies used, which are subject to individual differences.     

Hemispheric processing of lexical information in Chinese character recognition and its relationship to reading performance

Hemispheric predominance has been well documented in the visual perception of alphabetic words. However, the hemispheric processing of lexical information in Chinese character recognition and its relationship to reading performance are far from clear. In the divided visual field paradigm, participants were required to judge the orthography, phonology, or semantics of Chinese characters, which were presented randomly in the left or right visual field. The results showed a right visual field/left hemispheric superiority in the phonological judgment task, but no hemispheric advantage in the orthographic or semantic task was found. In addition, reaction times in the right visual field for phonological and semantic tasks were significantly correlated with the reading test score. These results suggest that both hemispheres involved in the orthographic and semantic processing of Chinese characters, and that the left lateralized phonological processing is important for Chinese fluent reading.     

Differential cerebral involvement in the cognitive functioning of bilinguals

The cognitive processing strategies of two groups of French-English bilinguals were studied by means of an auditory Stroop test designed to evaluate cerebral hemispheric involvement. An “early bilingual” group were bilingual before the age of 5, and a “late bilingual” group were bilingual after the age of 10. Stimuli were words uttered in pitches that were related to word meanings either congruently (as in the word “high” uttered in a high pitch) or incongruently (the word “haute” uttered in a low pitch). In one condition, subjects were to differentiate low from high pitches, disregarding meaning, while in a second condition, they were to disregard pitch and respond to word meanings. Measures of field independence were also taken. Results of data analyses suggest that male early bilinguals—the most field independent subgroup—process meaning efficiently in both cerebral hemispheres, but process pitch better in the right hemisphere. However, male late bilinguals and female bilinguals, both early and late, process meaning more rapidly in the right cerebral hemisphere and pitch equally rapidly in both hemispheres. The findings are interpreted as reflecting hemisphere-based strategy and sex differences in information-processing by the two bilingual groups.     

Handedness and hemispheric asymmetry of pattern reversal visual-evoked potentials

Pattern reversal visual-evoked potentials (EP) from temporal leads in the two hemispheres of 26 right-handed (14 right-eye-dominant and 12 left-eye-dominant) and 10 left-handed (left-eye-dominant) adults were recorded. Checkerboard patterns (check sizes: 5.7 and 17 min of arc) at 1 and 8 Hz were reversed. Stimuli (a) subtended 6 degrees of visual field, (b) subtended 1 degree of visual field (foveal condition), and (c) were restricted to the annular portion of the visual field around the fovea (peripheral condition). Larger EP amplitudes in right or left hemisphere in relation to handedness, temporal frequency, and visual field condition were recorded. Eye dominance of dextrals appeared to play a role in determining the hemispheric asymmetry. Previous literature data and present results in relation to the hypothesis of different hemispheric specialization for basic visual information are discussed.