Biomedical subjects

Michal Lavidor

Publications and source records attributed to Michal Lavidor.

At least 19 recordsLinked to original sources

PubMed2006

A TMS examination of semantic radical combinability effects in Chinese character recognition.

The proposal of human foveal splitting assumes a vertical meridian split in the foveal representation and the consequent contralateral projection of information in the two hemifields to the two hemispheres and has been shown to have important implications for visual word recognition. According to this assumption, in Chinese character recognition, the two halves of a centrally fixated character may be initially projected to and processed in different hemispheres. Here, we describe a repetitive transcranial magnetic stimulation (rTMS) investigation of hemispheric processing in Chinese character recognition, through examining semantic radical combinability effects in a character semantic judgment task. The materials used were a dominant type of Chinese character which consists of a semantic radical on the left and a phonetic radical on the right. Thus, according to the split fovea assumption, the semantic and phonetic radicals are initially projected to and processed in the right hemisphere and the left hemisphere, respectively. We show that rTMS over the left occipital cortex impaired the facilitation of semantic radicals with large combinability, whereas right occipital rTMS did not. This interaction between stimulation site and radical combinability reveals a flexible division of labor between the hemispheres in Chinese character recognition, with each hemisphere responding optimally to the information in the contralateral visual hemifield to which it has direct access. The results are also consistent with the split fovea claim, suggesting functional foveal splitting as a universal processing constraint in reading.

Adult↗

PubMed2006

Magnetic stimulation of the left visual cortex impairs expert word recognition.

One of the hallmarks of expert reading is the ability to identify arrays of several letters quickly and in parallel. Such length-independent reading has only been found for word stimuli appearing in the right visual hemifield (RVF). With left hemifield presentation (LVF), response times increase as a function of word length. Here we investigated the comparative efficiency with which the two hemispheres are able to recognize visually presented words, as measured by word length effects. Repetitive transcranial magnetic stimulation (rTMS) of the left occipital cortex disrupted expert processing of the RVF such that a length effect was created (Experiment 1). Right occipital rTMS, on the other hand, had no such effect on RVF words and nor did it modulate the length effect already present in the LVF. Experiment 2 explored the time course of these TMS-induced effects by applying single pulses of TMS at various stimulus-onset asynchronies for the same task. We replicated the TMS-induced length effect for RVF words, but only when a single pulse was applied to the left visual cortex 80 msec after target presentation. This is the first demonstration of TMS-induced impairment producing a word length effect, and as such confirms the specialization of the left hemisphere in word recognition. It is likely that anatomical differences in the pathway linking retinal input to higher level cortical processing drive this effect.

Adult↗

PubMed2005

Facilitative orthographic neighborhood effects: the SERIOL model account.

A large orthographic neighborhood (N) facilitates lexical decision for central and left visual field/right hemisphere (LVF/RH) presentation, but not for right visual field/left hemisphere (RVF/LH) presentation. Based on the SERIOL model of letter-position encoding, this asymmetric N effect is explained by differential activation patterns at the orthographic level. This analysis implies that it should be possible to negate the LVF/RH N effect and create an RVF/LH N effect by manipulating contrast levels in specific ways. In Experiment 1, these predictions were confirmed. In Experiment 2, we eliminated the N effect for both LVF/RH and central presentation. These results indicate that the letter level is the primary locus of the N effect under lexical decision, and that the hemispheric specificity of the N effect does not reflect differential processing at the lexical level.

Adolescent↗

PubMed2005

When phonology fails: orthographic neighbourhood effects in dyslexia.

Both cerebral hemispheres contain phonological, orthographic and semantic representations of words, however there are between-hemisphere differences in the relative engagement and specialization of the different representations. Taking orthographic processing for example, previous studies suggest that orthographic neighbourhood size (N) has facilitatory effects in the right but not the left hemispheres. To pursue the nature of this asymmetric N effect, in particular whether there are individual differences in such specialisation, we examined N in a case of developmental dyslexia, FM. We first describe the nature of his difficulties, which are mainly severe phonological deficits. Employing the divided visual field paradigm with FM revealed a greater sensitivity in the right than in the left hemisphere to orthographic variables, with a significant inhibitory N effect in the left, but not right hemisphere. Such inhibition, to a lesser degree, was found among a group of adults with dyslexia but not among age-matched normal readers. We argue that enhanced sensitivity to orthographic cues is developed in some cases of dyslexia when a normal, phonology-based left hemisphere word recognition processing is not achieved. The interpretation presented here is cast in terms of differences between people with dyslexia and typical readers that originate in the atypical way in which orthographic representations are initially set up.

Adult↗

PubMed2005

Word length effects in Hebrew.

Numerous lateralization studies have reported that word length has a stronger effect in the left visual field (LVF) than in the right visual field (RVF) for right-handed people due to hemispheric asymmetry for language processing. Alternatively, early perceptual learning theory argued that the length effects might depend on the frequency of having read words at various lengths displayed at different retinal locations. The two alternatives were tested with right-handers participants who were native speakers of Hebrew which is read from right to left, that is Hebrew readers have a different perceptual experience than English readers. We found the predicted interaction between word length and hemifield; however, longer latencies to longer letter strings were found at both visual fields. We argue that these results are best accounted by the SERIOL model of letter-position encoding.

Adult↗

PubMed2004

Magnetic stimulation and the crossed-uncrossed difference (CUD) paradigm: selective effects in the ipsilateral and contralateral hemispheres.

When a visual target is presented to one hemifield, manual responses made to the target using the ipsilateral hand (uncrossed responses) are faster than responses using the contralateral hand (crossed response), because there is no need for visuomotor information to be transferred between the hemispheres. This difference in response times is termed the crossed-uncrossed difference (CUD) and is a valuable means of estimating interhemispheric transfer time. We aimed to investigate the CUD by applying repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lateralized target-detection task. Eleven neurologically healthy subjects, all right-handed, participated in the study. Relative to sham TMS we increased the CUD, by inhibiting the crossed latencies, but only when rTMS was applied to the hemisphere receiving visual information. These results replicate and extend previous findings and suggest the inhibitory rTMS effect under the crossed condition might be because the weak visual output is unable to activate the crossed pathway.

Adult↗

PubMed2004

The cortical representation of foveal stimuli: evidence from quadrantanopia and TMS-induced suppression.

To address the extent to which the visual foveal representation is split, we examined a 29-year-old patient with a lower right quadrantanopia following surgical removal of the left occipital cortex above the calcarine sulcus and compared her performance with subjects receiving transcranial magnetic stimulation (TMS) over the occipital lobes. In a letter/digit classification task, the patient responded accurately to targets presented in the upper visual field, for all horizontal eccentricities. In the lower visual field, she failed to discriminate letters from digits when targets were presented in the right, but not the left visual field (RVF and LVF, respectively). This pattern was also true for the foveal targets, with poor performance to foveal-RVF (0.5 degrees to the right of fixation) but not foveal-LVF (0.5 degrees to the left of fixation) targets. Similar patterns of normal performance to LVF but not RVF or foveal-RVF targets were observed in a group of nine normal observers when TMS was applied over their left occipital cortex. Complementary impairments to LVF and foveal-LVF target classification were induced with TMS over the right occipital cortex. Thus, we have induced an hemianopic pattern in normal observers contralateral to the magnetically stimulated hemisphere. This correspondence between real and TMS-induced visual field defects is further evidence, in neurologically intact subjects, that the cortical representation of the fovea is split between the two hemispheres along the vertical meridian.

Adult↗

PubMed2004

Why word length only matters in the left visual field.

During visual word recognition, string length affects performance when stimuli are presented to the left visual field (LVF), but not when they are presented to the right visual field (RVF). Using a lexical-decision experiment, we investigated an account of this phenomenon based on the SERIOL model of letter-position encoding. Bottom-up activation patterns were adjusted via positional manipulations of letter contrast. This manipulation eliminated the LVF length effect by facilitating responses to longer words, thereby demonstrating that a length effect is not an inherent property of right-hemisphere processing. In contrast, the same manipulation slowed responses to longer words in the RVF, creating a length effect. These results show that hemisphere-specific activation patterns are the source of the asymmetry of the length effect.

Adolescent↗

PubMed2004

Hemispheric asymmetry and the mental number line: comparison of double-digit numbers.

To investigate the spatial representation of numbers and its possible hemispheric organisation, 46 participants were presented with a series of double-digit numbers in a fixed standard number comparison task using 65 (in Experiment 1) and 55 (in Experiment 2) as the standard. Visual field of presentation, distance from the target number and magnitude were manipulated. Reaction times decreased with increasing distance from the target number, in line with the well-established distance effect. There was no main effect of visual field, however the interaction between visual field, distance and magnitude was significant. For large distances, numbers of small magnitude were responded to faster in the left over the right visual field, while there was a right visual field advantage for large magnitude numbers. These results suggest that the representation of numbers has spatial qualities (i.e., smaller numbers are represented to the left of a comparison point, larger numbers to the right).

Adolescent↗

PubMed2004

Evaluating a split processing model of visual word recognition: effects of orthographic neighborhood size.

The split fovea theory proposes that visual word recognition of centrally presented words is mediated by the splitting of the foveal image, with letters to the left of fixation being projected to the right hemisphere (RH) and letters to the right of fixation being projected to the left hemisphere (LH). Two lexical decision experiments aimed to elucidate word recognition processes under the split fovea theory are described. The first experiment showed that when words were presented centrally, such that the initial letters were in the left visual field (LVF/RH), there were effects of orthographic neighborhood, i.e., there were faster responses to words with high rather than low orthographic neighborhoods for the initial letters ('lead neighbors'). This effect was limited to lead-neighbors but not end-neighbors (orthographic neighbors sharing the same final letters). When the same words were fully presented in the LVF/RH or right visual field (RVF/LH, Experiment 2), there was no effect of orthographic neighborhood size. We argue that the lack of an effect in Experiment 2 was due to exposure to all of the letters of the words, the words being matched for overall orthographic neighborhood count and the sub-parts no longer having a unique effect. We concluded that the orthographic activation found in Experiment 1 occurred because the initial letters of centrally presented words were projected to the RH. The results support the split fovea theory, where the RH has primacy in representing lead neighbors of a written word.

Adolescent↗

PubMed2004

Magnetic stimulation studies of foveal representation.

The right and left visual fields each project to the contralateral cerebral hemispheres, but the extent of the functional overlap of the two hemifields along the vertical meridian is still under debate. After presenting the spatial, temporal, and functional specifications of Transcranial Magnetic Stimulation (TMS), we show that TMS is particularly suitable to study the question of foveal representation as it allows one to study the effects of selective disruption of left or right hemisphere functions on the processing of centrally presented stimuli. Future strategies for the use of TMS in further tests of visual word recognition are suggested.

Fovea Centralis↗

PubMed2004

The nature of foveal representation.

A fundamental question in visual perception is whether the representation of the fovea is split at the midline between the two hemispheres, or bilaterally represented by overlapping projections of the fovea in each hemisphere. Here we examine psychophysical, anatomical, neuropsychological and brain stimulation experiments that have addressed this question, and argue for a shift from the current default view of bilateral representation to that of a split representation, to provide a greater understanding of higher visual processes.

Animals↗

PubMed2003

A magnetic stimulation examination of orthographic neighborhood effects in visual word recognition.

The split-fovea theory proposes that visual word recognition is mediated by the splitting of the foveal image, with letters to the left of fixation projected to the right hemisphere (RH) and letters to the right of fixation projected to the left hemisphere (LH). We applied repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lexical decision task to investigate the extent to which word recognition processes could be accounted for according to the split-fovea theory. Unilateral rTMS significantly impaired lexical decision latencies to centrally presented words, supporting the suggestion that foveal representation of words is split between the cerebral hemispheres rather than bilateral. Behaviorally, we showed that words that have many orthographic neighbors sharing the same initial letters ("lead neighbors") facilitated lexical decision more than words with few lead neighbors. This effect did not apply to end neighbors (orthographic neighbors sharing the same final letters). Crucially, rTMS over the RH impaired lead-, but not end-neighborhood facilitation. The results support the split-fovea theory, where the RH has primacy in representing lead neighbors of a written word.

Adult↗

PubMed2003

Interhemispheric integration of letter stimuli presented foveally or extra-foveally.

Two experiments are reported in which participants decided whether a single target letter presented below and to the left or right of fixation matched either of two probe letters presented above the fixation cross to the left and right. The level of matching required was either physical (A-A) or abstract (A-a). All three letters were presented either extra-foveally (Experiment 1) or within the fovea (Experiment 2). In both experiments, physical matching was faster than abstract matching. Physical matching was faster within than across visual fields while abstract matching showed the opposite pattern. Matching was faster when the matching probe letter was in the LVF than when it was in the RVF. Importantly, the pattern of results was the same for extra-foveal and foveal presentations, supporting the theory that the representation of the fovea is split down the middle into two visual fields rather then being bilateral. The practical implication following this study is that lateralization studies can be performed with closer to fixation stimuli presentation.

Adult↗

PubMed2003

Semantically convergent and semantically divergent priming in the cerebral hemispheres: lexical decision and semantic judgment.

The effects of semantically divergent and semantically convergent priming on the processing of lexical ambiguity by the two cerebral hemispheres were examined in two visual hemifield experiments. The experiments investigated the ability of the right hemisphere (RH) and the left hemisphere (LH) to summate activation from two single word primes followed by a laterally presented ambiguous target word. In a lexical decision task (experiment 1), the two priming words were either both related to the dominant meaning of the target (new, fresh-NOVEL), or to the subordinate meaning (story, book-NOVEL), or to one dominant and one subordinate meaning (new, story-NOVEL). Results indicated that the LH benefited most from semantically convergent primes that converged onto the dominant meaning of the ambiguous target word, whereas the RH benefited most from semantically mixed (divergent) primes, that diverge on alternate meanings of the ambiguous target word. We used the same stimuli in a semantic relatedness judgment task (experiment 2), and found that the facilitation in the RH was significantly larger when the primes were mixed than when both primes converged on a single (i.e. either dominant or subordinate) meaning of the to-be-presented target word. In contrast, the only facilitation found in the LH was when the two primes were associated with a single meaning (either dominant or subordinate) of the to-be-presented target word. When the primes were mixed, there was no facilitation in the LH. These results support previous findings indicating that during word recognition, the RH activates a broader range of related meanings than the LH, including alternate meanings of ambiguous words. Thus, by summating activation for seemingly incongruous elements, the RH may be critically involved in at least one important component of verbal creativity.

Adolescent↗

PubMed2003

Orthographic and phonological priming in the two cerebral hemispheres.

The patterns of activation invoked in the two cerebral hemispheres by written words may be different. Two lexical decision experiments investigated several aspects of such activation patterns. Experiment 1 tested phonological and orthographic priming in the hemispheres, manipulating two levels of phonological and two levels of orthographic similarity. Orthographic priming in the left visual field (LVF) was significantly larger than in the right visual field (RVF). In Experiment 2, primes were phonologically identical to the targets (homophones) but differed in their orthographic similarity. For LVF targets, only orthographic priming was significant, whereas for RVF targets, the phonological primes were effective regardless of their orthographic similarity. The results imply that orthographic activation, though maintained by both hemispheres, is more characteristic of right hemisphere word recognition processes, whereas phonological priming is more characteristic of left hemisphere processes.

Journal Article↗

PubMed2003

Handedness, measures of hemispheric asymmetry, and lateralised lexical decision.

The goals of the present study were to evaluate the differences between measures of lateralisation in the human brain derived from a dichotic listening (DL) task and from a task involving identification of visually presented consonant-vowel-consonant (CVC) nonwords, and to correlate these lateralisation indices with performance on a lexical decision task involving visually presented words and nonwords. Visual stimuli were presented either in a central position, or to the left or right of fixation. Left-handed and right-handed participants completed each of the three tasks. Lateralisation indices were derived from performance on the DL and CVC tasks, and latency and accuracy measures of lexical decision performance were obtained for targets presented centrally and parafoveally. Lateralisation indices for left-handed and right-handed participants differed significantly for both tasks, and the two lateralisation indices had different pattern of correlation with lexical decision performance for left- and right-handers. For left-handers, lateralisation indices derived from the (visual) CVC task were correlated with lexical decision response latency for targets in central and left visual field. For right-handers, lateralisation indices derived from the (auditory) DL task were correlated with lexical decision response latency for targets in central and right visual field. A possible explanation for this finding is that the two laterality measures reflect specialisation for different types of linguistic processing in the human brain.

Journal Article↗

PubMed2003

How sleep is related to fatigue.

OBJECTIVES: It is evident that sleep patterns have direct effects on fatigue. However, the multidimensionality of fatigue may imply that complex patterns of relationships exist between fatigue and sleep characteristics. We aimed to study the correlations between fatigue and quantitative and qualitative sleep measurements, while taking into consideration depression and somatization which are known to affect both sleep and fatigue. We predicted that sleep quality, unattained by the effects of somatization and depression, would affect perceived fatigue more than the quantitative characteristics of sleep. DESIGN: Employing a cross-sectional design, hypotheses were addressed using multiple hierarchical regression analyses according to established methods. METHODS: Data were gathered from a targeted, randomly selected adult sample (N = 278) by means of subjective sleep reports, a mental health inventory, somatization inventory, several fatigue questionnaires and a demographic questionnaire. RESULTS: Fatigue was significantly predicted by depression scores, somatization levels and subjective sleep quality, but not quantitative sleep characteristics such as sleep latency, nocturnal awakenings and early morning arousals. Depression levels were positively and significantly related to all aspects of fatigue except physical fatigue and fatigue that responds to rest and sleep. Physical fatigue was correlated with somatization, but not depression. CONCLUSIONS: The data further our understanding of the multifaceted nature of human fatigue and underline the greater importance of perceived sleep quality, compared to other sleep characteristics, in predicting fatigue.

Adult↗