On hemispheric specialisation and visual field effects in the perception of print: A comment on Jordan,Patching, and Thomas

Introduction Due to structural characteristics of the visual pathways, stimuli that are presented in the right half of the visual field (RVF) are initially projected to the left cerebral hemisphere, while those presented in the left half of the visual field (LVF) are projected to the right cerebral hemisphere. This anatomical feature has frequently been taken to support the notion that the well-documented RVF advantage in recognising printed words is a reflection of functional differences between the two hemispheres; notably that of the dominance of the left hemisphere for processing language. Word stimuli that are sent straight to the left hemisphere are believed to profit from more efficient processing than those sent initially to the right hemisphere, because the latter stimuli must follow a longer and more noisy pathway before reaching the language-dominant hemisphere. In the work by Jordan, Patching, and Thomas (2003) the above notion is further developed to speculate that the point of entry of visual information into the cortex may determine the procedure that will underlie the ensuing word recognition process: "... the left hemisphere can process words by mapping orthographic information in parallel onto lexical entries whereas the right hemisphere has a more rudimentary process, that can only map orthographic information sequentially" (p. 50).     

On hemispheric specialisation and visual field effects in the perception of print: A comment on Jordan, Patching, and Thomas

Introduction Due to structural characteristics of the visual pathways, stimuli that are presented in the right half of the visual field (RVF) are initially projected to the left cerebral hemisphere, while those presented in the left half of the visual field (LVF) are projected to the right cerebral hemisphere. This anatomical feature has frequently been taken to support the notion that the well-documented RVF advantage in recognising printed words is a reflection of functional differences between the two hemispheres; notably that of the dominance of the left hemisphere for processing language. Word stimuli that are sent straight to the left hemisphere are believed to profit from more efficient processing than those sent initially to the right hemisphere, because the latter stimuli must follow a longer and more noisy pathway before reaching the language-dominant hemisphere. In the work by Jordan, Patching, and Thomas (2003) the above notion is further developed to speculate that the point of entry of visual information into the cortex may determine the procedure that will underlie the ensuing word recognition process: "... the left hemisphere can process words by mapping orthographic information in parallel onto lexical entries whereas the right hemisphere has a more rudimentary process, that can only map orthographic information sequentially" (p. 50).     

Cross-language tests of hemispheric strategies in reading nonwords.

Four experiments explored the effects of specific language characteristics on hemispheric functioning in reading nonwords using a lateralized trigram identification task. Previous research using nonsense consonant-vowel-consonant (CVC) trigrams has shown that total error scores reveal a right visual field (RVF) advantage in Hebrew, Japanese, and English. Qualitative error patterns have shown that the right hemisphere uses a sequential strategy, whereas the left hemisphere uses a more parallel strategy in English but shows the opposite pattern in Hebrew. Experiment 1 tested whether this is due to the test language or to the native language of the participants. Results showed that native language had a stronger effect on hemispheric strategies than test language. Experiment 2 showed that latency to target letters in the CVCs revealed the same asymmetry as qualitative errors for Hebrew speakers but not for English speakers and that exposure duration of the stimuli affected misses differentially according to letter position. Experiment 3 used number trigrams to equate reading conventions in the 2 languages. Qualitative error scores still revealed opposing asymmetry patterns. Experiments 1-3 used vertical presentations. Experiment 4 used horizontal presentation, which eliminated sequential processing in both hemispheres in Hebrew speakers, whereas English speakers still showed sequential processing in both hemispheres. Comparison of the 2 presentations suggests that stimulus arrangement affected qualitative errors in the left visual field but not the RVF for English speakers and in both visual fields for Hebrew speakers. It is suggested that these differences result from orthographic and morphological differences between the languages: Reading Hebrew requires attention to be deployed to all the constituents of the stimulus in parallel, whereas reading English allows sequential processing of the letters in both hemispheres. Implications of cross-language studies for models of hemispheric function are discussed.     

Perceptual interactions between bilaterally presented words: what you get is often not what you see

Bilateral presentations of words, 1 in the left visual hemifield (LVF) and 1 in the right (RVF), are used widely in studies of hemispheric asymmetry. However, although words shown centrally (i.e., nonlaterally) produce perceptual interactions in which 1 word alters the perceived identity of the other, perceptual interactions between bilaterally presented words have never been reported. To investigate this issue, the authors used brief, bilateral displays of words (e.g., romp-ramp) presented simultaneously. An eye tracker and forced-choice task ensured appropriate presentation and testing. Report accuracy was greatest for RVF words. However, this was accompanied by perceptual interactions that occurred almost exclusively in responses to LVF words, indicating that RVF words often altered the perceived identity of LVF words but not vice versa.     

Lateralization of repetition effects in event-related potentials to words in left- and right-handed women

In our previous event-related potential (ERP) study [A. Nowicka, I. Szatkowska, Memory-induced modulation of event-related potentials in frontal cortex of human subjects: a divided visual field study, Neurosci. Lett. 359 (2004) 171-174], word repetition effects in right-handed males were observed only in case of the direct stimulation of the left (competent) hemisphere. Since the left-handedness and the factor of female gender may determine the lateralization of verbal functions, the goal of the present study was to test the sensitivity of the ERP repetition effects to the visual field of word presentation in the group of left- and right-handed women. ERPs were recorded from symmetrical sites over the left and right hemisphere. Target words were presented in the left (LVF) or right (RVF) visual hemifield. Subjects' were instructed to recognize the target word on a response card. A substantial portion of words was repeated twice. Words elicited different ERPs at frontal sites when presented the first and second times: ERPs were more positive to the repeated stimulus than to its first occurrence. However, in right-handed participants, repetition effects were only observed when target words were presented to the RVF whereas in left-handed participants, repetition effects were only observed when target words were presented to the LVF. These findings are indicative of the asymmetrical involvement of the two hemispheres in memory-induced modulation of brain activity related to verbal processing in left- and right-handed women.     

Visual field asymmetries in selective attention: evidence from a modified search paradigm

The present study investigated visual field differences in selective attention. Five stimuli were briefly presented and subjects were asked to identify a predefined target. The target/distractor physical similarity varied systematically (low, medium or high) in order to encourage attentional resolving. Right/left hemifield differences were examined in Experiment 1, temporal/nasal hemifield differences in Experiment 2, and upper/lower hemifield differences in Experiment 3. Visual field differences were found only in Experiment 1 suggesting a left/right hemispheric asymmetry in selective attention. These asymmetries appear with increasing stimuli similarity, and suggest that each hemisphere gets involved when attentional selection cannot be carried out without the mode of information processing that characterizes that hemisphere. The absence of other hemifield asymmetries is not in favor of neither a subcortical, nor a specific superior occipito-parietal involvement in attentional resolving and selectivity.     

Asymmetries and Eccentricities in Studies of Lateralised Word Recognition: A Response to Nazir

The anatomical arrangement of the human visual system offers considerable scope for investigating functional asymmetries in hemispheric processing. In particular, because each hemisphere receives information initially from the contralateral visual hemifield, visual stimuli presented to the left of a central fixation point can be projected directly to the right hemisphere and visual stimuli presented to the right of a central fixation point can be projected directly to the left hemisphere. Numerous studies using displays of this type suggest that, for the vast majority of individuals, written words produce different patterns of performance when presented to different hemifields and these findings have inspired considerable debate about the processes available for word recognition in each hemisphere.     

Asymmetries and eccentricities in studies of lateralised word recognition: a response to nazir

The anatomical arrangement of the human visual system offers considerable scope for investigating functional asymmetries in hemispheric processing. In particular, because each hemisphere receives information initially from the contralateral visual hemifield, visual stimuli presented to the left of a central fixation point can be projected directly to the right hemisphere and visual stimuli presented to the right of a central fixation point can be projected directly to the left hemisphere. Numerous studies using displays of this type suggest that, for the vast majority of individuals, written words produce different patterns of performance when presented to different hemifields and these findings have inspired considerable debate about the processes available for word recognition in each hemisphere.     

Mechanisms underlying the left visual-field advantage in the dual stream RSVP task: evidence from N2pc, P3, and distractor-evoked VEPs

When two letter streams containing two targets (T1 and T2) are presented left and right, T2 is better identified in the left hemifield. This study on 16 healthy participants used evidence from ERPs to decide whether this advantage is due to better processing in the right hemisphere or to overload of the left one. N2pc and P3 components evoked by T1 and T2 were measured, as well as the VEPs evoked by the stream of distractor stimuli. Already at the onset of the stream, these VEPs peaked earlier at the right than at the left hemisphere. N2pc was evoked earlier and P3 amplitudes were larger with left than with right T2. Previously reported side differences in T1-evoked N2pc were no longer obtained after correcting for constant hemispheric differences. The faster VEP latencies at the right hemisphere from the very beginning of the stimulus series may reflect an advantage in structuring fast sequences, which may cause the left visual-field advantage.     

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.     

Lateralized responses during covert attention are modulated by target eccentricity

Various studies have demonstrated that covert attention to different locations in the visual field can be used as a control signal for brain computer interfacing. It is well known that when covert attention is directed to the left visual hemifield, posterior alpha activity decreases in the right hemisphere while simultaneously increasing in the left hemisphere and vice versa. However, it remains unknown if and how the classical lateralization pattern depends on the eccentricity of the locations to which one attends. In this paper we study the effect of target eccentricity on the performance of a brain computer interface system that is driven by covert attention. Results show that the lateralization pattern becomes more pronounced as target eccentricity increases and suggest that in the current design the minimum eccentricity for having an acceptable classification performance for two targets at equal distance from fixation in opposite hemifields is about 6° of visual angle.     

Grammatical number agreement processing using the visual half-field paradigm: An event-related brain potential study

Despite indications in the split-brain and lesion literatures that the right hemisphere is capable of some syntactic analysis, few studies have investigated right hemisphere contributions to syntactic processing in people with intact brains. Here we used the visual half-field paradigm in healthy adults to examine each hemisphere's processing of correct and incorrect grammatical number agreement marked either lexically, e.g., antecedent/reflexive pronoun (“The grateful niece asked herself/*themselves…”) or morphologically, e.g., subject/verb (“Industrial scientists develop/*develops…”). For reflexives, response times and accuracy of grammaticality decisions suggested similar processing regardless of visual field of presentation. In the subject/verb condition, we observed similar response times and accuracies for central and right visual field (RVF) presentations. For left visual field (LVF) presentation, response times were longer and accuracy rates were reduced relative to RVF presentation. An event-related brain potential (ERP) study using the same materials revealed similar ERP responses to the reflexive pronouns in the two visual fields, but very different ERP effects to the subject/verb violations. For lexically marked violations on reflexives, P600 was elicited by stimuli in both the LVF and RVF; for morphologically marked violations on verbs, P600 was elicited only by RVF stimuli. These data suggest that both hemispheres can process lexically marked pronoun agreement violations, and do so in a similar fashion. Morphologically marked subject/verb agreement errors, however, showed a distinct LH advantage.     

Re-evaluating split-fovea processing in word recognition: effects of retinal eccentricity on hemispheric dominance

Several studies have claimed that hemispheric asymmetries affect word recognition right up to the point of fixation because each fovea is split precisely at its vertical midline and information presented either side of this midline projects unilaterally to different, contralateral hemispheres. To investigate this claim, four-letter words were presented to the left or right of fixation, either close to fixation entirely in foveal vision (0.15, 0.25, and 0.35 degrees from fixation) or further from fixation entirely in extrafoveal vision (2.00, 2.10, and 2.20 degrees from fixation). Fixation location and stimulus presentation were controlled using an eye-tracker linked to a fixation-contingent display and performance was assessed using a forced-choice task to suppress confounding effects of guesswork. A left hemisphere advantage was observed for words presented in extrafoveal locations but no hemisphere advantage (left or right) was observed for words presented in any foveal location. These findings support the well-established view that words encountered outside foveal vision project to different, contralateral hemispheres but indicate that this division for word recognition occurs only outside the fovea and provide no support for the claim that a functional split in hemispheric processing exists at the point of fixation.     

The influence of stimulus direction and eccentricity on pro- and anti-saccades in humans

We examined the sensory and motor influences of stimulus eccentricity and direction on saccadic reaction times (SRTs), direction-of-movement errors, and saccade amplitude for stimulus-driven (prosaccade) and volitional (antisaccade) oculomotor responses in humans. Stimuli were presented at five eccentricities, ranging from 0.5° to 8°, and in eight radial directions around a central fixation point. At 0.5° eccentricity, participants showed delayed SRT and increased direction-of-movement errors consistent with misidentification of the target and fixation points. For the remaining eccentricities, horizontal saccades had shorter mean SRT than vertical saccades. Stimuli in the upper visual field trigger overt shifts in gaze more easily and faster than in the lower visual field: prosaccades to the upper hemifield had shorter SRT than to the lower hemifield, and more anti-saccade direction-of-movement errors were made into the upper hemifield. With the exception of the 0.5° stimuli, SRT was independent of eccentricity. Saccade amplitude was dependent on target eccentricity for prosaccades, but not for antisaccades within the range we tested. Performance matched behavioral measures described previously for monkeys performing the same tasks, confirming that the monkey is a good model for the human oculomotor function. We conclude that an upper hemifield bias lead to a decrease in SRT and an increase in direction errors.     

Effects of eccentricity on the attention-related N2pc component of the event-related potential waveform

The N2pc ERP component has been widely used as a measure of lateralized visual attention. It is characterized by a negativity contralateral to the attended location or target, and it is thought to reflect contralaterally enhanced processing of attended information in intermediate to high levels of the ventral visual pathway. Given that the receptive fields in these areas often extend a few degrees into the ipsilateral hemifield, we might expect that near-midline stimuli would be processed by both the contralateral and ipsilateral hemispheres, resulting in a diminished N2pc. However, little is known about the effect of eccentricity on the N2pc component. To address this gap in knowledge, we recorded the EEG while participants performed a discrimination task with stimuli presented at one of five eccentricities (0°, 0.05°, 1°, 2°, 4° and 8° between the inner edge of the stimulus and the midline). We found that the N2pc amplitude remained relatively constant across eccentricities, including when the inner edge was at the midline, except that N2pc amplitude was reduced by more than 50% at the greatest eccentricity (8°). We also examined the contralateral positivity that often follows the N2pc. This positivity became progressively larger, and the transition from negative to positive occurred progressively later, as the eccentricity increased. These findings suggest that future experiments looking at the N2pc can use near-midline stimuli without compromising the N2pc amplitude, but should avoid large eccentricities. Implications about the neural generators of the N2pc are also discussed.     

Word position affects stimulus recognition: Evidence for early ERP short-term plastic modulation

The present study was aimed at investigating the short-term plastic changes that follow word learning at a neurophysiological level. The main hypothesis was that word position (left or right visual field, LVF/RH or RVF/LH) in the initial learning phase would leave a trace that affected, in the subsequent recognition phase, the Recognition Potential (i.e., the first negative component distinguishing words from other stimuli) elicited 220–240 ms after centrally presented stimuli. Forty-eight students were administered, in the learning phase, 125 words for 4 s, randomly presented half in the left and half in the right visual field. In the recognition phase, participants were split into two equal groups, one was assigned to the Word task, the other to the Picture task (in which half of the 125 pictures were new, and half matched prior studied words). During the Word task, old RVF/LH words elicited significantly greater negativity in left posterior sites with respect to old LVF/RH words, which in turn showed the same pattern of activation evoked by new words. Therefore, correspondence between stimulus spatial position and hemisphere specialized in automatic word recognition created a robust prime for subsequent recognition. During the Picture task, pictures matching old RVF/LH words showed no differences compared with new pictures, but evoked significantly greater negativity than pictures matching old LVF/RH words. Thus, the priming effect vanished when the task required a switch from visual analysis to stored linguistic information, whereas the lack of correspondence between stimulus position and network specialized in automatic word recognition (i.e., when words were presented to the LVF/RH) revealed the implicit costs for recognition. Results support the view that short-term plastic changes occurring in a linguistic learning task interact with both stimulus position and modality (written word vs. picture representation).     

Exploring cerebral asymmetries for the verb generation task

This study investigated potential right hemisphere involvement in the verb generation task. Six divided visual field experiments explored cerebral asymmetries for word retrieval in the verb generation task as well as in rhyme generation and immediate and delayed word pronunciation. The typical right visual field/left hemisphere (RVF/LH) advantage was observed for pronunciation and rhyme generation. For verb generation, the RVF/LH advantage was obtained only when stimulus items had a single prepotent response and not when there were multiple response alternatives. A semantic priming experiment suggested that activation for less common, related verbs was maintained for a longer time course within the right than within the left hemisphere. The authors suggest that the right hemisphere may play a role in continued activation of semantically related response alternatives in word generation and discuss methodological implications of their findings.