Visual field effects in the discrimination of sine-wave gratings

The time needed to decide whether the second of two successively presented sinusoidal gratings was of a higher or lower spatial frequency than the first was measured for spatial frequencies of 1, 2, 4, 8, and 12 cycles per degree (cpd) presented in either the left visual field (LVF) or right visual field (RVF). A LVF advantage was found for discriminating within the low-spatial-frequency range (i.e., 1 and 2 cpd), whereas a RVF advantage was found for discriminating within the high-spatial-frequency range (i.e., 4-12 cpd). These findings support the conclusion that hemispheric asymmetries in the processing of gratings arise when comparisons are made between the output of spatial-frequency channels.     

Hemispheric Differences for Visual Matrix Processing: Stimulus Size and Spatial Frequency Effects

Hemispheric processing differences were assessed by presenting square matrices that varied in size and the number of filled-in cells. Subjects judged whether the matrix contained an even or odd number of filled cells. Experiment 1 employed relatively small matrix sizes (2 x 2, 3 x 3, and 4 x 4), and Experiment 2 employed relatively large matrix sizes (4 x 4, 6 x 6, and 8 x 8). Response time was shorter and error rates lower for left visual field/right hemisphere (LVF/RH) presentations compared to right visual field/left hemisphere (RVF/LH) presentations, with the larger matrices demonstrating the strongest visual field/hemispheric effects. Increases in the number of filled cells contributed to increases for the LVF/RH response time advantage only for the larger arrays. Analysis of the data from both studies collapsed across the number of filled cells produced highly consistent LVF/RH advantages for both response time and error rate, with stronger LVF/RH advantages found for the larger matrix sizes of both studies. The findings suggest that visual stimulus spatial frequency is a key determinant of hemispheric processing advantages, but that this factor is constrained by stimulus size variation. Theoretical implications with respect to the hemispheric processing double filtering by frequency model are discussed.     

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.     

Backward masking of lateralized faces by noise, pattern, and spatial frequency

Experiments 1 and 2 measured the critical interstimulus interval at which a face presented to the right or left visual field escaped a trailing noise, pattern, or spatial-frequency mask. The function relating target duration to critical ISI was multiplicative in the noise and spatial-frequency condition, but additive at longer durations in the pattern mask condition. An advantage of about 8 msec for the left visual field and 2 msec for the right field was found in the pattern and spatial-frequency masking condition, respectively. No consistent visual field differences were found in the noise mask condition. Taken together, these results suggest that hemispheric difference in face recognition are either absent or inconsistent at early, peripheral, energy-sensitive stages of processing, but emerge strongly at higher order central stages. The results also suggest that the left and right hemispheres are not differentially sensitive to the output of high- and low-spatial-frequency channels, respectively. If it is assumed that the central face processor is functionally localized to the right hemisphere, one can infer from these results that interhemispheric transmission time is not greater than 8 msec, and the output of sensory analysis and/or relational features are transferred across the interhemispheric commissures.     

Hemispheric asymmetry and pun comprehension: when cowboys have sore calves

Event-related potentials (ERPs) were recorded as healthy participants listened to puns such as "During branding, cowboys have sore calves." To assess hemispheric differences in pun comprehension, visually presented probes that were either highly related (COW), moderately related (LEG), or unrelated, were presented in either the left or right visual half field (LVF/RVF). The sensitivity of each hemisphere to the different meanings evoked by the pun was assessed by ERP relatedness effects with presentation to the LVF and the RVF. In Experiment 1, the inter-stimulus interval between the pun and the onset of the visual probe was 0 ms; in Experiment 2, this value was 500 ms. In Experiment 1, both highly and moderately related probes elicited similar priming effects with RVF presentation. Relative to their unrelated counterparts, related probes elicited less negative ERPs in the N400 interval (300-600 ms post-onset), and more positive ERPs 600-900 ms post-onset, suggesting both meanings of the pun were equally active in the left hemisphere. LVF presentation yielded similar priming effects (less negative N400 and a larger positivity thereafter) for the highly related probes, but no effects for moderately related probes. In Experiment 2, similar N400 priming effects were observed for highly and moderately related probes presented to both visual fields. Compared to unrelated probes 600-900 ms post-onset, related probes elicited a centro-parietal positivity with RVF presentation, but a fronto-polar positivity with LVF presentation. Results suggest that initially, the different meanings evoked by a pun are both active in the left hemisphere, but only the most highly related meaning is active in the right hemisphere. By 500 ms, both meanings are active in both hemispheres.     

Evidence for scanning with unilateral visual presentation of letters

When letters and words are presented tachistoscopically, material from the right visual field (RVF) can be reported more accurately than that from the left visual field (LVF). The RVF superiority may reflect either left hemispheric dominance for language or directional scanning. Previous studies have deliberately focused on the cerebral asymmetry factor while "controlling" scanning and, thus, have cast some doubt on the potency of the scanning factor. Two experiments were conducted to show that scanning can induce a RVF superiority comparable to that often associated with cerebral asymmetry. The first experiment required bilingual subjects to report six English or six Hebrew letters, shown briefly in either the LVF or RVF, with order of report controlled. A RVF superiority found with English characters was matched by an equal but opposite LVF effect with Hebrew. In a second experiment, five English characters were shown briefly in either the LVF or RVF, and subjects had to identify a single character indicated by a post exposural cue. Using a spatial cue to by pass scanning, there were no field differences; with an ordinal position cue--a procedure thought to force scanning--there was a strong RVF superiority. The results show clearly that scanning can induce visual field differences.     

Automatic semantic priming in the left and right hemispheres

We investigated hemispheric differences and inter-hemispheric transfer of facilitation in automatic semantic priming, using prime-target pairs composed of words of the same category but not associated (e.g. skirt-glove), and a blank-target baseline condition. Reaction time and accuracy were measured at short (300 ms) intervals between prime and target onsets, using a go/no-go task to discriminate between word or non-word targets. Reaction times were facilitated more for target words presented in the right visual field (RVF) compared to the left visual field (LVF), and targets presented in RVF were primed in both visual fields, whereas targets presented in LVF were primed by primes in the LVF only. These results suggest that both hemispheres are capable of automatic priming at very short stimulus onset asymmetries (SOA), but cross-hemisphere priming occurs only in the left hemisphere.     

Investigating hemispheric specialization in a novel face-word Stroop task

We examined hemispheric specialization in a lateralized Stroop facial identification task. A 2 (presentation side: left or right visual field [LVF or RVF])x2 (picture emotion: happy or angry)x3 (emotion of distractor word: happy, angry, or blank) factorial design placed the right hemispheric specialization for emotional expression processing and the left hemispheric specialization for verbal processing in conflict. Faces (from ) and emotion words were briefly displayed, and participants responded with keypresses corresponding to the picture emotion. As predicted, greater Stroop interference in identification accuracy was found with incongruent displays of facial expression in the LVF and emotion words in the RVF, and females exhibited less Stroop interference. Reaction times were moderated by emotion and visual field.     

About face: left-hemisphere involvement in processing physiognomies

Three experiments were concluded to investigate the involvement of the two cerebral hemispheres in processing faces. Perceptual discrimination of pairs of faces was equally speedy overall when the stimuli were presented in the right visual field (RVF) or left visual field (LVF). For faces differing in one or two features, however, a qualitatively different pattern of results was obtained for the two visual fields, and an RVF advantage emerged when the difference lay in the upper part of the faces (Experiment 1). An examination of the discriminability of the facial features from which the faces were constructed (Experiment 2) showed that the processes involved in RVF comparisons of faces were not dependent on the saliency of the features but, rather, followed a top-to-bottom serial analysis of the stimuli; the speed of the processing involved in LVF presentations was a function of the degree of similarity of the different comparison faces. Evidence for a serial type of comparison faces were used (Experiment 3). It was concluded that even though comparisons were equally speedy overall in LVF and RVF presentations, qualitatively different processes take place in the two hemispheres, which prove competent at processing faces, each in its own way. Some methodological problems inherent in tachistoscopic studies are discussed, and it is proposed that the quality of the stimulus representation achieved or required for cognitive processing may be determinant in the emergence of functional hemispheric asymmetries.     

Qualitative hemispheric differences in semantic category matching.

A unilateral category matching task with words as stimuli was employed to investigate semantic processing in the right and left hemispheres (RH, LH). An overall right visual field (RVF)/LH dominance was observed and performances were better than chance, also in the left visual field (LVF)/RH. A qualitative analysis of reaction times with individual differences multidimensional scaling (INDSCAL) revealed that LVF/RH INDSCAL solutions were significantly more differentiated in structure than RVF/LH solutions in terms of number and size of dimensions. These findings support a depth of activation hypothesis of hemispheric processing, with the LH rapidly and focally and the RH slowly and diffusely activating the semantic network.     

The neural basis of the right visual field advantage in reading: an MEG analysis using virtual electrodes

Right-handed participants respond more quickly and more accurately to written words presented in the right visual field (RVF) than in the left visual field (LVF). Previous attempts to identify the neural basis of the RVF advantage have had limited success. Experiment 1 was a behavioral study of lateralized word naming which established that the words later used in Experiment 2 showed a reliable RVF advantage which persisted over multiple repetitions. In Experiment 2, the same words were interleaved with scrambled words and presented in the LVF and RVF to right-handed participants seated in an MEG scanner. Participants read the real words silently and responded "pattern" covertly to the scrambled words. A beamformer analysis created statistical maps of changes in oscillatory power within the brain. Those whole-brain maps revealed activation of the reading network by both LVF and RVF words. Virtual electrode analyses used the same beamforming method to reconstruct the responses to real and scrambled words in three regions of interest in both hemispheres. The middle occipital gyri showed faster and stronger responses to contralateral than to ipsilateral stimuli, with evidence of asymmetric channeling of information into the left hemisphere. The left mid fusiform gyrus at the site of the 'visual word form area' responded more strongly to RVF than to LVF words. Activity in speech-motor cortex was lateralized to the left hemisphere, and stronger to RVF than LVF words, which is interpreted as representing the proximal cause of the RVF advantage for naming written words.     

Hemisphere specialization of Go experts in visuospatial processing

To investigate hemisphere function of experts, Go experts and novices were given two Salthouse-type visuospatial tasks. In Experiment 1, stimuli of 4 digits in 6 cells were projected to the left (LVF) or right visual field (RVF). There was no prominent group difference in identification of digits and locations. In Experiment 2, stimuli of 4 digits in 16 cells were projected to the LVF or RVF. Go experts showed more accurate performance than novices. Both groups showed the same laterality, an RVF advantage, in the number identification. However, in the location identification, Go experts showed no visual field difference, whereas novices showed an RVF advantage. Based on these findings, the relationship between task demand and hemisphere function of experts is discussed.     

Asymmetry in infant hemispheric readiness after exposure to a visual stimulus

This experiment assessed whether prior exposure to one visual stimulus could result in differential hemispheric responsiveness to a subsequent visual stimulus. The latency of saccadic orientation to a star‐shaped stimulus in the left or right visual field was assessed for 24 infants (mean age 22 weeks, SD 4.5) after exposure to either an upright or inverted facial pattern in the central visual field. The response to the lateral stimulus was equivalent in either visual field after exposure to the inverted facial pattern, but was significantly slower (p = 0.043) in the LVF (RH) than the RVF (LH) following presentation of the upright facial pattern. The outcome confirms that the processing of one visual stimulus may lead to differential hemispheric readiness to engage with a subsequent visual stimulus.     

Quantitative and Qualitative Hemispheric Asymmetry for Processing Japanese Kana

Native Japanese speakers identified three-letter kana stimuli presented to the left visual field and right hemisphere (LVF/RH), to the right visual field and left hemisphere (RVF/LH), or to both visual fields and hemispheres simultaneously (BILATERAL trials). There were fewer errors on RVF/LH and BILATERAL trials than on LVF/RH trials. Qualitative analysis of error patterns indicated that there were many fewer errors of first-letter identification than of last-letter identification, suggesting top-to-bottom scanning of the kana characters. In contrast to similar studies presenting nonword letter trigrams to native English speakers, qualitative error patterns were identical for the three visual field conditions. Taken together with the results of earlier studies, the results of the present experiment indicate that the ubiquitous RVF/LH advantage reflects a left-hemisphere superiority for phonetic processing that generalizes across specific languages. At the same time, qualitative aspects of hemispheric asymmetry differ from one language to the next and may depend on such things as the way in which individual characters map onto the pronunciation of words and nonwords.     

Different methods of lexical access for words presented in the left and right visual hemifields

Right-handed adults were asked to identify by name bilaterally presented words and pronounceable nonwords. For words in the normal horizontal format, word length (number of letters) affected left visual hemifield (LVF) but not right visual hemifield (RVF) performance in Experiments 1, 2, 3, 5, and 6. This finding was made for words of high and low frequency (Experiment 6) and imageability (Experiment 5). It also held across markedly different levels of overall performance (Experiments 1 and 2), and across different relative positionings of short and long words in the LVF and RVF (Experiment 3). Experiment 4 demonstrated that the variable affecting LVF performance is the number of letters in a word, not its phonological length. For pronounceable nonwords (Experiment 7) and words in unusual formats (Experiment 8), however, length affected both LVF and RVF performance. The characteristics identified for RVF performance in these experiments also hold for the normal reading system. In this (normal) system the absence of length effects for horizontally formatted words is generally taken to reflect the processes involved in lexical access. Length effects in the normal reading system are thought to arise when lexical access for unusually formatted words and for the pronunciation of nonwords requires the short-term storage of information at a graphemic level of analysis. The characteristics of LVF performance indicate that horizontally formatted words presented to the right cerebral hemisphere can only achieve lexical access by a method that requires the short-term storage of graphemic information. This qualitative difference in methods of lexical access applies regardless of whether the right hemisphere is seen as accessing words in the left hemisphere's lexicon or words in a lexicon of its own.     

Words, hemispheres, and dissociable subsystems: the effects of exposure duration, case alternation, priming, and continuity of form on word recognition in the left and right visual fields

Three experiments explore aspects of the dissociable neural subsystems theory of hemispheric specialisation proposed by Marsolek and colleagues, and in particular a study by [Deason, R. G., & Marsolek, C. J. (2005). A critical boundary to the left-hemisphere advantage in word processing. Brain and Language, 92, 251–261]. Experiment 1A showed that shorter exposure durations for lower-case words (13 ms) are associated with reduced right visual field (RVF) advantages compared with longer exposure durations (144 ms). Experiment 1B compared report accuracy for lower case and mixed case words at the same exposure duration (144 ms). The RVF advantage was reduced for mixed case words due to case alternation having more of an adverse effect in the RVF than in the LVF. Experiment 2 tested a different prediction of dissociable neural subsystems theory. Four-letter words were presented in mixed case in the LVF or RVF for 100 ms. They were preceded at the same location by a prime which could be in the same word in the same alternation pattern (e.g., FlAg–FlAg), the same word in the opposite alternation pattern (e.g., fLaG–FlAg), or an unrelated letter string in the same or opposite case alternation pattern (WoPk–FlAg or wOpK–FlAg). Relative to performance in the letter string prime conditions, which did not differ significantly between the two visual fields, there was more of an effect of word primes in the RVF than in the LVF. Importantly, the benefit of a word prime was the same whether the prime was in the same alternation pattern or was in the opposition alternation pattern. We argue that these results run contrary to the predictions of dissociable neural subsystems theory and are more compatible with theories which propose that a left hemisphere word recognition system is responsible for identifying written words, whether they are presented in the LVF or the RVF, and that letters are processed to an abstract graphemic level of representation before being identified by that system.     

Hemispheric asymmetry in the processing of high and low spatial frequencies: a facial recognition task

The research investigated the relationship between spatial frequency and visual field in a facial recognition task. Faces of neutral affect (Ekman, 1979) were tachistoscopically presented to the right or left visual field. The faces were presented alone, or masked with square wave gratings of 1, 24, or 48 cycles/degree, for a duration of 10 msec. Accuracy in recognizing each target face from a group of five served as the dependent measure. Subjects were 15 males and 15 females. ANOVA results included a frequency x visual field interaction effect (p less than .001). As was hypothesized, LVF errors were highest in the absence of low spatial frequencies, while RVF errors were highest when a higher range of spatial frequencies was removed. These results confirm that the hemispheres show a differential efficiency in processing high and low spatial frequency information in faces. They also offer empirical evidence to support the clinical findings that both hemispheres contribute to facial recognition.     

Sequential processing in hemispheric word recognition: the impact of initial letter discriminability on the OUP naming effect

The cerebral hemispheres have been proposed to engage different word recognition strategies: the left hemisphere implementing a parallel, and the right hemisphere, a sequential, analysis. To investigate this notion, we asked participants to name words with an early or late orthographic uniqueness point (OUP), presented horizontally to their left (LVF), right (RVF), or both fields of vision (BVF). Consistent with past foveal research, Experiment 1 produced a robust facilitatory effect of early OUP for RVF/BVF presentations, indicating the presence of sequential processes in lexical retrieval. The effect was absent for LVF trials, which we argue results from the disadvantaged position of initial letters of words presented in the LVF. To test this proposition, Experiment 2 assessed the discriminability of various letter positions in the visual fields using a bar-probe task. The obtained error functions highlighted the poor discriminability of initial letters in the LVF and latter letters in the RVF. To confirm that this asymmetry in initial letter acuity was responsible for the absent OUP effect for LVF presentations, Experiment 3 replicated Experiment 1 using vertical stimulus presentations. Results indicated a marked facilitatory effect of early OUP across visual fields, supporting our contention that the lack of OUP effect for LVF presentations in Experiment 1 resulted from poor discriminability of the initial letters. These findings confirm the presence of sequential processes in both left and right hemisphere word recognition, casting doubt on parallel models of word processing.     

Strategies of semantic categorization in the cerebral hemispheres

Strategies of semantic categorization in intact cerebral hemispheres were studied in two experiments by presenting names of typical and atypical category instances to the left visual field (LVF) (right hemisphere) or to the right visual field (RVF) (left hemisphere). The results revealed that the typicality of instances had a large effect on categorization times in the LVF in both experiments, suggesting that the right hemisphere relies strongly on a holistic, similarity-based comparison strategy. In Experiment 1, the typicality effect was weaker in the RVF than in the LVF. In Experiment 2, a typicality effect in the RVF was observed for the "four-footed animal" category but not for the "bird" category. The hypothesis that the left hemisphere employs a strategy based on defining or necessary features is not supported by the observed typicality effect in the "four-footed animal" category. Instead, it is suggested that the left hemisphere may be able to categorize on the basis of prestored instance-category knowledge. When such knowledge is not available (e.g., as for four-footed animals), a similarity-based comparison strategy is employed by the left hemisphere.     

Visual hemispheric asymmetries depend on which spatial frequencies are task relevant

Observers classified sine-wave and square-wave gratings on the basis of fundamental frequency (Are the bars wide or narrow?) or on the basis of higher harmonic frequencies (Are the bars sharp or fuzzy?). Stimuli were presented in either the left (LVF) or right (RVF) visual field. When the classification was made on the basis of the fundamental frequencies (1 or 3 c/deg), there was a LVF/right hemisphere advantage. However, when the classification was on the basis of a sharp/fuzzy distinction which involved searching for the higher harmonic frequencies, then a RVF/left hemisphere advantage was found.