Conditions of visual verbal extinction: does the ipsilesional stimulus have to be identified?

Six left-neglect patients were presented with four-letter words in the left and/or right hemifield, in different contextual conditions: unilateral, bilateral-x in which one word appears on one side and a string of "x" appears on the other side (the side of "x' was not predictable), and bilateral-word (presentation of one word in each hemifield). In Experiment 1, left extinction occurred even if the right stimulus was an easily discriminable string of "x." Experiment 2 showed that increasing the size of the left stimuli reduced extinction when a string of "x" was presented on the right hemifield. However, extinction was stronger with bilateral-word presentation. These results indicate the presence of an early component in the extinction phenomenon, i.e., a "magnetic" attraction toward the ipsilesional hemifield, but are also in favor of some additional deficit, at a later stage of information processing.     

The effect of set size on hemifield asymmetries in letter recognition

Single letters were presented tachistoscopically in the left or right hemifield to right-handed observers. Superior recognition of letters on the right was found for letters chosen from subsets containing four items, but no differences were found when any letter, or any symmetrical letter, could appear. In a second experiment, the right hemifield advantage was found under conditions requiring the subject to determine the subset only when the subset chosen was appropriate. Results were discussed in terms of the adequacy of language-based explanations of hemifield asymmetries in letter recognition.     

Right hand presence modulates shifts of exogenous visuospatial attention in near perihand space

To investigate attentional shifting in perihand space, we measured performance on a covert visual orienting task under different hand positions. Participants discriminated visual shapes presented on a screen and responded using footpedals placed under their right foot. With the right hand positioned by the right side of the screen, mean cueing effects were significantly greater for targets presented on the right compared to the left side, at the shortest stimulus onset asynchrony. The right hand still affected attention when the left foot was used to respond and when the right hand was crossed over the midline, indicating that this effect is not restricted to the right hemifield and cannot be accounted for by greater stimulus–response compatibility with the right (responding) foot. These experiments provide preliminary evidence that the presence of the right hand can modulate shifts of visual attention but emphasise the importance of stimulus–response compatibility effects in such investigations.     

The visual basis of category effects in object identification: evidence from the visual hemifield paradigm

The basis for the category specific living things advantage in object recognition (i.e., faster and more accurate identification of living compared to nonliving things) was investigated in two experiments. It was hypothesised that the global shape of living things on average provides more information about their basic level identity than the global shape of nonliving things. In two experiments subjects performed name-picture or picture-name verification tasks, in which blurred or clear images of living and nonliving things were presented in either the right or the left visual hemifield. With blurred images, recognition performance was worst for nonliving things presented to the right visual field/left hemisphere, indicating that the lack of visual detail in the stimulus combined with a left hemisphere bias toward processing high frequency visual elements proved detrimental for processing nonliving stimuli in this condition. In addition, an overall living things advantage was observed in both experiments. This advantage was considerably larger with blurred images than with clear. These results are compatible with the global shape hypothesis and converge with evidence using other paradigms.     

Cerebral asymmetry for American Sign Language: The effects of moving stimuli

Previous studies of cerebral asymmetry for the perception of American Sign Language (ASL) have used only static representations of signs; in this study we present moving signs. Congenitally deaf, native ASL signers identified moving signs, static representations of signs, and English words. The stimuli were presented rapidly by motion picture to each visual hemifield. Normally hearing English speakers also identified the English words. Consistent with previous findings, both the deaf and the hearing subjects showed a left-hemisphere advantage to the English words; likewise, the deaf subjects showed a right hemisphere advantage to the statically presented signs. With the moving signs, the deaf showed no lateral asymmetry. The shift from right dominance to a more balanced hemispheric involvement with the change from static to moving signs is consistent with Kimura's position that the left hemisphere predominates in the analysis of skilled motor sequencing (Kimura 1976). The results also indicate that ASL may be more bilaterally represented than is English and that the spatial component of language stimuli can greatly influence lateral asymmetries.     

Hemispheric asymmetry in memory search for four-letter names and human faces

Two memory search experiments were conducted using vertically oriented four-letter names and human faces as stimuli. Subjects were required to indicate as quickly and as accurately as possible whether or not a single probe stimulus (presented for 150 msec to either the left or right visual field) was contained in a set of 2, 3, 4, or 5 items being held in short-term memory. The probe stimuli were presented alone (clear condition) or centrally embedded in a matrix of dots (degraded condition). In Experiment 1 (involving names), a right visual field/left hemisphere advantage was obtained and pinpointed at the encoding stage rather than at the memory comparison stage of the information-processing system. For Experiment 2 (involving human faces), no hemispheric advantage was readily observed. In each experiment, both the left hemisphere and the right hemisphere employed an abstract memory comparison operation from which the effects of probe degradation have been removed. These results are discussed in terms of their implications for various models of hemispheric asymmetry.     

Hemispheric, attentional, and processing speed factors in the treatment of developmental dyslexia

Aim of the study is to analyze the contributions of hemispheric, attentional, and processing speed factors to the effects of neuropsychological treatment of developmental dyslexia. Four groups of dyslexic children (M-type dyslexia) were treated over a period of four months. A first group (n = 9) underwent Bakker's Hemisphere-Specific Stimulation, with presentation of words in the right and left visual field. A second group (n = 7) received the same stimuli randomly in either visual hemifield. A third group (n = 8) received the same words presented centrally at fixation point. A fourth group (n = 6) received central stimuli with fixed presentation time (1500 ms). The children were tested before and after treatment on reading and spelling measures. All groups improved significantly after treatment on all variables. However, the group that was treated with centrally presented stimuli improved more than the other groups in spelling measures. A possible explanation is that rapid, simultaneous presentation to both hemispheres enhances interhemispheric exchange, which could produce an advantage in tasks requiring a high degree of integration between left and right hemispheric functions, such as spelling. The absence of significant differences in reading improvement may point to the role of memory functions or strategic factors characterizing all the treatment programs, possibly overweighing the effect of the other factors.     

Hemispheric differences in object identification

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

Interlateral asymmetry in the time course of the effect of a peripheral prime stimulus

Evidence exists that both right and left hemisphere attentional mechanisms are mobilized when attention is directed to the right visual hemifield and only right hemisphere attentional mechanisms are mobilized when attention is directed to the left visual hemifield. This arrangement might lead to a rightward bias of automatic attention. The hypothesis was investigated by testing male volunteers, wherein a "location discrimination" reaction time task (Experiments 1 and 3) and a "location and shape discrimination" reaction time task (Experiments 2 and 4) were used. Unilateral (Experiments 1 and 2) and unilateral or bilateral (Experiments 3 and 4) peripheral visual prime stimuli were used to control attention. Reaction time to a small visual target stimulus in the same location or in the horizontally opposite location was evaluated. Stimulus onset asynchronies (SOAs) were 34, 50, 67, 83 and 100 ms. An important prime stimulus attentional effect was observed as early as 50 ms in the four experiments. In Experiments 2, 3 and 4, this effect was larger when the prime stimulus occurred in the right hemifield than when it occurred in the left hemifield for SOA 100 ms. In Experiment 4, when the prime stimulus occurred simultaneously in both hemifields, reaction time was faster for the right hemifield and for SOA 100 ms. These results indicate that automatic attention tends to favor the right side of space, particularly when identification of the target stimulus shape is required.     

Change detection in complex scenes: hemispheric contribution and the role of perceptual and semantic factors

The perceptual salience and semantic relevance of objects for the meaning of a scene were evaluated with multiple criteria and then manipulated in a change-detection experiment that used an original combination of one-shot and tachistoscopic divided-visual-field paradigms to study behavioural hemispheric asymmetry. Coloured drawings that depicted meaningful situations were presented centrally and very briefly (120 ms) and only the changes were lateralised by adding an object in the right or in the left visual hemifield. High salience and high relevance improved both response times (RTs) and accuracy, although the overall contribution of salience was greater than that of relevance. Moreover, only for low-salience changes did relevance affect speed. RTs were shorter when a change occurred in the left visual hemifield, suggesting a right-hemisphere advantage for detection of visual change. Also, men responded faster than women. The theoretical and methodological implications are discussed.     

Mirror-reading in right- and left-handers

In mirror-reading, words are read from right to left and letters are read in a reverse right-to-left orientation. In one experiment we compared the ability of normal right- and left-handed subjects to mirror-read and found that the left-handers made fewer errors and could read mirror print more rapidly. In a second experiment we attempted to learn whether there is a hemifield superiority for reading mirror words and whether there are any differences between left- and right-handers in a hemifield. We found that although both right- and left-handers more rapidly detected mirror words projected to the left visual half-field, there were no differences between groups. However, in the right visual half-field, the performance of left-handers was superior to that of the right-handers. The results of the hemifield study suggest that left-handers may be superior at reading mirror words because they can more easily reverse their scanning pattern.     

Right hemisphere sensitivity to word- and sentence-level context: evidence from event-related brain potentials

Researchers using lateralized stimuli have suggested that the left hemisphere is sensitive to sentence-level context, whereas the right hemisphere (RH) primarily processes word-level meaning. The authors investigated this message-blind RH model by measuring associative priming with event-related brain potentials (ERPs). For word pairs in isolation, associated words elicited more positive ERPs than unassociated words with similar magnitudes and onset latencies in both visual fields. Embedded in sentences, these same pairs showed large sentential context effects in both fields. Small effects of association were observed, confined to incongruous sentences after right visual hemifield presentation but present for both congruous and incongruous sentences after left visual hemifield presentation. Results do not support the message-blind RH model but do suggest hemispheric asymmetries in the use of word and sentence context during real-time processing.     

Word matching and Lexical Decisions: A Visual Half-Field study

In unilateral Visual Half-Field tasks visuospatial and linguistic processing were compared. In a Word Matching task subjects judged the physical identity of simultaneously presented pairs of three-letter words or legal nonwords. No mainfield effects were found, but word pairs were recognized better and faster as "same" than nonword pairs. Latencies and errors in "different" pairs increased monotonically with position of letter change in the left but not in the right visual field (RVF), suggesting a serial, letter-by-letter way of processing for the right hemisphere and a whole word approach for the left. At this perceptual level the ability to store lexical information from the icon is stressed as a hemisphere-specific factor. In a Lexical Decision task the same subjects judged the same items on the word/nonword dimension. A RVF advantage associated with words as compared to nonwords occurred, as expected. Additional analysis suggests that order and difficulty of tasks may influence females' laterality, as compared to that of males.     

Right visual field advantage for perceived contrast: correlation with an auditory bias and handedness

Studies have suggested that supramodal attentional resources are biased rightward due to asymmetric spatial fields of the two hemispheres. This bias has been observed especially in right-handed subjects. We presented left and right-handed subjects with brief uniform grey visual stimuli in either the left or right visual hemifield. Consistent with the proposed asymmetry in attentional resources, right-handed subjects estimated right hemifield targets as having a higher contrast than physically identical stimuli presented in the left hemifield. Left-handed participants did not show a systematic rightward or leftward bias. However, the group of left-handed participants also took part in a dichotic listening experiment whose results showed that visual bias score correlated positively with ear-advantage in dichotic listening. Our results are consistent with the view that supramodal processing resources are biased towards the right hemispace, and that this bias is influenced by handedness.     

Mental rotation of laterally presented random shapes in males and females

Eighteen right-handed subjects (9 males, 9 female) were to decide if laterally presented random shapes were identical or a mirror image of a centrally presented standard shape. The lateral shapes were rotated over 0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees, or 300 degrees. For unrotated (0 degrees) mirror image stimuli, females showed a significant right visual-field advantage, whereas males showed no significant hemifield effect. The rate of rotation was equivalent for both sexes. Field of presentation did not affect the rotation rate either. The present results support a growing number of findings that indicate that the interpretation of mental rotation as a typical right-hemispheric spatial-processing task is questionable.     

Picture naming in young children: a developmental study on interhemispheric collaboration

The aim of the present study was to determine how interhemispheric collaboration and visual attention in basic lexical tasks develop during early childhood. Two- to 6-year-old children were asked to name two different pictures presented simultaneously either one in each visual hemifield (bilateral condition) or both in a single hemifield (either right or left, unilateral condition). In the bilateral condition, children were overall more accurate in naming right visual field than left visual field pictures. This difference was significant for 2- and 3- to 4-year-old children, but not for 5- to 6-year-old children. These results show that the right and left cerebral hemispheres do not develop naming competencies equally well in early childhood. A second analysis, based on the order of report, showed that when 2- and 3- to 4-year-old children named both the left and the right visual field pictures, they named the right visual field picture first. In contrast, at the age of 5-6 years, children named the left visual field picture first and overall naming performance reached a ceiling level. Several interpretations are proposed to explain this shift of visual attention at the age of 5-6 years. In the unilateral condition, no difference was found between naming accuracy in the right and left visual fields, presumably because interhemispheric pathways are functional: visual stimuli presented to the right hemisphere can be processed by the most competent left hemisphere without degradation of information. This result confirms previous findings on the development of interhemispheric collaboration.     

Location-specific versus hemisphere-specific adaptation of processing selectivity

Current theories of cognitive control assume that processing selectivity is adjusted according to the utility of processing task-irrelevant stimulus features. Consistently, interference evoked by flanker stimuli is reduced when the proportion of incompatible trials—in which flankers are associated with an incorrect response—is increased. Consistent with the idea that the cerebral hemispheres select processing strategies independently of each other, Corballis and Gratton (2003) demonstrated that flanker interference for stimuli presented in either the left or right visual hemifield is affected by the ratio of compatible and incompatible target-flanker pairings presented in the same—but not in the other—hemifield. Presenting stimuli at four different locations, we demonstrated independent effects of the ratio of compatible and incompatible trials for stimulus locations in different hemifields as well as for stimulus locations within the same hemifield. Independent selectivity adjustment regarding the left and right visual hemifields thus appears to be a special case of a larger class of location-specific adaptation effects and might not be informative regarding hemisphere-specific processing.     

A comparison of upper vs. lower and right vs. left visual fields using lexical decision

A series of experiments using the lexical decision task was conducted in order to investigate the functional differences between the upper and lower visual fields (UVF, LoVF) in word recognition. Word-nonword discrimination was swifter and more accurate for word stimuli presented in the UVF. Changing the eccentricity did not affect the UVF advantage over the LoVF. UVF superiority over LoVF was found to be equivalent for both right and left visual hemifield (RVF, LVF). In general, presenting related word primes enhanced all visual field differences in a similar manner (UVF over LoVF and RVF over LVF). However, primes consisting of semantically constraining sentences enhanced the RVF advantage over the LVF, but did not affect the UVF and LoVF differentially. The argument is made that UVF superiority cannot be due to perceptual or attentional differences alone, but must also reflect top-down information flow.     

Right visual field advantage for perceived contrast: Correlation with an auditory bias and handedness

Studies have suggested that supramodal attentional resources are biased rightward due to asymmetric spatial fields of the two hemispheres. This bias has been observed especially in right-handed subjects. We presented left and right-handed subjects with brief uniform grey visual stimuli in either the left or right visual hemifield. Consistent with the proposed asymmetry in attentional resources, right-handed subjects estimated right hemifield targets as having a higher contrast than physically identical stimuli presented in the left hemifield. Left-handed participants did not show a systematic rightward or leftward bias. However, the group of left-handed participants also took part in a dichotic listening experiment whose results showed that visual bias score correlated positively with ear-advantage in dichotic listening. Our results are consistent with the view that supramodal processing resources are biased towards the right hemispace, and that this bias is influenced by handedness.