The left intraparietal sulcus modulates the selection of low salient stimuli

Neuropsychological and functional imaging studies have suggested a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. In contrast, a recent transcranial magnetic stimulation study [Mevorach, C., Humphreys, G. W., & Shalev, L. Opposite biases in salience-based selection for the left and right posterior parietal cortex. Nature Neuroscience, 9, 740-742, 2006b] demonstrated that functional lateralization of selection in the parietal cortices on the basis of the relative salience of stimuli might provide an alternative explanation for previous results. In the present study, we applied a whole-brain analysis of the functional magnetic resonance signal when participants responded to either the local or the global levels of hierarchical figures. The task (respond to local or global) was crossed with the saliency of the target level (local salient, global salient) to provide, for the first time, a direct contrast between brain activation related to the stimulus level and that related to relative saliency. We found evidence for lateralization of salience-based selection but not for selection based on the level of processing. Activation along the left intraparietal sulcus (IPS) was found when a low saliency stimulus had to be selected irrespective of its level. A control task showed that this was not simply an effect of task difficulty. The data suggest a specific role for regions along the left IPS in salience-based selection, supporting the argument that previous reports of lateralized responses to local and global stimuli were contaminated by effects of saliency.     

Hemispheric asymmetries in hierarchical stimulus processing are modulated by stimulus categories and their predictability

Hemispheric dominance has been behaviourally documented for the local (left hemisphere, LH) or global (right hemisphere, RH) processing of hierarchical letters. However, Fink et al. (1997) indicated that stimulus category modulates this hemispheric asymmetry. The purpose of this study was to investigate the influence of the category (letters versus objects) on hemispheric specialisation for global and local processing using a visual half-field presentation in a task where participants ignored whether the target appeared at the global or local level. In Experiment 1 we replicated the classic hemispheric asymmetry for global/local processing of hierarchical letters. In Experiment 2, which consisted of hierarchical object processing, a RH dominance for the local level was observed. In Experiment 3 a within-participant design was used where anticipation about the stimulus category was precluded, resulting in the classic RH and LH specialisations for global and local processing for both letter-based and object-based stimuli. Taken together, these results suggest that the highly demanding local processing stage engages one hemisphere more than the other, according to the lateralisation of cerebral networks specialised for stimulus category. In addition, the direction of lateralisation for the local level was also modulated by the predictability of the stimulus category.     

Hemispheric asymmetries in hierarchical stimulus processing are modulated by stimulus categories and their predictability

Hemispheric dominance has been behaviourally documented for the local (left hemisphere, LH) or global (right hemisphere, RH) processing of hierarchical letters. However, Fink et al. (1997) indicated that stimulus category modulates this hemispheric asymmetry. The purpose of this study was to investigate the influence of the category (letters versus objects) on hemispheric specialisation for global and local processing using a visual half-field presentation in a task where participants ignored whether the target appeared at the global or local level. In Experiment 1 we replicated the classic hemispheric asymmetry for global/local processing of hierarchical letters. In Experiment 2, which consisted of hierarchical object processing, a RH dominance for the local level was observed. In Experiment 3 a within-participant design was used where anticipation about the stimulus category was precluded, resulting in the classic RH and LH specialisations for global and local processing for both letter-based and object-based stimuli. Taken together, these results suggest that the highly demanding local processing stage engages one hemisphere more than the other, according to the lateralisation of cerebral networks specialised for stimulus category. In addition, the direction of lateralisation for the local level was also modulated by the predictability of the stimulus category.     

Adaptation to leftward-shifting prisms reduces the global processing bias of healthy individuals

When healthy individuals are presented with peripheral figures in which small letters are arranged to form a large letter, they are faster to identify the global- than the local-level information, and have difficulty ignoring global information when identifying the local level. The global reaction time (RT) advantage and global interference effect imply preferential processing of global-level information in the normal brain. This contrasts with the local processing bias demonstrated following lesions to the right temporo-parietal junction (TPJ), such as those that lead to hemispatial neglect (neglect). Recent research from our lab demonstrated that visuo-motor adaptation to rightward-shifting prisms, which ameliorates many leftward performance deficits of neglect patients, improved the local processing bias of patients with right TPJ lesions (Bultitude, Rafal, & List, 2009). Here we demonstrate that adaptation to leftward-shifting prisms, which can induce neglect-like performance in neurologically healthy individuals, also reduces the normal global processing bias. Forty-eight healthy participants were asked to identify the global or local forms of hierarchical figures before and after adaptation to leftward- or rightward-shifting prisms. Prior to prism adaptation, both groups had greater difficulty ignoring irrelevant global information when identifying the local level (global interference) compared to their ability to ignore irrelevant local-level information when identifying the global level (local interference). Participants who adapted to leftward-shifting prisms showed a significant reduction in global interference, but there was no change in the performance of the rightward-shifting Prism Group. These results show, for the first time, that in addition to previously demonstrated effects on lateralised attention, prism adaptation can influence non-lateralised spatial attention in healthy individuals.     

Right hemisphere superiority for initial stages of letter processing

When single letters, which could be perfectly recognized when presented alone, were embedded in an overlapping masking stimulus, observers recognized more letters from the left than from the right visual field. This left visual field-right hemisphere advantage persisted over short time intervals between the letter and the mask, regardless of which stimulus occurred first. Such results suggest that the right cerebral hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded, even though letters are highly associated with language and, therefore, readily processed along verbal-analytic dimensions.     

Hemispheric differences in global versus local processing of hierarchical visual stimuli by normal subjects: new data and a meta-analysis of previous studies

Twenty-four normal right-handed male subjects classified hierarchical visual stimuli (small letters arranged to form larger letters), which were presented separately to each cerebral hemisphere. Previous studies using similar paradigms have yielded mixed evidence for the specialization of local, analytic processing in the left hemisphere and global, holistic processing in the right hemisphere. No statistically significant evidence for such a pattern of specialization was evident in the results of the present study. However, when meta-analytic techniques were used to combine the present results with those from previous studies, there was evidence that the hypothesized left-hemisphere/local, right-hemisphere/global specialization does in fact exist.     

Letter processing interferes with inhibition of return: evidence for cortical involvement

Inhibition of return (IOR) refers to the finding that, when the time lag between a cue and a target is prolonged, the reaction to the target, when it eventually appears, is actually slower than with no cue. This phenomenon is thought to make visual search more efficient, and it is subserved by the left inferior parietal cortex and the supramarginal gyrus bilaterally. Interestingly, the very same brain structures are also involved in letter processing. Accordingly, we asked whether the two mental processes interfere with each other when simultaneously probed. The first experiment used a typical IOR procedure, but the cue/target placeholders were either simple geometric shapes or English letters. The results show that, although IOR is approximately the same across visual fields when shape placeholders are used, it is significantly lessened in the right visual field when letters are used as cue and target placeholders. To examine if this finding was due to potential spatial frequency differences between the placeholders, a second experiment using shapes and Japanese letters was conducted, and no differences in IOR were found. The supramarginal gyrus appears to be the most likely locus for the letter-IOR interference effect because it is active bilaterally in IOR, but only in the left hemisphere during letter processing. These findings provide support for the notion that IOR is not simply due to subcortical processes but also involves processing from cortical structures.     

Reflexive and preparatory selection and suppression of salient information in the right and left posterior parietal cortex

Attentional cues can trigger activity in the parietal cortex in anticipation of visual displays, and this activity may, in turn, induce changes in other areas of the visual cortex, hence, implementing attentional selection. In a recent TMS study [Mevorach, C., Humphreys, G. W., & Shalev, L. Opposite biases in salience-based selection for the left and right posterior parietal cortex. Nature Neuroscience, 9, 740-742, 2006b], it was shown that the posterior parietal cortex (PPC) can utilize the relative saliency (a nonspatial property) of a target and a distractor to bias visual selection. Furthermore, selection was lateralized so that the right PPC is engaged when salient information must be selected and the left PPC when the salient information must be ignored. However, it is not clear how the PPC implements these complementary forms of selection. Here we used on-line triple-pulse TMS over the right or left PPC prior to or after the onset of global/local displays. When delivered after the onset of the display, TMS to the right PPC disrupted the selection of the more salient aspect of the hierarchical letter. In contrast, left PPC TMS delivered prior to the onset of the stimulus disrupted responses to the lower saliency stimulus. These findings suggest that selection and suppression of saliency, rather than being "two sides of the same coin," are fundamentally different processes. Selection of saliency seems to operate reflexively, whereas suppression of saliency relies on a preparatory phase that "sets up" the system in order to effectively ignore saliency.     

Attention and interference in the processing of global and local information: effects of unilateral temporal-parietal junction lesions

The processing of hierarchical stimuli was examined in patients with lesions in the temporal-parietal junction. In separate blocks of trials, subjects were instructed to identify the letter at the local or the global level of a hierarchical stimulus. Consistent with previous findings, reaction times for controls were faster in the globally than in the locally directed condition and reaction times to the local level were longer when the letters at the two levels were different (e.g. local "S"s forming a global "H") than when they were the same (e.g. local "S"s forming a global "S"). In other words, controls exhibited interference when locally directed. Patients with lesions centered in the rostral inferior parietal lobe (IPL) showed interference effects similar to controls. In contrast, patients with lesions centered in the posterior superior temporal gyrus and adjacent caudal inferior parietal lobe (STG) showed no interference. The data suggest that the posterior superior temporal plane and adjacent caudal inferior parietal lobe plays an important role in the integration of and/or attention to local and global level information.     

Feature similarity and laterality effects in visual masking

Observers attempted to recognize single letters presented to the left or right visual field when preceded or followed by a masking stimulus. When the masking stimulus contained features identical to those of the target letter, there was a left visual field (right hemisphere) advantage for target recognition. When the target and mask contained extremely different features, the opposite visual field advantage was obtained. Implications are discussed for models of hemispheric asymmetry and for interpreting visual laterality studies that use masks to reduce the overall level of performance.     

Hemispheric preponderance in categorical and coordinate visual processes.

Two experiments were conducted to address whether a left hemispheric bias would be observed for categorical processing in both 'what' and 'where' systems (experiment 1) while a reverse bias would characterize coordinate processing whatever the systems (experiment 2). Young normal subjects were tested using divided visual field tasks. The results of experiment 1 indicated that subjects made categorical judgments in both what and where systems faster when stimuli are presented to the left hemisphere. The results of experiment 2 showed a significant interaction between visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantage was observed when the task required easy processing whereas a right-hemisphere advantage was noted for difficult distinctions either in location (where system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothesis (1989) for the where system and suggests that the same left-hemisphere advantage also exists for the what system. Concerning coordinate processing, our findings highlight the influence of processing difficulty on the hemispheric lateralization and evidence a right hemispheric advantage for difficult coordinate processing and a left hemispheric advantage for easy coordinate processing. The results are discussed in terms of possible link between on the one hand difficulty and coordinate processing, and easiness and categorization on the other hand.     

Patterns of brain activity during visual imagery of letters.

Cortical signals associated with visual imagery of letters were recorded from 10 healthy adults with a whole-scalp 122-channel neuromagnetometer. The auditory stimulus sequence consisted of 20 different phonemes corresponding to single letters of the Roman alphabet and of tone pips (17%), delivered once every 1.5 sec in a random order. The subjects were instructed to visually imagine the letter corresponding to the auditory stimulus and to examine its visuospatial properties: The associated brain activity was compared with activity evoked by the same stimuli when the subjects just detected the intervening tones. All subjects produced broad imagery-related responses over multiple cortical regions. After initial activation of the auditory cortices, the earliest imagery-related responses originated in the left prerolandic area 320 msec after the voice onset. They were followed within 70 msec by signals originating in the posterior parietal lobe close to midline (precuneus) and, 100 msec later, in the posterior superior temporal areas, predominantly in the left hemisphere. The activations were sustained and partially overlapping in time. Imagery-related activity in the left lateral occipital cortex was observed in two subjects, and weak late activity in the calcarine cortex in one subject. Real audiovisually presented letters activated multiple brain regions, and task-induced visuospatial processing of these stimuli further increased activity in some of these regions and activated additional areas: Some of these areas were activated during imagery as well. The results suggest that certain brain areas involved in high-level visual perception are activated during visual imagery and that the extent of imagery-related activity is dictated by the requirements of the stimuli and the task.     

Event-related functional magnetic resonance imaging study of the extrastriate cortex response to a categorically ambiguous stimulus primed by letters and familiar geometric figures.

Functional neuroimaging studies have suggested a specific role of the extrastriate cortex in letter string and visual word form processing. However, this region has been shown to be involved in object recognition and its specificity for the processing of linguistic stimuli may be questioned. The authors used an event-related functional magnetic resonance imaging design with category priming to record the response elicited by the passive viewing of single letters, geometric figures, and of the categorically ambiguous stimulus "O" that pertains to both sets of familiar symbols. Bilateral activations in the extrastriate cortex were found, with a left predominance particularly pronounced for the ambiguous stimulus. Individual analysis of spatial extent and signal intensity showed a priming x stimulus x hemisphere interaction. When primed by the congruous categoric set, a bilateral decrease in activation was observed for letters and geometric figures. The ambiguous stimulus behaved as a letter for the left hemisphere, with decreased activation when primed by letters, whereas in the right hemisphere, an adaptation effect occurred when primed by geometric figures. These priming effects suggest that, for the ambiguous stimulus, letter processing was systematically involved in the left extrastriate cortex. The current results support the existence of a neural substrate for the abstract category of letters.     

Neural substrates differentiating global/local processing of bilateral visual inputs

We investigated neural substrates of global/local processing of bilateral hierarchical stimuli using functional magnetic resonance imaging (fMRI). Subjects were presented with two compound letters that were displayed simultaneously in the left and right visual fields, respectively. In a steady-state, block-design paradigm, hemodynamic responses were recorded while subjects detected infrequent global or local targets presented in one hemifield in separate epochs of trials. While behavioural responses were more accurate and faster to global than local targets, attention to the global level of bilateral visual inputs induced stronger activations in the left and right temporal cortex relative to attention to the local level. However, attention to the local level generated stronger activations in bilateral superior parietal cortex compared with attention to the global level. The results suggest that distinct neural substrates in the temporal and parietal cortices are preferentially engaged in the global and local processing of bilateral visual inputs, respectively.     

Effects of lesions of temporal-parietal junction on perceptual and attentional processing in humans

When stimuli with larger forms (global) containing smaller forms (local) are presented to subjects with large lesions in the right hemisphere, they are more likely to miss the global form than the local form, whereas subjects with large lesions in the left are more likely to miss the local than the global form. The present study tested whether the global/local impairment in subjects with posterior lesions was due to deficits in controlled attentional processes, passive perceptual processes, or both. Attentional control was examined by measuring reaction time changes when the probability of a target appearing at either the global or local level was varied. Patients with unilateral right or left lesions centered in temporal-parietal regions and age-matched controls served as subjects. Because neurophysiological and neuropsychological evidence have implicated temporal regions in visual discrimination and inferior parietal regions in the allocation of attention to locations in the visual field, patients with left hemisphere lesions were further subdivided into those with lesions centered in the superior temporal gyrus (LSTG) or rostral inferior parietal lobule (LIPL). Patients with right hemisphere injury could not be analogously subdivided. The results revealed that the LSTG group was able to control the allocation of attention to global and local levels normally, while the LIPL group was not. In contrast, the LSTG group showed a strong baseline reaction time advantage toward global targets, while normals and the LIPL group showed no advantage toward one level or the other. Finally, the perceptual component was affected differentially by lesions in the right hemisphere and LSTG, with lesions in the left favoring global targets and lesions in the right favoring local targets. These findings indicate that the hemispheric global/local asymmetry is due to a perceptual mechanism with a critical anatomical locus centered in the STG.     

The fate of global information in dorsal simultanagnosia

Abstract

We report a patient with simultanagnosia and Bálint-Holmes syndrome, following bilateral parieto-occlpital lesions, who exhibits a selective disturbance of global processing in everyday situations and in clinical tasks. For hierarchical Navon stimuli, where global letters are formed by the layout of smaller local letters, she could report only local shapes. She could identify the global form via proprioceptive input, when her finger was passively moved to trace the global shape, provided her eyes were closed. However, with her eyes open, the local visible form dominated once again even when the global shape was traced. This demonstrates dominance of pathological vision over intact proprioception, and shows that local capture persists even when the global information can still be processed by the patient, through another modality. This raises the possibility that some visual global processing might likewise still take place despite the local capture. In accordance wlth this notion, additional experiments showed that the patient was faster at naming local shapes if the global shape had the same identity rather than an incongruent identity. Moreover, a visual-search task found that parallel searching for unique features was preserved across the visual field. Taken together, our experiments suggest that the dominance of the local scale for KB is not due to a total inability to code any global information, but rather to an attentional bias towards salient local details following her brain damage.     

Lateral asymmetry: Hard or simple-minded?

A letter or a three-dimensional shape was presented in the center of the visual field. Following the off-set of this stimulus either a comparison letter or a three-dimensional shape was flashed briefly in either the right or left visual field. The subject's task was to respond SAME, or DIFFERENT. The stimuli could be in the same plane, rotated in two dimensions (letters) or in three dimensions (three-dimensional shapes). The left visual field presentations (right hemisphere) of same-pair matches for letters only produced faster reaction times and fewer errors. In all other conditions reaction time measures showed no hemisphere effects. By contrast, error score data indicated that the left hemisphere was overwhelmingly more accurate.     

Cerebral lateralization of frontal lobe language processes and lateralization of the posterior visual word processing system

The brain areas involved in visual word processing rapidly become lateralized to the left cerebral hemisphere. It is often assumed this is because, in the vast majority of people, cortical structures underlying language production are lateralized to the left hemisphere. An alternative hypothesis, however, might be that the early stages of visual word processing are lateralized to the left hemisphere because of intrinsic hemispheric differences in processing low-level visual information as required for distinguishing fine-grained visual forms such as letters. If the alternative hypothesis was correct, we would expect posterior occipito-temporal processing stages still to be lateralized to the left hemisphere for participants with right hemisphere dominance for the frontal lobe processes involved in language production. By analyzing event-related potentials of native readers of French with either left hemisphere or right hemisphere dominance for language production (determined using a verb generation task), we were able to show that the posterior occipito-temporal areas involved in visual word processing are lateralized to the same hemisphere as language production. This finding could suggest top-down influences in the development of posterior visual word processing areas.     

Schizophrenia Patients Show Deficits in Shifts of Attention to Different Levels of Global-Local Stimuli: Evidence for Magnocellular Dysfunction

Abnormalities of attention and visual perception are well documented in schizophrenia. The global-local task is a measure of attention and perceptual organization that utilizes visual stimuli comprised of large letters (global level) made up of smaller letters (local level). Subjects identify target letters appearing at either the global or local level of the stimulus. In this study, we used a version of the global-local task specifically designed to examine lateralized hemispheric processing and attention shifting in 30 schizophrenia patients and 24 normal controls. Global-local stimuli were presented in couplets (consecutive pairs). Reaction time for the second target in a couplet was compared under conditions in which the target remained at the same level (global-global, local-local) and when the target changed levels (global-local, local-global). Level-specific priming (ie, global to global and local to local) and the local-to-global level shift were similar in both groups. Schizophrenia patients were significantly slower, however, shifting attention from the global to the local level. These results implicate an impairment in shifting attentional resources from predominantly right lateralized magnocellular/dorsal stream processing of global targets to predominantly left lateralized parvocellular/ventral stream processing of local targets. Local interference effects in global processing provide further support for impaired magnocellular processing in schizophrenia patients.     

Hemispheric asymmetries in visual search

We conducted two visual search experiments, and found that target-detection accuracy and speed were better when the target was projected to the right hemisphere in the feature search condition and better when the target was projected to the left hemisphere in the feature-conjunction search condition. We propose that the highly efficient, so-called parallel search performance characteristic of feature search is enabled by a broadly distributed, global view of the visual field, and the right hemisphere is more efficient than the left in such global processing. On the other hand, the less-efficient performance characteristic of conjunction search (demonstrated by the set-size effect) involves serial shifts of focused attention, and the left hemisphere is more efficient than the right in such localised attentional processing. We suggest that hemispheric asymmetries observed in visual search are related to the attentional demands of the task, and that we adjust our attentional distribution to fit task difficulty. When the target is very distinct, a global, low-resolution attentional distribution is sufficient, and enables parallel search; but a localised, narrow-aperture attentional distribution is sometimes necessary to find targets that either require feature binding, or are very similar to other “distractor” stimuli in terms of compositional attributes.