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.     

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.     

Directed and divided attention during hierarchical processing in patients with visuo-spatial neglect and matched healthy volunteers

Hierarchically organized figures (for example, a large E made up of smaller N's) are frequently used to investigate directed and divided attention. Investigations of neurological and psychiatric patients, and also tachistoscopic and functional neuroimaging studies on healthy subjects, typically find the right hemisphere to be specialized for the processing of global stimuli and the left hemisphere to be specialized for the processing of local stimuli. In the current study, a group of 12 patients with visuo-spatial neglect (NP) after right hemisphere lesions and 12 age and sex-matched control subjects (CO) performed a directed and a divided attention task with hierarchically organized letters. As expected, faster reaction times were found for control subjects than for neglect patients, especially for the directed global attention task. Lower error rates were found for CO and NP for local than for global targets during the divided attention condition. Local on global interference was found for both groups in reaction times. These local processing advantages for older healthy adults have been reported previously. Additionally, an impairment in the divided attention task was found in both groups, but especially for global targets in NP. This impairment is consistent with other evidence of difficulty in disengaging attention shown by patients with visuo-spatial neglect.     

Differential processing of hierarchical visual stimuli in young and older healthy adults: implications for pathology

Hierarchical figures in which large (global) forms are constructed from smaller (local) forms (Navon, 1977) have proved valuable in studies of perceptual organisation and hemispheric specialisation in both healthy volunteers and a wide range of neurological and psychiatric patients. In studies using Navon figures, normal young adults typically identify global forms faster than local forms. When the global and local forms are incongruent (e.g., a large E made of smaller Rs), global forms often interfere with local form identification more than vice versa. In two conditions on the same subjects, we contrasted the performance of young (mean age 22 years) and older (mean age 58 years) healthy volunteers on global and local processing. In the directed attention task, subjects were instructed to detect a target letter that occurred at the prespecified local or global level. The young subjects showed, as expected, faster reaction times (RTs) to detect global targets. In contrast, the older subjects showed significantly faster RTs to the local targets. Likewise, in a divided attention task, in which subjects were instructed to detect a target letter that could occur at either the local or the global level, the young adults were slightly quicker to detect the global targets and the older subjects were significantly quicker to detect the local targets. Error rates were generally low and there was no significant speed/accuracy trade-off in either condition. The observed local precedence effects in healthy older subjects were unexpected and are discussed in reference to previous work on differential hemispheric aging. That work has suggested that the left hemisphere is preferentially biased toward local processing and ages relatively slowly while the right hemisphere is biased toward global processing and ages relatively quickly. The implications of such putative differential aging for the interpretation of pathological local/global processing in neurological and psychiatric diseases are also emphasised.     

Hemisphere priming through practice in a musical chords task

In a study of selective hemisphere activation, the performance of 30 subjects given either a "local" or "global" priming activity before and after a monoaurally presented chord analysis task was compared with an unprimed control group. The hypothesis was that ear advantage scores on the chords task would show increased left hemisphere involvement following local priming and increased right hemisphere involvement following global priming. The results failed to support this hypothesis. All subjects, however, regardless of priming condition, showed a very strong practice effect represented by a shift from a weak initial left ear (right hemisphere) advantage towards a significant right ear (left hemisphere) advantage (P less than 0.00006). The findings suggest that although the local/global priming activity did not lead to selective hemisphere activation, repeated exposure to the chords task resulted in increased use of analytic left hemisphere processing strategies as subjects became familiar with its processing requirements.     

Attentional and executive dysfunctions in schizophrenia and depression: evidence from dichotic listening performance.

BACKGROUND: We compared performance on a dichotic listening (DL) task between schizophrenic, depressed, and healthy control subjects. A variant of the traditional DL paradigm was used in which the subjects were required to focus attention either on the left (forced-left condition) or right (forced-right condition) ear stimulus. METHODS: The subjects were 51 patients with a DSM-IV diagnosis of schizophrenia, 49 patients with recurrent unipolar major depression, and 49 healthy control subjects. They were tested with a consonant-vowel syllables dichotic listening task under three attentional instructions. RESULTS: There was a significant overall right ear advantage during the nonforced condition, which increased dramatically during the forced-right condition and was eliminated during the forced-left condition. The depressed patients showed no signs of impairments compared with the healthy control group. Thus, they showed a right ear advantage during the nonforced and forced-right conditions, which was shifted to a left ear advantage during the forced left condition. The schizophrenic patients, however, were impaired on the forced-left condition compared with the healthy control and depressed subjects. CONCLUSIONS: The results are discussed in terms of separating attentional and inhibitory executive impairments in schizophrenia and depression, taking into consideration illness duration and information-processing demands.     

Hemispheric differences in global and local processing with orientation classification tasks

Hemispheric differences in global and local processing were examined in one experiment with hierarchical stimuli. The figures consisted of large squares with the right or left side missing made up of small squares with the right or left side missing. The subjects were asked to decide the opening (left/right) of the square either at the global level or at the local level. The findings showed that with the task and stimuli used here global judgements were as fast and accurate as local judgements, the interference was bidirectional and symmetrical and, finally, that the right hemisphere and the left hemisphere had the same ability to manage with global and local information. So, the experiment does not provide evidence for hemispheric specialisation in global and local processing.     

Normal global-local analysis in patients with dorsolateral frontal lobe lesions.

The role of prefrontal cortex in the analysis of global and local levels of a visual stimulus was assessed by measuring reaction time to identify a target at one level or the other. Unlike patients with temporal-parietal lesions (STG), there were no global-local performance deficits in right or left prefrontal groups (RFL or LFL). Reallocation of attention to global and local levels was measured by examining changes in performance when the probability of a target appearing at one level or the other varied. While patients with lateral parietal lesions (IPL) have been shown to have deficits in these conditions, both RFL and LFL showed normal changes in performance. In sum, the performance of prefrontal groups differed from both IPL and STG groups but not from normal controls. These results strengthen previous arguments that posterior association cortex is crucial in responding to global and local levels of a pattern.     

Effects of saliency, not global dominance, in patients with left parietal damage

Neuropsychological and functional imaging studies have shown a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. There is also evidence for the left parietal lobe being important for switching attention between local and global levels. Here we examined whether the left parietal lobe is associated with another aspect of attentional control over hierarchical visual processing; namely, ignoring the irrelevant aspect of the stimulus when it is more salient than the target attribute. In experiment 1 a group of left parietal patients were abnormally affected by a salient local stimulus that significantly interfered with their ability to identify global shapes. This effect was reversed in experiment 2 when small sized compound letters were used (where the global shape was more salient than the local letters). The patients then had difficulty ignoring the global shape. Experiments 3 and 4 demonstrated that the failure to accurately identify global form in experiment 1 could not be attributed to a difficulty in spreading attention across a large area. Finally, in experiment 4 the effect of a salient irrelevant stimulus was significantly attenuated when the irrelevant level did not map onto a response. The data indicate that damage to the left parietal lobe disrupts the ability to select attributes of stimuli that have low salience when other attributes have high salience for the task.     

An ERP study of the global precedence effect: the role of spatial frequency.

OBJECTIVE: This study investigated the neural mechanisms underlying the effects of removal of low spatial frequency (SF) contents from stimulus displays on the processing of global and local properties of compound stimuli. METHODS: Event-related brain potentials (ERPs) were recorded from 16 subjects who selectively attended to the global or local features of compound letters, which were either white on a gray background containing broadband SFs or were contrast-balanced (CB) to eliminate low SFs, and were randomly presented in the left or right visual fields. ERPs were analyzed to examine how global/local attention modulations of neural substrates were influenced by SF manipulations. RESULTS: We found that an early process of global recognition was indexed by a negativity peaking at 190 ms over contralateral occipito-temporal cortex and was eliminated by contrast balancing. The late stage of global recognition was reflected in a late negativity peaking at 300 ms and was only retarded by contrast balancing. Global-to-local interference was characterized by enhanced occipito-temporal negativities and was evident for both broadband and CB stimuli. CONCLUSIONS: The results clarify distinct cognitive and neural mechanisms underlying the global precedence and interference effects, which were different in terms of the independence of low SFs in compound stimuli.     

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.     

Processing of illusion of length in spatial hemineglect: a study of line bisection

Bisection of horizontal lines and of the Brentano form of the Müller-Lyer illusion was investigated in six right brain-damaged patients with left spatial hemineglect, and in six control subjects. Patients bisected the lines to the right of the objective mid-point. Comparable illusory effects on line bisection were however found in both patients and control subjects. Relative to the baseline condition, in both groups the subjective midpoint was displaced towards the side expanded by the illusion, both leftwards and rightwards. By contrast, line length and spatial position of the stimulus had differential effects. In neglect patients, the rightward bisection error increased disproportionately with line length, and when the stimulus was located in the left, neglected, side of egocentric space. Control subjects showed no such effects. The suggestion is made that the visual, non-egocentric, processes underlying these illusory effects of length may be spared in patients with left spatial neglect. The possible neural basis of this dissociation is discussed.     

EEG alpha oscillations in the preparation for global and local processing predict behavioral performance

Visual attention can be directed either to the global features of a display or to the local elements that make up the display. We investigated whether oscillatory brain responses to globally or locally directed cue stimuli predict behavioral performance in subsequent target processing. Induced alpha band (8-12 Hz) amplitudes in the pre-stimulus interval were measured separately for the global and the local level, where individual trials were assigned to one of three groups according to the response speed towards incongruent stimuli. Fast responses to local features were associated with high alpha amplitudes in the right centro-parietal cortex, whereas fast responses to global forms were associated with high alpha in left centro-parietal cortex. For trials with slower responses, the pattern of hemispheric differences was diminished or even reversed. It is interpreted that the left and the right parietal cortex exert top-down control over hierarchical processing by inhibiting stimulus representations in one hemisphere.     

Electrophysiological evidence for aging effects on local contextual processing

We used event-related potentials to investigate how aging affects local contextual processing. Local context was defined as the occurrence of a short predictive series of visual stimuli before delivery of a target event. Stimuli were presented to either the left or right visual field and consisted of 15% targets (downward facing triangle) and 85% of equal numbers of three types of standards (triangles facing left, upwards and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Predictive local context affected target detection by reducing the duration of stimulus evaluation compared to detection of non-predictive random targets comparably for both young and older adults, as shown by a P3b latency shift. The peak of an earlier latency context positivity, which was seen only in the predicted compared to the random target condition, was prolonged in the older population compared to young adults. Finally, older subjects elicited a late sustained positivity in the predictive condition, not seen in the younger subjects. Taken together, these findings suggest that local contextual effects on target detection processes are altered with age.     

Visuo-spatial cognition in schizophrenia: confirmation of a preference for local information processing

During visuo-spatial cognitive tasks, patients with schizophrenia show a preference for local (detailed) rather than global (holistic) information processing. The efficiency of such information processing is influenced by task difficulty. We tested whether patients' preference for local processing would persist if task demands favored global or local processing. Twenty-four stabilized patients with schizophrenia (SZ) and 25 healthy, matched controls (C) were tested in a mental mirroring task. Task difficulty was manipulated while stimulus surface structures were maintained unchanged. Information processing was assessed by recording eye movements. SZ were slower than C in the easiest condition but they made more errors than C in the more difficult conditions. Further, SZ did not adapt their average fixation duration to task demands resulting in longer fixation duration in the easiest condition and shorter fixation duration in the most difficult condition compared to C. These findings suggest that patients employ local information processing even when it is maladaptive for task demands. That is, patients do not adapt their fixation duration to task demands implicating (i) a preference for scanning local stimuli features and (ii) information processing inflexibility. These features need to be taken into account when evaluating visuo-spatial cognitive performance in schizophrenia.     

Interactions between damaged brain hemisphere and mode of presentation on the recognition of faces and figures

Visual recognition performance of right or left hemispheric damaged subjects and of a healthy control group was investigated under three modes of stimulus presentation: normal, sequential, or fragmentary. Stimuli were faces or not easily verbalizable so-called Perret-figures. A dissociation between side of brain damage and performance under sequential vs normal item presentation was found: similar to control subjects, left hemispheric damaged patients remembered previously seen stimuli better under normal presentation, while right brain damaged subjects were superior in performance under sequential compared to normal stimulus presentation. These results may be of relevance for therapeutical interventions with respect to learning and remembering figural material in right brain damaged subjects.     

Effect of optokinetic stimulation in patients with visual neglect

Three groups of subjects, normal controls, right brain damaged patients with and without heminattention, were required to bisect a line in presence of a fixed or a moving background. The stimulus moving horizontally toward the left or the right induced an optokinetic nystagmus with a slow phase coherent with the direction of the movement: together with the optokinetic nystagmus, a displacement of the subjective midpoint, as compared to the condition with a fixed background, was observed in all three groups of subjects. Within the right brain damaged with heminattention the displacement of the line bisection was great for stimuli moving toward the right, but a significant reduction of the left bias was present. In the same group of patients the effect of the optokinetic stimulation was present in a very large proportion of cases and proved to be relatively stable in a test-retest presentation. Theoretical relevance of these data and their potential importance for rehabilitation are discussed.     

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.     

Differences in semantic category priming in the left and right cerebral hemispheres under automatic and controlled processing conditions.

The contribution of each cerebral hemisphere to the generation of semantic category meanings at automatic and strategic levels of processing was investigated in a priming experiment where prime and target words were independently projected to the left or right visual fields (LVF or RVF). Non-associated category exemplars were employed as related pairs in a lexical decision task and presented in two experimental conditions. The first condition was designed to elicit automatic processing, so related pairs comprised 20% of the positive set, stimulus pairs were temporally separated by a stimulus onset asynchrony (SOA) of 250 ms, and there was no allusion to the presence of related pairs in the instructions to subjects. The second condition, designed to invoke controlled processing, incorporated a relatedness proportion of 50%, stimulus pairs separated by an SOA of 750 ms, and instructions which informed subjects of the presence and use of category exemplar pairs in the stimulus set. In the first condition, a prime directed to either visual field facilitated responses to categorically related targets subsequently projected to the RVF, while in the second condition a prime directed to either visual field facilitated responses to related targets projected to the LVF. The facilitation effects obtained in both conditions appeared to reflect automatic processes, while strategic processes were invoked in the left, but not the right hemisphere in the second condition. The results suggest that both hemispheres have automatic access to semantic category meanings, although the timecourse of activation of semantic category meanings is slower in the right hemisphere than in the left.