Gestalt perception and the decline of global precedence in older subjects

Our visual world is hierarchically organized. Hierarchical processing is frequently investigated using Navon figures (large letters made up of smaller ones). In young adults, many studies reported faster reaction times (RT) to target letters presented at the global level [i.e., global precedence (GP)]. Furthermore, an age-related decline of this GP has been reported. We tested whether deficits in perceptual grouping via Gestalt laws (Gestalt principles of Proximity and Continuity) might contribute to this decline. In a directed attention task with valid and invalid cues, 20 young (mean age 22) and 20 older (mean age 57) male subjects had to indicate whether a target letter appeared at the global or local level of a Navon figure. The number of local letters forming the global figure was modulated in 5 steps. As expected, during valid trials, young adults showed a GP that linearly increased with increasing numbers of local letters (i.e., GP enhancement). This suggests that GP is related to perceptual grouping via Gestalt laws. By contrast, the group of older subjects demonstrated no precedence effect in RT and a non-significant trend toward GP in error rates (ER). No GP enhancement with an increasing number of local elements was observed. Exploratory analysis revealed that individual insensitivity to the modulation of matrix density, as revealed by a lack of global RT acceleration, was restricted to subjects that showed an overall local precedence (LP). Because older subjects tended to more frequently display an insensitivity to matrix modulation and an LP, we conclude that deficient Gestalt detection as indicated by non-enhanced global RT might contribute to the RT-related decline of GP with age.     

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.     

Visual half-field tests of lateralized letter presentations: differences between preliterate and literate children

In the present study, hemispheric differences in global and analytic processing were investigated in preliterate and literate children, using the "dual-letter" matching task paradigm (Navon, 1977). The stimuli consisted of lateralized visual presentations of large uppercase letters made up of small uppercase letters. The task of the subject was to decide on each trial if the large or small letters were the same or not. Vocal reaction time (VRT) and error-frequency were used as dependent measures. In Experiment 1, 28 right-handed 8-years old children participated. The children were split into a preliterate and a literate group depending on teachers evaluations, and on scores on a reading test. The results showed longer VRTs for the preliterate children when the stimuli were initially presented to the right hemisphere and especially when the subjects were required to match the small letters. These results were followed up in Experiment 2, where the preliterate subjects were split into "fast" and "slow" readers. The results showed that the "slow" readers were more impaired in processing the letter stimuli when the stimuli were initially presented to the right hemisphere. It is concluded that hemispheric asymmetry for letter processing interacts with the development of normal reading ability in children.     

The interference of local over global information processing in children with attention deficit hyperactivity disorder of the inattentive type

A classic finding in perception of compound patterns is normal individuals cannot skip global analysis in local-oriented processing, but they can successfully resist local analysis in global-oriented processing-the so-called global interference [1]. Recently, studies examining the role of brain hemisphere activity in the Navon task have indicated that the processing of global and local information can be, respectively, attributed to the right and left hemispheres. Moreover, many neuroimaging researches have revealed that certain core symptoms of attention deficit hyperactivity disorder (ADHD) are related to dysfunction of right hemisphere. These findings imply that global interference will be substantially less evident, and possibly even replaced by local interference in ADHD. The present study compared the performance of children with and without attention deficit hyperactivity disorder of the inattentive type (ADHD-I) in the processing of global and local information to examine the local interference hypothesis in ADHD. An ADHD-I group (n=15) and a paired control group (n=19) completed tasks using two versions of the Navon task, one requiring divided attention, in which no information was given to participants regarding the level at which a target would appear, and the other requiring selective attention, in which participants were instructed to attend to either the local or the global level. The results showed that children with ADHD-I exhibited local interference, regardless of which attention procedure was used. These results support the weak right hemisphere hypothesis in ADHD, and provide evidence against the deficit hypotheses for ADHD in the DSM-IV criteria [29], which postulates that inattention symptoms may manifest as a failure to provide close attention to details.     

What forgetting tells us about remembering: The influence of top–down control on hemispheric asymmetries in verbal memory

It has been suggested that left hemisphere (LH) advantages in verbal processing is due to superior top–down control of verbal information. It is not clear how top–down mechanisms affect the encoding and retrieval of verbal information from hemispheric memory and whether they only influence activation or also encompass the inhibition of verbal information. The directed forgetting method, in conjunction with divided visual field presentation, was used to examine the influence of top–down control mechanisms on hemispheric asymmetries in verbal memory. Participants were cued to remember or forget words. Cues were presented either simultaneously with targets or after a short delay. A recognition memory test using divided visual field presentation was then given. Response times (RTs) revealed effects of cue timing in the LH. With simultaneous cues, RTs were faster to “Remember” words compared to “Forget” words. With delayed cues, RTs for “Remember” and “Forget” words were equivalent. In the right hemisphere (RH), “Remember” words were consistently faster than “Forget” words, regardless of cue timing. These data provide evidence that top–down mechanisms influenced LH verbal memory retrieval more than RH verbal memory retrieval. Finally, there was little evidence to suggest the hemispheres differ in inhibitory processing.     

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.     

Covert orienting of visuospatial attention in the early stages of aging

Electrophysiological and behavioral responses were recorded in healthy young (19-23 years) and older (56-66 years) subjects during the execution of a visuospatial attention task. The objective was to test whether covert orienting of visuospatial attention (COVAT) is sensitive to the early stages of aging. All subjects responded faster to targets following valid than invalid cues. The amplitude of the P1 component of visual event-related potentials (ERP) was larger to targets following central valid cues at all SOAs. Subtle age-related changes were observed in P1 amplitude under peripheral cueing. Furthermore, older subjects presented longer reaction times (RTs) and lower P1 amplitudes regardless of the attention condition.     

Modulation of non-spatial attention and the global/local processing bias

Amelioration of the rightward spatial attention bias in patients with hemispatial neglect following manipulations of non-spatial attention suggests that spatial attention and mechanisms related to the regulation of attention are interrelated. Studies in normal, healthy subjects have shown similar modulation in spatial bias following tonic and phasic changes in attention suggesting that this interaction is a general mechanism of attention rather than a curiosity of the neglect disorder. The current study examined this attentional interaction to determine if perceptual processes favoring one hemisphere over the other are affected by this relationship. Participants first made rapid discriminations of Navon figures presented at central fixation. As expected, when participants attended to either the local or global dimension, incongruence in the orthogonal dimension resulted in longer reaction times for accurate discrimination compared to congruent trials. However, following a brief (16-min) continuous performance task designed to elicit behaviors associated with greater tonic and phasic alertness, participants showed significantly less local interference when attending the global dimension and more global interference when attending the local dimension on the Navon discrimination task compared to a control task condition. The results indicate that exercising tonic and phasic alertness produces a global processing bias.     

Modulation of Global and Local Processing Biases in Adults with Autistic-like Traits

Previous work shows that doing a continuous performance task (CPT) shifts attentional biases in neurotypical individuals towards global aspects of hierarchical Navon figures by selectively activating right hemisphere regions associated with global processing. The present study examines whether CPT can induce similar modulations of attention in individuals with high levels of autistic traits who typically show global processing impairments. Participants categorized global or local aspects of Navon figures in pre- and post-CPT blocks. Post-CPT, high trait individuals showed increased global interference during local categorization. This result suggests that CPT may be useful for temporarily enhancing global processing in individuals with high levels of autistic traits and possibly those diagnosed with autism.     

Varying the scope of attention alters the encoding of categorical and coordinate spatial relations

Two types of representations can be used to specify spatial relations: Coordinate spatial relations representations specify the precise distance between two objects, whereas categorical spatial relations representations assign a category (such as above or below) to specify a spatial relation between two objects. Computer simulation models suggest that coordinate spatial relations representations should be easier to encode if one attends to a relatively large region of space, whereas categorical spatial relations should be easier to encode if one attends to a relatively small region of space. We tested these predictions. To vary the scope of attention, we asked participants to focus on the local or global level of Navon letters, and immediately afterwards had them decide whether a dot was within 2.54 cm of a bar (coordinate judgment) or was above or below the bar (categorical judgment). Participants were faster in the coordinate task after they had just focused on the global level of a Navon letter whereas they were faster in the categorical task after they had just focused on the local level. Although we did not test the hemispheric lateralization of these effects, these findings have direct implications for theories of why the cerebral hemispheres differ in their relative ease of encoding the two kinds of spatial relations.     

Transient interference of right hemispheric function due to automatic emotional processing

We examined the effects of emotional stimuli on right and left hemisphere detection performance in a hemifield visual discrimination task. A group of 18 healthy subjects were asked to discriminate between upright and inverted triangles (target). Targets were randomly presented in the left or right visual hemifield (150 ms target duration). A brief emotional picture (pleasant or unpleasant; 150 ms stimulus duration) or neutral picture selected from the International Affective Picture System was randomly presented either in the same (47%) or the opposite (47%) spatial location to the subsequent target. Emotional or neutral stimuli offset 150 ms prior to the subsequent target. Subjects were instructed to ignore the pictures and respond to the targets as quickly and accurately as possible. Independent of field of presentation, emotional stimuli prolonged reaction times (P < 0.01) to LVF targets, with unpleasant stimuli showing a greater effect than pleasant stimuli. The current study shows that brief emotional stimuli selectively impair right hemispheric visual discrimination capacity. The findings suggest automatic processing of emotional stimuli captures right hemispheric processing resources and transiently interferes with other right hemispheric functions.     

Global-local processing in schizophrenia: hemispheric asymmetry and symptom-specific interference.

The hypothesis of atypical functional hemispheric asymmetry in schizophrenia is tested using the directed global-local paradigm, a lateralizing measure of visual perception. Results indicate low error rates (< 2%) for schizophrenia and normal control groups, but longer response times for the schizophrenia group. In the normal group, detection speed of global and local forms did not differ. In contrast, the schizophrenia group responded significantly faster to local relative to global forms, which supports the asymmetry hypotheses of left hemisphere overactivity-right hemisphere underactivity in schizophrenia. The normal group exhibited a global interference effect (slowed response latency to the local target in the presence of a dissimilar global distractor). When the schizophrenia group was examined according to symptom type and severity, high positive symptom severity was associated with local interference (slowed response latency to the global target in the presence of dissimilar local distractors). Negative symptoms were not associated with interference from the competing local or global forms. Patients with a combination of high positive and low negative symptoms showed significantly greater local interference than patients with high negative and low positive symptoms. Interconnected temporal and frontal systems are postulated to contribute to this pattern of perceptual processing efficiency and distractibility in schizophrenia.     

Seeing trees OR seeing forests in simultanagnosia: attentional capture can be local or global

Patients with simultanagnosia often demonstrate 'local capture', meaning that they identify only the local elements of stimuli that contain a hierarchy of both local and global structures. Recent studies, however, have found that these patients may implicitly process the global form. We examined the general applicability of the concept of local capture, and specifically whether the global level of stimuli can be explicitly reported by patients with simultanagnosia. We tested a patient with simultanagnosia with globally biased stimuli such as hierarchical Arcimboldo faces and small, dense Navon letters. With Arcimboldo faces our patient often reported only the face and not the local elements--the first demonstration of global rather than local capture. With Navon letters, the patient's ability to report the global letter varied with stimulus density and inversely with stimulus size, so that local capture was found only with large and sparse stimuli. With both faces and letters, the likelihood of global capture by the patient was related to the ease of global reporting in controls, as indexed by their reaction times. This suggests that the patient's global perception is influenced by the same factors operating in healthy individuals. We conclude that attentional capture in simultanagnosia can be either global or local. Capture likely occurs because of a pathological restriction and/or rigidity of attention, but the type of capture depends upon the competitive balance between global and local salience.     

The effect of aging on event-related potentials and behavioral responses: comparison of tonal, phonologic and semantic targets.

OBJECTIVE: To investigate age-related changes in speech perception by measuring event-related potentials (ERPs) elicited by auditory stimuli varying in their linguistic characteristics from pure tones to words. METHODS: ERPs were recorded from 64 subjects in three age groups (young, middle age and elderly) to auditory target stimuli, using an oddball paradigm. Three different tasks and stimuli were used: tonal, phonological and semantic. RESULTS: N100 latency to tonal targets was significantly shorter than to both types of speech targets. P300 latency to tonal targets was significantly shorter than to phonological targets, which in turn was shorter than to semantic targets. P300 amplitude recorded to the speech targets was significantly larger over the left hemisphere than over the right hemisphere in the young subjects. However, the reverse pattern of asymmetry, favoring the right hemisphere was found in the elderly subjects. The pattern of the hemispheric distribution for the middle aged was somewhere in between the young and elderly. CONCLUSIONS: The data indicate possible progressive changes in left-right asymmetry in language processing with aging. SIGNIFICANCE: Findings may indicate an increased use of compensatory mechanisms for speech processing, or alternatively, an increased use of different generators as individuals age.     

Interaction between lateralization of memory and probe stimulus in the recognition of verbal and spatial visual stimuli

The aim of the study was to sort out whether in normal subjects hemispheric asymmetries in memory encoding arise at the input or at the recognition stage of verbal (diagrams of letters) and spatial patterns (diagrams of stars) in a 3×3 virtual matrix. Target and probe stimuli were tachistoscopically presented to the same or the opposite hemisphere. Letters, but not stars, were better recognized in a crossed condition in which targets were flashed to the right and probes to the left hemisphere. The finding suggests that the right hemisphere (but not the left) is capable of adding specific, possibly mnestic, resources to the first stage of letter processing.     

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.     

Does "callosal relay" explain ear advantage in dichotic monitoring?

Pairs of consonant-vowel (CV) syllables were dichotically presented to a large sample of healthy subjects (n = 263) who were instructed to monitor for the presence of a target CV (/ta/) which could occur in either ear. The subjects responded by pressing a response button, allowing the recording of reaction times (RTs) and number of correct responses. The investigated sample comprised consistent right-handers (CRH), consistent left-handers (CLH), and mixed-handers (MH). It was found that right-ear targets were detected more frequently and faster than left-ear targets, both in CRH and MH subjects. CLH subjects, on the other hand, responded faster to targets presented to the left ear but there was no ear advantage in terms of the correct responses. The RT data were used to examine whether they are compatible with the callosal relay model of language lateralisation. It was found that the predictions made by the callosal relay model were supported by the RT data for all groups.     

Does "callosal relay" explain ear advantage in dichotic monitoring?

Pairs of consonant-vowel (CV) syllables were dichotically presented to a large sample of healthy subjects (n = 263) who were instructed to monitor for the presence of a target CV (/ta/) which could occur in either ear. The subjects responded by pressing a response button, allowing the recording of reaction times (RTs) and number of correct responses. The investigated sample comprised consistent right-handers (CRH), consistent left-handers (CLH), and mixed-handers (MH). It was found that right-ear targets were detected more frequently and faster than left-ear targets, both in CRH and MH subjects. CLH subjects, on the other hand, responded faster to targets presented to the left ear but there was no ear advantage in terms of the correct responses. The RT data were used to examine whether they are compatible with the callosal relay model of language lateralisation. It was found that the predictions made by the callosal relay model were supported by the RT data for all groups.     

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.