Tactual recognition of cognitive stimuli: roles of hemisphere and lobe.

Four groups of focal brain-damaged patients, involving left and right hemisphere damage with or without the involvement of parietal lobe, and a control group of general medical patients, were required (a) dichaptically to perceive two cognitive stimuli (letter letter, figure figure, or letter figure), and (b) haptically match the target stimulus from a set of test stimuli. The number of trials taken to match the target stimulus correctly was the dependent measure. The significant main effect of lobe suggested that the parietal lobe is the primary locus for tactual recognition. All interaction effects involving lobe were nonsignificant. The main effect of hemisphere was nonsignificant, although the interaction of hemisphere x lobe was significant. Left and right hemisphere damaged patients required more trials to recognize "letter" and "figure", respectively.     

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.     

Effects of attention on dichotic listening: an 15O-PET study

The present study investigated the effect of attention on brain activation in a dichotic listening situation. Dichotic listening is a technique to study laterality effects in the auditory sensory modality. Two different stimuli were presented simultaneously, one in each ear. Twelve subjects listened to lists of consonant-vowel syllables, or short musical instrument passages, with the task of detecting a "target" syllable or musical instrument by pressing a button. The target stimulus appeared an equal number of times in the left and right ear. The subjects were instructed to either concentrate on the stimuli presented in both ears, or only on the left or right ear stimulus. Brain activation was measured with 15O-PET, and significant changes in regional normalized counts (rNC) were evaluated using statistical parametric mapping (SPM96) software. Concentrating on either the right or left ear stimulus significantly decreased activity bilaterally in the temporal lobes compared to concentrating on both ear stimuli, at the expense of an increased activation in the right posterior and inferior superior parietal lobe. The CV-syllables activated areas corresponding to the classic language areas of Broca and Wernicke. The musical instrument stimuli mainly activated areas in visual association cortex, cerebellum, and the hippocampus. An interpretation of the findings is that attention has a facilitating effect for auditory processing, causing reduced activation in the primary auditory cortex when attention is explicitly recruited. The observed activations in the parietal lobe during the focused attention conditions could be part of a modality non-specific "attentional network".     

Transcranial magnetic stimulation to the parietal lobes reduces detection of contralateral somatosensory stimuli

OBJECTIVES: Neglect has been described in patients with lesions of the parietal cortex and has been interpreted as a disorder of the allocation of spatial attention. The persistence of neglect has been linked to poor rehabilitation outcome in patients suffering from acute stroke. Transcranial magnetic stimulation (TMS) applied to the parietal cortex has been shown to induce changes in the perception of stimuli including tactile stimulation of the fingers contra- and ipsilateral to the stimulated hemisphere. MATERIAL AND METHODS: In the current study, eleven normal young subjects performed a detection task for cutaneous electrical stimuli to the left or right forearm that had been precued by a preceding visual warning stimulus. To investigate the role of the parietal cortical areas for attentional processes TMS was applied to frontal and parietal scalp sites of each hemisphere in the cue-target interval before the somatosensory stimulus. RESULTS: Right and left parietal stimulation led to reduced detection sensitivity for near threshold stimuli to the forearm contralateral to the stimulated hemisphere without hemispheric differences. Ipsilateral tactile perception was not influenced by parietal TMS and there was no change in perception after frontal stimulation to left or right scalp sites. CONCLUSION: This pattern of results is consistent with a role of the right and left parietal lobe in the distribution of spatial attention and provides an experimental basis for possible therapeutical application of TMS to improve attentional deficits in stroke patients.     

Somatoperceptual asymmetry for cognitive stimuli: support from experimental and clinical observations

Two studies, one experimental with intact-brain subjects (Study I), and one clinical with left or right hemisphere-damaged and non-neurological general medical patients (Study II), were conducted to examine somatolateral asymmetry in the recognition of verbal (letter) and visuospatial (nonsense shapes) stimuli. Blindfolded subjects were required to explore two stimuli dichhaptically, and were instructed to haptically identify the target stimulus in a set of test stimuli. The findings of Study I indicated that fewer trials were required to recognise nonsense shapes when they were presented to the left hand, whereas fewer trials were required for letters when they were presented to the right hand. The findings of Study II indicated that performance of the hand ipsilateral to the damaged left hemisphere was better for nonsense shapes than for letters, and performance of the hand ipsilateral to the damaged right hemisphere was better for letters than for nonsense shapes.     

Brain activations associated with probability matching

Previously, in a simple probability-matching experiment with two split-brain patients that involved having the participant predict which of two events will happen on the next trial, we found that the left hemisphere tended to look for patterns and match the frequency of previous occurrences but not the right hemisphere [Wolford, G., Miller, M. B., & Gazzaniga, M. S. (2000). The left hemisphere's role in hypothesis formation. Journal of Neuroscience, 20(RC64), 1-4]. In this study, we examined those findings in normal subjects using fMRI. Subjects alternated between blocks of trials in which they predicted the location of a stimulus and those in which they detected the location of a stimulus. Previous investigators using similar paradigms reported mostly right hemisphere activations, including activations in the right dorsolateral and ventrolateral prefrontal cortex, the medial prefrontal cortex, and the right lateral parietal lobe. We also found mostly right hemisphere activations, but we found that some of the activations in the dorsolateral prefrontal and parietal cortices were sensitive to individual differences in the tendency to look for patterns in random sequences. Further, we found that, by controlling for the working memory component of the predicting task, all brain activations in the normal brain associated with looking for patterns were related to the task demands of working memory processes underlying probability matching and predicting.     

Neural correlates of conscious and unconscious vision in parietal extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.     

Neural Correlates of Conscious and Unconscious Vision in Parietal Extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.     

Right hemisphere specialization for mental rotation in normals and brain damaged subjects

Hemisphere specialization for mental rotation was investigated utilizing Shepard's (1971) paradigm. In each of two experiments, the procedure involved presenting pairs of novel non-verbal stimuli at various angles of disparity. Subjects were instructed to construct a mental image of one stimulus, rotate this image, and judge whether or not the image was a congruent match with its mate. Both response time and accuracy were measured. In Experiment 1, the testing of right-handed normals revealed a significant left visual field advantage for accuracy (p less than .0001) and response time (p less than .05). In Experiment 2, a comparison of right parietal lesioned patients with both left parietal lesioned patients and matched normal controls likewise revealed significant right lesion effects for accuracy (p greater than .0001) and response time (p greater than .01). Right hemisphere specialization for mental rotation was documented for both normals and brain damaged subjects.     

Neural consequences of somatosensory extinction

There are currently two main interpretations proposing mechanisms underlying tactile extinction: sensory and attention deficit hypotheses. Kinsbourne proposed an opponent processor model to support the attention deficit hypothesis. He insisted that bilateral hemispheres interact reciprocally through contralaterally oriented vectors, and in patients presenting extinction, balance is impaired, causing inattention. From Kinsbourne's point of view, extinction is not caused by sensory disturbance but inattention, therefore even in extinction patients, simultaneous bilateral stimuli should reach the bilateral primary sensory cortices (SI). Using functional magnetic resonance imaging (fMRI), tactile stimuli were administered to both hands of healthy subjects as well as a tactile extinction patient. The patient with tactile extinction extinguished right palm stimuli following simultaneous palm stimulation. During the fMRI study, we gave tactile stimuli to the right palm, the left palm, and simultaneously to both palms. In normal subjects, simultaneous bilateral stimuli activated the bilateral SI and bilateral secondary sensory cortices (SII). In the patient with right tactile extinction, simultaneous bilateral stimuli activated the bilateral SI along with the bilateral SII and right superior parietal lobule. Our study suggests that activation of SI is insufficient to engender an awareness of sensory stimuli. From the view point of Kinsbourne, stimulus driven activity in one hemisphere suppresses activity in the other hemisphere via callosal connections. Our results support the notion that an undamaged superior parietal lobule in the patient with tactile extinction suppresses the damaged parietal lobe function and causes extinction.     

Hemispheric preference in visuospatial processing: a complementary approach with fMRI and lesion studies

Historically, the left cerebral hemisphere has been considered specialized for language, whereas the right cerebral hemisphere is aligned with spatial processes. However, studies have called into question adherence to this model and suggested that both hemispheres participate in language and spatial cognition. Using functional Magnetic Resonance Imaging (fMRI) and human brain lesion studies, we determined whether these complementary techniques could clarify issues of hemispheric dominance. Using a modified Benton Judgement of Line Orientation (JLO) test, considered a relatively pure spatial processing task, we found robust and significant (p < 0.0005) bilateral superior parietal lobe activation on fMRI in ten right-handed male adult volunteers. This was corroborated by lesion data in a cohort of 17 patients who showed significant JLO impairments after either right or left parietal lobe damage, with right parietal damage associated with somewhat more severe deficit. Detailed wavelet analysis of the fMRI time-series did, however, reveal a more dominant role of the right parietal lobe in "kick-starting" the task. To our knowledge, this is a novel way of using fMRI to address functional hemispheric differences in a cognitive task that is known to have bilateral representation.     

Visual-field superiority as a function of stimulus type and content

The objective of this study was to examine whether hemispheric superiority is determined more by stimulus type (facial, lexical) or content (neutral, emotional). A split, visual-field experiment was designed using a computer-based program with bilateral presentation (left visual-field, right visual-field) for four sets of stimuli: face (neutral, emotional) x word (neutral, emotional), N = 40. The dependent measures were the frequency of correct response and response latency of correct responses. The visual-field effect was nonsignificant for correct responses; however, the interaction of stimulus type x content was found significant. The interaction of visual-field x stimulus type was significant with response time as the dependent measure. Facial stimuli were processed faster in the left visual-field (a right hemispheric function) and lexical stimuli were processed faster in the right visual-field (a left hemispheric function). No hemispheric effect was observed for stimulus content.     

Cingulate cortex activation and competing responses: the role of preparedness for competition

Regional cerebral blood flow (rCBF) was studied in a task, where a preparatory stimulus (S1) cued for an imperative second stimulus (S2) which was associated with a response. Two preparatory stimuli cued unequivocally each for one response. In contrast, a third preparatory stimulus cued for two response alternatives which appeared for the same ratio (each in 50% of all trials) introducing response competition. In a first experimental condition, non-arbitrary, unambiguous stimuli were used as S1 to enable the subjects to prepare their responses. In a second and third scan, arbitrary preparatory stimuli were used during different stages of awareness for the S1-S2 association. Subjects performed this task "naive" without knowledge about the S1-S2 association and also in an experimental condition being aware of the S1-S2 association. Button presses after unambiguous, non-arbitrary preparatory stimuli activated the right middle frontal gyrus and inferior parietal lobe if S1 was associated with a definite response. When the subjects did not know the S1-S2 relation, left prefrontal cortex activation was associated with trials including definite responses. Performing the same S1-S2 response condition after subjects knew their relation right prefrontal and left parietal areas became additionally engaged. However, in the first experimental condition using unambiguous, non-arbitrary stimuli and in the third, "aware" experimental condition when S1 was coupled with two response alternatives, the anterior cingulate cortex was activated. As these experimental conditions have in common, that the preparatory stimulus shares information about the upcoming competing response alternatives they highlight the evaluative role of the anterior cingulate cortex for competing actions.     

Is it mine? Hemispheric asymmetries in corporeal self-recognition

The aim of this study was to investigate whether the recognition of "self body parts" is independent from the recognition of other people's body parts. If this is the case, the ability to recognize "self body parts" should be selectively impaired after lesion involving specific brain areas. To verify this hypothesis, patients with lesion of the right (right brain-damaged [RBD]) or left (left brain-damaged [LBD]) hemisphere and healthy subjects were submitted to a visual matching-to-sample task in two experiments. In the first experiment, stimuli depicted their own body parts or other people's body parts. In the second experiment, stimuli depicted parts of three categories: objects, bodies, and faces. In both experiments, participants were required to decide which of two vertically aligned images (the upper or the lower one) matched the central target stimulus. The results showed that the task indirectly tapped into bodily self-processing mechanisms, in that both LBD patients and normal subjects performed the task better when they visually matched their own, as compared to others', body parts. In contrast, RBD patients did not show such an advantage for self body parts. Moreover, they were more impaired than LBD patients and normal subjects when visually matching their own body parts, whereas this difference was not evident in performing the task with other people's body parts. RBD patients' performance for the other stimulus categories (face, body, object), although worse than LBD patients' and normal subjects' performance, was comparable across categories. These findings suggest that the right hemisphere may be involved in the recognition of self body parts, through a fronto-parietal network.     

VISUAL-FIELD SUPERIORITY AS A FUNCTION OF STIMULUS TYPE AND CONTENT

The objective of this study was to examine whether hemispheric superiority is determined more by stimulus type (facial, lexical) or content (neutral, emotional). A split, visual-field experiment was designed using a computer-based program with bilateral presentation (left visual-field, right visual-field) for four sets of stimuli: face (neutral, emotional) &#50 word (neutral, emotional), N = 40. The dependent measures were the frequency of correct response and response latency of correct responses. The visual-field effect was nonsignificant for correct responses; however, the interaction of stimulus type &#50 content was found significant. The interaction of visual-field &#50 stimulus type was significant with response time as the dependent measure. Facial stimuli were processed faster in the left visual-field (a right hemispheric function) and lexical stimuli were processed faster in the right visual-field (a left hemispheric function). No hemispheric effect was observed for stimulus content.     

Cortical and subcortical contributions to the representation of temporal information

Converging evidence suggests that temporal representations of brief durations are derived subcortically. We tested split-brain patient JW in order to investigate whether these representations project bilaterally or unilaterally to cortex. Using visual stimuli to signal time intervals, JW was asked to compare the duration of a pair of standard stimuli that were presented bilaterally with a comparison stimulus that was presented to either the left or right visual field. Assuming the hand of response is controlled by the contralateral cerebral hemisphere, a hand by visual field interaction was predicted if the representation of stimulus duration was restricted to the cerebral hemisphere receiving the lateralized stimulus. However, we failed to observe this interaction for two different ranges of stimulus durations, both in the hundred (Experiment 2) to hundreds (Experiment 1) of milliseconds range. Instead, there was a consistent right hemisphere advantage in task performance. When the task then required a discrimination based on the physical size of the stimuli rather than their duration, an interaction between response hand and visual field was obtained (Experiment 3). Taken together, these results suggest that (1) even though the comparison stimulus was presented unilaterally, the representation of its duration was available to both cerebral hemispheres, and (2) a right hemisphere advantage in psychophysical tasks requiring the comparison of successive stimuli is observed for temporal and non-temporal judgments.     

Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI

OBJECTIVE: To use functional MRI (fMRI) to determine which brain regions are implicated when normal volunteers judge whether pretransected horizontal lines are correctly bisected (the Landmark test). BACKGROUND: Manual line bisection and a variant thereof involving perceptual judgments of pretransected lines (the Landmark test) are widely used to assess unilateral visuospatial neglect in patients with neurologic disease. Although unilateral (left) neglect most often results from lesions to right temporoparietal cortex, the normal functional anatomy of the Landmark test has not been convincingly demonstrated. METHODS: fMRI was carried out in 12 healthy right-handed male volunteers who judged whether horizontal lines were correctly prebisected. In the control task, subjects detected whether the horizontal lines contained a transection mark irrespective of the position of that mark. Response was by two-choice key press: on half the trials, subjects used the right, and on half, the left hand. Statistical analysis of evoked blood oxygenation level-dependent responses, measured with echoplanar imaging, employed statistical parametric mapping. RESULTS: Performing the Landmark task showed neural activity (p < 0.05, corrected) in the right superior posterior and right inferior parietal lobe, early visual processing areas bilaterally, the cerebellar vermis, and the left cerebellar hemisphere. Only the latter area showed a significant interaction with hand used. CONCLUSIONS: The right hemispheric dominance observed in inferior parietal cortex is consistent with the results of lesion studies. Right superior parietal cortex, vermis, and left cerebellar hemisphere have not been implicated in neglect, but all appear to play a cognitive role in the Landmark task.     

Neural activation during response competition

The flanker task, introduced by Eriksen and Eriksen [Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143--149], provides a means to selectively manipulate the presence or absence of response competition while keeping other task demands constant. We measured brain activity using functional magnetic resonance imaging (fMRI) during performance of the flanker task. In accordance with previous behavioral studies, trials in which the flanking stimuli indicated a different response than the central stimulus were performed significantly more slowly than trials in which all the stimuli indicated the same response. This reaction time effect was accompanied by increases in activity in four regions: the right ventrolateral prefrontal cortex, the supplementary motor area, the left superior parietal lobe, and the left anterior parietal cortex. The increases were not due to changes in stimulus complexity or the need to overcome previously learned associations between stimuli and responses. Correspondences between this study and other experiments manipulating response interference suggest that the frontal foci may be related to response inhibition processes whereas the posterior foci may be related to the activation of representations of the inappropriate responses.     

Stimulus type affects Wada memory performance

The effects of amytal injection side, seizure focus laterality, and stimulus type (real and line-drawn objects, printed words, and faces) on recognition memory were studied during the Wada procedure. To-be-remembered stimuli were presented during cerebral anesthesia to 35 patients with left temporal lobe epilepsy (LTLE) and 28 patients with right temporal lobe epilepsy (RTLE), all with left hemisphere language dominance. In both groups, recognition of real and line-drawn objects was best after anesthetization of the lesional hemisphere. Recognition of faces was poor after either injection in patients with RTLE, but only after right injection in patients with LTLE. Conversely, recognition of words by patients with LTLE was impaired equally after either injection, but more so after left than right injection in patients with RTLE. The findings suggest that (1) real and line-drawn objects are "dually encoded" and memory accuracy depends on seizure focus laterality, and (2) accuracy in recognition of words and faces is related to seizure focus laterality, but may also depend on the language dominance of the hemisphere being assessed.     

Cerebral specialization for speech and non-speech stimuli in infants

Early cerebral specialization and lateralization for auditory processing in 4-month-old infants was studied by recording high-density evoked potentials to acoustical and phonetic changes in a series of repeated stimuli (either tones or syllables). Mismatch responses to these stimuli exhibit a distinct topography suggesting that different neural networks within the temporal lobe are involved in the perception and representation of the different features of an auditory stimulus. These data confirm that specialized modules are present within the auditory cortex very early in development. However, both for syllables and continuous tones, higher voltages were recorded over the left hemisphere than over the right with no significant interaction of hemisphere by type of stimuli. This suggests that there is no greater left hemisphere involvement in phonetic processing than in acoustic processing during the first months of life.