Subregions of human parietal cortex selectively encoding object orientation

Computation of object orientation could be an independent process from those of other object features, but currently neither the location of human brain areas selectively coding orientation information nor an optimum experimental paradigm have yet been established. In this study, functional magnetic resonance imaging was used to investigate brain activation in the parietal cortices related to object orientation. Using an Arabic digit whose spatial attributes were carefully manipulated, we found parietal areas exclusively sensitive to object orientation, but not to general spatial attention. It seems that, by excluding confounds such as mental manipulation or working memory as well as inherent spatial information within the stimuli, functional segregation within the parietal lobe can be effectively probed.     

Functional clusters in the human parietal cortex as revealed by an observer-independent meta-analysis of functional activation studies

The human parietal cortex is a highly differentiated structure consisting of cytoarchitectonically defined subareas that are specifically connected with other cortical and subcortical areas. Based on evidence from neurophysiological studies in subhuman primates these subareas are supposed to be functionally highly specialized. Here, we reviewed 51 different neuroimaging studies on healthy subjects with activation of the parietal lobe in statistical parametric maps. Running a cluster analysis on the stereotactic coordinates of the centers of gravity of the activation areas and plotting them into Talairach space showed a high consistency of the mean activation foci for similar paradigms across different laboratories and functional imaging modalities. Our meta-analysis exposed seven distinct pairs of quite symmetrically distributed subareas of the parietal cortex of each hemisphere as well as three unpaired regions that are critically involved in the generation of limb and eye movements in egocentric and allocentric coordinates, but also in attention, memory and cognitive problem solving. These data highlights the modular organization of the human parietal lobe. By its locally interspersed distributed circuits it orchestrates specialized cognitive subfunctions interfacing perception and action. Our meta-analysis provides a new framework for understanding information processing in the human parietal cortex.     

Striate cortex in humans demonstrates the relationship between activation and variations in visual form

Electrophysiologic and functional imaging studies have shown that the visual cortex produces differential responses to the presence or absence of structure within visual textures. To further define and characterize regions involved in the analysis of form, functional magnetic resonance imaging (fMRI) was used to detect changes in activation during the viewing of four levels of isodipole textures. The texture levels systematically differed in the density of visual features such as extended contours and blocks of solid color present within the images. A linear relationship between activation level and density of structure was observed in the striate cortex of human subjects. This finding suggests that a special subpopulation of striate cortical neurons participates in the ability to extract and process structural continuity within visual stimuli. Received: 8 July 1998 / Accepted: 25 May 1999     

Combined visual attention and finger movement effects on human brain representations

Sensory and motor systems interact in complex ways; visual attention modifies behavior, neural encoding, and brain activation; and dividing attention with simultaneous tasks may impede performance while producing specific brain activation patterns. We hypothesized that combining voluntary movement with visual attention would yield unique brain representations differing from those occurring for movement or visual attention alone. Hemodynamic signals in humans were obtained with functional magnetic resonance imaging (MRI) while participants performed one of four tasks that required only a repetitive finger movement, only attending to the color of a visual stimulus, simultaneous finger movement and visual attention, or no movement and no visual attention. The movement-alone task yielded brain activation in structures commonly engaged during voluntary movement, including the primary motor cortex, supplementary motor area, and cerebellum. Visual attention alone resulted in sparse cerebral cortical and substantial bilateral cerebellar activation. Simultaneous performance of visual attention and finger movements yielded widespread cerebral cortical, cerebellar, and other subcortical activation, in many of the same sites activated for the movement or attention tasks. However, the movement-related plus attention-related activation extended beyond the movement-alone or attention-alone activation sites, indicating a novel activation pattern related to the combined performance of attention and movement. Additionally, the conjoint effects of visual attention and movement upon brain activation were probably not simple gain effects, since we found activation-related interactions in the left superior parietal lobule, the right fusiform gyrus, and left insula, indicating a potent combinatory role for visual attention and movement for activation patterns in the human brain. In conclusion, performing visual attention and movement tasks simultaneously, even though the tasks had no specific interrelationship, resulted in novel activation patterns not predicted by performing movements or visual attention alone. Electronic Publication     

Case mixing and the right parietal cortex: evidence from rTMS

We investigated the necessary role of the right parietal lobe in visual word recognition using transcranial magnetic stimulation (TMS). TMS was applied to the right posterior parietal lobe and to a control area as participants read aloud words presented either in lower case or in mIxEd-cAsE. The words were presented either with unlimited duration and high contrast (Experiment 1), or with brief presentation and low-contrast (Experiments 2 and 3). In all three experiments, TMS over the parietal area disrupted reading, and in Experiments 2 and 3 this effect was most pronounced for mIxEd-cAsE words. This suggests that the right parietal lobe mediates the recognition of words in unfamiliar formats.     

Local brain atrophy accounts for functional activity differences in normal aging

Functional brain imaging studies of normal aging typically show age-related under- and overactivations during episodic memory tasks. Older individuals also undergo nonuniform gray matter volume (GMv) loss. Thus, age differences in functional brain activity could at least in part result from local atrophy. We conducted a series of voxel-based blood oxygen level-dependent (BOLD)-GMv analyses to highlight whether age-related under- and overrecruitment was accounted for by GMv changes. Occipital GMv loss accounted for underrecruitment at encoding. Efficiency reduction of sensory-perceptual mechanisms underpinned by these areas may partly be due to local atrophy. At retrieval, local GMv loss accounted for age-related overactivation of left dorsolateral prefrontal cortex, but not of left dorsomedial prefrontal cortex. Local atrophy also accounted for age-related overactivation in left lateral parietal cortex. Activity in these frontoparietal regions correlated with performance in the older group. Atrophy in the overrecruited regions was modest in comparison with other regions as shown by a between-group voxel-based morphometry comparison. Collectively, these findings link age-related structural differences to age-related functional under- as well as overrecruitment.     

Interactions of cognitive reserve with regional brain anatomy and brain function during a working memory task in healthy elders

Cognitive reserve (CR) defines the capacity of the adult brain to cope with pathology in order to minimize symptomatology. Relevant lifetime social, cognitive and leisure activities represent measurable proxies of cognitive CR but its underlying structural and functional brain mechanisms remain poorly understood. We investigated the relationship between CR and regional gray matter volumes and brain activity (fMRI) during a working memory task in a sample of healthy elders. Participants with higher CR had larger gray matter volumes in frontal and parietal regions. Conversely, a negative correlation was observed between CR and fMRI signal in the right inferior frontal cortex, suggesting increased neural efficiency for higher CR individuals. This latter association however disappeared after adjusting for gray matter images in a voxel-based manner. Altogether, present results may reflect both general and specific anatomofunctional correlates of CR in the healthy elders. Thus, whereas heteromodal anterior and posterior gray matter regions correspond to passive (i.e. morphological) correlates of CR unrelated to functional brain activation during this particular cognitive task, the right inferior frontal area reveals interactions between active and passive components of CR related to the cognitive functions tested in the fMRI study.     

Activation of the human posterior parietal cortex by median nerve stimulation

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70–110 ms.     

Decoding task and stimulus representations in face-responsive cortex

Observers can deliberately attend to some aspects of a face (e.g. emotional expression) while ignoring others. How do internal goals influence representational geometry in face-responsive cortex? Participants watched videos of naturalistic dynamic faces during MRI scanning. We measured multivariate neural response patterns while participants formed an intention to attend to a facial aspect (age, or emotional valence), and then attended to that aspect, and responses to the face's emotional valence, independent of attention. Distinct patterns of response to the two tasks were found while forming the intention, in left fronto-lateral but not face-responsive regions, and while attending to the face, in almost all face-responsive regions. Emotional valence was represented in right posterior superior temporal sulcus and medial prefrontal cortex, but could not be decoded when unattended. Shifting the focus of attention thus alters cortical representation of social information, probably reflecting neural flexibility to optimally integrate goals and perceptual input.     

Own-sex effects in emotional memory for faces

The amygdala is known to be critical for the enhancement of memory for emotional, especially negative, material. Importantly, some researchers have suggested a sex-specific hemispheric lateralization in this process. In the case of facial expressions, another important factor that could influence memory success is the sex of the face, which could interact with the emotion depicted as well as with the sex of the perceiver. Whether this is the case remains unknown, as all previous studies of sex difference in emotional memory have employed affective pictures. Here we directly explored this question using functional magnetic resonance imaging in a subsequent memory paradigm for facial expressions (fearful, happy and neutral). Consistent with our hypothesis, we found that the hemispheric laterality of the amygdala involvement in successful memory for emotional material was influenced not only by the sex of the subjects, as previously proposed, but also by the sex of the faces being remembered. Namely, the left amygdala was more active for successfully remembered female fearful faces in women, whereas in men the right amygdala was more involved in memory for male fearful faces. These results confirm the existence of sex differences in amygdala lateralization in emotional memory but also demonstrate a subtle relationship between the observer and the stimulus in this process.     

Damage to superior parietal cortex impairs pointing in the sagittal Plane

Neurophysiology and neuroimaging research implicates distinct regions of posterior parietal cortex for reaching versus grasping and for completing these movements in central versus peripheral space. Typically, visuomotor tasks only examine movements made in the frontoparallel plane. We examined a patient with a right superior parietal lesion encompassing the parietal-occipital junction, the intraparietal sulcus and the putative human homologue of V6A on pointing tasks in the sagittal or frontoparallel planes. The patient did not demonstrate a speed-accuracy trade-off, but did show larger times post-peak velocity for all movement directions. Her movements in the sagittal axis were more disordered than movements in the frontoparallel plane. These data indicate a role for superior parietal cortex in fine tuning of visually guided movements and more particularly for movements made back towards the body.

Cortical processing of lateral skin stretch stimulation in humans

Direction discrimination of a moving tactile stimulus requires intact dorsal columns and provides a sensitive clinical test of somatosensory dysfunction. Cortical mechanisms are poorly understood. We have applied tangential skin pulls to the right lower leg during functional magnetic resonance imaging. Healthy subjects judged the direction of the skin pulls (task experiment, n = 7) or received skin pulls passively (no task experiment, n = 8). Second somatosensory cortex (S2) was activated in the task as well as no task experiment, and there was no significant difference in cortical activation between the two experiments. Within S2 nearly all subjects had prominent activations in the caudal and superficial part, i.e., in the opercular parietal (OP) area 1. S1 was activated in only one of the subjects. Thus, S2 and especially OP 1 seems to be important for processing of lateral skin stretch stimulation. The finding suggests that a lesion of this area might cause a disturbance in tactile direction discrimination which should be relevant for clinical testing.     

Hemispheric asymmetry in visuospatial attention assessed with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) was used to study visuospatial attention processing in ten healthy volunteers. In a forced choice recognition task the subjects were confronted with two symbols simultaneously presented during 120 ms at random positions, one in the left and the other in the right visual field. The subject had to identify the presented pattern out of four possible combinations and to press the corresponding response key within 2 s. Double-pulse TMS (dTMS) with a 100-ms interstimulus interval (ISI) and an intensity of 80% of the stimulator output (corresponding to 110-120% of the motor threshold) was applied by a non-focal coil over the right or left posterior parietal cortex (PPC, corresponding to P3/P4 of the international 10-20 system) at different time intervals after onset of the visual stimulus (starting at 120 ms, 270 ms and 520 ms). Double-pulse TMS over the right PPC starting at 270 ms led to a significant increase in percentage of errors in the contralateral, left visual field (median: 23% with TMS vs 13% without TMS, P=0.0025). TMS applied earlier or later showed no effect. Furthermore, no significant increase in contra- or ipsilateral percentage of errors was found when the left parietal cortex was stimulated with the same timing. These data indicate that: (1) parietal influence on visuospatial attention is mainly controlled by the right lobe since the same stimulation over the left parietal cortex had no significant effect, and (2) there is a vulnerable time window to disturb this cortical process, since dTMS had a significant effect on the percentage of errors in the contralateral visual hemifield only when applied 270 ms after visual stimulus presentation.     

Functional properties of neurones in the parietal retroinsular cortex in awake monkey

The parietal retroinsular cortex of three, awake, behaving macaque monkeys was investigated using transdural microelectrode recording technique. All 73 cells identified responded to somatosensory stimuli. Most of the neurones (51) were activated by compression of the skin in an on, off or on-off fashion; these cells were unresponsive to light touching or stroking of the skin. The rest of the cells responded to light touching, rotation of a joint or palpation of a muscle belly. All body parts were represented in this area. Of the cells 30% responded to stimulation of both sides of the body. The results indicate that the parietal retroinsular cortex participates in the analysis of skin compression. This information is used, i.e. in the control of manipulative movements and in discrimination of supported weights. It is possible that ablation of the area examined would impair these functions.     

Functional properties of neurons in the temporo-parietal association cortex of awake monkey

Summary The temporo-parietal association cortex around the caudal end of the Sylvian fissure was studied with the single cell recording technique in three awake behaving Macaca speciosa-monkeys. Of the 197 cells isolated, 5% were active only during the monkey's own movements, mostly during head rotation, and 95% were responsive to sensory stimulation: 54% to auditory stimuli, 24% to somatosensory stimuli, 13% to both of these and 4% to visual stimuli. Some cells, classified as responsive to somatosensory stimuli, were activated only by passive rotation of the head on the cervical axis; it is possible that they were driven by vestibular stimuli. Half of the cells were activated by stimuli on both sides of the monkey, and almost all the rest, only by stimuli on the side contralateral to the hemisphere recorded.Of the acoustically drivable cells, 95% responded to natural sounds, such as, rubbing hands together, rustle of clothes, clicks or jingles (sounds with noise spectrum and rapid intensity transitions). Most of these neurons were also examined with pure tones of 0.2–20 kHz: various inhibitory or excitatory responses were elicited in half of them, usually over a wide range of frequencies. The responses of most acoustically drivable cells (62%) depended on the location of the sound source with reference to the monkey's head so that the maximal response was elicited by sounds with a certain angle of incidence, usually on the contralateral side.The present results suggest that the area studied participates in the analysis of the temporal pattern of a sound, the location of the sound source and in spatial control of head movements.     

Two different streams form the dorsal visual system: anatomy and functions

There are two radically different views on the functional role of the dorsal visual stream. One considers it as a system involved in space perception. The other is of a system that codes visual information for action organization. On the basis of new anatomical data and a reconsideration of previous functional and clinical data, we propose that the dorsal stream and its recipient parietal areas form two distinct functional systems: the dorso-dorsal stream (d-d stream) and the ventro-dorsal stream (v-d stream). The d-d stream is formed by area V6 (main d-d extrastriate visual node) and areas V6A and MIP of the superior parietal lobule. Its major functional role is the control of actions "on line". Its damage leads to optic ataxia. The v-d stream is formed by area MT (main v-d extrastriate visual node) and by the visual areas of the inferior parietal lobule. As the d-d stream, v-d stream is responsible for action organization. It, however, also plays a crucial role in space perception and action understanding. The putative mechanisms linking action and perception in the v-d stream is discussed.     

Optokinetic nystagmus deficits following parieto-occipital cortex lesions in monkeys

Summary Optokinetic nystagmus (OKN) was induced in six monkeys by rotation of a full field drum. After unilateral lesions of the inferior parietal lobule and prestriate cortex (IPL-PS lesion), three monkeys had diminished velocity of slow components when the drum rotated toward the side of the lesion. OKN slow components appeared normal when the drum rotated in the opposite direction, while fast components appeared normal in both directions. The severity of the slow component deficit was greater at higher rates of stimulus rotation than at lower ones. Recovery occurred within 7–10 days for two monkeys; no recovery was evident after 2 weeks for the third. Subsequent bilateral IPL-PS lesions in two of these monkeys reduced OKN slow phase velocity in both directions. Two monkeys with unilateral lesions limited to the inferior parietal lobule (IPL lesions) had only very mild and transient deficits lasting 1–3 days. Bilateral IPL lesions also produced only slight OKN deficits. One monkey had a lesion which destroyed most of the lateral striate cortex of one hemisphere and had no discernable OKN deficit. These results demonstrate that, in contrast to earlier reports, lesions of parieto-occipital cortex in monkeys produce deficits which are qualitatively similar to, although of shorter duration than, those OKN deficits which commonly are associated with posterior parietal damage in humans.Supported by NIH grants EY-2640, EY-5318, 5-S01-RR-5530-14, and a grant from the Mayo Foundation     

Transcranial magnetic stimulation in the visual system. II. Characterization of induced phosphenes and scotomas

Transcranial magnetic stimulation (TMS) induces phosphenes and disrupts visual perception when applied over the occipital pole. Both the underlying mechanisms and the brain structures involved are still unclear. In the first part of this study we show that the masking effect of TMS differs to masking by light in terms of the psychometric function. Here we investigate the emergence of phosphenes in relation to perimetric measurements. The coil positions were measured with a stereotactic positioning device, and stimulation sites were characterized in four subjects on the basis of individual retinotopic maps measured by with functional magnetic resonance imaging. Phosphene thresholds were found to lie a factor of 0.59 below the stimulation intensities required to induce visual masking. They covered the segments in the visual field where visual suppression occurred with higher stimulation intensity. Both phosphenes and transient scotomas were found in the lower visual field in the quadrant contralateral to the stimulated hemisphere. They could be evoked from a large area over the occipital pole. Phosphene contours and texture remained quite stable with different coil positions over one hemisphere and did not change with the retinotopy of the different visual areas on which the coil was focused. They cannot be related exclusively to a certain functionally defined visual area. It is most likely that both the optic radiation close to its termination in the dorsal parts of V1 and back-projecting fibers from V2 and V3 back to V1 generate phosphenes and scotomas.     

Brain regions involved in spatial frequency discrimination: evidence from fMRI

The cortical areas underlying successive spatial-frequency discrimination were explored using functional magnetic resonance imaging (fMRI). In a steady-state, block-design paradigm, 12 subjects viewed a single fixation cross during a rest period, followed by an activation period consisting of the presentation of horizontal (distractors) and vertical (targets) sinewave gratings. Two tasks were performed: in the control task, subjects pressed a button after the second vertical grating was presented within each trial; in the discrimination task, subjects decided which target grating had the higher spatial frequency. Post-processing consisted of off-line image registration to correct for head motion, spatial and temporal smoothing, and cross-correlation between each voxel time course and a phase-shifted stimulus time profile. The results indicate that striate, extrastriate, parietal, and prefrontal areas show significant BOLD (blood oxygen level dependent) effects during both discrimination and control tasks, with consistently higher activity levels in the discrimination task.     

Modified expression of Bcl-2 and SOD1 proteins in lymphocytes from sporadic ALS patients

The main goal of this functional magnetic resonance imaging (fMRI) study was to identify the neural correlates of a mystical experience. The brain activity of Carmelite nuns was measured while they were subjectively in a state of union with God. This state was associated with significant loci of activation in the right medial orbitofrontal cortex, right middle temporal cortex, right inferior and superior parietal lobules, right caudate, left medial prefrontal cortex, left anterior cingulate cortex, left inferior parietal lobule, left insula, left caudate, and left brainstem. Other loci of activation were seen in the extra-striate visual cortex. These results suggest that mystical experiences are mediated by several brain regions and systems.