A PET study of axis orientation discrimination

Evolution of Subjective Visual Vertical Perturbation After Stroke

Objective. The perception of visual verticality is often perturbed after stroke and might be an underlying component of imbalance. The aim of this study was to describe the evolution of visual vertical (VV) perturbation and to investigate the factors affecting it.

Methods. Thirty patients with hemiplegia after a single hemispheric stroke (17 left lesioned [LL] and 13 right lesioned [RL]) were studied. Visual verticality was tested within 45 days of stroke, and then at 3 and 6 months. Subjects sat in a dark room and adjusted a luminous rod to the vertical position. The differences between patients’ adjustments and vertical were calculated. The effects on VV evolution of the side, size, type, and location of the lesion were tested.

Results. Sixty percent of the recent stroke patients had an initial inaccurate perception of verticality, and 39% of these patients recovered during the 1st 3 months after stroke. The evolution of VV tilt depended on the side of the lesion ( P = 0.01), with better recovery in LL patients. None of the other factors studied affected VV normalization.

Conclusions. The poorer recovery of vertical perception after right-side stroke might be due to the predominant role of the right hemisphere in spatial cognition, and might be involved in the poorer recovery of balance after stroke in RL patients.

Effects of Feedback-Based Visual Line-Orientation Discrimination Training for Visuospatial Disorders After Stroke

Background. Patients with right or more rarely left parietotemporal lesions after stroke may have profound visuospatial disorders that impair activities of daily living (ADL) and long-term outcome. Clinical studies indicate improvements with systematic training of perception. Studies of perceptual learning in healthy persons suggest rapid improvements in perceptual learning of spatial line orientation with partial transfer to nontrained line orientations. Objective. The authors investigated a novel feedback-based perceptual training procedure for the rehabilitation of patients after stroke. Methods. In an uncontrolled trial, 13 participants showing profound deficits in line orientation and related visuospatial tasks within 12 to 28 weeks of onset performed repetitive feedback-based, computerized training of visual line orientation over4 weeks of treatment. Visual line-orientation discrimination and visuospatial and visuoconstructive tasks were assessed before and after training. Results. The authors found ( a) rapid improvements in trained but also in nontrained spatial orientation tests in all 13 participants, partially up to a normal level; ( b) stability of the obtained improvements at 2-month follow-up; ( c) interocular transfer of training effects to the nontrained eye in 2 participants suggesting a central, postchiasmatic locus for this perceptual improvement; and ( d) graded transfer of improvements to related spatial tasks, such as horizontal writing, analog clock reading, and visuoconstructive capacities but no transfer to unrelated measures of visual performance. Conclusions. These results suggest the potential for treatment-induced improvements in visuospatial deficits by feedback-based, perceptual orientation training as a component of rehabilitation after stroke.

Localization of human intraparietal areas AIP, CIP, and LIP using surface orientation and saccadic eye movement tasks

In monkeys, areas in the intraparietal sulcus (IPS) play a crucial role in visuospatial information processing. Despite many human neuroimaging studies, the location of the human functional homologs of some IPS areas is still a matter of debate. The aim of the present functional magnetic resonance imaging (fMRI) study was to identify the distinct locations of specific human IPS areas based on their functional properties using stimuli adapted from nonhuman primate experiments, in particular, surface orientation discrimination and memory guided saccadic eye movements (SEM). Intersubject anatomical variability likely accounts for much of the debate. By applying subject by subject analysis, we can demonstrate that sufficient intersubject anatomical and functional commonalities exist. Both the lateral bank of the anterior part of IPS, the putative human homolog of the area AIP, and the caudal part of the IPS (putative CIP) showed activation related to spatial discrimination of surface orientation. Eye tracking conducted during fMRI data acquisition allowed us to show that both areas were separated by an area related to SEM. This area was located in the middle region of the IPS (most probably including LIP), i.e., similar to the location observed in nonhuman primates. In 10 of 11 subjects our putative CIP activation was located in a medial side branch of the posterior part of the IPS, on the opposite side as described in nonhuman primates, making this landmark a useful anatomical marker for the location of CIP.

Effects of right parietal transcranial magnetic stimulation on object identification and orientation judgments

We investigated the role played by the right parietal lobe in object identification and the ability to interpret object orientation, using transcranial magnetic stimulation (TMS) to momentarily interfere with ongoing cortical activity. Short trains of TMS pulses (12 Hz) were applied to a site overlying the right intraparietal sulcus/inferior parietal lobe while subjects performed either object identification tasks (i.e., picture-word verification and categorizing objects as natural or manufactured) or object orientation judgment tasks (i.e., picture-arrow verification and deciding whether an object was rotated clockwise or counterclockwise). Across different tasks, right parietal TMS impaired orientation judgments, but facilitated object identification, compared to TMS applied to a brain vertex control site. These complementary findings demonstrate that the right parietal lobe--a region belonging to the dorsal visual stream--is critical for processing the spatial attributes of objects, but not their identity. The observed improvement in object recognition, however, suggests an indirect role for the right parietal lobe in object recognition. We propose that this involves the creation of a spatial reference frame for the object, which allows interaction with the object and the individuation of specific viewing instances.

Dysfunction of right-hemisphere attentional networks in attention deficit hyperactivity disorder

Although differential right-hemisphere dysfunction has been implicated in attention deficit hyperactivity disorder (ADHD) for more than 15 years, this relation remains controversial. Neuroimaging studies suggest asymmetric dysfunction, but neuropsychological evidence in support of this is rather inconsistent. This study examined attentional asymmetry in ADHD adults with a psychophysical extinction task. The interference from right- or left-hemifield distractors with contralateral orientation sensitivity was determined. In a previous study using this paradigm, right brain-damaged patients with left neglect displayed asymmetric distractor interference, meaning a significant interference from a right distractor with left-hemifield orientation sensitivity but no interference from a left distractor with right-hemifield orientation sensitivity. A similar but less pronounced asymmetry was observed in a group of ADHD adults (n = 16). These results indicate dysfunction of right-hemisphere attentional circuits in ADHD.

Sex differences in visuo-spatial processing: an fMRI study of mental rotation

Following the theoretical framework of coordinate and categorical principals for visuo-spatial processing, originally formulated by [Kosslyn, S. M. (1987). Seeing and imagining in the cerebral hemispheres: AQ computational approach. Psychological Review, 94, 148-175], we present data from an fMRI study on mental rotation, using the classic [Shepard, R. N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703] task, comparing males and females. Subjects were presented with black-and-white drawings of 3-D shapes taken from the set of 3-D perspective drawings developed by [Shepard, R. N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703], alternated with 2-D white bars as control stimuli. The drawings were presented pairwise, as black and white drawings against a black circular background. On half of the trials, the two 3-D shapes were congruent but portrayed with different orientation, in the other half the two shapes were incongruent. Analysis of response accuracy and reaction times did not reveal any significant differences between the sexes. However, clusters of significant neuronal activation were found in the superior parietal lobule (BA 7), more intensely over the right hemisphere, and bilaterally in the inferior frontal gyrus (BA 44/45). Males showed predominantly parietal activation, while the females, in addition, showed inferior frontal activation. We suggest that males may be biased towards a coordinate processing approach, and females biased towards a serial, categorical processing approach.

Cytoarchitectonic identification and probabilistic mapping of two distinct areas within the anterior ventral bank of the human intraparietal sulcus

Anatomical studies in the macaque cortex and functional imaging studies in humans have demonstrated the existence of different cortical areas within the intraparietal sulcus (IPS). Such functional segregation, however, does not correlate with presently available architectonic maps of the human brain. This is particularly true for the classical Brodmann map, which is still widely used as an anatomical reference in functional imaging studies. The aim of this cytoarchitectonic mapping study was to use previously defined algorithms to determine whether consistent regions and borders can be found within the cortex of the anterior IPS in a population of 10 post-mortem human brains. Two areas, the human intraparietal area 1 (hIP1) and the human intraparietal area 2 (hIP2), were delineated in serial histological sections of the anterior, lateral bank of the human IPS. The region hIP1 is located posterior and medial to hIP2, and the former is always within the depths of the IPS. The latter, on the other hand, sometimes reaches the free surface of the superior parietal lobule. The delineations were registered to standard reference space, and probabilistic maps were calculated, thereby quantifying the intersubject variability in location and extent of both areas. In the future, they can be a tool for analyzing structure-function relationships and a basis for determining degrees of homology in the IPS among anthropoid primates. We conclude that the human IPS has a more finely grained parcellation than shown in Brodmann's map.

The functional organization of the intraparietal sulcus in humans and monkeys

In macaque monkeys, the posterior parietal cortex (PPC) is concerned with the integration of multimodal information for constructing a spatial representation of the external world (in relation to the macaque's body or parts thereof), and planning and executing object-centred movements. The areas within the intraparietal sulcus (IPS), in particular, serve as interfaces between the perceptive and motor systems for controlling arm and eye movements in space. We review here the latest evidence for the existence of the IPS areas AIP (anterior intraparietal area), VIP (ventral intraparietal area), MIP (medial intraparietal area), LIP (lateral intraparietal area) and CIP (caudal intraparietal area) in macaques, and discuss putative human equivalents as assessed with functional magnetic resonance imaging. The data suggest that anterior parts of the IPS comprising areas AIP and VIP are relatively well preserved across species. By contrast, posterior areas such as area LIP and CIP have been found more medially in humans, possibly reflecting differences in the evolution of the dorsal visual stream and the inferior parietal lobule. Despite interspecies differences in the precise functional anatomy of the IPS areas, the functional relevance of this sulcus for visuomotor tasks comprising target selections for arm and eye movements, object manipulation and visuospatial attention is similar in humans and macaques, as is also suggested by studies of neurological deficits (apraxia, neglect, Bálint's syndrome) resulting from lesions to this region.

Functional properties and interaction of the anterior and posterior intraparietal areas in humans

In the monkey the lateral bank of the anterior part of the intraparietal sulcus (area AIP), contains neurons that are involved in visually guided, object-related hand movements. It has also been shown that neurons in the caudal part of the intraparietal sulcus (area CIP) preferentially respond to 3D surface orientation. According to these results, it has been hypothesized that neurons in area CIP primarily encode the 3D features of an object and forwards this information to area AIP. AIP then utilizes this information for appropriate hand actions towards the object. Based on analogies to these primate studies, recent neuroimaging studies have suggested human homologues of areas AIP and CIP, however, the functional interaction between these areas remains unclear. Our event related fMRI study was designed to address specifically the question, how CIP and AIP interact in the process of adjustment of hand orientation towards objects. Volunteers were asked to perform three tasks: discrimination of surface orientation, imaging of visually guided hand movements and execution of visually guided hand movements. Our data show that the human AIP was activated both during discrimination of surface orientation and during the subsequent spatial adjustment of the thumb and index finger position towards the surface orientation. In contrast, human CIP was activated by the surface orientation but not by spatial adjustment of finger position. These data clearly indicate that the function of human CIP is more involved in coding 3D features of the objects, whereas human AIP is more involved in visually guided hand movements, similar to its role in the monkey.

Actions speak louder than functions: the importance of manipulability and action in tool representation

PET was used to investigate the neural correlates of action knowledge in object representations, particularly the left lateralized network of activations previously implicated in the processing of tools and their associated actions: ventral premotor cortex (VPMCx), posterior middle temporal gyrus (PMTG), and intraparietal sulcus (IPS). Judgments were made about the actions and functions associated with manipulable man-made objects (e.g., hammer); this enabled us to measure activations in response to both explicit and implicit retrieval of knowledge about actions associated with manipulable tools. Function judgments were also made about nonmanipulable artifacts (e.g., traffic light) providing a direct comparison for manipulable objects. Although neither the left VPMCx nor the left PMTG were selective for tool stimuli (nonmanipulable objects also activated these areas relative to a visual control condition), both regions responded more strongly to manipulable objects, suggesting a role for these cortical areas in the processing of knowledge associated with tools. Furthermore, these activations were insensitive to retrieval task, suggesting that visually presented tools automatically recruit both left VPMCx and left PMTG in response to action features that are inherent in tool representations. In contrast, the IPS showed clear selectivity for explicit retrieval of action information about manipulable objects. No regions of cortex were more activated by function relative to action judgments about artifacts. These results are consistent with the brain's preferential responsiveness to how we interact with objects, rather than what they are used for.

Functional MRI of self-controlled stereoscopic depth perception

Stereoscopic depth perception was studied in healthy young adults using fMRI imaging at 2.0 T. In a novel paradigm we compared the cortical activation elicited by single-image stereograms which create alternating 2D and 3D percepts (event-related analysis triggered on the self-controlled switches between the two percepts) with the activation caused by a more conventional approach contrasting pairs of stereoscopic images with pairs of identical images (block design). The data show a distributed network of cortical areas embedded within the visual pathways that included about one-quarter of the cortical surface activated by 2D visual stimulation and about one-half of the area activated by 3D percepts based on stereoscopic image pair. 3D perception recruited mostly neuronal populations in higher order visual areas: whereas about 40% of the visually activated locations along the intraparietal sulcus were also activated by 3D perception based on single-image stereograms (resp. 90% stereoscopic images), only 10% such overlap was found in striate cortex. The study revealed no sup-port for a right-hemispheric lateralization of depth perception.

Irrelevant digits affect feature-based attention depending on the overlap of neural circuits

Feature-based attention was investigated by examining the effect of irrelevant information on the processing of relevant information. In all experiments, irrelevant information consisted of digits whose semantic information is known to be processed in parietal areas. Between experiments we varied the degree of parietal involvement in the processing of the relevant feature. The influence of the irrelevant digit on the binary manual response task on the relevant feature was measured by the SNARC effect, a spatial numerical association of response codes demonstrating faster left than right hand responses for small numbers and faster right than left hand responses for large numbers. When processing of the relevant feature depended on parietal cortex, as is the case for orientation processing (exps. 1 and 4), there was an effect of the digit's semantic value on response times. Conversely, there was no effect of the irrelevant digit on the processing of color (exps. 2 and 3) or shape (exp. 5), which rely only minimally on parietal resources. After ruling out alternative explanations we conclude that the efficiency of feature-based attention is determined by the degree of neural overlap of structures dedicated to process relevant and irrelevant information.

Parametrically dissociating speech and nonspeech perception in the brain using fMRI

Candidate brain regions constituting a neural network for preattentive phonetic perception were identified with fMRI and multivariate multiple regression of imaging data. Stimuli contrasted along speech/nonspeech, acoustic, or phonetic complexity (three levels each) and natural/synthetic dimensions. Seven distributed brain regions' activity correlated with speech and speech complexity dimensions, including five left-sided foci [posterior superior temporal gyrus (STG), angular gyrus, ventral occipitotemporal cortex, inferior/posterior supramarginal gyrus, and middle frontal gyrus (MFG)] and two right-sided foci (posterior STG and anterior insula). Only the left MFG discriminated natural and synthetic speech. The data also supported a parallel rather than serial model of auditory speech and nonspeech perception.

Identifying rate-limiting nodes in large-scale cortical networks for visuospatial processing: an illustration using fMRI

With the advent of functional neuroimaging techniques, in particular functional magnetic resonance imaging (fMRI), we have gained greater insight into the neural correlates of visuospatial function. However, it may not always be easy to identify the cerebral regions most specifically associated with performance on a given task. One approach is to examine the quantitative relationships between regional activation and behavioral performance measures. In the present study, we investigated the functional neuroanatomy of two different visuospatial processing tasks, judgement of line orientation and mental rotation. Twenty-four normal participants were scanned with fMRI using blocked periodic designs for experimental task presentation. Accuracy and reaction time (RT) to each trial of both activation and baseline conditions in each experiment was recorded. Both experiments activated dorsal and ventral visual cortical areas as well as dorsolateral prefrontal cortex. More regionally specific associations with task performance were identified by estimating the association between (sinusoidal) power of functional response and mean RT to the activation condition; a permutation test based on spatial statistics was used for inference. There was significant behavioral-physiological association in right ventral extrastriate cortex for the line orientation task and in bilateral (predominantly right) superior parietal lobule for the mental rotation task. Comparable associations were not found between power of response and RT to the baseline conditions of the tasks. These data suggest that one region in a neurocognitive network may be most strongly associated with behavioral performance and this may be regarded as the computationally least efficient or rate-limiting node of the network.

Surface orientation discrimination activates caudal and anterior intraparietal sulcus in humans: an event-related fMRI study

Perception of surface orientation is an essential step for the reconstruction of the three-dimensional (3D) structure of an object. Human lesion and functional neuroimaging studies have demonstrated the importance of the parietal lobe in this task. In primate single-unit studies, neurons in the caudal part of the intraparietal sulcus (CIP) were found to be active during the extraction of surface orientation through monocular (two-dimensional) cues such as texture gradients and linear perspective as well as binocular (3D) cues such as disparity gradient and orientation disparity. We used event-related fMRI to study the functional neuroanatomy of surface orientation discrimination using stimuli with monocular depth cues in six volunteers. Both posterior (CIP) and anterior (AIP) areas within the intraparietal sulcus showed a stronger activation during surface orientation as compared with a control (color discrimination) task using identical stimuli. Furthermore, the signal changes in CIP showed a greater performance effect than those in AIP, suggesting that CIP is tightly linked to the discrimination task.

Orientation discrimination of objects and gratings compared: an fMRI study

We used functional magnetic resonance imaging to compare the human brain regions involved in orientation discrimination of two-dimensional (2D) objects and gratings. The orientation discrimination tasks, identification and successive discrimination, were contrasted to a dimming detection control condition with identical retinal input. Regions involved in orientation discrimination were very similar for the two types of tasks and for the two types of stimuli and both belonged to the dorsal and ventral visual pathways. They included posterior occipital, lingual, posterior fusiform, inferior temporal, dorsal intraparietal and medial parietal regions. The main difference between the two types of stimuli was a larger activation of precuneus when 2D objects were used compared to gratings. The main difference between discrimination tasks was an enhanced activity, at the group level, in superior frontal sulcus in identification compared to successive discrimination, and at least at the single subject level, a larger activity in right fusiform cortex in successive discriminations compared to identification. Thus, in contradiction to generally accepted views, orientation discrimination of gratings and objects involve largely similar networks including both ventral and dorsal visual regions.

Multimodal spatial orientation deficits in left-sided visual neglect

Patients with right-sided temporo-parietal lesions often show contralesional neglect. However, neglect patients may also show spatial-perceptual deficits beyond the bisection and space exploration deficits frequently assessed in the horizontal plane, that is, deficits in the judgment of the subjective visual vertical or horizontal. In a recent study (Kerkhoff, G. & Zoelch, C.. Disorders of visuo-spatial orientation in the frontal plane in patients with visual neglect following right or left parietal lesions. Exp. Brain Res., 1998;122:108-120) we found significant perturbations in the perception of these three visual spatial axes in patients with contralesional neglect from right or left parietal lesions. To examine if this finding extends also to another modality we investigated how neglect patients perform tasks of visual- and tactile-spatial judgments of axis-orientation in the frontal plane. Visual-spatial and tactile-spatial judgments of the subjective vertical, horizontal and a right oblique orientation were obtained from patients with and without neglect as well as from normal subjects. Patients with left neglect showed a significant, contraversive tilt of all three visual-spatial axes (+5.6 degrees to +9.5 degrees, counterclockwise), and of the three tactile-spatial axes as well (+5.2 degrees to +10.5 degrees, counterclockwise). In contrast, right and left hemisphere lesioned control patients without neglect and normal control subjects showed unimpaired visual and tactile-spatial judgments (constant errors: < 1.0 degree). Difference thresholds in the visual-spatial tasks and unsigned errors in the tactile-spatial tasks were selectively elevated in the neglect group in contrast to all other subject groups. Spatial orientation deficits were significantly associated with the severity of clinical neglect (r = 0.55-0.88), and with the patients' ambulation performance (r = 0.45-0.70). Furthermore, crossmodal axis orientation tests in two neglect patients showed a similar counterclockwise tilt of +5 degrees to +15 degrees, suggesting a similar spatial deficit in both modalities. Orientation judgments were significantly aggravated by a 25 degree-tilt of the head to the left, as tested in one neglect patient, while a comparable rightward head-tilt improved spatial judgments in both modalities. This suggests that spatial orientation judgments are significantly modulated by gravitational input in neglect patients. Together these results are interpreted as evidence for multisensory spatial orientation deficits in neglect patients which are modulated by head-position and are related to their accompanying postural impairment.

A fronto-parietal circuit for object manipulation in man: evidence from an fMRI-study

Functional magnetic resonance imaging (fMRI) was used to localize brain areas active during manipulation of complex objects. In one experiment subjects were required to manipulate complex objects for exploring their macrogeometric features as compared to manipulation of a simple smooth object (a sphere). In a second experiment subjects were asked to manipulate complex objects and to silently name them upon recognition as compared to manipulation of complex not recognizable objects without covert naming. Manipulation of complex objects resulted in an activation of ventral premotor cortex [Brodmann's area (BA) 44], of a region in the intraparietal sulcus (most probably corresponding to the anterior intraparietal area in the monkey), of area SII and of a sector of the superior parietal lobule. When the objects were covertly named additional activations were found in the opercular part of BA 44 and in the pars triangularis of the inferior frontal gyrus (BA 45). We suggest that a fronto-parietal circuit for manipulation of objects exists in humans and involves basically the same areas as in the monkey. It is proposed that area SII analyses the intrinsic object characteristics whilst the superior parietal lobule is related to kinaesthesia.