Biomedical subjects

James Danckert

Publications and source records attributed to James Danckert.

12 recordsLinked to original sources

PubMed2006

Exaggerated leftward bias in the mental number line of patients with schizophrenia.

Several visuo-motor tasks can be used to demonstrate biases towards left hemispace in schizophrenic patients, suggesting a minor right hemineglect. Recent studies in neglect patients used a new number bisection task to highlight a lateralized defect in their visuo-spatial representation of numbers. To test a possible lateralized representational deficit in schizophrenia, we used the number bisection task in 11 schizophrenic patients compared to 11 healthy controls. Participants were required to orally indicate the central number of an interval orally presented. Whereas healthy subjects showed no significant bias, schizophrenic patients presented a significant leftward bias. Therefore, these results suggest an impairment in higher order representations of the number space in patients with schizophrenia, an impairment that is qualitatively similar to the deficit described in neglect patients.

Adult↗

PubMed2006

Differential influences of prism adaptation on reflexive and voluntary covert attention.

Recent research has demonstrated some beneficial effects in patients with neglect using rightward shifting prismatic lenses. Despite a great deal of research exploring this effect, we know very little about the cognitive mechanisms underlying prism adaptation in neglect. We examined the possibility that prism adaptation influences visual attention by having healthy participants complete either a reflexive or a voluntary covert visual attention cuing paradigm before and after adaptation to leftward, rightward, or sham (no shift) prisms. The results for reflexive orienting demonstrated that a subset of participants with large cuing effects before prism adaptation were faster to reorient attention away from an invalid cue on the side of space opposite the prismatic shift post adaptation. For voluntary orienting, left prisms increased the efficiency of voluntary attention in both left and right visual space in participants with a small cuing effect before prism adaptation. In contrast, right prisms decreased the efficiency of voluntary attention in both left and right space for participants with a large cuing effect before prism adaptation. No significant effects were observed in the sham prism groups. These results suggest that prism adaptation may exert a variety of influences on attentional orienting mechanisms.

Adaptation, Physiological↗

PubMed2006

Neuropsychological profile of acute alcohol intoxication during ascending and descending blood alcohol concentrations.

Numerous studies have investigated the effects of alcohol on motor processes during rising and declining blood alcohol concentrations (BAC), however, relatively little research has examined the alcohol-induced impairment of cognitive performance on the two limbs of the BAC curve. This experiment administered a neuropsychological test battery to assess the degree to which rising and declining BACs during an acute dose of alcohol impair nine cognitive processes within an individual. In all, 20 healthy male social drinkers (university students) were assigned to one of two groups (n = 10) who received a beverage containing either 0.0 g/kg (placebo) or 0.65 g/kg alcohol and performed the test battery when BAC was increasing and was decreasing. Comparisons of alcohol and placebo groups revealed impairment (slower response and/or increased errors) in seven of the cognitive processes: long-term verbal memory; information processing; declarative memory; inhibitory control; short-term visual memory; long-term visual memory, and visual-spatial working memory. However, some processes were impaired only during rising BACs whereas the impairment of others during declining BACs was evident only by an increase in errors. These results show cognitive tasks performed by an individual are not similarly affected by rising and declining BACs, and call attention to the importance of assessing both speed and accuracy on both limbs of the BAC curve. The particular cognitive processes differentially affected by rising vs declining BACs raised the possibility that acute alcohol intoxication may impair one cerebral hemisphere to a greater degree than the other, and this could be explored by neuroimaging techniques.

Adult↗

PubMed2005

Revisiting unilateral neglect.

Unilateral neglect, a neurological disorder in which patients fail to detect or respond to contralesional stimuli, has long been considered a failure of attentional orienting mechanisms. This review provides a selective overview of the prominent biases in spatial orienting and exploratory motor behaviour observed in these patients before considering the impact of other factors on the presentation of the disorder and how those factors might inform current neurological models of neglect. In the latter part of the review, we intend to suggest that neglect is likely to be a combination of distinct but interacting impairments including biases in attentional orienting, exploratory motor behaviours and a deficit of spatial working memory. That is, we suggest that the cardinal symptom of neglect - a loss of awareness for contralesional stimuli and events - arises as a result of a combination of these impairments rather than being associated solely with the more dramatic and immediately evident biases in spatial attention.

Animals↗

PubMed2005

Lost in space--the fate of memory representations for non-neglected stimuli.

Typically, spatial neglect after right-hemisphere brain damage is defined as a failure to orient towards or attend to stimuli located towards the contralesional, in this case the left, side of space. Here, we report that neglect patients have difficulty maintaining the spatial locations of vertically arranged stimuli on the right side of space. This indicates that neglect is associated with a severe deficit in the maintenance of spatial information even on the ipsilesional "good" side.

Aged↗

PubMed2005

Blindsight in action: what can the different sub-types of blindsight tell us about the control of visually guided actions?

Blindsight broadly refers to the paradoxical neurological condition where patients with a visual field defect due to a cortical lesion nevertheless demonstrate implicit residual visual sensitivity within their field cut. The aim of this paper is twofold. First, through a selective review of the blindsight literature we propose a new taxonomy for the subtypes of residual abilities described in blindsight. Those patients able to accurately act upon blind field stimuli (e.g. by pointing or saccading towards them) are classified as having 'action-blindsight', those whose residual functions can be said to rely to some extent upon attentive processing of blind field stimuli are classified as demonstrating 'attention-blindsight', while finally, patients who have somewhat accurate perceptual judgements for blind field stimuli despite a complete lack of any conscious percept, are classified as having 'agnosopsia'--literally meaning 'not knowing what one sees'. We also address the possible neurological substrates of these residual sensory processes. Our second aim was to investigate the most striking subtype of blindsight, action-blindsight. We review the data relevant to this subtype and the hypotheses proposed to account for it, before speculating on how action-blindsight may inform our normal models of visuomotor control.

Attention↗

PubMed2004

Attention, motor control and motor imagery in schizophrenia: implications for the role of the parietal cortex.

Many recent models of schizophrenia have attempted to explain the so-called first-rank symptoms in terms of a breakdown in the self-monitoring of thoughts and behaviours. These models have focused on the most common symptom of schizophrenia auditory hallucinations-suggesting that they may represent disordered self-monitoring of internal speech. As such, much attention has been given to the role of the temporal and frontal cortices in the clinical presentation of patients with schizophrenia. In this review, we examine the role of the posterior parietal cortex (PPC) in schizophrenia within the context of recent models of self-monitoring deficits in these patients. Attentional dysfunctions and certain impairments of motor control and motor imagery all point towards the involvement of the parietal cortex in the disorder. In particular, we suggest that patients experiencing passivity phenomena (e.g., delusions of control) may have particular impairments of parietal function related to poor utilisation of forward models of intended actions. We also present a novel hypothesis that suggests differential impairments of the left and right parietal cortices in schizophrenia may help explain many of the first-rank symptoms of the disorder.

Attention↗

PubMed2004

Spared somatomotor and cognitive functions in a patient with a large porencephalic cyst revealed by fMRI.

To date functional magnetic resonance imaging (fMRI) has not been extensively used in presurgical evaluation of patients with intractable epilepsy. Patient S.P. presented with left frontal originating seizures, secondary to a large porencephalic cyst that encompassed much of his occipital and temporal cortex and a substantial portion of ipsilateral parietal cortex. Nevertheless, S.P. did not demonstrate any gross impairments of praxis or speech. Scalp electroencephalogram (EEG) telemetry revealed reduced background activity in the left hemisphere, an absence of identifiable normal sleep states, and confirmed the left frontal origin of his seizures with a prolonged postictal state, suggesting that the remaining cortex in S.P.'s left hemisphere did not function normally despite his apparently normal appearance. Dichotic listening results also suggested that S.P. had an atypical language representation suggestive of either bilateral or right hemisphere speech representation. Surgical intervention to remove the remaining left hemisphere cortex was a serious consideration for treatment of S.P.'s seizures. We used fMRI to evaluate whether or not the remaining cortex in S.P.'s left hemisphere supported any cognitive or motor functions. Even though the volume of cerebral cortex was severely reduced and displaced in the left hemisphere, fMRI revealed significant activation in this remaining tissue in response to motor, somatosensory, and word generation tasks. In other words, we were able to demonstrate using fMRI that the remaining tissue in S.P.'s left hemisphere continued to support some motor and cognitive functions. The possible implications of these findings in terms of functional reorganisation are discussed briefly.

Brain Diseases↗

PubMed2003

Measuring unconscious actions in action-blindsight: exploring the kinematics of pointing movements to targets in the blind field of two patients with cortical hemianopia.

We tested two patients with posterior cerebral lesions on two pointing tasks. In the first task, the patients pointed to targets presented on a touch screen monitor and pointing accuracy was recorded. One patient (JR) demonstrated good localisation of targets presented to her blind field while the other patient (YP) did not. Movement kinematics were measured in the second task to compare the kinematics of movements made to sighted field targets with those made to blind field targets. For this version of the task both patients demonstrated above chance localisation of blind field targets although the slope of the relationship between the end of pointing movements and the target locations was significantly steeper for JR than for YP. Furthermore, JR showed a kinematic profile for movements made to blind field targets that mirrored the profile of kinematics to sighted field targets. That is, both peak velocity and time to peak velocity increased with increasing target eccentricity for movements made to blind and sighted field targets alike. Although patient YP now showed more reliable spatial localisation on this pointing task when compared with the touch screen task, his kinematics for movements made to targets in his blind field were quite different from those made to targets in his sighted field. Based on the patients' CT scans, we suggest that the superior performance of patient JR is a consequence of greater sparing of her parietal cortex in the damaged hemisphere.

Adult↗

PubMed2002

Exploring imagined movements in patients with schizophrenia.

Patients with schizophrenia demonstrate impairments indicative of an inability to accurately monitor internally generated images. Motor imagery measures the ability to generate internal images of intended but not executed motor movements. Ten patients with schizophrenia completed actual and imagined versions of a pointing task with a well defined speed-accuracy trade-off function. For controls, movement time increases as target size decreases for both actual and imagined movements. Despite showing the expected speed-accuracy trade-off for actual movements, the imagined movements of schizophrenics showed no reliable relationship to target size. This was true for each patient and appeared to be independent of symptom profile. These results suggest that patients with schizophrenia are unable to generate accurate internal images of their own motor movements.

Adult↗

PubMed2002

Grasping two-dimensional images and three-dimensional objects in visual-form agnosia.

Visually guided prehension is controlled by a specialized visuomotor system in the posterior parietal cortex. It is not clear how this system responds to visual stimuli that lack three-dimensional (3D) structure, such as two-dimensional (2D) images of objects. We asked a neurological patient with visual-form agnosia (patient D.F.) to grasp 3D objects and 2D images of the same objects and to estimate their sizes manually. D.F.'s grip aperture was scaled to the sizes of the 2D and 3D target stimuli, but her manual estimates were poorly correlated with object size. Control participants demonstrated appropriate size-scaling in both the grasping and manual size-estimation tasks, but tended to use a smaller peak aperture when reaching to grasp 2D images. We conclude that: (1) the dorsal stream grasping system does not discriminate in a fundamental way between 2D and 3D objects, and (2) neurologically normal participants might adopt a different visuomotor strategy for target objects that are recognized to be ungraspable. These findings are consistent with the view that the dorsal grasping system accesses a pragmatic, spatial representation of the target object, whereas the ventral system accesses a more comprehensive, volumetric description of the object.

Adult↗

PubMed2002

Selective, non-lateralized impairment of motor imagery following right parietal damage.

Using variants of a visually guided pointing task, in which subjects make pointing movements towards targets of varying sizes, we explored motor imagery in a patient with visual neglect. When this patient actually pointed towards targets of different sizes he showed the normal correlation between movement duration (MD) and target size, such that MD increased as target size decreased. In contrast, his imagined movements did not show the same speed-accuracy trade-off observed for actual movements. This was true regardless of the hand used or the initial direction of movement (left versus right). The patient performed normally on several tasks of visual imagery, including size estimation, perceptual discrimination and localization of cities on an imagined map. This patient's performance suggests that the networks in the right parietal lobe play an important role in the generation of internal models of motor movements regardless of the hand used to perform the task.

Aged↗