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Biomedical subjects

G Rees

Publications and source records attributed to G Rees.

At least 19 recordsLinked to original sources
PubMed2006

Extra-classical receptive field effects measured in striate cortex with fMRI.

The aim of this study was to measure the contextual influence of globally coherent motion on visual cortical responses using functional magnetic resonance imaging. Our motivation was to test a prediction from representational theories of perception (i.e. predictive coding) that primary visual responses should be suppressed by top-down influences during coherent motion. We used a sparse stimulus array such that each element could not fall within the same classical receptive field of primary visual cortex neurons (i.e. precluding lateral interactions within V1). This enabled us to attribute differences, in striate cortex responses, to extra-classical receptive field effects mediated by backward connections. In accord with theoretical predictions we were able to demonstrate suppression of striate cortex activations to coherent relative to incoherent motion. These results suggest that suppression of primary visual cortex responses to coherent motion reflect extra-classical effects mediated by backward connections.

Adult↗
PubMed2006

Plaque biofilms: the effect of chemical environment on natural human plaque biofilm architecture.

The architecture of microbial biofilms especially the outer regions have an important influence on the interaction between biofilm and local environment particularly on the flux of materials into and out of biofilm compartments and as a consequence, biofilm metabolic behaviour. In the case of dental plaque biofilms, architecture will determine access of nutrients including acidogenic substrates and therapeutic materials to the microbial biomass and to the underlying tooth surface. Manipulation of this architecture may offer a means of altering mass transfer into the whole biofilm and biomass and raises the possibility of improving access of therapeutics. Plaque biofilms formed in vivo on human enamel were subjected to a number of different chemical conditions while under observation by confocal laser scanning microscopy in reflection mode. In this way the outer 50-100 microm or so of the biofilms was examined. Density and distribution of biomass were recorded as degree of reflectance. The amount and density of biofilm biomass increased from the plaque saliva interface towards the interior. Plaque biofilms were robust and little affected by mechanical manipulation, high ionic strength or low pH (2.5). Detergent (SLS), however, often appeared to either remove biomass and/or dramatically reduce its density.

Biofilms↗
PubMed2006

Sound alters activity in human V1 in association with illusory visual perception.

When a single brief visual flash is accompanied by two auditory bleeps, it is frequently perceived incorrectly as two flashes. Here, we used high field functional MRI in humans to examine the neural basis of this multisensory perceptual illusion. We show that activity in retinotopic visual cortex is increased by the presence of concurrent auditory stimulation, irrespective of any illusory perception. However, when concurrent auditory stimulation gave rise to illusory visual perception, activity in V1 was enhanced, despite auditory and visual stimulation being unchanged. These findings confirm that responses in human V1 can be altered by sound and show that they reflect subjective perception rather than the physically present visual stimulus. Moreover, as the right superior temporal sulcus and superior colliculus were also activated by illusory visual perception, together with V1, they provide a potential neural substrate for the generation of this multisensory illusion.

Acoustic Stimulation↗
PubMed2005

Cognitive processes in saccade generation.

The analysis of saccades offers an opportunity to study a number of different cognitive processes, such as visuospatial attention, working memory, and volitional conflict. A study of saccades in patients with visuospatial hemineglect, who performed a visual search task, showed repeated fixations on targets previously discovered, yet they often failed to retain the information that a particular target had previously been discovered. High-resolution structural brain scanning showed that this abnormality was due either to a lesion in the right intraparietal sulcus or the right inferior frontal lobe. Detailed analysis of the scanpaths suggested that the former location was associated with an accumulating impairment in remapping target locations across saccades or impaired memory of previously inspected target locations, whereas the latter location was more consistent with a failure to inhibit responses to rightward locations. When combined with a spatial bias to the right, such deficits might explain why many neglect patients often reexamine rightward targets, at the expense of items to their left. The functions of the supplementary eye field (SEF), in the medial frontal lobe, in relation to saccade generation are controversial. A series of studies in a patient with a focal lesion of the right SEF has indicated an important role for the SEF in the rapid self-control of saccadic eye movements and in set-switching (i.e., implementing control in situations of response conflict when ongoing saccadic plans have to be changed rapidly), rather than monitoring errors. In a recent fMRI study of normal subjects, it was shown that the SEF is involved in implementing the resolution of any volitional conflict, whereas other presupplementary motor areas are involved in the generation of volitional plans and processing volitional conflict.

Brain Mapping↗
PubMed2004

Nasopharyngeal pH monitoring in chronic sinusitis patients using a novel four channel probe.

OBJECTIVE: To determine the prevalence of acid reflux into the nasopharynx in patients with chronic sinusitis. STUDY DESIGN: Prospective study of patients presenting to a specialist rhinology practice with chronic sinusitis. METHODS: Forty patients with chronic sinusitis underwent ambulatory 24 hour pH testing. The mean age of the patients was 56.3 years (25 Female, 15 Male). The studies were performed using a specially developed bifurcated 4 channel pH probe, incorporating 2 circumferential sensors positioned at the naso- and hypo-pharynx, and 2 unidirectional sensors positioned at the proximal and distal esophagus. RESULTS: The circumferential band sensors yielded a stable recording largely free of artefact allowing meaningful recordings to be obtained from thirty-seven patients. Twelve patients (32.4%) were diagnosed with gastroesophageal reflux. A total of 809 reflux episodes were recorded. Of these, 596 (73.7%) reached the distal esophagus, with 187 (23.1%) and 24 (3.0%) reaching the proximal esophagus and hypopharynx respectively. Only 2 episodes (0.2%) were recorded in the nasopharynx. This occurred in 2 of 37 patients (5%). CONCLUSIONS: Acid reflux into the nasopharynx is a rare event in patients with chronic sinusitis even though a significant proportion (32.4%) have abnormal 24 hour pH studies. It is likely that alternative mechanisms other than direct acid contact are involved in the pathogenesis of chronic sinusitis.

Adult↗
PubMed2002

The internet.

The growing use of email and the world wide web (WWW), by the public, academics, and clinicians-as well as the increasing availability of high quality information on the WWW-make a working knowledge of the internet important. Although this article aims to enhance readers' existing use of the internet and medical resources on the WWW, it is also intelligible to someone unfamiliar with the internet. A web browser is one of the central pieces of software in modern computing: it is a window on the WWW, file transfer protocol sites, networked newsgroups, and your own computer's files. Effective use of the internet for professional purposes requires an understanding of the best strategies to search the WWW and the mechanisms for ensuring secure data transfer, as well as a compendium of online resources including journals, textbooks, medical portals, and sites providing high quality patient information. This article summarises these resources, available to incorporate into your web browser as downloadable "Favorites" or "Bookmarks" from www.jnnp.com, where there are also freely accessible hypertext links to the recommended sites.

Attitude to Computers↗
PubMed2001

Attention, adaptation, and the motion aftereffect.

Activation of the human visual motion area V5/MT was previously thought to be the basis of the motion aftereffect. New findings suggest that previous observations were confounded by attention and arousal, providing evidence that adaptation of directionally selective neurons in area V5/MT represents the fundamental substrate for the motion aftereffect.

Adaptation, Physiological↗
PubMed2001

An fMRI study of anticipation and learning of smooth pursuit eye movements in humans.

We studied the neural substrate of anticipation and learning of smooth pursuit eye movements in humans using fMRI. Both predictable and non-predictable eye movements, compared to baseline, activated a common network previously associated with oculomotor function. The temporal dynamics of activity in a subset of these areas suggested a strong correlation with type of condition. Specifically, differential decreases in activity were seen in dorsolateral prefrontal cortex and the intraparietal sulcus during the predictable condition. During the non-predictable condition the same areas exhibited evidence of high levels of activity that further increased throughout the condition. In contrast, differential increases associated with the predictable condition were seen in anterior cingulate and preSMA cortex regions. These changes in activity mirror the time course of the short-term learning of eye movements seen behaviourally, and are congruent with learning-related changes that have been reported for other motor paradigms.

Adult↗
PubMed2001

The role of working memory in visual selective attention.

The hypothesis that working memory is crucial for reducing distraction by maintaining the prioritization of relevant information was tested in neuroimaging and psychological experiments with humans. Participants performed a selective attention task that required them to ignore distractor faces while holding in working memory a sequence of digits that were in the same order (low memory load) or a different order (high memory load) on every trial. Higher memory load, associated with increased prefrontal activity, resulted in greater interference effects on behavioral performance from the distractor faces, plus increased face-related activity in the visual cortex. These findings confirm a major role for working memory in the control of visual selective attention.

Adult↗
PubMed2001

Functional magnetic resonance imaging and evoked potential correlates of conscious and unconscious vision in parietal extinction patients.

We describe recent functional magnetic resonance imaging (fMRI) and event-related potential (ERP) studies of visual extinction in patients with right parietal damage who can detect isolated visual stimuli on either side, yet often miss contralesional (left) stimuli during bilateral stimulation. We consider the neural fate of such extinguished visual stimuli and how neural responses differ for consciously detected versus extinguished stimuli. fMRI findings indicate that extinguished stimuli evoke activity in striate and ventral extrastriate visual cortex, despite escaping awareness. Activations for extinguished stimuli can be found even in category-specific (face-responsive) areas of the fusiform gyrus. On the other hand, activations in visual cortex are stronger for consciously detected versus extinguished stimuli, with parietal and frontal areas of the intact left hemisphere also implicated in this comparison. Recent ERP data likewise suggest differential neural responses for consciously detected versus extinguished stimuli. We discuss these findings in relation to current speculations about the neural basis of conscious and unconscious perception.

Attention↗
PubMed2001

Processing of irrelevant visual motion during performance of an auditory attention task.

The extent to which irrelevant perception of visual motion distractors can be modulated by manipulating auditory load in a relevant task was assessed with Positron Emission Tomography (PET) and behavioural experiments. Subjects performed an auditory task and ignored an irrelevant visual motion stimulus, under two conditions. In a low load condition, subjects were asked to detect words spoken in a loud voice among words spoken in a quiet voice, while in a high load condition they attempted to detect bisyllabic words among monosyllabic and trisyllabic words. We found that motion-related visual areas were strongly activated by the irrelevant motion stimulus, compared to a static stimulus, under both conditions of load in the auditory task. In a second behavioural experiment, the duration of the motion after-effect was similarly unaffected by adaptation under low or high auditory load. These results are in clear contrast with the strong modulation of irrelevant motion processing by visual load, as reflected in the duration of the motion after effect (Section 6) and neural responses in motion-related visual areas (Rees et al., Science, (1997) 278, 338). These findings support the claim that attentional capacity is restricted within but not between sensory modalities, and indicate that processing of visual distractors may occur whenever there is sufficient visual capacity to process them, despite being task- and modality-irrelevant.

Adult↗
PubMed2001

What can functional imaging reveal about the role of attention in visual awareness?

This review focuses on neuroimaging studies that address the relationship between selective attention, neural activity and visual awareness. Withdrawing attention from particular visual stimuli reduces modality-specific processing in posterior visual cortex, and when attention is fully engaged elsewhere, even highly salient but task-irrelevant stimuli can fail to evoke activity and reach awareness. However, the link between visual attention and awareness extends beyond posterior visual cortex to also encompass regions of parietal and prefrontal cortex. Activity in the posterior visual cortex may be necessary but not sufficient for awareness, without a contribution from frontal and parietal cortex. Consistent with this, enhanced interactions between parietal, frontal and posterior visual cortex are observed as a function of both visual attention and visual awareness; and lesions of parietal cortex disrupt both visual attention and awareness. Taken together, these data suggest that distributed interactions between modality-specific posterior visual cortex and frontoparietal areas subserve both visual attention and visual awareness.

Attention↗
PubMed2001

A family history of breast cancer: women's experiences from a theoretical perspective.

Individuals at increased risk of developing breast cancer due to their family history of the disease face a number of uncertainties. Personal cancer risk estimates are imprecise and current methods for early detection or prevention are not 100% effective. It is therefore not surprising that adverse psychosocial outcomes have been described within this population. Research attempting to predict the incidence of distress and dysfunction in individuals at increased risk of cancer has been largely a-theoretical and has overlooked a number of potentially important predictive variables. In particular, the influence of personal experience of cancer through involvement with affected relatives has been neglected. There are strong theoretical grounds for hypothesising that dimensions of personal experience may influence response to cancer risk. This paper discusses the potential impact of personal experience on risk perception, illness representations and decision-making. Systematic research in this area may improve predictions of outcome of cancer genetic counselling and inform the clinical process.

Breast Neoplasms↗
PubMed2001

Neuroimaging of visual awareness in patients and normal subjects.

The immediacy and directness of our visual experience belies the complexity of the underlying neural mechanisms, which remain incompletely understood. Recent neuroimaging studies suggest that activity in ventral visual cortex is necessary but not sufficient for visual awareness. Experiments in both patients and normal subjects indicate that parietal and frontal areas make an important contribution to visual awareness, suggesting that reciprocal interactions between dorsal frontoparietal areas and ventral visual cortex may provide a fundamental neural substrate for conscious visual experience.

Animals↗
PubMed2001

Neural correlates of change detection and change blindness.

Functional magnetic resonance imaging (fMRI) of subjects attempting to detect a visual change occurring during a screen flicker was used to distinguish the neural correlates of change detection from those of change blindness. Change detection resulted in enhanced activity in the parietal and right dorsolateral prefrontal cortex as well as category-selective regions of the extrastriate visual cortex (for example, fusiform gyrus for changing faces). Although change blindness resulted in some extrastriate activity, the dorsal activations were clearly absent. These results demonstrate the importance of parietal and dorsolateral frontal activations for conscious detection of changes in properties coded in the ventral visual pathway, and thus suggest a key involvement of dorsal-ventral interactions in visual awareness.

Adult↗