Biomedical subjects

V Walsh

Publications and source records attributed to V Walsh.

At least 19 recordsLinked to original sources

PubMed1999

Short-term memory for colour following posterior hemispheric lesions in man.

Short-term memory for colour was studied in five patients with circumscribed posterior hemispheric lesions. It was impaired independently of colour discrimination in one and more than colour discrimination in two patients. Two patients were normal in colour short-term memory, one with normal and one with deficient colour discrimination performance. Deficient performance in colour short-term memory was associated with bilateral lesions of the inferior occipitotemporal junction including the lateral part of the fusiform gyrus or with a unilateral lesion of the left parieto-occipital convexity. An additional colour constancy deficit was found in the former but not the latter condition. Thus, colour short-term memory can be affected independently of colour discrimination or colour constancy, and may depend on at least two distinct neural circuits.

Adult↗

PubMed1999

Cortical function: jump-starting the brain.

Magnetic stimulation as used in studies of the human brain may not merely disrupt cognitive functions, but also enhance them. The direction of the effect may depend on the frequency of stimulation as much as the area of the brain that is stimulated.

Animals↗

PubMed1999

The role of the parietal cortex in visual attention--hemispheric asymmetries and the effects of learning: a magnetic stimulation study.

Our previous studies of the role of the parietal cortex in visual learning and attention showed that the right parietal cortex is required for normal performance on conjunction visual search tasks but that its role depends on whether subjects are naive or trained on the task. Here we extend these findings in two Experiments. Experiment 1 shows that magnetic stimulation of the left parietal cortex also impairs performance (measured as reaction time) on conjunction visual search tasks, but only when the target is present in the right (contralateral) visual field. Stimulation of the same region on a feature detection task speeds up performance significantly when the target is in the left (ipsilateral) visual field. Experiment 2 explores further the role of the right parietal cortex in learning conjunction search tasks. Stimulation of the right parietal cortex in subjects who had already trained on some visual search tasks did not impair performance on a novel motion/form conjunction task even though the search was clearly serial. Stimulation of area V5, however, severely disrupted performance on the same task. These data indicate that the role of the parietal cortex may change much earlier in the course of training than initially thought.

Adult↗

PubMed1999

A primer of magnetic stimulation as a tool for neuropsychology.

Transcranial magnetic stimulation (TMS) offers the neuropsychologist a 'virtual lesion' method of investigating the effects of cortical dysfunction. Classical neuropsychology relies on patients with irreversible, and often diffuse brain lesions and these factors place limitations on the inferences that can be drawn about normal brain function. Thus the neuropsychologist is constrained by the extent to which the damaged brain undergoes reorganisation and by the inability to address questions regarding the timing of cognitive functions. TMS can disrupt cognitive functions for a few tens of milliseconds (although some effects of TMS can be seen for longer), with spatial resolution in the order of a centimetre and therefore allows one to study the role of brain areas without the masking effects of cortical reorganisation. The spatial and temporal resolutions are not unique to TMS but because TMS can be used as a temporary interference technique, it has a functional resolution with which one can address questions beyond the range of other neuroimaging and patient studies. Here we outline how TMS produces transitory 'lesion' effects, examine how the effects of stimulation spread in depth and breadth across the cortex and discuss the principles of the use of TMS in neuropsychology. Finally, we also itemise some issues of safety.

Cerebral Cortex↗

PubMed1998

Task-specific impairments and enhancements induced by magnetic stimulation of human visual area V5.

Transcranial magnetic stimulation (TMS) can be used to simulate the effects of highly circumscribed brain damage permanently present in some neuropsychological patients, by reversibly disrupting the normal functioning of the cortical area to which it is applied. By using TMS we attempted to recreate deficits similar to those reported in a motion-blind patient and to assess the specificity of deficits when TMS is applied over human area V5. We used six visual search tasks and showed that subjects were impaired in a motion but not a form 'pop-out' task when TMS was applied over V5. When motion was present, but irrelevant, or when attention to colour and form were required, TMS applied to V5 enhanced performance. When attention to motion was required in a motion-form conjunction search task, irrespective of whether the target was moving or stationary, TMS disrupted performance. These data suggest that attention to different visual attributes involves mutual inhibition between different extrastriate visual areas.

Adult↗

PubMed1998

Brain mapping: Faradization of the mind.

Electromagnetic induction of focal currents in the brain - 'transcranial magnetic stimulation' - can be used to study cortical development and plasticity, as well as the organization of sensory and cognitive functions. It may also prove to be useful tool in the treatment of depression.

Animals↗

PubMed1998

Colour constancy impairments in patients with lesions of the prestriate cortex.

Colour matching and colour constancy were studied in seven patients and 46 control subjects. Subjects were required to match Munsell Colour Chips presented under either identical or different illumination. Three of the patients had deficits in colour constancy, i.e. failure to compensate for the change in the wavelength composition of the illumination. Two of the patients with defective constancy had suffered bilateral cortical damage to the posterior lingual and fusiform gyri, and one patient had a lesion restricted to the same regions of the right hemisphere. Our observations indicate that these cortical areas, which include part of putative human area V4, play an important role in colour constancy.

Adult↗

PubMed1998

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation.

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task during and after learning it. Here, we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Electric Stimulation↗

PubMed1998

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation.

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task while learning it and after learning it. Here we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Attention↗

PubMed1998

Perceptual learning in visual search: some evidence of specificities.

To test a recent suggestion that perceptual learning in visual search is non-specific, two groups of subjects were trained on visual search tasks and tested for transfer of learning to new tasks. One group was trained on parallel ("pop-out") tasks and transferred to serial, conjunction tasks and the other group trained on conjunction and transferred to pop-out. Some (not all) tasks which are initially serial, rapidly became parallel. Some transfer occurred between the different types of tasks. Under some conditions transfer was either absent or even negative. The specificities observed may reflect the roles of the brain regions involved in learning.

Color Perception↗

PubMed1998

Identification of transcription factors expressed during ATRA-induced neutrophil differentiation of HL60 cells.

A recent clinical therapeutic initiative has been the use of chemical agents which induce the leukaemic cells to overcome their block in differentiation. In order to understand this block the cascade of molecular events needs to be characterized. Haemopoietic differentiation is ultimately controlled at the level of gene transcription which is mediated by an array of transcription factors. Many transcription factors contain similar structural protein sequences, and we have used an RT-PCR-based approach to isolate sequences, from transcription factor gene families which share similar domains. Degenerate primers corresponding to the TFIIIA zinc-finger consensus amino acid sequences and to the POU-homeodomain and POU-specific domain were used to amplify genes on the basis that they contained similarities in structural motifs shared within these families of transcription factors. A serum-independent HL60 cell line was induced towards the neutrophil lineage by treatment with all-trans retinoic acid (ATRA) for 24 h. CD38+ cells committed towards this lineage were enriched and a population of these cells treated with dihydroxyvitamin D3 to induce neutrophil maturation. RNA extracted from uninduced, ATRA-induced CD38+ cells, and vitamin D3 treated maturing cell cultures were amplified using the degenerate primers. PCR fragments were cloned, sequenced, clustered into homologous groups, and the group sequences searched on the GenBank database. The Oct 1 transcription factor, and a very close homologue, KIAA0144, was identified using the POU family primers. The zinc-finger primers identified three zinc-finger genes. The pattern of gene expression was suggested from the number of clones in each group at neutrophil commitment and maturation. The differential expression of the genes in the zinc finger and POU families will lead to a better understanding of the cascade of gene expression which occurs following ATRA-induced differentiation.

Cell Differentiation↗

PubMed1997

Perceptual learning: insight in sight.

The Hebbian synapse and Hebb learning rule are familiar to those working on biological and machine learning. But Hebb's insights from over fifty years ago carry many other lessons in learning and may contribute to a more parsimonious taxonomy of the mechanisms involved.

Animals↗

PubMed1997

Higher-level cortical processing of colour.

Wavelength information serves both to organise shapes and to provide the appearance of surfaces. These two roles are considered within a model that outlines the role of colour in picture naming (Davidoff and de Bleser, 1993). An introduction to the papers in this issue is given within the framework of the model. The topics in this issue cover the role of colour in scene segmentation, colour constancy and the memory for the colours of objects.

Cerebral Cortex↗

PubMed1997

Temporal aspects of visual search studied by transcranial magnetic stimulation.

Transcranial magnetic stimulation was applied over the parietal visual cortex of subjects while they were performing 'pop-out' or conjunction visual search tasks in arrays containing eight distractors. Magnetic stimulation had no detrimental effect on the performance of pop-out search, but did significantly increase reaction times on conjunction search when stimulation was applied over the right parietal cortex 100 msec after the onset of the visual display for trials when the target was present. Target absent reaction times were elevated when stimulation was applied 160 msec after array onset. Stimulation had no effect on the number of errors made. The results suggest that a sub-region of the right parietal lobe is important for conjunction search but not for pre-attentive pop-out. The result from target present trials is consistent with timing data from studies of single cells in monkeys and the hypothesis that parietal areas generate a signal that projects back to extrastriate visual areas to enhance the processing of features in a restricted part of the visual field. The timing of the effect indicates that transcranial stimulation disrupts the mechanisms underlying the focal attention necessary for feature binding in conjunction search. The effects of TMS on target absent trials are interpreted in terms of fronto-parietal connections and the role of frontal cortex in decision-making. The results also highlight the efficacy of transcranial magnetic stimulation as a complement to other spatial and temporal imaging techniques.

Humans↗

PubMed1997

To see but not to read; the magnocellular theory of dyslexia.

Developmental dyslexics often complain that small letters appear to blur and move around when they are trying to read. Anatomical, electrophysiological, psychophysical and brain-imaging studies have all contributed to elucidating the functional organization of these and other visual confusions. They emerge not from damage to a single visual relay but from abnormalities of the magnocellular component of the visual system, which is specialized for processing fast temporal information. The m-stream culminates in the posterior parietal cortex, which plays an important role in guiding visual attention. The evidence is consistent with an increasingly sophisticated account of dyslexia that does not single out either phonological, or visual or motor deficits. Rather, temporal processing in all three systems seems to be impaired. Dyslexics may be unable to process fast incoming sensory information adequately in any domain.

Attention↗

PubMed1997

The detection and discrimination of categorical yellow.

Threshold detection of most spectral lights presented on a white background is subserved by colour opponent mechanisms which produce distinct percepts of colours. Five ranges of wavelengths can be discriminated: red, yellow, green, blue and violet [Mullen and Kulikowski (1990) Wavelength discrimination at detection threshold. J. Opt. Soc. Am. A7, 733-742]. However, under most viewing conditions yellow can also be detected and discriminated by activating the achromatic mechanism. Using 2-alternative forced-choice we studied detection and discrimination between spots (1 deg) which were either spectral colours or white (matching the background in colour temperature), with and without masking the achromatic mechanisms. For low colour temperatures of white (2700 K), yellow could be discriminated from white at slight suprathreshold levels of detection. However, at a colour temperature of 6800 K and with masking, the yellow-white discrimination threshold for 565-574 nm was also close to detection threshold (as for other wavelengths). We conclude that it is possible to demonstrate the role of the blue-yellow opponent mechanism in categorical perception of yellow (at threshold) by sensitizing its yellow branch and by suppressing the achromatic mechanism.

Color Perception↗

PubMed1996

Reflections on mirror images. Neuropsychology.

About half of the readers of this article would not be able to tell whether a picture of the Mona Lisa is correct or mirror-reversed. Neuropsychological studies may help explain this striking dissociation between the ability to identify images and memory for their handedness.

Animals↗

PubMed1996

Perception: the seeing ear.

Some people perceive colours when they hear words--a phenomenon known as synaesthesia. Brain imaging techniques have revealed the areas of the cortex involved in this rare mingling of the senses.

Animals↗