Biomedical subjects

Ian J Murray

Publications and source records attributed to Ian J Murray.

10 recordsLinked to original sources

PubMed2006

Heterogeneous oxygen extraction in the visual cortex during activation in mild hypoxic hypoxia revealed by quantitative functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) techniques were used to study haemodynamic and metabolic responses in human visual cortex during varying arterial blood oxygen saturation levels (Y(sat), determined by pulse-oximeter) and stimulation with contrast-reversing checkerboards. The visual-evoked potential amplitude remained constant at lowered Y(sat) of 0.82+/-0.03. Similarly, fMRI cerebral blood flow (CBF) responses were unchanged during reduced Y(sat). In contrast, visual cortex volume displaying blood oxygen level-dependent (BOLD) fMRI response decreased as a function of Y(sat), but the BOLD signal change of 3.6%+/-1.4% was constant. Oxygen extraction ratio (OER) during visual activation showed values of 0.26+/-0.03 for normal Y(sat). At lowered Y(sat), two OER patterns were observed. Firstly, a reduced OER of 0.14+/-0.03 in the visual cortex structures showing BOLD in hypoxia was observed. Secondly, signs of much higher OER in other parts of visual cortex were obtained. T2*-weighted magnetic resonance imaging revealed signal increases by 0.8%+/-0.4% with visual activation during lowered Y(sat) in the visual cortex structures, which showed BOLD of 3.6% in magnitude under normoxia. Because the CBF response in the visual cortex was quantitatively similar during stimulation in normoxia and hypoxia, attenuated T2*-weighted signal increase in parts of visual cortex indicated high OER during visual activation in hypoxia, which was close to that encountered in the resting brain. These spatially localised regions of tissue oxygen extraction and metabolism argue for dissociation between CBF and BOLD fMRI signals in mild hypoxia. The findings point to heterogeneity with regard to oxygen requirement and its coupling to the haemodynamic response in the brain.

Adult↗

PubMed2006

Variant and invariant color perception in the near peripheral retina.

Perceived shifts in hue that occur with increasing retinal eccentricity were measured by using an asymmetric color matching paradigm for a range of chromatic stimuli. Across nine observers a consistent pattern of hue shift was found; certain hues underwent large perceived shifts in appearance with increasing eccentricity, while for others little or no perceived shift was measured. In separate color naming experiments, red, blue, and yellow unique hues were found to be correlated with those hues that exhibited little or no perceptual shift with retinal eccentricity. Unique green, however, did not exhibit such a strong correlation. Hues that exhibited the largest perceptual shifts in the peripheral retina were found to correlate with intermediate hues that were equally likely to be identified by adjacent color naming mechanisms. However, once again the correlation was found to be weakest for the green mechanism. These data raise the possibility that perceptually unique hues are linked to color signals that represent the most reliable (minimally variant) chromatic information coming from the retina.

Adult↗

PubMed2005

Colour matching of isoluminant samples and backgrounds: a model.

A cone-opponent-based vector model is used to derive the activity in the red-green, yellow-blue, and achromatic channels during a sequential asymmetric colour-matching experiment. Forty Munsell samples, simulated under illuminant C, were matched with their appearance under eight test illuminants. The test samples and backgrounds were photometrically isoluminant with each other. According to the model, the orthogonality of the channels is revealed when test illuminants lie along either red-green or yellow blue cardinal axes. The red green and yellow-blue outputs of the channels are described in terms of the hue of the sample. The fact that the three-channel model explains the data in a colour-matching experiment indicates that an early form of colour processing is mediated at a site where the three channels converge, probably the input layer of V1.

Case-Control Studies↗

PubMed2005

Colour matching of isoluminant samples and backgrounds: a dimming effect.

Sequential asymmetrical colour matching of forty Munsell samples simulated under illuminant C and one of eight test illuminants was carried out. The subjects matched the appearance of each sample under illuminant C with its appearance under the test illuminant. Samples and background (N7) were presented for 1 s under the test illuminant and were isoluminant with each other. Subjects adjusted hue, chroma, and value under illuminant C. The experiments distinguished two groups of subjects; some observers needed to reduce the luminance of the sample to make a match while others did not. This 'dimming' occurred when the matches were close to cardinal axes, especially the tritanopic confusion line. A model of luminance and cone-opponent mechanisms contributing to brightness can account for the dimming effect. Details of analysis in cone-opponent space (L - M, L + M - S, L + M) are presented in the companion paper (Stanikunas et al, 2005 Perception 34 this issue).

Adaptation, Ocular↗

PubMed2005

Magnocellular channel subserves the human contrast-sensitivity function.

There is evidence that the human contrast-sensitivity function (CSF) is mediated by the spatiotemporal characteristics of magno and parvo neurons early in the visual pathway. In this study we use a measure of contrast gain derived from simple reaction times, to investigate the neural substrates of suprathreshold performance. The results reveal the activity of two mechanisms having distinctly different contrast-gain characteristics. Comparing these to neurophysiological data, we find that the magnocellular system dominates close-to-threshold detection and probably forms the basis of the achromatic CSF, whereas the parvocellular system dominates detection at higher contrasts, when the magnocellular system saturates.

Adult↗

PubMed2005

The role of retinal adaptation in night driving.

PURPOSE: Driving is essentially a visuomotor task, and there is now compelling evidence that the disproportionate number of road accidents under night driving conditions is linked to changes in visual performance resulting from reduced lighting. The objective of this article is to establish the extent to which vision is either rod-or cone-dominated under night driving conditions. METHODS: Visual thresholds are measured under lighting conditions that simulate urban lighting. Dark adaptation curves are obtained under three ambient lighting conditions ranging from low (0.1 cd/m) to high (5 cd/m) mesopic levels of retinal adaptation using circular discs of different sizes (1 degree, 2 degrees, 3 degrees, and 5 degrees) presented at retinal eccentricities of 0 degrees, 10 degrees, 20 degrees, 30 degrees, and 40 degrees. RESULTS: The dark adaptation curves exhibit the classic inflection point between rod and cone activity for the lower levels of ambient illumination but a simple monophasic function for the high mesopic levels (>0.5 lux). Adaptation rates are four times faster for the higher compared with the lower illumination level and twice as fast for central compared with peripheral presentation. CONCLUSIONS: The data suggest that vision is mediated by cone pathways at 5 lux and by rod pathways at 0.5/0.1 lux. This shift does not profoundly affect sensitivity, but because rod pathways are known to be slower than cone pathways, it will certainly affect observers' ability to respond to rapidly changing viewing conditions such as are encountered when driving at night.

Adult↗

PubMed2004

Plasma and macular responses to lutein supplement in subjects with and without age-related maculopathy: a pilot study.

There is a growing body of evidence which suggests that macular pigment (MP), which is entirely of dietary origin, protects against age-related maculopathy. We evaluated the effect of a daily 20 mg lutein ester (equivalent of 10 mg/day free lutein) supplement in patients with early age-related maculopathy (ARM), in terms of macular pigment optical density (MPOD) and plasma concentrations of lutein. MPOD was measured using a flicker photometric technique in seven ARM sufferers and six age-matched controls over a period of supplementation which lasted 18-20 weeks. Plasma lutein increased from a mean (SD) baseline concentration of 182 (127)ng ml(-1) to a peak of 1077 (165)ng ml(-1) in ARM patients, and from 152 (57) to 1110 (605)ng ml(-1) in control subjects. Mean MPOD had increased significantly from baseline of 0.24 to a peak of 0.31 in ARM sufferers. This mean increment of 0.07 was the same for the age-matched controls (baseline: 0.20; peak: 0.27). The augmentation of MP, and plasma concentrations of lutein, following supplementation in subjects with ARM provides the first evidence the disease is not associated with intestinal malabsorption of the relevant macular carotenoids, and that a diseased macula can accumulate and stabilise lutein and/or zeaxanthin. Furthermore, these results suggest that the beneficial effects of lutein supplementation, if any, may be extended to subjects with established ARM.

Aged↗

PubMed2003

Spectral sensitivity in eyes with macular holes and their fellow eyes.

BACKGROUND: This prospective study was undertaken to investigate whether spectral sensitivity can be useful in determining the prognosis of fellow eyes of eyes with macular holes. METHODS: Spectral sensitivity measurements using a one degree test spot presented at a rate of 1 Hz and 25 Hz on a bright (1000 td) white background were carried out on 10 patients aged between 67 and 74 years (mean age 70.3 +/- 2.6 years). Each patient had a full thickness macular hole in one eye and a normal contralateral fellow eye. The spectral sensitivity measurements were made with eccentric fixation in the eyes with macular holes and with central fixation in the normal fellow eye. A year later, the patient files were reviewed to look at the patient's ocular condition. Another 10 subjects between the ages of 50 and 80 years (mean age 69.5 +/- 4.2 years) were also seen. These control group subjects had visual acuities of 6/9 or better with minimal ocular media changes and no ocular or systemic pathology that could affect colour vision. RESULTS: The 1 Hz and 25 Hz spectral sensitivities of all patients were reduced for both eyes. Despite the good eye without a macular hole having a VA of 6/6, the spectral sensitivity was similar to that of the eye with the macular hole and markedly reduced visual acuity. CONCLUSION: The present investigation enabled us to examine the chromatic and achromatic mechanisms by testing spectral sensitivity at 1 Hz and 25 Hz, respectively. The data revealed that both chromatic and achromatic processing could be damaged in the eye with a macular hole. Surprisingly, the spectral sensitivities of both 1 Hz and 25 Hz are equally reduced in the good fellow eye with no macular hole. A one-year follow-up showed that two of the 10 patients (20 per cent) did eventually develop a macular hole in the normal fellow eye. This indicates that there is some subclinical foveal dysfunction in the normal fellow eye, the nature of which is unclear.

Aged↗

PubMed2003

Simple reaction times in color space: the influence of chromaticity, contrast, and cone opponency.

PURPOSE: This study examined the influence of stimulus chromaticity on simple reaction times (RTs) to determine the stage of chromatic processing that is most influential in their generation. METHODS: Simple RTs were measured in response to the cosinusoidally ramped onset of small, equiluminant, colored Gaussian spots. The chromaticity of these stimuli was varied, to modulate along a series of vectors in color space that included red-green (L-M) and blue-yellow (S-[L+M]) opponent axes. RESULTS: RTs are highly sensitive to small departures from subjective equiluminance. They are also dependent on stimulus chromaticity. The longest RTs are generated in response to equiluminant stimuli that isolate S-cone activity, whereas the shortest are generated by stimuli that modulate the L-M opponent axis. However, temporal processing differences are highly dependent on how the chromatic stimuli are scaled in relation to one another. The differences are reduced when scaling is based on detection threshold. The relationship between chromatic contrast and RT can be described by the modified Piéron equation RT = RT(0) + k x C(-1). CONCLUSIONS: Simple RTs generated in this study conform to the idea that they are largely determined by cone-opponency mechanisms. The use of cone contrast as a metric for scaling chromatic stimuli exaggerates differences between the temporal responsiveness of L-M and S-(L+M) opponency mechanisms.

Adult↗

PubMed2002

Synphilin in normal human brains and in synucleinopathies: studies with new antibodies.

Mutations in the gene encoding alpha-synuclein (alpha-syn) have recently been linked to rare hereditary forms of Parkinson's disease. A yeast two-hybrid screen with alpha-synuclein (alpha-syn) identified synphilin as an alpha-syn-interacting protein, potentially implicating synphilin in the pathogenesis of synucleinopathies. Co-transfection of synphilin and the central (NAC) region of alpha-syn in HEH293 cells resulted in synuclein inclusions. Furthermore, synphilin immunoreactivity has been observed in Lewy bodies (LBs) and glial cytoplasmic inclusions of synucleinopathies. To further characterize synphilin, we utilized two new anti-synphilin antibodies for biochemical and immunohistochemical studies in normal and disease brain tissues. In normal brain tissue, synphilin localized predominantly to large neurons, such as substantia nigra neurons, hippocampal pyramidal and cerebellar Purkinje cells. However, in a few pathological cases synphilin immunoreactivity was present in glial cells and a small percentage of cortical and nigral LBs. In brain extracts, synphilin was observed primarily as a 90-kDa band but protein bands of 50 and 65 kDa were also present in both soluble (high salt) and lipid (Triton X-100) fractions. Additionally, less abundant higher molecular mass species, including a 120-kDa band of similar size to that of synphilin expressed in transiently transfected cells were recovered in 8 M urea-solubilized pellets after sequential extraction of brain tissue with buffers of increasing strengths. The presence of the synphilin of higher molecular mass was detected regardless of alpha-syn pathology and may represent an immature form of synphilin. Thus, although synphilin may be an alpha-syn-interacting protein present in some alpha-syn lesions, it still remains to be determined whether synphilin plays a critical role in mechanisms of brain degeneration in human synucleinopathies.

Adult↗