Correlations between MRI white matter lesion location and executive function and episodic memory.

OBJECTIVES: MRI white matter hyperintensity (WMH) volume is associated with cognitive impairment. We hypothesized that specific loci of WMH would correlate with cognition even after accounting for total WMH volume. METHODS: Subjects were identified from a prospective community-based study: 40 had normal cognition, 94 had mild impairment (defined here as a Clinical Dementia Rating [CDR] score of 0.5 without dementia), and 11 had mild Alzheimer's dementia. Factor analysis of a 22-item neuropsychological battery yielded 4 factors (episodic memory, executive function, spatial skills, and general knowledge). MRI WMH segmentation and analysis was performed using FreeSurfer software. RESULTS: Higher WMH volume was independently associated with lower executive function and episodic memory factor scores. Voxel-based general linear models showed loci where WMH was strongly inversely associated with specific cognitive factor scores (p < 0.001), controlling for age, education, sex, APOE genotype, and total WMH volume. For episodic memory, clusters were observed in bilateral temporal-occipital and right parietal periventricular white matter, and the left anterior limb of the internal capsule. For executive function, clusters were observed in bilateral inferior frontal white matter, bilateral temporal-occipital and right parietal periventricular white matter, and the anterior limb of the internal capsule bilaterally. CONCLUSIONS: Specific WMH loci are closely associated with executive function and episodic memory, independent of total WMH volume. The anatomic locations suggest that WMH may cause cognitive impairment by affecting connections between cortex and subcortical structures, including the thalamus and striatum, or connections between the occipital lobe and frontal or parietal lobes.

Evaluation of muscle oxidative potential by 31P-MRS during incremental exercise in old and young humans.

The purpose of this study was to compare muscle oxidative capacity between moderately active young and old humans by measuring intracellular threshold (IT) during exercise with 31P-magnetic resonance spectroscopy (31P-MRS). Changes in phosphocreatine, inorganic phosphate, and intracellular pH were measured by 31P-MRS during a progressive unilateral ankle plantar flexion exercise protocol in groups of moderately active old (n = 12, mean age 66.7 years) and young (n = 13, mean age 26.2 years) individuals. From muscle biopsy samples of the lateral gastrocnemius, citrate synthase (CS) activity was determined in six subjects from each group, and fibre type composition was determined in nine old and ten young subjects. The old group had a lower IT for pH, as a percentage of peak work rate (P < 0.05), despite a similar CS activity compared to the young. IT was significantly correlated with CS activity (R = 0.59; P < 0.05), but not with fibre type composition. It was concluded that metabolic responses to exercise are affected by ageing, as indicated by a lower IT in old compared to young individuals.

The effects of age on kinetics of oxygen uptake and phosphocreatine in humans during exercise.

We compared the kinetics of oxygen uptake (VO2) and phosphocreatine (PCr) during the adjustment to and recovery from plantar flexion exercise in moderately active older (n = 10, 66.9 years) and younger (n = 10, 27.5 years) individuals. VO2 kinetics were similar in the two groups, with time constants (tau) averaging 46.3 +/- 10.2 s (younger, on-transient), 38.1 +/- 14.4 s (younger, off-transient), 46.3 +/- 17.8 (older, on-transient) and 40.7 +/- 19.2 s (older, off-transient). These were similar to corresponding PCr kinetics, measured by 31P nuclear magnetic resonance spectroscopy, which averaged 50.6 +/- 24.0 s (younger, on-transient), 42.0 +/- 16.1 s (younger, off-transient), 39.8 +/- 22.0 s (older, on-transient) and 37.6 +/- 21.6 s (older, off-transient). On-transient tau values for VO2 and PCr were correlated, for combined groups (r = 0.53; P = 0.015). We conclude that: (1) VO2 and PCr kinetics during exercise of a muscle group accustomed to daily activity are not compromised in physically active older humans, and (2) PCr kinetics reflect the kinetics of muscle O2 consumption, and are expressed at the lung (VO2 kinetics) after a transit delay.

Kinetics of pulmonary oxygen uptake and muscle phosphates during moderate-intensity calf exercise.

The purpose of this study is to directly compare the dynamic responses of phosphocreatine (PCr) and P(i) to those oxygen uptake (VO2) measured at the lung during transitions to and from moderate-intensity exercise. Changes in PCr and P(i) were measured by 31P-nuclear magnetic resonance spectroscopy, and changes in VO2 were measured breath by breath by mass spectroscopy during transitions to and from moderate-intensity square-wave ankle plantar flexion exercise in 11 subjects (7 men and 4 women; mean age 27 yr). Three repeated transitions were averaged for improvement in signal-to-noise ratio of phosphate data, and 12 transitions were averaged for VO2 measurements. Averaged transitions were fit with a monoexponential curve for determination of the time constant (tau) of the responses. Mean tau values for on transients of PCr, P(i), and phrase 2 VO2 were 47.0, 57.7, and 44.5 s, respectively, whereas means tau values for off transients were 44.8, 42.1, and 33.4 s, respectively. There were no significant differences between tau values for phosphate- and VO2-measured transients or on and off transients. The similarity of on and off kinetics supports linear first-order respiratory control models. Measurement of phase 2 pulmonary VO2 kinetics to and from moderate-intensity small-muscle-mass exercise reflect muscle phosphate kinetics (and muscle oxygen consumption).

Geometrical optical analysis of photorefractive methods.

Photorefractive methods allow rapid measures of the refractive and accommodative state of infants and young children whose brief attention and co-operation limit the use of more traditional methods such as retinoscopy and autorefraction. Three methods have been defined: orthogonal, isotropic and eccentric. We provide a common geometrical optical analysis for these three methods where the photorefractive pattern is defined at the plane of focus of the camera. Since this plane is conjugate with the detector plane of the camera then the critical optical parameters can be defined without reference to the design of the camera by simply determining the relative magnification of the projected image of an object photographed at the camera's plane of focus. Specifically the pattern width CF (mm) over a photorefractor's working range can be defined for the isotropic and orthogonal methods as: [formula: see text] and for the eccentric method as: [formula: see text] where: K is a myopic refractive error of the eye (dioptres) (K Less than 0); P the distance of the flash source to the eye (dioptres); L the distance in front of the eye of the camera's plane of focus (dioptres) (L less than 0); GH the pupil diameter (mm); e the eccentricity of the flash source from the camera aperture (mm); M magnification of the image measured relative to the camera plane of focus.

Coaxial photorefractive methods: an optical analysis.

We provide a novel geometrical optical analysis of two coaxial photorefractive methods (isotropic and orthogonal). The size of the photorefractive pattern is defined in terms of the critical optical parameters without reference to specific camera parameters. A set of equations is derived that defines the following: a working range where the photorefractive pattern increases linearly with refractive error and pupil size, a dead zone where changes in refractive error do not influence pattern size, and critical values where vignetting by the camera lens becomes important. From this analysis optical parameters can be systematically adjusted to vary a photorefractor's working range. Small discrepancies found between measurements taken with model eyes indicated threshold and blooming effects that require calibration.