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At least 19 recordsLinked to original sources
PubMed1987

Evidence for spatially local computations underlying discrimination of periodic patterns in fovea and periphery.

Human visual sensitivity for discriminating between, on the one hand stimuli composed of components (F) and (F + 3F) (compound detection), and on the other hand (F + 3F) and (F - 3F) (phase discrimination), was measured as a function both of stimulus contrast and eccentricity. Performance under these particular conditions was found to depend upon whether the (F) or (3F) component was dominant in the pattern. When the (F) component was high in contrast, visual performance was well modelled by an edge-blur discrimination, whereas when the (3F) component was high in contrast, visual performance was well modelled by a contrast discrimination involving local spatial features within each waveform. These conclusions were valid for both foveal and peripheral vision. The finding that these suprathreshold compound stimuli are discriminated on the basis of the local spatial features, and not on differences in their phase spectra as previously thought, allows a reinterpretation of the importance of phase coding in normal vision and of the selective loss of these discriminations that have been previously reported for peripheral vision and in amblyopia.

Adult↗
PubMed1992

The spatial localization deficit in amblyopia.

There have now been numerous reports of a spatial localization deficit in amblyopia but none so far have tackled (1) the relationship between the contrast sensitivity and spatial localization deficits and (2) whether the spatial localization deficit is best described in units of visual angle or in terms of the underlying filter size. These issues are germane because they lie at the very heart of our understanding of the underlying deficit in amblyopia. To answer these questions we use spatially bandpass stimuli so that we can readily compare detection and localization for the same stimuli at each of a number of spatial scales. For some amblyopes (all strabismics and a minority of anisometropes) the contrast sensitivity defect neither underlies nor covaries with the spatial localization deficit. In the majority of anisometropic amblyopes, the contrast sensitivity loss is a complete description. The spatial localization deficit in amblyopia is of two independent kinds; positional inaccuracy and positional distortion. The positional inaccuracy deficit which can occur in varying degrees in both strabismic and anisometropic amblyopia, affects all spatial scales equally and therefore is best thought of in terms of a constant fraction of the underlying filter size in the space-frequency plane. The positional distortion deficit which can also occur to varying degrees in both strabismic and anisometropic forms can not be easily understood within this metric at least for strabismics.

Amblyopia↗
PubMed1990

Transmural high energy phosphate distribution and response to alterations in workload in the normal canine myocardium as studied with spatially localized 31P NMR spectroscopy.

Spatially localized phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy has been applied to the study of the normal canine myocardium to measure the relative content of high energy phosphates across the left ventricular wall. Transmural NMR data were acquired in five voxels spanning the wall of the left ventricle using the FLAX-ISIS technique. The validity of the FLAX-ISIS approach in acquiring localized spectra for transmural studies and in providing quantitative information from the localized spectra was examined rigorously by studies involving phantoms, intact rats, and the canine myocardium in vivo. The results indicated that (1) this technique yields spatially resolved spectra with partial overlap between adjacent voxels and virtually no overlap between every other voxel; (2) in the canine heart, signals from subepicardium, midwall, and subendocardium can be detected separately without cross contamination; and (3) relative metabolite contents within a voxel and among voxels can be quantitated. Transmural 31P NMR spectra were acquired with cardiac gating on 29 separate animals either at early systole or late diastole, and at three different workloads with the heart rate peak systolic pressure product (RPP) increasing from 6000 mmHg/min to 35,000 mmHg/min. The data revealed that in the normal canine myocardium, the creatine phosphate (CP) content and the CP/ATP ratio was significantly lower in the subendocardium than in the subepicardium. ATP levels were transmurally constant. Both the CP content and the CP/ATP ratio measured for each voxel remained unaltered in relation to either the phase of the cardiac cycle or approximately fourfold increase in workload. Free ADP levels calculated for each voxel showed that ADP was relatively higher in the subendocardium than the subepicardium, and in all transmural layers was higher than its apparent Km for oxidative phosphorylation. In this domain changes in ADP content with workload and MVO2 are not expected and were not observed.

Adenosine Diphosphate↗
PubMed1987

Spatial localization in NMR spectroscopy in vivo.

Spatial localization techniques are necessary for in vivo NMR spectroscopy involving heterogeneous organisms. Localization by surface coil NMR detection alone is generally inadequate for deep-lying organs due to contaminating signals from intervening surface tissues. However, localization to preselected planar volumes can be accomplished using a single selective excitation pulse in the presence of a pulsed magnetic field gradient, yielding depth-resolved surface coil spectra (DRESS). Within selected planes, DRESS are spatially restricted by the surface coil sensitivity profiles to disk-shaped volumes whose radii increase with depth, notwithstanding variations in the NMR signal density distribution. Nevertheless, DRESS is a simple and versatile localization procedure that is readily adaptable to spectral relaxation time measurements by adding inversion or spin-echo refocusing pulses or to in vivo solvent-suppressed spectroscopy of proton (1H) metabolites using a combination of chemical-selective RF pulses. Also, the spatial information gathering efficiency of the technique can be improved to provide simultaneous acquisition of spectra from multiple volumes by interleaving excitation of adjacent planes within the normal relaxation recovery period. The spatial selectivity can be improved by adding additional selective excitation spin-echo refocusing pulses to achieve full, three-dimensional point resolved spectroscopy (PRESS) in a single excitation sequence. Alternatively, for samples with short spin-spin relaxation times, DRESS can be combined with other localization schemes, such as image-selected in vivo spectroscopy (ISIS), to provide complete gradient controlled three-dimensional localization with a reduced number of sequence cycles.

Animals↗
PubMed1989

Proton-decoupled, Overhauser-enhanced, spatially localized carbon-13 spectroscopy in humans.

Spatially localized, natural abundance, carbon (13C) NMR spectroscopy has been combined with proton (1H) decoupling and nuclear Overhauser enhancement to improve 13C sensitivity up to five-fold in the human leg, liver, and heart. Broadhand-decoupled 13C spectra were acquired in 1 s to 17 min with a conventional 1.5-T imaging/spectroscopy system, an auxiliary 1H decoupler, an air-cooled dual-coil coplanar surface probe, and both depth-resolved surface coil spectroscopy (DRESS) and one-dimensional phase-encoding gradient NMR pulse sequences. The surface coil probe comprised circular and figure-eight-shaped coils to eliminate problems with mutual coupling of coils at high decoupling power levels applied during 13C reception. Peak decoupler RF power deposition in tissue was computed numerically from electromagnetic theory assuming a semi-infinite plane of uniform biological conductor. Peak values at the surface were calculated at 4 to 6 W/kg in any gram of tissue for each watt of decoupler power input excluding all coil and cable losses, warning of potential local RF heating problems in these and related experiments. The average power deposition was about 9 mW/kg per watt input, which should present no systemic hazard. At 3 W input, human 13C spectra were decoupled to a depth of about 5 cm while some Overhauser enhancement was sustained up to about 3 cm depth, without ill effect. The observation of glycogen in localized natural abundance 13C spectra of heart and muscle suggests that metabolites in the citric acid cycle should be observable noninvasively using 13C-labeled substrates.

Carbon Isotopes↗
PubMed1991

Effect of differential saturation on the spatial localization performance of depth pulses.

Computer simulations of Depth pulse B (theta; (2 theta [+/- x, +/- y])2; acquire) and other Depth pulses, verified by experimental surface coil NMR studies utilizing phantom samples, reveal that the spatial localization performance of Depth pulses degrades when the repetition time is short relative to T1 because of differential saturation, i.e. T1 discrimination effects. Simulations of Depth pulse A (theta; 2 theta [+/- x, +/- y]; acquire) and Depth pulse B indicate that there is no phase-cycled pulse sequence delivery order which negates the untoward effect of T1 discrimination on spatial localization performance. The results of this study demonstrate the importance of consistent magnetization preparation prior to the delivery of each phase-cycled multiple pulse sequence in a Depth pulse cycle for obtaining optimal spatial localization performance. The untoward effects of inconsistent magnetization preparation, resulting from T1 discrimination, may be ameliorated by the application of many Depth pulse cycles.

Computer Simulation↗
PubMed1991

New spatial localization method using pulsed high-order field gradients (SHOT: Selection with High-Order gradienT).

A new spatial localization method using an additional set of high-order magnetic field gradients is described. The method uses a nonlinear part of high-order magnetic field gradient patterns which allows us to select a volume in conjunction with the selective radiofrequency (RF) pulse. Unlike the other existing volume selection methods such as ISIS or SPARS, the proposed selection method requires only one RF-gradient pulse pair to select a volume in two directions. The center of the selected volume can be moved to any arbitrary location within the body by the addition of precalculated lower order gradients which are simultaneously pulsed with the high-order gradient. The method also has the potential for localized spectroscopy from the FID signal which can be realized by using oscillating second- and first-order gradients for 3D selection with a single RF pulse. By using the proposed localization method, it is possible to design more flexible pulse sequences, e.g., the shorter echo-time spectroscopic pulse sequence. We have designed and constructed a six-loop r2 (or x2 + y2) gradient coil for initial application. By simultaneously applying this second-order gradient and proper x, y, and/or z gradients, 2D selections were achieved in arbitrarily selected positions in conjunction with a single selective RF pulse. Phantom and animal experiments have been performed and the results appear promising, especially in areas of NMR spectroscopic imaging applications where spatial localization is essential.

Animals↗
PubMed1992

Asymmetries in the spatial localization of transformed targets.

This study was designed to examine the contribution of the right cerebral hemisphere in the spatial localization of visual targets for manual aiming. Visual targets were briefly presented to the right and left fields and subjects were required to point either to the target location, or a "mirror" image of the target location with their right or left index finger. Whereas reaction times were faster for left-hand pointing than for right-hand pointing, there was no differential effect of the mirror image transformation. This suggests that left-hand reaction time advantages are more related to right hemisphere involvement in the spatial parameterization of the movement than spatial localization of the target.

Adult↗
PubMed1976

[Contingent negative variation in 10-year-old children. Relations with the type of performance in a task of spatial localization].

CNV magnitude was studied in a task involving spatial localization and discrimination Subjects were 18 children 10 years of age. The CNV amplitude of half the subjects increased when the performance decreased and, in the other half, CNV amplitude decreased when the performance decreased. However, it seems that CNV magnitude increases as task difficulty increases but only above a minimal threshold varying with the subject. The results suggest that CNV amplitude cannot be related to a single psychological factor.

Age Factors↗
PubMed1991

Spatially localized magnetic resonance spectroscopy of the brains of normal and asphyxiated newborns.

Phase-modulated rotating frame imaging is a modification of magnetic resonance spectroscopy, which uses a linear radiofrequency field gradient to obtain spatially localized biochemical information. Phase-modulated rotating frame imaging was used to study regional cerebral energy metabolism in the brains of 9 normal newborns and 25 newborns after birth asphyxia. Relative concentrations of phosphorus-containing metabolites and intracellular pH were determined for brain tissue at three specified depths below the brain surface for all neonates. Wide variations in metabolite ratios were seen among normal neonates, and considerable metabolic heterogeneity was demonstrated in individual neonates by depth-resolved spectroscopy. Asphyxiated neonates with severe hypoxic-ischemic encephalopathy and a poor neurodevelopmental outcome showed the expected rise in inorganic orthophosphate and fall in phosphocreatine concentrations in both global and spatially localized spectra. Phase-modulated rotating frame imaging showed that metabolic derangement was less in superficial than in deeper brain tissue. The inorganic orthophosphate-adenosine triphosphate ratio from 1 to 2 cm below the brain surface was more accurate than any global metabolite ratio for the identification of neonates with a poor short-term outcome. These data are consistent with the known vulnerability of subcortical brain tissue to hypoxic-ischemic injury in the full-term neonate.

Adenosine Triphosphate↗
PubMed1988

Local spatial scale for three-dot alignment acuity.

Three-dot alignment discrimination thresholds were determined for blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. Thresholds were determined as a function of the blur parameter of the stimuli. This was done for a range of eccentricities in the visual field (from 45 degrees nasal to 65 degrees temporal). The thresholds were corrected for variations of the stimulus extent with the blur parameter. The results were used to estimate the local spatial scale for three-dot alignment acuity. This was done by a method recently introduced by Watson (1987). It was found that the local spatial scale for three-dot alignment acuity is approximately linearly proportional to eccentricity.

Adult↗
PubMed1992

Spatially localized in vivo 1H magnetic resonance spectroscopy of an intracerebral rat glioma.

Surface coil MRI combined with spatially localized spectroscopy was used to noninvasively detect 1H signals from metabolites within an intracerebral malignant glioma in rats. The MRS pulse sequence was based upon two-dimensional ISIS, which restricted 1H signals to a column-shaped volume, combined with one-dimensional spectroscopic imaging, which further resolved the signals into 8 or 16 slices along the major axis of the column. All experiments were executed with adiabatic pulses which induced uniform spin excitation despite the inhomogeneous radiofrequency field distribution produced by the surface coil transmitter. Surface coil MRI and MRS experiments were performed on phantom samples, normal rat brains, and rat brains harboring malignant gliomas. Spatially resolved in vivo 1H spectra of intracerebral gliomas revealed significantly decreased concentrations of N-acetyl-aspartate and creatine and increased lactic acid (or lipids) as compared to the contralateral hemisphere. These results demonstrate that metabolic abnormalities in intracerebral rat gliomas can be spatially resolved in a noninvasive manner using localized in vivo 1H MRS.

Animals↗
PubMed1992

Topography of the evoked potential to spatial localization cues.

Visual tasks that are perceptually diverse might be expected to elicit unique evoked-potential waveforms that exhibit differing topographic maps. To investigate this possibility, multichannel visual-evoked potentials (VEPs) were recorded in response to several dot spatial localization stimuli that are physically similar yet produce different percepts (vernier offsets, steroscopic disparity, bisection, orientation, and relative displacement) to determine if the unique percepts arising from these stimuli reflect the activation of different cortical neural populations. The resulting evoked potentials were all similar in waveform, although the stereoscopic VEPs were relatively delayed. Topographic maps of the evoked-potential activity to each stimulus revealed a late major component with two independent foci: one 7 or more centimeters above the inion lateral to the midline, and the other at least 6 cm lateral to OZ. The scalp localization of both peaks was independent of both the position of the stimulus in the visual field and the particular stimulus cue presented. An asymmetric response to pattern appearance vs. disappearance indicated strong pattern specificity for each stimulus type except unreferenced motion. The timing of the VEP responses and relative insensitivity to retinal locus of stimulation suggest the involvement of higher cortical areas. The two map foci might be interpreted as activation of inferotemporal and parietal cortices whose roles are thought to be visual object interpretation and spatial attention and localization, respectively.

Evoked Potentials, Visual↗
PubMed1991

Spatial localization across channels.

We have studied vernier acuity for patterns in which the stimuli to be aligned either are similar in their spatial and color characteristics or differ in these properties. The question which we address is whether spatial localization is independent of the channels being stimulated by the patterns to be aligned. We found that the precision of vernier alignment of Gabor patches was very similar irrespective of whether the patches were the same or different in spatial frequency, orientation, or color. It appears that the visual system extracts very precise location information independent of the similarity or dissimilarity of the spatio-chromatic selectivity of the channels carrying that information.

Color Perception↗
PubMed1977

Spatial localization with paralyzed eye muscles.

Four subjects suffering from a unilateral peripheral paralysis of the 3rd or the 6th nerves have been studied in spatial localization tasks, with their normal eye occluded. When peripheral targets were presented in the hemifield corresponding to the paralysis, the saccadic eye movements (recorded from the normal occluded eye) were of an exaggerated amplitude. 'Staircase' oculomotor patterns, closely similar to those occurring in 'open-loop' visual stimulation, could also be observed. Our patients also presented the classical hypermetric misreaching when attempting to point by hand at visual targets in an otherwise dark room. This effect (past-pointing) was likely to be due to a monitoring of the exaggerated oculomotor signal: in one subject past-pointing disappeared when reaching at the targets on the basis of the sole retinal cues. Finally, the classically described illusory visual effects of ocular paralysis were limited to a feeling of instability during self-motion.

Abducens Nerve↗
PubMed1990

Spatial localization for motion-rejected NMR imaging: SLO-MOTION.

A new method of eliminating NMR image artifacts caused by random or sporadic motion is presented. The method uses only NMR techniques. A combination of a spatial localization method and a standard 2DFT imaging sequence is employed and consequently named SLO-MOTION. Alternatively, the sequence may be used to aid motion artifact reduction by postprocessing methods. The feasibility of the method is demonstrated on a phantom.

Humans↗