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Biomedical subjects

R Hylton

Publications and source records attributed to R Hylton.

7 recordsLinked to original sources
PubMed1988

Vitamin E in the treatment of tardive dyskinesia: the possible involvement of free radical mechanisms.

One of the major problems associated with long-term neuroleptic treatment is persistent tardive dyskinesia (TD), for which there is no satisfactory treatment. We have recently proposed that some cases of TD are associated with neuronal dysfunction resulting from excess free radical production occurring during catecholamine metabolism. We therefore decided to assess the efficacy of a powerful free radical scavenging agent, alpha-tocopherol (vitamin E), on the clinical signs of TD. We treated 15 patients with persistent TD with alpha-tocopherol and matched placebo in a randomized crossover design. Patients demonstrated a significant overall reduction in scores on the Abnormal Involuntary Movements Scale (AIMS) after treatment with alpha-tocopherol, but not after placebo. The mean reduction in the AIMS score with alpha-tocopherol was 43 percent, with seven patients showing a greater than 50 percent reduction in their dyskinesia. There was also a trend for a decrease in scores on the Brief Psychiatric Rating Scale, but no change was observed in scores on the Simpson-Angus Scale for Extrapyramidal Side Effects. Our findings are consistent with the possibility that alpha-tocopherol is beneficial in the treatment of some patients with TD, but further research is necessary to establish the efficacy of this agent.

Antipsychotic Agents↗
PubMed1985

Spatial-frequency discrimination at low frequencies: evidence for position quantization by receptive fields.

In a recent study of spatial-frequency discrimination [J. Opt. Soc. Am. 72, 1367 (1982)], we reported evidence for the quantization of position in the human visual system with the fundamental unit of quantization being the photoreceptor spacing for spatial frequencies greater than 2 cycles per degree (c/deg). In this paper we extend our measurements to lower spatial frequencies, between 0.3 and 2, which we call the mid-spatial-frequency band, and find that this band of spatial frequencies shows similar evidence for spatial quantization. However, the spacing of the fundamental unit is about 0.056 deg or approximately seven or eight times the spacing of the foveal photoreceptor instead of about 0.008 deg or approximately equal to the spacing of foveal cones. We interpret this as evidence for a class of retinal receptive fields that we call spatial-sampling fields that perform a dual function of neural blurring and spatial sampling. We propose a scaled lattice model of spatial vision that allows apparently scale-free spatial processing while retaining the advantages of linear maps. The model generalizes the notion of hyperacuity to low-resolution tasks for which neural interpolation occurs not only on the photoreceptor sampling lattice to achieve hyperacuity but also on receptive-field sampling lattices to achieve lower resolutions.

Discrimination, Psychological↗
PubMed1984

Quality of the primate photoreceptor lattice and limits of spatial vision.

Quantitative analysis of a primate photoreceptor lattice shows that the foveal lattice is a highly regular hexagonal structure with a positional correlation length of at least 130 photoreceptors. This result indicates that the photoreceptor lattice is not sufficiently disordered to prevent aliasing in the fovea, but rather could provide the metric with which the visual system determines spatial separation even for tasks involving hyperacuity .

Animals↗
PubMed1984

Orientation dependence of visual hyperacuity contains a component with hexagonal symmetry.

Recently we reported that hyperacuity thresholds for separation-discrimination tasks were not smooth functions of spatial variables but rather were regularly segmented with sharp transitions between the segments. In this study we have measured the orientation dependence of two tasks involving visual hyperacuity, the discrimination of spatial frequencies and the discrimination of vernier offsets, in the vicinity of a transition between segments. The orientation dependence of both tasks contains a periodic component with a period of 60 degrees. We interpret this as evidence that the cortical substrate that processes spatial information has a hexagonal two-dimensional organization, preserving the hexagonal packing of the foveal photoreceptors.

Female↗
PubMed1982

Simultaneous recognition of two spatial-frequency components.

A two-frequency two-response paradigm was employed. The stimulus on any trial was a compound grating containing two sinusoidal components of different spatial frequencies where the contrast in one or both components could be zero. The observer gave two responses each indicating confidence that one of the two components had been presented with contrast greater than zero. If these responses are assumed to be related to outputs of spatial-frequency channels according to specified recognition linking hypotheses, then channel properties such as bandwidth, negative influences, correlation of noise, and additivity can be inferred. Possible modifications of spatial frequency channel models are discussed.

Form Perception↗
PubMed1982

Limits of spatial-frequency discrimination as evidence of neural interpolation.

We have studied the ability of observers to discriminate between suprathreshold vertical sinusoidal spatial-frequency gratings on the basis of spatial frequency. The results show that spatial-frequency discrimination is not a smooth function of spatial frequency but instead appears regularly segmented. Similar results were also obtained in an experiment in which observers discriminated between pairs of narrow vertical lines on the basis of their separation. Angular resolutions achieved for both discrimination tasks were less than the spacing between photo-receptors, requiring some type of neural interpolation. The similarity between the two sets of data indicates that discrimination between spatial-frequency gratings is probably based on the separation between two features exactly one cycle apart. We suggest that the segmentation reflects the existence of neural-image representations with discrete levels of spatial accuracy.

Humans↗