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Biomedical subjects

D W Heeley

Publications and source records attributed to D W Heeley.

10 recordsLinked to original sources
PubMed1992

Directional acuity for drifting plaids.

Orientation discrimination thresholds were estimated for symmetrical "plaids", constructed by the super-imposition of two, independent drifting sine-wave gratings of 2.5 c/deg. Experiments were conducted when the apparent direction of drift was on the two principal meridians (vertical and horizontal), and when the apparent direction of drift was at the two oblique orientations (45 and 135 deg). Acuity for the direction of drift for these stimuli is of the same order of precision as orientation acuity for static or drifting gratings, and exhibits a meridional anisotropy that favours the principal meridians. This anisotropy follows a pattern that is determined by the apparent direction of drift, and not the orientation of the underlying elements. Acuity for orientation is lowest for oblique drift directions, even though both of the elements are oriented on the principal meridians. This finding was confirmed when the orientation of the underlying elements was subject to a random variation. The results are not easily accommodated by models that propose that the individual elements of the plaid are analysed separately. Second, the data are incompatible with models of the oblique effect of orientation discrimination that are based on an axis dependent, differential sampling of the retinal image.

Discrimination, Psychological↗
PubMed1992

Orientation acuity estimated with simultaneous and successive procedures.

Orientation discrimination thresholds for medium contrast sinewave grating stimuli were estimated by a forced-choice procedure for the two principal, and the two main oblique axes. The results obtained for simultaneous presentation with a centre/surround stimulus arrangement are compared with the threshold estimates obtained when the two stimuli to be discriminated do not overlap in time. We find that orientation acuity depends on the temporal relationship of the discriminanda. Estimates of orientation threshold with simultaneous presentation are lower on all axes tested than those obtained with successive presentation; the greatest difference in orientation acuity was found at obliques. The data support the hypothesis that the meridional anisotropy in orientation discrimination that is found with conventional, two-interval forced-choice methods is not entirely attributable to anatomical and physiological changes at the level of the striate cortex.

Discrimination Learning↗
PubMed1991

Perception and action in 'visual form agnosia'.

A single case study of a patient with 'visual form agnosia' is presented. A severe visual recognition deficit was accompanied by impairments in discriminating shape, reflectance, and orientation, although visual acuity and colour vision, along with tactile recognition and intelligence, were largely preserved. Neuropsychological and behavioural investigations have indicated that the patient is able to utilize visual pattern information surprisingly well for the control of hand movements during reaching, and can even read many whole words, despite being unable to make simple discriminative judgements of shape or orientation. She seems to have no awareness of shape primitives through Gestalt grouping by similarity, continuity or symmetry. It is proposed that many of these perceptual disorders might be the combined result of (1) a selective loss of the cortical elaboration of the magnocellular visual processing stream, and (2) a selective output disconnection from a central processor of visual boundaries and shape primitives in the occipital cortex.

Adult↗
PubMed1991

Spatial frequency difference thresholds depend on stimulus area.

Spatial frequency difference thresholds were estimated for vertical, high contrast 5.0 cpd sinewave gratings that had been multiplied by a two dimensional Gaussian window function of contrast. A range of different elliptical window configurations were tested that varied in their relative height and width. Spatial frequency acuity improved as the area of the target was increased. The data are compatible with a model of spatial frequency discrimination that is based on statistical averaging of spatial intervals interpolated between local retinal signs.

Adult↗
PubMed1990

Recognition of stimulus orientation.

The threshold for detecting a deviation in the orientation of a test stimulus from vertical and from horizontal was measured with a single test interval. The test stimuli were step edges, thin lines, and sine wave gratings of 2.5, 5.0 and 10.0 c/deg. Recognition thresholds were found to be independent of the frequency content and nature of the test stimulus. The average threshold value on the principal meridians was found to be approx. 0.6 deg. Additional testing was conducted with two interval forced choice designs, both with and without random perturbation of the orientation of the stimuli. Random perturbation of orientation reduces acuity estimates. The results are compared with those obtained by testing on the oblique meridia. We conclude that the meridional anisotropy of orientation discrimination, which favours the vertical and horizontal is a result, in part, of the influence of non-visual mechanisms.

Adolescent↗
PubMed1989

The effect of stationary, random phase discontinuities, on spatial frequency discrimination.

We measured spatial frequency discrimination for sine wave gratings that had stationary, periodic, random phase shifts. Testing was conducted at 2.5, 5.0 and 10.0 c/deg. The effects of introducing phase shifts that occurred after a phase continuity in the range 1-8 cycles of the parent sine wave were examined in different experiments. We found that the Weber fraction for spatial frequency was an inverse linear function of the logarithm of the number of cycles of the sine wave in continuous phase. Discrimination performance improved as the number of cycles before the phase shift was increased. The results are compatible with models of spatial frequency discrimination that are based on the averaging of estimates of the spatial intervals between characteristic luminance features (Heeley, 1987a, b), and show also that performance on this task is related to the amount of information available in the retinal image.

Adult↗
PubMed1989

Spatial frequency discrimination at different orientations.

Hirsch and Hylton (1984) have reported a meridional anisotropy for spatial frequency discrimination with a periodicity containing both square and hexagonal components. We measured the difference limen for spatial frequency for sine-wave gratings whose orientations varied over a 180 deg range and whose nominal spatial frequency varied between 2.5 and 10.0 c.deg.-1. Although some periodicity was evident at the lowest spatial frequency, it was not consistent between observers and it did not contain a hexagonal component. At the higher spatial frequencies there was no evidence for any meridional anisotropy.

Adult↗
PubMed1989

Width discrimination for band-pass stimuli.

There is a disagreement in the literature about whether the discrimination function for width is a continuous or a segmented function of target size (Hirsch & Hylton, 1982; Mayer & Kim, 1986). We determined the Weber fraction for width for difference of Gaussian (DOG) luminance distributions. Data for seven different standard widths were collected simultaneously. Although the value of the Weber fraction varied with target size, there was no evidence for any discontinuities in the discrimination function, either in the fovea or at 2.0 deg eccentricity.

Humans↗
PubMed1987

Spatial frequency discrimination for sinewave gratings with random, bandpass frequency modulation: evidence for averaging in spatial acuity.

Spatial frequency difference thresholds for vertical, high contrast sinewave gratings were estimated at 1.25, 2.5, 5.0 and 10.0 cyc deg-1. Within an experiment two independent manipulations of the stimulus were employed: (1) the number of cycles of the sinewave grating was varied over a range of 2.0 to 15.0 cycles; and (2) a stationary, random frequency modulation was imposed on the sinewave. The probability density function of the frequency modulation was a Gaussian whose dispersion coefficient was varied, in different experiments, in the range of 0 to 10% of the frequency of the parent sinewave. Both of these experimental variables were found to affect the precision with which spatial frequency discrimination could be performed. The Weber fraction increased both as the number of cycles present was decreased and as the dispersion coefficient of the modulating function was increased. These two effects were independent. The data support previous psychophysical findings that spatial frequency discrimination involves averaging over the total area of the stimulus and are compatible with spatial primitive models of spatial contrast vision. The data are not compatible with those of Hirsch and Hylton (J. opt. Soc. Am. 72, 1367-1374) which suggest that spatial interval discrimination occurs solely by operation of foveal mechanisms.

Adult↗