Biomedical subjects

D MacVeigh

Publications and source records attributed to D MacVeigh.

7 recordsLinked to original sources

PubMed1992

Spatial scaling of vernier acuity tasks.

Vernier acuity thresholds for two abutting lines and for a two-dot stimulus were measured as a function of stimulus magnification at eccentricities of 0, 5, 10 and 15 deg using a spatial scaling technique in which all stimuli are simply magnified versions of each other. The advantage of such a technique is that no prior knowledge of a suitable magnification factor with which to increase the size of peripheral stimuli is required. Thresholds for the line stimulus could be successfully scaled by the application of a magnification factor with an E2 value of 1.23-1.78 deg. Further, provided that the effects of dot separation and eccentricity were dissociated, spatial scaling with an E2 value of 1.06-1.96 deg was also successful in removing eccentricity dependence for two-dot vernier thresholds.

Fovea Centralis↗

PubMed1992

Sequential mapping of weighting functions for visual location.

The effect upon perceived location of adding an extra dot offset from the centre of a cluster of pseudorandom dots was investigated using a vernier acuity task. With this technique, weighting functions showing the extent to which the added dot pulls the apparent location of the entire cluster can be defined as a function of distance from the centre of the cluster. When dot density within the cluster is high, the weighting functions approximate to what would be expected on the basis of centroid alignment. With low dot densities, it appears that performance is determined by aligning the outermost dots within each cluster. The peak amplitudes of these weighting functions are proportional to the square root of dot density within the clusters. The results are consistent with the view that each vernier element is localised in an orthoaxial direction prior to discrimination of the vernier offset.

Humans↗

PubMed1992

Variations in hyperacuity performance with age.

The term hyperacuity has been applied to a group of stimuli which produce spatial thresholds smaller than those expected given the relatively large receptor spacing and the retinal image quality of the human eye. It is not yet firmly established whether hyperacuity performance declines with increasing age in the same way as most other measures of visual ability. This is perhaps due to the use of varying task configurations and criterion-dependent psychophysical techniques. The present study examines age-related performance in three different hyperacuity tasks using a criterion independent forced-choice method. Both displacement and bisection thresholds were found to increase with age, but there was no significant change in vernier acuity. This indicates that age has a differential effect on thresholds for various hyperacuities depending upon the task requirements. No significant age-related trend was observed in hyperacuity bias, which represents the difference between subjective and true physical alignment.

Adolescent↗

PubMed1991

Interaction of spatial frequency and separation in vernier acuity.

There are two general theories which attempt to explain the ability to judge the relative positions of objects in the visual field. The first is the "local-sign" model which assigns location to individual features of the stimulus. The other is the "channel" model involving pooling of responses of overlapping filters having different spatial frequency and orientation tuning. We set out to investigate these using vernier stimuli whose frequency content could be controlled. The stimuli consisted of vertically orientated sinusoidal gratings placed one above the other with a variable separation between them. Thresholds for high frequency gratings increased rapidly with separation while low frequencies were little affected. In addition the cycle width of gratings which could just be distinguished as being in or out of phase and of gratings which gave optimum vernier acuity both increased linearly with separation. Results are discussed in terms of a model based on spatial frequency filters and we suggest that our findings place significant constraints upon a local-sign model.

Contrast Sensitivity↗

PubMed1991

Spatial summation determines the contrast response of displacement threshold hyperacuity.

The effect of line length on displacement threshold hyperacuity at various levels of contrast was investigated. At high contrasts there was no significant effect of line length, but as contrast was reduced thresholds for shorter line lengths increased rapidly. Thresholds at longer line lengths demonstrated a form of contrast saturation which differed from vernier acuity whose thresholds increased consistently with a reduction in contrast. As line length in the displacement threshold task was reduced, the amount of contrast saturation became less and displacement thresholds therefore resembled the contrast response of vernier acuity. Results are explained in terms of spatial summation along the length of the lines, the effects of which depend upon the spatial configuration of the hyperacuity stimulus under consideration.

Contrast Sensitivity↗

PubMed1990

Displacement thresholds for various types of movement: effect of spatial and temporal reference proximity.

Displacement thresholds for unidirectional stop-go-stop and continuous types of movement were measured as a function of duration of movement. A circular stationary reference of varying radius surrounding the stimulus was either present throughout the whole duration of movement, disappeared just before movement began, or was totally absent. In the absence of the reference, thresholds increased with duration according to a constant velocity prediction. When present continuously, thresholds for all durations of movement were reduced. For references of close proximity, displacement thresholds became independent of movement duration. Results are discussed in terms of direct mechanisms of movement detection and also spatio-temporal hyperacuity analysis mediating the detection of relative positional change.

Humans↗

PubMed1990

Neural contribution to spatiotemporal contrast sensitivity decline in healthy ageing eyes.

Contrast sensitivity thresholds were measured over a range of spatial and temporal frequencies in both a group of young and older observers. Results demonstrate a significant reduction in contrast sensitivity of the older age group at all but the lowest combinations of spatial and temporal frequencies investigated. The senile miosis and reduced optical transmission of the older eye was then mimicked using the younger observers as subjects. This combined effect of reducing retinal illumination produced no significant change in sensitivity. These findings are discussed in terms of neural loss within the visual pathways with increasing age.

Adult↗