Biomedical subjects

R Demanins

Publications and source records attributed to R Demanins.

7 recordsLinked to original sources

PubMed1999

A deficit in strabismic amblyopia for global shape detection.

Using a task which relied upon the detection of sinusoidal deformations from circularity, we show that strabismic amblyopes exhibit deficits which are not critically dependent on either the scale of deformation or the spatial frequency characteristics of the stimulus (circular D4) itself. We show that this loss is not due to the restricted passband of the amblyopic eye. Furthermore, in a pedestal distortion experiment, we show that the suprathreshold form of this loss is consistent with an elevated level of 'intrinsic noise' rather than a loss in 'sampling efficiency'.

Amblyopia↗

PubMed1999

The neural deficit in strabismic amblyopia: sampling considerations.

In order to understand the nature of the neural loss in strabismic amblyopia, we have applied a technique which has been used in the normal periphery to psychophysically probe the sampling properties of the neuronal population. We ask whether there is a 'sampling' deficit and if so whether it is based on either an absolute loss of neurons (i.e. spatial undersampling) or an irregular arrangement of a normal number of neurons (i.e. irregular sampling). Our results suggest that neural pooling restricts the spatial frequency region where sampling considerations are important to a very small part of the visible high spatial frequency range. Within this limited region, irregular sampling rather than spatial undersampling is the greater contributor to the strabismic amblyope deficit.

Amblyopia↗

PubMed1999

The orientation discrimination deficit in strabismic amblyopia depends upon stimulus bandwidth.

We show that the previously reported orientation deficit in amblyopia (Skottun, B. C., Bradley, A., & Freeman, R. D. (1986). Orientation discrimination in amblyopia. Investigative Ophthalmology and Visual Science, 30, 532-537) also occurs for arrays of randomly positioned Gabor micropatterns for which explanations based on either neural disarray or local neural interactions would not hold. Furthermore, when using Gabors, we show that the deficit varies with the spatial frequency and orientational bandwidth of the stimuli used to measure it. We discuss two competing explanations for this, one based on a broader underlying detector bandwidth in amblyopia (both orientation and spatial frequency) and the other based on a selective deficit of first-order, as opposed to second-order orientation processing in strabismic amblyopia. Our results favour the latter interpretation.

Amblyopia↗

PubMed1998

Contour integration in anisometropic amblyopia.

Contour integration was measured in a group of anisometropic amblyopes to test the idea recently put forward that positional uncertainty sets a fundamental limit to contour integration in amblyopia. Anisometropic amblyopia, unlike strabismic amblyopia, has little or no positional uncertainty once the initial filtering loss has been taken into account. Therefore, according to the explanation put forward to explain strabismic amblyopia, anisometropes should exhibit normal contour integration. We show that this expectation is realized for five of our six anisometropic amblyopes.

Adult↗

PubMed1997

A reduced motion aftereffect in strabismic amblyopia.

The motion aftereffect was measured using both static and dynamic test stimuli in a group of normal observers and a group of strabismic amblyopes. Amblyopes exhibited a reduced direct aftereffect for both static and dynamic stimuli and only two of the eight amblyopes exhibited any measurable interocular transfer for either test stimulus. It is hard to explain these results in terms of either the known spatial (contrast sensitivity and positional sensitivity) or motion deficits previously reported in amblyopia. These results suggest a primary motion deficit in amblyopia affecting both the static and dynamic motion aftereffects.

Adaptation, Ocular↗

PubMed1996

Effect of exposure duration on spatial uncertainty in normal and amblyopic eyes.

Previous studies have found that the spatial uncertainty of amblyopes is critically dependent on temporal factors. These studies claim that the spatial uncertainty is much greater at short exposure durations. We have reassessed the effect of exposure duration on the spatial uncertainty of normal and amblyopic eyes using a task in which we can compensate for the loss in contrast sensitivity which inevitably occurs as exposure duration is shortened. Our task involved a three-element alignment task, where each of the elements were spatial Gabors at two different separations. We ensured that our stimuli were always displayed at a fixed ratio above contrast detection thresholds at each exposure duration. Our results show that for normal subjects, for well separated equivisible stimuli, there is only a weak effect of exposure duration. A similar dependence is found for the dominant and amblyopic eyes of a group of strabismic amblyopes. Dominant eyes of strabismic amblyopes show increased spatial uncertainty compared with normal subjects. Amblyopic eyes of strabismic amblyopes show increased spatial uncertainty compared with their dominant fellow eye which is invariant with exposure duration. Some subjects show a larger positional deficit at short durations when the stimuli are almost abutting.

Adolescent↗

PubMed1996

Positional loss in strabismic amblyopia: inter-relationship of alignment threshold, bias, spatial scale and eccentricity.

In order to understand the spatial loss in strabismic amblyopia and its relationship to the contrast sensitivity deficit, we measured alignment performance for a three element vertical alignment task in which the elements were equi-visible, spatial Gabors. We derived the threshold and bias and compared these for stimuli of different spatial scale and eccentricity. Our results suggest that: (1) the deficits for alignment thresholds and bias are uncorrelated; (2) in the majority of strabismic amblyopes, both deficits are scale invariant; (3) the form of the regional distribution depends on the spatial measure used and the scale at which it is measured; and (4) there is a poor correlation between the deficit for either spatial measure and the contrast sensitivity loss.

Amblyopia↗