Disruptions to human speed perception induced by motion adaptation and transcranial magnetic stimulation.

To investigate the underlying nature of the effects of transcranial magnetic stimulation (TMS) on speed perception, we applied repetitive TMS (rTMS) to human V5/MT+ following adaptation to either fast- (20 deg/s) or slow (4 deg/s)-moving grating stimuli. The adapting stimuli induced changes in the perceived speed of a standard reference stimulus moving at 10 deg/s. In the absence of rTMS, adaptation to the slower stimulus led to an increase in perceived speed of the reference, whilst adaptation to the faster stimulus produced a reduction in perceived speed. These induced changes in speed perception can be modelled by a ratio-taking operation of the outputs of two temporally tuned mechanisms that decay exponentially over time. When rTMS was applied to V5/MT+ following adaptation, the perceived speed of the reference stimulus was reduced, irrespective of whether adaptation had been to the faster- or slower-moving stimulus. The fact that rTMS after adaptation always reduces perceived speed, independent of which temporal mechanism has undergone adaptation, suggests that rTMS does not selectively facilitate activity of adapted neurons but instead leads to suppression of neural function. The results highlight the fact that potentially different effects are generated by TMS on adapted neuronal populations depending upon whether or not they are responding to visual stimuli.

Compound grating discrimination in extrafoveal and amblyopic vision.

Compound grating discrimination was measured in normal foveal and extrafoveal vision as well as in central vision of amblyopic subjects. Two types of discrimination were examined: 0 degrees versus 180 degrees and 90 degrees versus 270 degrees phase shift of the second harmonic relative to the fundamental. In common with several previous studies, we found that both 0/180 and 90/270 discriminations are possible in extrafoveal vision. However, we show that differences in foveal and extrafoveal sensitivity can be eliminated for both types of discrimination by scaling the stimulus size appropriately. The extent of spatial magnification necessary to equate foveal and extrafoveal performance differed markedly, with 90/270 discriminations requiring much more magnification. In the amblyopic subjects, the magnitude of the 90/270 deficit was greater than the 0/180 deficit in all six amblyopes tested. In common with previous investigations, we suggest that the visual system adopts a discrimination strategy, based upon differences in local features, between the patterns to be discriminated. One process registers positional relationships, while a second process registers local contrast differences. In this context, the reduced ability of the normal periphery and amblyopic fovea to perform mirror-symmetric discriminations is explained in terms of losses in positional acuity.

Vernier acuity with compound gratings: the whole is equal to the better of its parts.

To evaluate the relative importance of local feature and spatial filter models for hyperacuity, vernier thresholds were determined for gratings consisting of a fundamental (Fo) and third harmonic (3F) presented alone, and added together in compound gratings where the relative phase offsets of 3F to Fo was 0, 90, 180 and 270 degrees. Thresholds were determined for a range of spatial frequencies of Fo (0.5-16 c deg-1) for abutting and non-abutting stimuli. Compound grating vernier performance was found to be: (i) invariant with relative phase offset for the abutting and non-abutting conditions; and (ii) predictable from the vernier thresholds for the individual grating components making up the compound stimulus. The results support a view that supra-threshold components in a multi-frequency stimulus act independently and it is the spatial frequency content, not the local feature characteristics, which limit vernier performance.

Discriminating mirror symmetry in foveal and extra-foveal vision.

The ability to discriminate perfect from imperfect mirror symmetry was examined at the fovea and at eccentricities out to 10 degrees in the nasal visual field. A 2-AFC method of constant stimuli was employed in which a bilaterally symmetric pattern was presented in one interval and a degraded version of this symmetric pattern in the other. The subject's task was to decide which interval contained the perfectly symmetric pattern. Pattern size was varied by changing the viewing distance. Probit analysis revealed the degree of asymmetry corresponding to 75% correct performance. Given sufficient size scaling, perfectly symmetric stimuli can be discriminated from degraded symmetric stimuli in extra-foveal vision. Spatial scaling with an E2 value similar to that for positional acuity was successful in removing the eccentricity dependence for the task.

No effect of age on spatial interval discrimination as a function of eccentricity or separation.

PURPOSE: Performance on positional acuity tasks exhibits marked resistance to the effects of optical image degradation. This places positional acuity tasks in a unique position for psychophysical examination of the effects of age upon visual performance because any observed age-related changes reflect losses in retinal/neural function. One positional acuity task with important consequences for "real-world" vision is spatial interval discrimination, a task in which the subject is required to detect changes in the size of the gap between two objects. In the present study we examine spatial interval discrimination in young and elderly observers as a function of separation and eccentricity. METHODS: Stimuli were two Gaussian-modulated luminance patches placed side by side around an imaginary iso-eccentric arc in the upper visual field, allowing eccentricity and separation to be varied independently. Changes in separation were achieved by moving the stimuli around the arc, while eccentricity was varied by changing the radius of the arc. Thresholds were obtained for healthy young and elderly observers using a forced-choice method of constants at two eccentricities (1.25 degrees and 10 degrees) and five separations. RESULTS: When thresholds, expressed as Weber fractions, are plotted as a function of the geometric ratio of the stimuli (separation/eccentricity) the data from the young and elderly groups collapse to a single function. Performance is independent of age and eccentricity, and depends only on the geometric ratio of the stimuli. CONCLUSIONS: No effect of age was found for spatial interval discrimination. Our results suggest that spatial interval discrimination belongs to the wider group of positional acuities whose neural substrates are unaffected by ageing.

Grating detection and orientation discrimination in amblyopia.

PURPOSE: We examined whether the misperceptions associated with amblyopic visual perception can be revealed under natural viewing conditions by comparing the ability to detect the presence of a grating with the ability to identify the grating orientation. METHODS: Grating detection and orientation discrimination performance (horizontal versus vertical) were determined, using stimuli that consisted of sinusoidal gratings of fixed contrast (75%) but with variable spatial frequency. A total of four amblyopic subjects (two strabismic and two non-strabismic) and four age-matched normals participated in the experiment. RESULTS: Psychometric functions for grating detection and orientation identification were found to be closely matched in the normal subjects and in all four amblyopic subjects, indicating that orientation could be correctly identified at detection threshold. CONCLUSIONS: The absence of orientation uncertainty in the psychophysical data for the amblyopic observers is not consistent with the several previous reports of spatial aliasing in the central field of amblyopes. Our results suggest that non veridical visual perception in central amblyopic vision can not be revealed under natural viewing conditions by comparing the ability to detect the presence of a grating with the ability to identify its orientation. Possible reasons for the failure of this technique to reveal spatial aliasing in amblyopes are discussed.

Centroid analysis predicts visual localization of first- and second-order stimuli.

Perceived alignment of asymmetric Gaussian-windowed stimuli was measured in an attempt to differentiate between stimulus characteristics which might underlie visual localization. These asymmetric stimuli have the advantage of being continuous in the spatial domain and of possessing well-defined spatial characteristics in which centroid, points of inflexion and peak can be separated from each other. Results for both luminance- and contrast-defined stimuli are reasonably well described on the basis that the centroid of the stimulus envelope represents the primitive which determines perceived visual location. Centroid location is inherent in the output of filters which are large enough to cover the object of interest.

Anterior chamber depth measurement in clinical practice.

This study compares the validity of anterior chamber depth estimates provided by a slit-lamp technique with those obtained from pachometry and ultrasonography. The technique provides depth estimates which are valid to within +/- 0.33 mm relative to standard pachometry, and to within +/- 0.42 mm relative to ultrasonography. The technique is simple to perform, requires no extra slit-lamp attachments, and may be carried out on any slit-lamp which has a calibrated variable length of slit.

Assessing retinal/neural function in the presence of ocular media opacities.

Techniques currently available for investigating retinal/neural function in the presence of media opacities have traditionally been assessed in terms of their ability to predict the level of Snellen acuity which will be achieved following therapy. We propose an alternative method of assessment in which techniques are evaluated in terms of their ability to pre-operatively distinguish patients with reduced retinal/neural function from normals. In order to demonstrate this method of assessment, the data from a paper comparing the predictive abilities of a white-light and a laser interferometer in patients undergoing capsulotomy [21] have been re-analyzed. The approach enables fair comparison of the predictive abilities of the various techniques and will therefore help to resolve the issue of which technique provides the best means of assessing retinal/neural function in the presence of media opacities.

Clinical comparison of three techniques for evaluating visual function behind cataract.

The aim of the study was to compare the ability of displacement threshold hyperacuity to identify reduced retinal/neural function in cataract patients with that of the potential acuity meter (PAM) and laser interferometer. Fifty-two patients (mean age 72.0 +/- 10.8 years) admitted for extracapsular cataract extraction were randomly selected for inclusion in the study. Patients underwent routine refraction and logMAR acuity measurement on the eve of cataract surgery. Following pupillary dilation and in random order, the oscillatory displacement threshold (ODT; defined as the smallest detectable displacement of an object) was determined, and predictions of the post-operative acuity were obtained using the PAM and Rodenstock Retinometer (RR). Patients were re-examined approximately 7 weeks post-operatively when logMAR acuities and ODTs were remeasured. On the basis of the results of an independent fundus examination, patients were assigned to either a normal or a pathology group. Increasing cataract density adversely affected the predictions provided by the PAM (p = 0.0001) and ODT (p = 0.0001) techniques, but not the RR (p = 0.137). One-factor ANOVA revealed that pre-operative ODT (p = 0.0001), PAM (p = 0.0001) and RR (p = 0.0004) measures successfully distinguished patients with retinal/neural pathology from normals. Pre-operative cut-off levels to distinguish normal from pathology cases were retrospectively determined for each technique. The cut-off values for the PAM and ODT techniques yielded higher sensitivity and specificity values than was the case for the RR. The results suggest that the predictive abilities of the PAM and ODT techniques are similar, and superior to that of the RR. Further work involving larger numbers of patients with posterior segment pathology is necessary to reveal which technique provides the best means of assessing retinal/neural function in the presence of ocular media opacities.

Assessing retinal/neural function in patients with cataract using oscillatory displacement thresholds.

Oscillatory displacement threshold (ODT) measurement has been proposed as a useful means of assessing retinal/neural function behind cataract. In order to examine the qualitative predictive ability of this technique, the influence of age, simulated cataract, and various posterior segment disorders upon ODT's were investigated. In a separate investigation, thresholds were measured in 30 cataract patients on the eve of surgery, and again 6 to 8 weeks postoperatively. ODT's were found to increase with age (p < 0.001) and were highly resistant to the effects of image degradation as produced by simulated cataract. In addition, thresholds were sensitive to the presence of retinal/neural disorders (p << 0.001). Although not completely independent of the presence of cataract, higher ODT's were measured in cataract patients with coexistent retinal/neural dysfunction than was the case for patients with normal posterior segments (p < 0.001). Preoperative ODT measurement may offer a means of distinguishing between the optical and retinal/neural components of visual impairment in patients with cataract.

Effects of posterior segment disorders on oscillatory displacement thresholds, and on acuities as measured using the potential acuity meter and laser interferometer.

Oscillatory displacement thresholds (ODTs) were measured in 67 subjects with clear ocular media, but known posterior segment disorders. The ability of these thresholds to assess reduced visual function, as quantified by visual acuity deficits, was compared with that of the Potential Acuity Meter (PAM) and the Rodenstock Retinometer (RR). ODTs were found to be sensitive to acuity deficits which were retinal/neural in origin. Despite the fact that interferometric acuities are, unlike ODTs, a measure of resolution, the RR proved no more accurate in assessing the level of letter acuity which existed than did ODT measurement. As would be expected, the PAM provided for the most accurate means of assessing non-optical visual acuity deficits. However, the PAM proved to be less useful in eyes with: very poor levels of acuity, and certain anomalous conditions. The results have implications for the relative ability of each of the three tests to identify reduced visual function behind cataract.