167 research outputs found
Crowding and visual acuity measured in adults using paediatric test letters, pictures and symbols
Crowding refers to the degradation of visual acuity for target optotypes with, versus without, surrounding
features. Crowding is important clinically, however the effect of target-flanker spacing on acuity for
symbols and pictures, compared to letters, has not been investigated. Five adults with corrected-tonormal
vision had visual acuity measured for modified single target versions of Kay Pictures, Lea
Symbols, HOTV and Cambridge Crowding Cards, tests. Single optotypes were presented in isolation
and with surrounding features placed 0–5 stroke-widths away. Visual acuity measured with Kay
Picture optotypes is 0.13–0.19 logMAR better than for other test optotypes and varies significantly across
picture. The magnitude of crowding is strongest when the surrounding features abut, or are placed 1
stroke-width away from the target optotype. The slope of the psychometric function is steeper in the
region just beyond maximum crowding. Crowding is strongest and the psychometric function steepest,
with the Cambridge Crowding Cards arrangement, than when any single optotype is surrounded by a
box. Estimates of crowding extent are less variable across test when expressed in units of strokewidth,
than optotype-width. Crowding for single target presentations of letters, symbols and pictures
used in paediatric visual acuity tests can be maximised and made more sensitive to change in visual acuity,
by careful selection of optotype, by surrounding the target with similar flankers, and by using a closer
target-flanker separation than half an optotype-width
Very few exclusive percepts for contrast-modulated stimuli during binocular rivalry
Binocular rivalry properties for contrast-modulated (CM) gratings were examined to gain insight into
their locus of processing. Two orthogonally orientated gratings were presented, one to each eye.
Perceptual change rates, proportions of exclusivity and mixed percepts, and mean durations were calculated.
Stimuli were noiseless luminance-defined (L), luminance-modulated noise (LM) and contrastmodulated
noise (CM) gratings with sizes of 1, 2 and 4 deg and spatial frequencies of 4, 2 and 1 c/deg,
respectively. For the LM and CM gratings, binary noise was fully correlated between eyes. Maximum producible
modulations were used (1.0 for CM, 0.78 for LM and 0.98 for L stimuli). In a control experiment,
contrasts of LM gratings were reduced until the multiples over detection threshold were similar to those
of CM stimuli. Trial durations of 120 s were analyzed. Exclusive visibility decreased with increasing stimulus
size regardless of the stimulus type. Even with visibilities at similar multiples above detection
threshold, significantly lower proportions of exclusive percepts and perceptual changes were found for
CM, compared to LM gratings. The results obtained with dichoptically presented orthogonal CM gratings
are significantly different from those obtained for orthogonal gratings presented to one eye. CM stimuli
therefore do engage in binocular rivalry but with different characteristics to those found for LM stimuli.
These results suggest that CM stimuli are processed by a mechanism that promotes binocular combination
rather than rivalry, and therefore may involve cells in a higher visual area than those that initially
process LM information
Monocular microsaccades are visual-task related
During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks
Assessment of compensation and simulation filters for colour vision deficiency
[Excerpt] Compensation filters may improve perception in humans with
Red-Green colour vision deficiency (CVD) whilst simulation filters
may mimic CVD in colour vision normals (CVN). A previous study
(Lillo et al. 2014) showed that Variantor simulation filters mimic
protanopia in a discrimination task when tested only at pseudoachromatic confusion axes. Studies (e.g. Gómez-Robledo et al.
2018) using EnChroma compensation filters (EnChroma, Berkeley,
USA) have not found statistically significant improvements in
colour vision. We aimed to assess effects of simulation (Variantor)
and compensation (EnChroma) filters on perceptual performance
in discrimination and colour naming tasks.- This work was supported by an Evelyn Trust Grant (to SJW) and HEFCE QR (Quality Related) Funds (to Anglia Vision Research) to support a Postdoctoral Research Fellow (LA) and visits (LA and JMML) to the laboratories of Anglia Vision Research, to facilitate completion of this project. The authors thank Ashley Gray (a research assistant supported by the QR fund), Emily Mailman and Laura Douds (who were undergraduate Optometry students) for their help with data collection
Foveal visual acuity is worse and shows stronger contour interaction effects for contrast-modulated than luminance-modulated Cs
Contrast-modulated (CM) stimuli are processed by spatial mechanisms that operate at larger spatial scales than those processing luminance-modulated (LM) stimuli and may be more prone to deficits in developing, amblyopic, and aging visual systems. Understanding neural mechanisms of contour interaction or crowding will help in detecting disorders of spatial vision. In this study, contour interaction effects on visual acuity for LM and CM C and bar stimuli are assessed in normal foveal vision. In Experiment 1, visual acuity is measured for all-LM and all-CM stimuli, at ~3.5× above their respective modulation thresholds. In Experiment 2, visual acuity is measured for Cs and bars of different type (LM C with CM bars and vice versa). Visual acuity is degraded for CM compared with LM Cs (0.46 ± 0.04 logMAR vs. 0.18 ± 0.04 logMAR). With nearby bars, CM acuity is degraded further (0.23 ± 0.01 logMAR or ~2 lines on an acuity chart), significantly more than LM acuity (0.11 ± 0.01 logMAR, ~1 line). Contour interaction for CM stimuli extends over greater distances (arcmin) than it does for LM stimuli, but extents are similar with respect to acuities (~3.5× the C gap width). Contour interaction is evident when the Cs and bars are defined differently: it is stronger when an LM C is flanked by CM bars (0.17 ± 0.03 logMAR) than when a CM C is flanked by LM bars (0.08 ± 0.02 logMAR). Our results suggest that contour interaction for foveally viewed acuity stimuli involves feature integration, such that the outputs of receptive fields representing Cs and bars are combined. Contour interaction operates at LM and CM representational stages, it can occur across stage, and it is enhanced at the CM stage. Greater contour interaction for CM Cs and bars could hold value for visual acuity testing and earlier diagnosis of conditions for which crowding is important, such as in amblyopia
Contour interaction for foveal acuity targets at different luminances
Single-letter visual acuity is impaired by nearby flanking stimuli, a phenomenon known as contour interaction. We showed previously that when foveal acuity is degraded by a reduction of letter contrast, both the magnitude and angular spatial extent of foveal contour interaction remain unchanged. In this study, we asked whether contour interaction also remains unchanged when foveal visual acuity is degraded by a reduction of the target’s background luminance.
Percent correct letter identification was measured for isolated, near-threshold black Sloan letters and for letters surrounded by 4 flanking bars in 10 normal observers, 5 at Anglia Ruskin University, UK (ARU) and 5 at Palacky University, Czech Republic (PU). A stepwise reduction in the background luminance over 3 log units resulted in an approximately threefold increase in the near-threshold letter size. At each background luminance, black flanking bars with a width equal to 1 letter stroke were presented at separations between approximately 0.45 and 4.5 min arc (ARU) or 0.32 and 3.2 min arc (PU).
The results indicate that the angular extent of contour interaction remains unchanged at approximately 4 min arc at all background luminances. On the other hand, the magnitude of contour interaction decreases systematically as luminance is reduced, from approximately a 50% reduction to a 30% reduction in percent correct. The constant angular extent and decreasing magnitude of contour interaction with a reduction of background luminance suggest foveal contour interaction is mediated by luminance-dependent lateral inhibition within a fixed angular region
More superimposition for contrast-modulated than luminance-modulated stimuli during binocular rivalry
Luminance-modulated noise (LM) and contrast-modulated noise (CM) gratings were presented with interocularly correlated, uncorrelated and anti-correlated binary noise to investigate their contributions to mixed percepts, specifically piecemeal and superimposition, during binocular rivalry. Stimuli were sine-wave gratings of 2c/deg presented within 2 deg circular apertures. The LM stimulus contrast was 0.1 and the CM stimulus modulation depth was 1.0, equating to approximately 5 and 7 times detection threshold, respectively. Twelve 45s trials, per noise configuration, were carried out. Fifteen participants with normal vision indicated via button presses whether an exclusive, piecemeal or superimposed percept was seen. For all noise conditions LM stimuli generated more exclusive visibility, and lower proportions of superimposition. CM stimuli led to greater proportions and longer periods of superimposition. For both stimulus types, correlated interocular noise generated more superimposition than did anti- or uncorrelated interocular noise. No significant effect of stimulus type (LM vs CM) or noise configuration (correlated, uncorrelated, anti-correlated) on piecemeal perception was found. Exclusive visibility was greater in proportion, and perceptual changes more numerous, during binocular rivalry for CM stimuli when interocular noise was not correlated. This suggests that mutual inhibition, initiated by non-correlated noise CM gratings, occurs between neurons processing luminance noise (first-order component), as well as those processing gratings (second-order component). Therefore, first- and second-order components can contribute to overall binocular rivalry responses. We suggest the addition of a new well to the current energy landscape model for binocular rivalry that takes superimposition into account
Foveal contour interaction on the edge: Response to 'Letter-to-the-Editor' by Drs. Coates and Levi
Recently, we reported that, when considered as a function of the edge-to-edge target-toflanker
separation in min arc, the spatial extent of foveal contour interaction is the same for high and
low contrast acuity targets. This result resolved an apparent discrepancy in the literature, which
suggested that foveal contour interaction was absent or reduced for low contrast targets. In
commenting on our results, Drs. Coates and Levi suggest a two-mechanism model for foveal crowding that depends on the center-to-center separation between the acuity target and flanking stimuli, and is based in part on a reanalysis of data from our recent work and a number of other studies. In our reply, we show that the spatial extent of foveal contour interaction for both high and low contrast targets is essentially unchanged by the width of the flanking targets when the target-to-flanker separation is depicted in terms of edge-to-edge separation, but varies systematically when depicted in terms of center-to-center separation. We therefore conclude that for foveal contour interaction in the range of a few min arc, edge-to-edge target-to-flanker separation is the more appropriate metric
Coloured filters can simulate colour deficiency in normal vision but cannot compensate for congenital colour vision deficiency
Red-green colour vision deficiency (CVD) affects ~ 4% of Caucasians. Notch filters exist to simulate CVD when worn by colour vision normal (CVN) observers (simulation tools), or to improve colour discrimination when worn by CVD observers (compensation tools). The current study assesses effects of simulation (Variantor) and compensation (EnChroma) filters on performance in a variety of tasks. Experiments were conducted on 20 CVN and 16 CVD participants under no-filter and filter conditions (5 CVN used Variantor; 15 CVN and 16 CVD used EnChroma). Participants were tested on Ishihara and Farnsworth-Munsell 100 hue tests, CVA-UMinho colour discrimination and colour naming tasks and a board-game colour-sorting task. Repeated-measures ANOVAs found Variantor filters to significantly worsen CVN performance, mimicking protanopia. Mixed-model and repeated-measures ANOVAs demonstrate that EnChroma filters do not significantly enhance performance in CVD observers. Key EnChroma results were replicated in 8 CVD children (Ishihara test) and a sub-sample of 6 CVD adults (CVA-UMinho colour discrimination and colour naming tasks) for a smaller stimulus size. Pattern similarity exists across hue for discrimination thresholds and naming errors. Variantor filters are effective at mimicking congenital colour vision defects in CVN observers for all tasks, however EnChroma filters do not significantly compensate for CVD in any
Foveal contour interaction for low contrast acuity targets
Previous investigators reported the impairment of foveal visual acuity by nearby flanking targets (contour interaction) is reduced or eliminated when acuity is measured using low contrast targets. Unlike earlier studies, we compared contour interaction for high and low contrast acuity targets using flankers at fixed angular separations, rather than at specific multiples of the acuity target’s stroke width. Percent correct letter identification was determined in 4 adult observers for computer generated, high and low contrast dark Sloan letters surrounded by 4 equal contrast flanking bars. Two low contrast targets were selected to reduce each observer’s visual acuity by 0.2 and 0.4 logMAR. The crowding functions measured for high and low contrast letters are very similar when percent correct letter identification is plotted against the flanker separation in min arc. These results indicate that contour interaction of foveal acuity targets occurs within a fixed angular zone of a few min arc, regardless of the size or contrast of the acuity target
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