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

Dioptric blur affects grating acuity more than letter acuity for contrast-modulated stimuli

It is valuable to consider the effects of blur, eccentricity and amblyopia on different measures of visual acuity, in particular grating and Snellen acuity (eg. Levi & Klein, 1982; McKee et al., 2003). For luminance stimuli, blur and amblyopia reduces grating acuity less than letter acuity. We examined the effects of dioptric blur on luminance, luminance-modulated (LM) and contrast-modulated (CM) noise gratings and C letter acuity to gain further insight into the visual processing of CM stimuli. Modulation sensitivity functions for standard luminance, LM and CM Gabor patches (0.5–32 c/deg) were measured and cut-off spatial frequencies estimated. Luminance-modulated and contrast-modulated stimuli were created from background dynamic binary noise, which was unscaled (angular size of checks constant for all frequencies) or scaled (6 checks/cycle of modulator). Square C acuity was also determined. Modulation and acuity thresholds were measured using different levels of blur (0–4D) and a method of constant stimuli with 2AFC and 4AFC paradigms. Dioptric blur reduces modulation sensitivity to LM Gabors in a similar fashion to standard Gabors. CM modulation sensitivity is much lower, and the effect of blur is greater, particularly for unscaled noise. The effect of blur on C acuity though is similar for LM and CM stimuli, although the acuity threshold for CM stimuli is about 0.3 logMAR higher at all levels of blur. When comparing grating and letter (crowded or uncrowded) acuities for different levels of blur, whereas for LM stimuli the slope falls from 0.5 to 1.0, for CM stimuli, it can be more than 2.0. That is, blur affects grating acuity more than letter acuity for CM stimuli. This finding is new and suggests that different limits affect CM than LM detection. However letter acuities are similarly affected by blur, suggesting that once extracted, letter acuity is affected by a common limit

Effects of dioptric blur on foveal acuity and contour interaction for noisy Cs

Contour interaction for visual acuity has been found to dissipate with imposed dioptric blur of +1.50D using standard luminance letters and bars (Simmers et al, 1999). It is unclear how dioptric blur affects visual acuity or contour interaction for letters imbedded in noise. We examined the effects of dioptric blur on luminance-modulated and contrast-modulated noise Cs with and without surrounding contours. Stimuli were constructed from random dot dynamic noise, added or multiplied to a square-wave profile. We measured foveal acuity for a square-shaped C with and without four flanking bars separated by 0 to 2 letter widths under different levels of dioptric blur (0-2D with flanks; 0-4D without). Acuity was measured using a method of constant stimuli and 4AFC paradigm for approximately equally visible luminance-modulated and contrast-modulated noisy Cs. Acuity thresholds for isolated contrast-modulated Cs are about 0.3 logMAR higher than for luminance-modulated Cs at all blur levels. The effect of increasing blur on acuity is ~0.3 logMAR/D (from 0 to 4D). The relative peak magnitude of contour interaction for the contrast-modulated C is greater (by ~0.1 logMAR) and more extensive, than for the luminance-modulated C. Dioptric blur up to 2D raises both types of isolated C acuity thresholds similarly, but affects their contour interaction parameters differently. The magnitude of the peak effect is directly related to the unflanked logMAR threshold (or letter size). However with increasing blur, the extent of interaction is maintained for the contrast-modulated C, but reduced significantly for the luminance-modulated C (as for standard luminance letters; Simmers et al, 1999). For these blur levels, contrast-modulated Cs are more subject to contour interaction effects than luminance-modulated Cs. This could be a result of larger integration areas for contrast-modulated stimuli and a differential effect of blur on contrast and luminance modulation sensitivity functions

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

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

Effect of blur in colour discrimination

[Excerpt] Blur impairs colour discrimination but this impairment has not been systematically investigated across hue. In this work we obtained colour discrimination thresholds for a number of hue angles, stimulus sizes and blur levels. A colour discrimination task (Linhares et al., 2016 JOSA A 33(3): A178-A83) was performed on a calibrated CRT monitor controlled by a ViSaGe-MKII. A square chromatic target was presented on an achromatic static luminance noise background. Observers indicated the location of the square (right or left). [...]info:eu-repo/semantics/publishedVersio

The effect of photopigment optical density on the color vision of the anomalous trichromat

AbstractWe present a theoretical model to estimate the influence of photopigment optical density (OD) on the color vision of anomalous trichromats. Photopigment spectral sensitivities are generated using the Lamb (1995) template, which we correct for OD and pre-receptoral filters. Sixteen hyperspectral images (Foster, Nascimento, & Amano, 2004; Nascimento, Ferreira, & Foster, 2002) are analyzed, and the signals produced in the post-receptoral channels calculated. In the case of anomalous trichromats whose two longer-wavelength cones have peak sensitivities that lie close together in the spectrum, color vision can be substantially enhanced if the cones differ in optical density by a realistic amount