Ocular accommodation and wavelength: The effect of longitudinal chromatic aberration on the stimulus–response curve by Fernandez-Alonso M, Finch AP, Love GD, Read JCA, FernandezAlonsoEA2024.pdf (4.8 MiB) - The longitudinal chromatic aberration (LCA) of the eye creates a chromatic blur on the retina that is an important cue for accommodation. Although this mechanism can work optimally in broadband illuminants such as daylight, it is not clear how the system responds to the narrowband illuminants used by many modern displays. Here, we measured pupil and accommodative responses as well as visual acuity under narrowband light-emitting diode (LED) illuminants of different peak wavelengths. Observers were able to accommodate under narrowband light and compensate for the LCA of the eye, with no difference in the variability of the steady-state accommodation response between narrowband and broadband illuminants. Intriguingly, our subjects compensated more fully for LCA at nearer distances. That is, the difference in accommodation to different wavelengths became larger when the object was placed nearer the observer, causing the slope of the accommodation response curve to become shallower for shorter wavelengths and steeper for longer ones. Within the accommodative range of observers, accommodative errors were small and visual acuity normal. When comparing between illuminants, when accommodation was accurate, visual acuity was worst for blue narrowband light. This cannot be due to the sparser spacing for S-cones, as our stimuli had equal luminance and thus activated LM-cones roughly equally. It is likely because ocular LCA changes more rapidly at shorter wavelength and so the finite spectral bandwidth of LEDs corresponds to a greater dioptric range at shorter wavelengths. This effect disappears for larger accommodative errors, due to the increased depth of focus of the eye.
Understanding accommodative control in the clinic: Modeling latency and amplitude for uncorrected refractive error, presbyopia and cycloplegia by Read JCA, Maus G, Schor CM, ReadMausSchor2024.pdf (3.9 MiB) - Accommodation is the process of adjusting the eye's optical power so as to focus at different distances. Uncorrected refractive error and/or functional presbyopia mean that sharp focus may not be achievable for some distances, so observers experience sustained defocus. Here, we identify a problem with current models of accommodative control: They predict excessive internal responses to stimuli outside accommodative range, leading to unrealistic adaptation effects. Specifically, after prolonged exposure to stimuli outside range, current models predict long latencies in the accommodative response to stimuli within range, as well as unrealistic dynamics and amplitudes of accommodative vergence innervation driven by the accommodative neural controller. These behaviors are not observed empirically. To solve this issue, we propose that the input to blur-driven accommodation is not retinal defocus, but correctable defocus. Predictive models of accommodative control already estimate demand from sensed defocus, using a realistic “virtual plant” to estimate accommodation. Correctable defocus can be obtained by restricting this demand to values physically attainable by the eye. If we further postulate that correctable defocus is computed using an idealized virtual plant that retains a young accommodative range, we can explain why accommodative–convergence responses are observed for stimuli that are too near—but not too far—to focus on. We model cycloplegia as a change in gain, and postulate a form of neural myopia to explain the additional relaxation of accommodation often seen with cycloplegia. This model produces plausible predictions for the accommodative response and accommodative convergence signal in a wide range of clinically relevant situations
Peripheral Flicker Fusion at High Luminance: Beyond the Ferry–Porter Law by Fernandez-Alonso M, Innes W, Read JCA, FernandezAlonsoInnesRead2023.pdf (0.9 MiB) - The relationship between luminous intensity and the maximum frequency of flicker that can be detected defines the limits of the temporal-resolving ability of the human visual system, and characterizing it has important theoretical and practical applications; particularly for determining the optimal refresh rate for visual displays that would avoid the visibility of flicker and other temporal artifacts. Previous research has shown that this relationship is best described by the Ferry–Porter law, which states that critical flicker fusion (CFF) increases as a linear function of log retinal illuminance. The existing experimental data showed that this law holds for a wide range of stimuli and up to 10,000 Trolands; however, beyond this, it was not clear if the CFF continued to increase linearly or if the function saturated. Our aim was to extend the experimental data available to higher light intensities than previously reported in the literature. For this, we measured the peripheral CFF at a range of illuminances over six orders of magnitude. Our results showed that for up to 104 Trolands, the data conformed to the Ferry–Porter law with a similar slope, as previously established for this eccentricity; however, at higher intensities, the CFF function flattens and saturates at ~90 Hz for a target size of 5.7 degrees, and at ~100 Hz for a target of 10 degrees of angular size. These experimental results could prove valuable for the design of brighter visual displays and illumination sources that are temporally modulated.
Extending the Human Foveal Spatial Contrast Sensitivity Function to High Luminance Range by Kaspiris-Rousellis C, Fernandez-Alonso M, Read JCA, KaspirisRousellisFernandezAlonsoRead2019.pdf (3.4 MiB) - The human contrast sensitivity function (CSF) is the most general way of quantifying what human vision can perceive. It predicts which artifacts will be visible on a display and what changes to hardware will result in noticeable improvements. Contrast sensitivity varies with luminance, and as new technology is producing higher luminance range displays, it is becoming essential to understand how the CSF behaves in this regime. Following this direction, we investigated the effect of adaptation luminance on contrast sensitivity for sine-wave gratings over a large number of CSF measurements in the literature. We examined the validity of the linear to DeVries-Rose to Weber region transition that is usually assumed to predict this relationship. We found a gradual transition among the three regions with steeper/flatter slopes for higher/lower frequencies and lower/higher retinal illuminance. A further decreasing region was located at low to intermediate frequencies, which was consistent across studies. Based on this theoretical construct, we adopted a CSF model consisting of central elements in the human visual signal processing and three limiting internal noise components corresponding to each region. We assessed the model’s performance on the measured contrast sensitivities and proposed an eight-parameter form to describe the contrast sensitivity surface in the spatial frequency-luminance domain.
Assessment of Psychophysical Methods for Measuring the Critical Flicker Fusion Frequency in Yes/No Tasks by Fernandez-Alonso M, Kaspiris-Rousellis C, Innes W, Read JCA, FernandezAlonsoKaspirisRousellisInnesRead2019.pdf (0.5 MiB) - The Critical Flicker Fusion (CFF) threshold is widely used to evaluate the limits of visual temporal processing and has important practical applications in the field of display technologies. In this study, we evaluate the suitability of a novel adaptive psychophysical procedure for measuring CFF thresholds in a YES/NO task. Our results indicate that while the adaptive staircase procedure has high repeatability and is of shorter
duration when compared to the more robust constant stimuli method, its accuracy is lower, giving thresholds that were significantly higher (p<0.01) by approximately 15Hz.
ASTEROID: A New Clinical Stereotest on an Autostereo 3D Tablet by Vancleef K, Serrano-Pedraza I, Sharp C, Slack G, Black C, Casanova T, Hugill J, Rafiq S, Burridge J, Puyat V, Ewane Enongue J, Gale H, Akotei H, Collier Z, Haggerty H, Smart K, Powell C, Taylor K, Clarke MP, Morgan G, Read JCA, VancleefEA_ASTEROIDMethods.pdf (1.9 MiB) - Purpose: To describe a new stereotest in the form of a game on an autostereoscopic
tablet computer designed to be suitable for use in the eye clinic and present data on
its reliability and the distribution of stereo thresholds in adults.
Methods: Test stimuli were four dynamic random-dot stereograms, one of which
contained a disparate target. Feedback was given after each trial presentation. A
Bayesian adaptive staircase adjusted target disparity. Threshold was estimated from the
mean of the posterior distribution after 20 responses. Viewing distance was monitored
via a forehead sticker viewed by the tablet’s front camera, and screen parallax was
adjusted dynamically so as to achieve the desired retinal disparity.
Results: The tablet must be viewed at a distance of greater than ~35 cm to produce a
good depth percept. Log thresholds were roughly normally distributed with a mean
of 1.75 log10 arcsec ¼ 56 arcsec and SD of 0.34 log10 arcsec ¼ a factor of 2.2. The
standard deviation agrees with previous studies, but ASTEROID thresholds are
approximately 1.5 times higher than a similar stereotest on stereoscopic 3D TV or on
Randot Preschool stereotests. Pearson correlation between successive tests in same
observer was 0.80. Bland-Altman 95% limits of reliability were 60.64 log10 arcsec ¼ a
factor of 4.3, corresponding to an SD of 0.32 log10 arcsec on individual threshold
estimates. This is similar to other stereotests and close to the statistical limit for 20
responses.
Conclusions: ASTEROID is reliable, easy, and portable and thus well-suited for clinical
stereoacuity measurements.
Translational Relevance: New 3D digital technology means that research-quality
psychophysical measurement of stereoacuity is now feasible in the clinic.
True stereoscopic 3D cannot be simulated by shifting 2D content off the screen plane by Hands P, Read JCA, HandsRead2017.pdf (0.9 MiB) - money by including brief sections of 2D content displayed with a uniform disparity, i.e. the 2D image is
geometrically shifted behind the screen plane. This manipulation is believed to produce an illusion of
depth which, while not as powerful as true S3D, is nevertheless more compelling than simple 2D. Our
study examined whether this belief is correct. 30 s clips from a nature documentary were shown in
the original S3D, in ordinary 2D and in shifted versions of S3D and 2D. Participants were asked to determine
the impression of depth on a 7 point Likert scale. There was a clear and highly significant difference
between the S3D depth perception (mean 6.03) and the shifted 2D depth perception (mean 4.13)
(P = 0.002, ANOVA). There was no difference between ordinary 2D presented on the screen plane, and
the shifted 2D. We conclude that the shifted 2D method not only fails to mimic the depth effect of true
S3D, it in fact has no benefit over ordinary 2D in terms of the depth illusion created. This could impact
viewing habits of people who notice the difference in depth quality.
Avoiding monocular artifacts in clinical stereotests presented on column-interleaved digital stereoscopic displays by Serrano-Pedraza I, Vancleef K, Read JCA, SerranoPedrazaVancleefRead.pdf (1.5 MiB) - New forms of stereoscopic 3-D technology offer vision
scientists new opportunities for research, but also
come with distinct problems. Here we consider
autostereo displays where the two eyes’ images are
spatially interleaved in alternating columns of pixels
and no glasses or special optics are required. Columninterleaved
displays produce an excellent stereoscopic
effect, but subtle changes in the angle of view can
increase cross talk or even interchange the left and
right eyes’ images. This creates several challenges to
the presentation of cyclopean stereograms (containing
structure which is only detectable by binocular vision).
We discuss the potential artifacts, including one that is
unique to column-interleaved displays, whereby scene
elements such as dots in a random-dot stereogram
appear wider or narrower depending on the sign of
their disparity. We derive an algorithm for creating
stimuli which are free from this artifact.We show that
this and other artifacts can be avoided by (a) using a
task which is robust to disparity-sign inversion—for
example, a disparity-detection rather than
discrimination task—(b) using our proposed algorithm
to ensure that parallax is applied symmetrically on the
column-interleaved display, and (c) using a dynamic
stimulus to avoid monocular artifacts from motion
parallax. In order to test our recommendations, we
performed two experiments using a stereoacuity task
implemented with a parallax-barrier tablet. Our
results confirm that these recommendations eliminate
the artifacts. We believe that these recommendations
will be useful to vision scientists interested in running
stereo psychophysics experiments using parallaxbarrier
and other column-interleaved digital displays
Viewing 3D TV over two months produces no discernible effects on balance, coordination or eyesight. by Read JCA, Godfrey A, Bohr I, SImonotto J, Galna B, Smulders TV, ReadGodfreyBohrSimonottoGalnaSmulders2016.pdf (2.2 MiB) - With the rise in stereoscopic 3D media, there has been concern that viewing stereoscopic 3D (S3D) content could have long-term adverse effects, but little data are available. In the first study to address this, 28 households who did not currently own a 3D TV were given a new TV set, either S3D or 2D. The 116 members of these households all underwent tests of balance, coordination and eyesight, both before they received their new TV set, and after they had owned it for 2 months. We did not detect any changes which appeared to be associated with viewing 3D TV. We conclude that viewing 3D TV does not produce detectable effects on balance, coordination or eyesight over the timescale studied. Practitioner Summary: Concern has been expressed over possible long-term effects of stereoscopic 3D (S3D). We looked for any changes in vision, balance and coordination associated with normal home S3D TV viewing in the 2 months after first acquiring a 3D TV. We find no evidence of any changes over this timescale.
