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.

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.