Should prospective dental students be screened for colour vision deficits?

A perennial question in dental education is: what skills and aptitudes should be assessed in prospective dental students? Intellectual capacity and manual dexterity understandably rank highly, but are there minimum thresholds for visual perception that applicants need be able to demonstrate before they enter the profession? We have recently flagged this issue with regard to the thresholds of stereoscopic acuity required for a dentist when operating on teeth. In the present article, we highlight the issue of identifying a minimum acceptable level of colour vision

Vision Is Adapted to the Natural Level of Blur Present in the Retinal Image

Background The image formed by the eye's optics is inherently blurred by aberrations specific to an individual's eyes. We examined how visual coding is adapted to the optical quality of the eye.Methods and Findings We assessed the relationship between perceived blur and the retinal image blur resulting from high order aberrations in an individual's optics. Observers judged perceptual blur in a psychophysical two-alternative forced choice paradigm, on stimuli viewed through perfectly corrected optics (using a deformable mirror to compensate for the individual's aberrations). Realistic blur of different amounts and forms was computer simulated using real aberrations from a population. The blur levels perceived as best focused were close to the levels predicted by an individual's high order aberrations over a wide range of blur magnitudes, and were systematically biased when observers were instead adapted to the blur reproduced from a different observer's eye.Conclusions Our results provide strong evidence that spatial vision is calibrated for the specific blur levels present in each individual's retinal image and that this adaptation at least partly reflects how spatial sensitivity is normalized in the neural coding of blur.This work was supported by the following: Ministerio de Ciencia e Innovación (MICINN), Formación de Personal Investigador (FPI) Predoctoral Fellowship to LS; Consejo Superior de Investigaciones Científicas (CSIC) I3P Predoctoral Fellowship to PdG; EY-10834 to MW; MICINN FIS2008-02065 and EURYI-05-102-ES (European Heads of Research Councils-European Science Foundation EUROHORCs-ESF) to SM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewe

Habitual wearers of colored lenses adapt more rapidly to the color changes the lenses produce

The visual system continuously adapts to the environment, allowing it to perform optimally in a changing visual world. One large change occurs every time one takes off or puts on a pair of spectacles. It would be advantageous for the visual system to learn to adapt particularly rapidly to such large, commonly occurring events, but whether it can do so remains unknown. Here, we tested whether people who routinely wear spectacles with colored lenses increase how rapidly they adapt to the color shifts their lenses produce. Adaptation to a global color shift causes the appearance of a test color to change. We measured changes in the color that appeared “unique yellow”, that is neither reddish nor greenish, as subjects donned and removed their spectacles. Nine habitual wearers and nine age-matched control subjects judged the color of a small monochromatic test light presented with a large, uniform, whitish surround every 5 s. Red lenses shifted unique yellow to more reddish colors (longer wavelengths), and greenish lenses shifted it to more greenish colors (shorter wavelengths), consistent with adaptation “normalizing” the appearance of the world. In controls, the time course of this adaptation contained a large, rapid component and a smaller gradual one, in agreement with prior results. Critically, in habitual wearers the rapid component was significantly larger, and the gradual component significantly smaller than in controls. The total amount of adaptation was also larger in habitual wearers than in controls. These data suggest strongly that the visual system adapts with increasing rapidity and strength as environments are encountered repeatedly over time. An additional unexpected finding was that baseline unique yellow shifted in a direction opposite to that produced by the habitually worn lenses. Overall, our results represent one of the first formal reports that adjusting to putting on or taking off spectacles becomes easier over time, and may have important implications for clinical management

Visual adaptation alters the apparent speed of real-world actions

The apparent physical speed of an object in the field of view remains constant despite variations in retinal velocity due to viewing conditions (velocity constancy). For example, people and cars appear to move across the field of view at the same objective speed regardless of distance. In this study a series of experiments investigated the visual processes underpinning judgements of objective speed using an adaptation paradigm and video recordings of natural human locomotion. Viewing a video played in slow-motion for 30seconds caused participants to perceive subsequently viewed clips played at standard speed as too fast, so playback had to be slowed down in order for it to appear natural; conversely after viewing fast-forward videos for 30seconds, playback had to be speeded up in order to appear natural. The perceived speed of locomotion shifted towards the speed depicted in the adapting video (‘re-normalisation’). Results were qualitatively different from those obtained in previously reported studies of retinal velocity adaptation. Adapting videos that were scrambled to remove recognizable human figures or coherent motion caused significant, though smaller shifts in apparent locomotion speed, indicating that both low-level and high-level visual properties of the adapting stimulus contributed to the changes in apparent speed

Assessment of #TheDress With Traditional Color Vision Tests: Perception Differences Are Associated With Blueness

Based on known color vision theories, there is no complete explanation for the perceptual dichotomy of #TheDress in which most people see either white-and-gold (WG) or blue-and-black (BK). We determined whether some standard color vision tests (i.e., color naming, color matching, anomaloscope settings, unique white settings, and color preferences), as well as chronotypes, could provide information on the color perceptions of #TheDress. Fifty-two young observers were tested. Fifteen of the observers (29%) reported the colors as BK, 21 (40%) as WG, and 16 (31%) reported a different combination of colors. Observers who perceived WG required significantly more blue in their unique white settings than those who perceived BK. The BK, blue-and-gold, and WG observer groups had significantly different color preferences for the light cyan chip. Moreland equation anomaloscope matching showed a significant difference between WG and BK observers. In addition, #TheDress color perception categories, color preference outcomes, and unique white settings had a common association. For both the bright and dark regions of #TheDress, the color matching chromaticities formed a continuum, approximately following the daylight chromaticity locus. Color matching to the bright region of #TheDress showed two nearly distinct clusters (WG vs. BK) along the daylight chromaticity locus and there was a clear cutoff for reporting WG versus BK. All results showing a significant difference involved blue percepts, possibly due to interpretations of the illuminant interactions with the dress material. This suggests that variations in attributing blueness to the #TheDress image may be significant variables determining color perception of #TheDress.Fil: Feitosa-Santana, Claudia. Universidade Federal do ABC; BrasilFil: Lutze, Margaret. Depaul University; Estados UnidosFil: Barrionuevo, Pablo Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Investigación en Luz, Ambiente y Visión. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Instituto de Investigación en Luz, Ambiente y Visión; ArgentinaFil: Cao, Dingcai. University of Illinois; Estados Unido

Visual adaptation and face perception

The appearance of faces can be strongly affected by the characteristics of faces viewed previously. These perceptual after-effects reflect processes of sensory adaptation that are found throughout the visual system, but which have been considered only relatively recently in the context of higher level perceptual judgements. In this review, we explore the consequences of adaptation for human face perception, and the implications of adaptation for understanding the neural-coding schemes underlying the visual representation of faces. The properties of face after-effects suggest that they, in part, reflect response changes at high and possibly face-specific levels of visual processing. Yet, the form of the after-effects and the norm-based codes that they point to show many parallels with the adaptations and functional organization that are thought to underlie the encoding of perceptual attributes like colour. The nature and basis for human colour vision have been studied extensively, and we draw on ideas and principles that have been developed to account for norms and normalization in colour vision to consider potential similarities and differences in the representation and adaptation of faces

Color perception in anomalous trichromats: Neuroimaging investigations of neural compensation for losses in spectral sensitivity

Anomalous trichromats have reduced sensitivity to the L-M dimension of color space due to the reduced separation between the spectral sensitivities of their L and M cones. Despite this, previous work suggests that these observers may perceive the world to be much more colorful than their cone sensitivities would predict, potentially because of long-term adaptation that amplifies the weakened chromatic signals provided by the cones. Most of the evidence for this gain adjustment rests on subjective measures of color appearance or color salience. In the present study, we tested for neural correlates of color compensation by using fMRI to compare the cortical responses to chromatic stimuli in normal and anomalous observers. Thresholds were collected for a total of 7 anomalous trichromats (3 deuteranomals and 4 protanomals), and 6 color normal controls. Initial results showed that chromatic thresholds for the L-M axis did not predict BOLD responses, indicating neural compensation in early visual areas. In an additional experiment, we used an attentionally demanding task to ensure that top-down influences were limited. We also collected retinotopic mapping in order to independently define early visual areas (V1, V2, V3, and hV4). In this case, the group-averaged BOLD responses to L-M stimuli were not significantly greater than responses predicted by threshold, but individual participants did show evidence of compensation. The same was true when responses were normalized to responses to S-axis stimuli. Our results thus provide evidence for compensatory amplification, but suggest that the degree of compensation varies across individuals

Coding Strategies Underlying Visual Processing

Acquiring and representing knowledge about our environment involves a variety of core neural computations. The coding strategies underlying visual perception highlight many of these processes, and thus reveal general design principles in perception and cognition. I will review three studies where I have used different computational frameworks and analyses to address open questions in visual coding. The first project uses factor analyses of individual differences in perception to demonstrate fundamentally different representational structures for the stimulus features of color and motion. In the second project, I have explored visual adaptation in the context of population coding to address controversies regarding which coding schemes are implicated by different patterns of adaptation aftereffects. In the third, I have explored these adaptation effects in the context of Bayesian inference. This approach accounts for the full gamut of known aftereffects within the context of physiologically plausible models and provides principled quantitative predictions for why and how much the system should adapt. Together, these projects draw on the power of formal computational approaches both for analyzing neural representations and for revealing the computations and coding principles on which they are based

Adaptation and the Perception of Radiological Images

Radiologists must classify and interpret medical images on the basis of visual inspection. We examined how an observer's visual sensitivity and perception might change as they view and thus adapt to the characteristic properties of radiological scans. Measurements were focused on the effects of adaptation to images of normal mammograms, and were tested primarily in observers who were not trained radiologists. Mammograms have steeper power spectra (slopes of ~-3) than natural images (~-2) and thus are physically blurry. Adapting to them produced shifts in the perceived spectrum of filtered noise consistent with adaptation to blur, even though this adaptation does not lead to measurable changes in the contrast sensitivity function. Strong aftereffects in the appearance of the images were also found when observers judged the perceived texture of the images. For example, tissue density in mammograms is routinely classified and ranges from "dense" to "fatty." Adaptation to dense images caused an intermediate image to appear more fatty and vice versa. Our results thus suggest that observers can selectively adapt to the properties of radiological images, and this could potentially be an important factor in the perception and learning of radiological images. In a further study we explored whether adaptation could enhance visual inspection of radiological images, specifically to aid observers in identifying abnormalities by adapting out or discounting the expected visual characteristics of the background. Observers searched for simulated lesions (Gaussian targets) added at random locations in the images. Prior adaptation to the images allowed the targets to be located more quickly, and this performance gain was selective for the tissue type and thus the visual texture defining the background. These improvements in visual search provide a novel demonstration of the advantages of spatial pattern adaptation within contexts that closely mimic routine visual tasks and settings. Finally, we explored the neural correlates of these adaptation aftereffects by measuring ERP's while observers adapted to the different textural properties of the mammogram images (dense or fatty). There was no significant difference of adapt condition in the component waveforms when tasked with categorizing the scans based upon their density classifications. In contrast, there was a significant effect of adaptation when observers were signaling target presence or absence. This significant difference was characterized by an enhancement of the neural response at early timepoints in occipital areas. Additionally, following adaptation we observed a divergence in the target present and absent waveforms at approximately 370 ms post-stimulus onset in frontal recording sites. These results suggest that target detection involves a form of the P300 component. Taken together these studies represent the first comprehensive analysis of the influence of adaption on the critically important visual judgments involved in interpreting and inspecting medical images. &#8195

Object Image Size Is a Fundamental Coding Dimension in Human Vision: New Insights and Model

In previous psychophysical work we found that luminance contrast is integrated over retinal area subject to contrast gain control. If different mechanisms perform this operation for a range of superimposed retinal regions of different sizes, this could provide the basis for size-coding. To test this idea we included two novel features in a standard adaptation paradigm to discount more pedestrian accounts of repulsive size-aftereffects. First, we used spatially jittering luminance-contrast adaptors to avoid simple contour displacement aftereffects. Second, we decoupled adaptor and target spatial frequency to avoid the well-known spatial frequency shift aftereffect. Empirical results indicated strong evidence of a bidirectional size adaptation aftereffect. We show that the textbook population model is inappropriate for our results, and develop our existing model of contrast perception to include multiple size mechanisms with divisive surround-suppression from the largest mechanism. For a given stimulus patch, this delivers a blurred step-function of responses across the population, with contrast and size encoded by the height and lateral position of the step. Unlike for textbook population coding schemes, our human results (N = 4 male, N = 4 female) displayed two asymmetries: (i) size aftereffects were greatest for targets smaller than the adaptor, and (ii) on that side of the function, results did not return to baseline, even when targets were 25% of adaptor diameter. Our results and emergent model properties provide evidence for a novel dimension of visual coding (size) and a novel strategy for that coding, consistent with previous results on contrast detection and discrimination for various stimulus sizes. [Abstract copyright: Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.