136 research outputs found
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
How conspicuous are peacock eyespots and other colorful feathers in the eyes of mammalian predators?
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Anomalous trichromacy : external enhancement of colour signals, individual differences and diagnosis
Anomalous trichromacy is known to conceal a substantial range in perceptual ability, but this is not typically considered in assessments of corrective aids and diagnostic tests. In addition to the diversity caused by genetic polymorphisms, perceptual ability is thought to be influenced by little understood postreceptoral mechanisms, adding to the need for research focusing on this population.
A modelling and behavioural investigation establishes the effectiveness of EnChroma filters in enhancing anomalous colour vision. Paper 1 (not yet published) employs a physiologically accurate model of colour vision to estimate the enhancements in cone-opponent signals conferred by the filters. Paper 2 (not yet published) presents behavioural validation of the modelâs predictions, showing that notch filters can result in enhanced perceived saturation for deuteranomalous observers, with effects for suprathreshold perception and partial effects at absolute threshold.
Paper 3 (submitted for publication) uses the physiologically accurate model of colour vision to investigate the impact of variation in edition and lighting conditions on the effectiveness of the Ishihara plates test in identifying those with mild anomalous trichromacy. The model predicts a significant impact of plate and illuminant, but no influence of edition, which is supported by the findings of a behavioural investigation.
This thesis provides the first direct evidence that altering the input to the visual system using filter-based aids can impact the cone-opponent signals available to anomalous trichromats, and that this change in signal is useable by the visual system, resulting in changes to perceived saturatio
Modeling age-related ocular media changes using the Farnsworth Munsell (FM) 100-Hue Test
Introduction: Evaluation of chromatic discrimination can help diagnose and monitor diseases and disorders of the visual system; however, normal age-related changes can make diagnosing colour discrimination losses challenging. This problem holds especially for the Farnsworth Munsell 100 Hue test (FM100 Hue). Identifying specific contributions of areas of the visual system that influence the FM100 Hue performance could aid in establishing standardized interpretation of test results and help understand the mechanisms contributing to the age-related changes.
Purpose: This study aimed to examine the theoretical changes in the FM100 Hue scores produced by age-related changes in the ocular media transmittances. These changes were examined with, and without, a von Kries type chromatic adaptation to determine the role of this adaptation process on age-related changes in hue discrimination.
Materials and methods: We calculated the CIECAM02 chromaticity coordinates of the FM100 Hue caps for 32-year-old and 74-year-old as ideal observers. These values were then used to predict the ordering of the FM100 Hue caps. The chromaticity coordinates were based on the spectral distribution of Illuminant D65, the spectral reflectances of the individual caps, and the CIE 1931 2o standard observer colour matching functions.
In calculating the values of the old observers, we modified the spectral distributions of the D65 light sources using Pokorny et al.âs and van de Kraats and van Norrenâs model of media transmittance to account for the relative change in transmittance from a 32-year-old observer to a 74-year-old observer. We also accounted for the decrease in retinal illumination in the older observers due to pupil miosis and the decrease in ocular media luminous transmittance.
The order of the caps was based on the minimum colour differences (ÎE) between nearby caps. The mean and standard deviation of the colour differences for each tray was calculated. The mean colour difference was also determined for the caps in the blue-yellow (BY) and red-green (RG) quadrants.
Results: With complete and partial adaptation, the Total Error Score (TES) increased from 8 for the younger observer to12 for both older observer models of media transmittance. The ordering for the caps along the blue-yellow axis was unchanged from the younger observer. Without adaptation, the error score for the older observer model increased further. The increase was primarily for caps along the red-green axis for van de Kraats and van Norrenâs model, whereas the increase for Pokorny et al.âs model was along both the red-green and blue-yellow axis.
The mean colour differences across trays for the young observer model were marginally larger than the old observer models for complete and partial adaptation. When the caps were grouped in the RG and BY quadrants, the mean differences for the older observers were still lower than the young observer. Differences between the BY and RG error scores were also larger for the older observers. Without adaptation, the mean colour differences for the older observer models were uniformly lower than with complete or partial adaptation.
Conclusion: The predicted effect of age-related changes in media transmittances on the ordering of the FM100 Hue test showed an increase in the TES, as expected. The increase, however, was primarily due to an increase in the RG partial error score, which disagrees with the psychophysical data showing a larger increase in the BY error score. This discrepancy suggests that age-related neural changes are also occurring, which is consistent with conclusions from other psychophysical studies. The two different models for age-related media changes produced similar changes in the FM100 error scores. Chromatic adaptation may compensate for age-related media changes
Peripheral factors affecting human colour perception
Human colour perception is mediated by multiple factors. These include: the external environment, physiological structures within the eye, and the neuronal pathways that originate in the eye. The aim of this thesis was to further investigate the impact of three main factors on both the perception and cortical representation of colour. These factors were: the external, changing seasonal environment, genetically determined differences in the number of photoreceptor types, and spatial filters inherent to cortical and pre-cortical luminance and chromatic pathways.
Novel findings and methods were demonstrated in this thesis:
1) For the first time, it was found that natural seasonal changes in the chromatic environment (in York, UK) affect the perception of unique yellow; this finding supports the existence of a slow normalisation mechanism, which is governed by changes in the average chromatic environment.
2) Genetically atypical individuals, who have fewer photoreceptor types (dichromats), demonstrated no differences in achromatic contrast discrimination thresholds compared to colour-normal trichromats. Therefore, for this particular measure, dichromats do not appear to benefit from increased neuronal resources from âunusedâ chromatic pathway populations. A multi-channel LED system was developed to allow the isolation of photoreceptor responses in individuals with an additional photoreceptor type (tetrachromats). Modelling of this system indicated that precision in the cone spectra used to generate the stimulus, relative to the observerâs actual cone sensitivities (i.e. peak wavelength sensitivities), is crucial for successful isolation of the cones.
3) fMRI-based population receptive field (pRF) mapping was used to measure pRF sizes in the pre-cortical channels. Between the pathways, no differences in pRF sizes were found, however, differences in fMRI measures of spatial frequency sensitivity were observed. These data indicate that spatial frequency tuning in early visual cortex may be decoupled from population receptive field sizes
What horses and humans see: a comparative review
Adaptations of the mammalian eye have tailored each to its own particular ecological niche. On the one hand, it would appear that the horse is best served by a system that can keep "half an eye" on everything, while the human benefits from focussing on more specific aspects of the visual array. By adapting a range of techniques, originally used to assess human visual ability, it has been possible to compare the human visual experience with that of the horse. In general, the results of the majority of these comparative studies indicate that the visual capabilities of the horse are broadly inferior to the human equivalents in acuity, accommodation, and colour vision. However, both the horse and human abilities to judge distance and depth perception may be quite comparable while equine vision is certainly superior to that of human's under scotopic conditions. Individual variation in visual ability, which is routinely taken for granted in humans, is also likely to occur in the horse. Such variation would undoubtedly affect equine performance, particularly in terms of expectation of athletic competitive outcomes in modern equitation
Coloured filters can simulate colour deficiency in normal vision but cannot compensate for congenital colour vision deficiency
Red-green colour vision deficiency (CVD) affectsâ~â4% 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
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