Healthy aging impairs face discrimination ability.

Face images enable individual identities to be discriminated from one another. We aimed to quantify age-related changes in different aspects of face identity discrimination. Face discrimination sensitivity was measured with a memory-free "odd-one-out" task. Five age groups (N = 15) of healthy adults with normal vision were tested: 20, 50-59, 60-69, 70-79, and 80-89. Sensitivity was measured for full-face images (all features visible), external features (head-shape, hairline), internal features (nose, mouth, eyes, and eyebrows) and closed-contour shapes (control object). Sensitivity to full-faces continuously declined by approximately 13% per decade, after 50 years of age. When age-related differences in visual acuity were controlled, the effect of age on face discrimination sensitivity remained. Sensitivity to face features also deteriorated with age. Although the effect for external features was similar to full-faces, the rate of decline was considerably steeper (approximately 3.7 times) for internal, relative to external, features. In contrast, there was no effect of age on sensitivity to shapes. All age groups demonstrated the same overall pattern of sensitivity to different types of face information. Healthy aging was associated with a continuous decline in sensitivity to both full-faces and face features, although encoding of internal features was disproportionately impaired. This age-related deficit was independent of differences in low-level vision. That sensitivity to shapes was unaffected by age suggests these results cannot be explained by general cognitive decline or lower-level visual deficits. Instead, healthy aging is associated with a specific decline in the mechanisms that underlie face discrimination.

Assessing the effect of Independent Prescribing for community optometrists and referral rates to Hospital Eye Services in Scotland.

INTRODUCTION: Since 2010, General Ophthalmic Services (GOS) legislation and Independent Prescribing (IP) enable community optometrists to manage primary eye conditions. No studies have assessed the effect of IP. We wished to determine the distribution of IP optometrists and associated hospital referral rates across Scotland. METHODS: In 2019, FOI requests (General Optical Council and NHS Education Scotland) identified all registered IP optometrists in Scotland and their registered postcodes. Data regarding community eye examinations and referrals to HES since 2010 were gathered via Information Services Division of NHS Scotland. RESULTS: As of March 2019, there were 278 IP optometrists in Scotland (278/1189; 23.4%). Two hundred eighteen IP optometrists work in 293 practices across 11 of Scotland's 14 health boards. There was a strong correlation (r = +0.96) between population density and number of IP optometrists. Fifty-six percent of IP optometrists work in the two most deprived quintiles. Since IP's introduction, there has been a marked increase in anterior segment supplementary visits (+290%). Optometry referrals to GPs have reduced by 10.5%, but referrals to HES have increased by 118% (to 96,315). There was no correlation between quantity of IP optometrists and referral rates to HES (r = -0.06, 95% CI -0.64 to 0.56, p = 0.86). CONCLUSIONS: This is the first analysis of IP optometrists and associated referral rates in Scotland. Despite good geographical distribution and increased supplementary attendances, optometric referrals to HES have doubled and continue to rise. We propose a ratio of primary, supplementary, non-referral and referral rates to discern the true impact of IP versus non-IP community optometric behaviour.

The Effect of Age-Related Macular Degeneration on Components of Face Perception.

Purpose: Patients with age-related macular degeneration (AMD) experience difficulty with discriminating between faces. We aimed to use a new clinical test to quantify the impact of AMD on face perception and to determine the specific aspects that are affected. Methods: The Caledonian face test uses an adaptive procedure to measure face discrimination thresholds: the minimum difference required between faces for reliable discrimination. Discrimination thresholds were measured for full-faces, external features (head-shape and hairline), internal features (nose, mouth, eyes, and eyebrows) and shapes (non-face task). Participants were 20 patients with dry AMD (logMAR VA = 0.14 to 0.62), 20 patients with wet AMD (0.10 to 0.60), and 20 age-matched control subjects (-0.18 to +0.06). Results: Relative to controls, full-face discrimination thresholds were, on average, 1.76 and 1.73 times poorer in participants with dry and wet AMD, respectively. AMD also reduced sensitivity to face features, but discrimination of the internal, relative to external, features was disproportionately impaired. Both distance VA and contrast sensitivity were significant independent predictors of full-face discrimination thresholds (R2 = 0.66). Sensitivity to full-faces declined by a factor of approximately 1.19 per 0.1 logMAR reduction in VA. Conclusions: Both dry and wet AMD significantly reduce sensitivity to full-faces and their component parts to similar extents. Distance VA and contrast sensitivity are closely associated with face discrimination sensitivity. These results quantify the extent of sensitivity impairment in patients with AMD and predict particular difficulty in everyday tasks that rely on internal feature information, including recognition of familiar faces and facial expressions.

The processing of compound radial frequency patterns.

Radial frequency (RF) patterns can be combined to construct complex shapes. Previous studies have suggested that such complex shapes may be encoded by multiple, narrowly-tuned RF shape channels. To test this hypothesis, thresholds were measured for detection and discrimination of various combinations of two RF components. Results show evidence of summation: sensitivity for the compounds was better than that for the components, with little effect of the components' relative phase. If both RF components are processed separately at the point of detection, they would combine by probability summation (PS), resulting in only a small increase in sensitivity for the compound compared to the components. Summation exceeding the prediction of PS suggests a form of additive summation (AS) by a common mechanism. Data were compared to predictions of winner-take-all, where only the strongest component contributes to detection, a single channel AS model, and multi-channel PS and AS models. The multi-channel PS and AS models were modelled under both Fixed and Matched Attention Window scenarios, the former assuming a single internal noise source for both components and compounds or different internal noise sources for components and compounds respectively. The winner-take-all and single channel models could be rejected. Of the remaining models, the best performing one was an AS model with a Fixed Attention Window, consistent with detection being mediated by channels that are efficiently combined and limited by a single source of noise for both components and compounds.

From individual features to full faces: Combining aspects of face information.

We investigated how information from face features is combined by comparing sensitivity to individual features with that for external (head shape, hairline) and internal (nose, mouth, eyes, eyebrows) feature compounds. Discrimination thresholds were measured for synthetic faces under the following conditions: (a) full-faces; (b) individual features (e.g., nose); and (c) feature compounds (either external or internal). Individual features and feature compounds were presented both in isolation and embedded within a fixed, task irrelevant face context. Relative to the full-face baseline, threshold elevations for the internal feature compound (2.41x) were comparable to those for the most sensitive individual feature (nose = 2.12x). External features demonstrated the same pattern. A model that incorporated all available feature information within a single channel in an efficient way overestimated sensitivity to feature compounds. Embedding individual features within a task-irrelevant context reduced discrimination sensitivity, relative to isolated presentation. Sensitivity to feature compounds, however, was unaffected by embedding. A loss of sensitivity when embedding features within a fixed-face context is consistent with holistic processing, which limits access to information about individual features. However, holistic combination of information across face features is not efficient: Sensitivity to feature compounds is no better than sensitivity to the best individual feature. No effect of embedding internal feature compounds within task-irrelevant external face features (or vice versa) suggests that external and internal features are processed independently.

Distribution of optometric practices relative to deprivation index in Scotland.

Background: The UK National Health Service aims to provide universal availability of healthcare, and eye-care availability was a primary driver in the development of the Scottish General Ophthalmic Services (GOS) model. Accordingly, a relatively equal distribution of optometry practices across socio-economic areas is required. We examined practice distribution relative to deprivation. Methods: 672 practices were sampled from nine Health Boards within Scotland. Practices were assigned a deprivation ranking by referencing their postcode with the Scottish Index of Multiple Deprivation (SIMD) tool (Scottish Executive National Statistics: General Report. 2016). Results: Averaged across Health Boards, the share of practices for the five deprivation quintiles was 25, 33, 18, 14 and 11% from most to least deprived area, respectively. Although there was some variation of relative practice distribution in individual Health Boards, 17 of the 45 regions (nine Health Boards, five quintiles) had a close balance between population and share of practices. There was no clear pattern of practice distribution as a function of deprivation rank. Analysis revealed good correlation between practice and population share for each Health Board, and for the combined data (R2 = 0.898, P < 0.01). Conclusion: Distribution of optometry practices is relatively balanced across socio-economic areas, suggesting that differences in eye-examination uptake across social strata are unrelated to service availability.

Contributions of individual face features to face discrimination.

Faces are highly complex stimuli that contain a host of information. Such complexity poses the following questions: (a) do observers exhibit preferences for specific information? (b) how does sensitivity to individual face parts compare? These questions were addressed by quantifying sensitivity to different face features. Discrimination thresholds were determined for synthetic faces under the following conditions: (i) 'full face': all face features visible; (ii) 'isolated feature': single feature presented in isolation; (iii) 'embedded feature': all features visible, but only one feature modified. Mean threshold elevations for isolated features, relative to full-faces, were 0.84x, 1.08, 2.12, 3.34, 4.07 and 4.47 for head-shape, hairline, nose, mouth, eyes and eyebrows respectively. Hence, when two full faces can be discriminated at threshold, the difference between the eyes is about four times less than what is required when discriminating between isolated eyes. In all cases, sensitivity was higher when features were presented in isolation than when they were embedded within a face context (threshold elevations of 0.94x, 1.74, 2.67, 2.90, 5.94 and 9.94). This reveals a specific pattern of sensitivity to face information. Observers are between two and four times more sensitive to external than internal features. The pattern for internal features (higher sensitivity for the nose, compared to mouth, eyes and eyebrows) is consistent with lower sensitivity for those parts affected by facial dynamics (e.g. facial expressions). That isolated features are easier to discriminate than embedded features supports a holistic face processing mechanism which impedes extraction of information about individual features from full faces.

Imagining circles--Empirical data and a perceptual model for the arc-size illusion.

An essential part of visual object recognition is the evaluation of the curvature of both an object's outline as well as the contours on its surface. We studied a striking illusion of visual curvature--the arc-size illusion (ASI)--to gain insight into the visual coding of curvature. In the ASI, short arcs are perceived as flatter (less curved) compared to longer arcs of the same radius. We investigated if and how the ASI depends on (i) the physical size of the stimulus and (ii) on the length of the arc. Our results show that perceived curvature monotonically increases with arc length up to an arc angle of about 60°, thereafter remaining constant and equal to the perceived curvature of a full circle. We investigated if the misjudgment of curvature in the ASI translates into predictable biases for three other perceptual tasks: (i) judging the position of the centre of circular arcs; (ii) judging if two circular arcs fall on the circumference of the same (invisible) circle and (iii) interpolating the position of a point on the circumference of a circle defined by two circular arcs. We found that the biases in all the above tasks were reliably predicted by the same bias mediating the ASI. We present a simple model, based on the central angle subtended by an arc, that captures the data for all tasks. Importantly, we argue that the ASI and related biases are a consequence of the fact that an object's curvature is perceived as constant with viewing distance, in other words is perceptually scale invariant.

The Caledonian face test: A new test of face discrimination.

This study aimed to develop a clinical test of face perception which is applicable to a wide range of patients and can capture normal variability. The Caledonian face test utilises synthetic faces which combine simplicity with sufficient realism to permit individual identification. Face discrimination thresholds (i.e. minimum difference between faces required for accurate discrimination) were determined in an "odd-one-out" task. The difference between faces was controlled by an adaptive QUEST procedure. A broad range of face discrimination sensitivity was determined from a group (N=52) of young adults (mean 5.75%; SD 1.18; range 3.33-8.84%). The test is fast (3-4 min), repeatable (test-re-test r(2)=0.795) and demonstrates a significant inversion effect. The potential to identify impairments of face discrimination was evaluated by testing LM who reported a lifelong difficulty with face perception. While LM's impairment for two established face tests was close to the criterion for significance (Z-scores of -2.20 and -2.27) for the Caledonian face test, her Z-score was -7.26, implying a more than threefold higher sensitivity. The new face test provides a quantifiable and repeatable assessment of face discrimination ability. The enhanced sensitivity suggests that the Caledonian face test may be capable of detecting more subtle impairments of face perception than available tests.

Distinct lower visual field preference for object shape.

Humans manipulate objects chiefly within their lower visual field, a consequence of upright posture and the anatomical position of hands and arms.This study tested the hypothesis of enhanced sensitivity to a range of stimuli within the lower visual field. Following current models of hierarchical processing within the ventral steam, discrimination sensitivity was measured for orientation, curvature, shape (radial frequency patterns), and faces at various para-central locations (horizontal, vertical, and main diagonal meridians) and eccentricities (5° and 10°). Peripheral sensitivity was isotropic for orientation and curvature. By contrast, observers were significantly better at discriminating shapes throughout the lower visual field compared to elsewhere. For faces, however, peak sensitivity was found in the left visual field, corresponding to the right hemispheric localization of human face processing. Presenting head outlines without any internal features (e.g., eyes, mouth) recovered the lower visual field advantage found for simple shapes. A lower visual field preference for the shape of an object, which is absent for more localized information (orientation and curvature) but also for more complex objects (faces), is inconsistent with a strictly feed-forward model and poses a challenge for multistage models of object perception. The distinct lower visual field preference for contour shapes is, however, consistent with an asymmetry at intermediate stages of visual processing, which may play a key role in representing object characteristics that are particularly relevant to visually guided actions.

Probing intermediate stages of shape processing.

The visual system provides a representation of what and where objects are. This entails parsing the visual scene into distinct objects. Initially, the visual system encodes information locally. While interactions between adjacent cells can explain how local fragments of an object's contour are extracted from a scene, such computations are ill suited to capture extended objects. This article reviews some of the evidence in favor of intermediate-level computations, tuned to the shape of an object, in the transformation from discrete local sampling to representation of complex objects. Two main paradigms, employed to study how information about the position and orientation of local signals are combined at intermediate levels, are considered here: a shape detection task (measuring the number of signal elements required to detect a shape in noise) and a shape discrimination task (requiring observers to discriminate between shapes). Results support the notion of global mechanisms that integrate information beyond neighboring cells and are optimally tuned to a range of different shapes. These intermediate processing stages appear vulnerable to damage. Diverse clinical conditions (amblyopia, macular disease, migraine, premature birth) show specific deficits for these tasks. Taken together, evidence is converging in favor of intermediate levels of processing, at which sensitivity to the global shape of objects emerges.

Adding rotation to translation: percepts and illusions.

This study investigated how the perception of a translating object is affected by rotation. Observers were asked to judge the motion and trajectory of objects that rotated around their centroid while linearly translating. The expected percept, consistent with the actual dynamics used to generate the movie sequences, is that of a translating and rotating object, akin to a tumbling rugby ball. Observers, however, do not always report this and, under certain circumstances, perceive the object to translate on an illusory curved trajectory, similar to a car driving on a curved road. The prevalence of veridical versus nonveridical percepts depends on a number of factors. First, if the object's orientation remains within a limited range relative to the axis of translation, the illusory, curved percept dominates. If the orientation, at any point of the movie sequence, differs sufficiently from the axis of translation, the percept switches to linear translation with rotation. The angle at which the switch occurs is dependent upon a number of factors that relate to an object's elongation and, with it, the prominence of its orientation. For an ellipse with an aspect ratio of 3, the switch occurs at approximately 45 degrees. Higher aspect ratios increase the range; lower ratios decrease it. This applies similarly to rectangular shapes. A line is more likely to be perceived on a curved trajectory than an elongated rectangle, which, in turn, is more likely seen on a curved path than a square. This is largely independent of rotational and translational speeds. Measuring perceived directions of motion at different instants in time allows the shape of the perceived illusory curved path to be extrapolated. This results in a trajectory that is independent of object size and corresponds closely to the actual object orientation at different points during the movie sequence. The results provide evidence for a perceptual transition from an illusory curved trajectory to a veridical linear trajectory (with rotation) for the same object. Both are consistent with special real-world cases such as objects rotating around a centre outside of the object so that their orientation remains tangent to the trajectory (cheetahs running along a curve, sailboats) or objects tumbling along simple trajectories (a monkey spinning in air, spinning cars on ice). In certain cases, the former is an illusion.

Cerebral visual dysfunction in prematurely born children attending mainstream school.

PURPOSE: Although premature birth is recognised as a cause of cerebral visual impairment (CVI), which can include cerebral visual dysfunction (CVD), the incidence and nature of CVD in prematurely born children is not known. METHODS: A prospective, controlled investigation was undertaken of forty-six, mainstream primary school children, prematurely born with gestations of 24.0-34.6 weeks, and of 130 control (term-born) children. Assessments were made of IQ, ophthalmic functions, visual perception and visual attention. Structured history-taking seeking evidence of behavioural features of CVI used a question inventory. Obstetric, neonatal and paediatric medical histories were documented from case records. RESULTS: Fifteen out of forty-six (33 %) of the prematurely born children-"cluster A"-revealed behaviours corresponding with CVD on cluster analysis of the CVI inventory. The whole prematurely born group performed worse than controls on all visual perception tests and all four visual attention tests. Children in cluster A were responsible for this effect, performing worse than controls on all visual perception and visual attention tests except visual closure, while cluster B prematurely born children performed no differently to controls. CONCLUSIONS: The prevalence of CVD in these prematurely born children is between 21-47 % (95 % CI), with a pattern similar to "dorsal stream dysfunction". Currently available perceptual tests appear to be unable to identify the specific pattern of problems noted in this group. Many studies have provided evidence of cognitive and intellectual dysfunction in prematurely born children, and it is possible that CVD is a contributor. The CVI inventory is a potential means of identifying and characterising the condition, which can be ameliorated with simple strategies.

Motion trajectories and object properties influence perceived direction of motion.

Judging the motion of objects is a fundamental task that the visual system executes in everyday life in order for us to navigate and interact safely with our surroundings. A number of strategies have been suggested to explain how the visual system uses motion information from different points of an object to compute veridical directions of motion. These include combining ambiguous signals from object contours via a vector summation (VS) or intersection of constraints (IOC) calculation, pooling information using a maximum likelihood or tracking object features. We measured the perceived direction of motion for a range of cross-shaped stimuli (composed of two superimposed lines) to test how accurately humans perceive their motion and compared data to predictions from these strategies. Crosses of different shapes (defined by the angle between the component lines) translated along 16 directions of motion with constant speed. The crosses either moved along one of their symmetry axes (balanced conditions with line components equidistant to the direction of motion) or had their symmetry axis tilted relative to the motion (unbalanced conditions) Data show reproducible differences between observers, including occasional bimodal behaviour, and exhibit the following common patterns. There is a general dependence on direction of motion: For all conditions, when motion is along cardinal axes (horizontal and vertical), perception is largely veridical. For non-cardinal directions, biases are typically small (<10 deg) when crosses are balanced but large biases occur (≥30 deg) when crosses are tilted relative to their direction of motion. Factors influencing the pattern of biases are the shape and tilt of the cross as well as the proximity of its direction of motion to cardinal axes. The dependence of the biases on the direction of motion is inconsistent with any isotropic mechanisms including VS, IOC, maximum likelihood or feature tracking. Instead, perception is biased by a number of intrinsic properties of the cross and external references. The strength of these cues depends on the type, with elongation producing the strongest weight, and their proximity to the direction of motion. This suggests that the visual system may rely on a number of static cues to improve the known low precision for non-cardinal directions of motion, a process which can, however, result in large perceptual biases in certain circumstances.

Set-size effects for sampled shapes: experiments and model.

The location of imperfections or heterogeneities in shapes and contours often correlates with points of interest in a visual scene. Investigating the detection of such heterogeneities provides clues as to the mechanisms processing simple shapes and contours. We determined set-size effects (e.g., sensitivity to single target detection as distractor number increases) for sampled contours to investigate how the visual system combines information across space. Stimuli were shapes sampled by oriented Gabor patches: circles and high-amplitude RF4 and RF8 radial frequency patterns with Gabor orientations tangential to the shape. Subjects had to detect a deviation in orientation of one element ("heterogeneity"). Heterogeneity detection sensitivity was measured for a range (7-40) of equally spaced (2.3-0.4°) elements. In a second condition, performance was measured when elements sampled a part of the shapes. We either varied partial contour length for a fixed (7) set-size, co-varying inter-element spacing, or set-size for a fixed spacing (0.7°), co-varying partial contour length. Surprisingly, set-size effects (poorer performance with more elements) are rarely seen. Set-size effects only occur for shapes containing concavities (RF4 and RF8) and when spacing is fixed. When elements are regularly spaced, detection performance improves with set-size for all shapes. When set-size is fixed and spacing varied, performance improves with decreasing spacing. Thus, when an increase in set-size and a decrease in spacing co-occur, the effect of spacing dominates, suggesting that inter-element spacing, not set-size, is the critical parameter for sampled shapes. We propose a model for the processing of simple shapes based on V4 curvature units with late noise, incorporating spacing, average shape curvature, and the number of segments with constant sign of curvature contained in the shape, which accurately accounts for our experimental results, making testable predictions for a variety of simple shapes.

Detecting shapes in noise: tuning characteristics of global shape mechanisms.

The proportion of signal elements embedded in noise needed to detect a signal is a standard tool for investigating motion perception. This paradigm was applied to the shape domain to determine how local information is pooled into a global percept. Stimulus arrays consisted of oriented Gabor elements that sampled the circumference of concentric radial frequency (RF) patterns. Individual Gabors were oriented tangentially to the shape (signal) or randomly (noise). In different conditions, signal elements were located randomly within the entire array or constrained to fall along one of the concentric contours. Coherence thresholds were measured for RF patterns with various frequencies (number of corners) and amplitudes ("sharpness" of corners). Coherence thresholds (about 10% = 15 elements) were lowest for circular shapes. Manipulating shape frequency or amplitude showed a range where thresholds remain unaffected (frequency ≤ RF4; amplitude ≤ 0.05). Increasing either parameter caused thresholds to rise. Compared to circles, thresholds increased by approximately four times for RF13 and five times for amplitudes of 0.3. Confining the signals to individual contours significantly reduced the number of elements needed to reach threshold (between 4 and 6), independent of the total number of elements on the contour or contour shape. Finally, adding external noise to the orientation of the elements had a greater effect on detection thresholds than adding noise to their position. These results provide evidence for a series of highly sensitive, shape-specific analysers which sum information globally but only from within specific annuli. These global mechanisms are tuned to position and orientation of local elements from which they pool information. The overall performance for arrays of elements can be explained by the sensitivity of multiple, independent concentric shape detectors rather than a single detector integrating information widely across space (e.g. Glass pattern detector).

Global shape processing deficits are amplified by temporal masking in migraine.

PURPOSE: Individuals with migraine show subtle defects in a range of visual tasks compared to nonmigraineurs. Increased neuronal noise can account for some of these deficits. To examine the generality of increased noise in migraine, masking effects were compared in migraineurs and headache-free controls using a shape discrimination task, thought to involve processing in extrastriate cortical areas. METHODS: Nine migraineurs with aura, nine migraineurs without aura, and nine headache-free controls participated. observers had to detect deviations in circular shapes with or without a larger contour mask. The nonoverlapping mask was presented at five temporal intervals (stimulus onset asynchronies, SOA): 0 (simultaneous), 66, 100, 133, and 250 ms. RESULTS: Migraineurs with aura performed worse in all tests than migraineurs without aura and controls. Both migraine groups performed poorer than controls at discriminating shapes without masks. Typical masking functions were obtained from all groups, but they were steeper for migraineurs than controls with thresholds raised most dramatically (2.1 and 4.4 times for migraineurs without and with aura relative to controls, respectively) at SOAs where masks had their most detrimental effect (66-100 ms). Modeling the effect of masking showed that raised internal noise alone is insufficient to explain these deficits. Rather, an abnormal nonlinear transducer function (e.g., as part of gain-control) together with increased multiplicative noise is required to capture the data. CONCLUSIONS: The findings are consistent with an extrastriate deficit in migraine that cannot be explained completely by defective inhibition.