Degrees Of Visual Angle Research Articles

TRNS effects on visual contrast detection

In recent years, transcranial electrical stimulation (tES) has been used to improve cognitive and perceptual abilities and to boost learning. In the visual domain, transcranial random noise stimulation (tRNS), a type of tES in which electric current is randomly alternating in between two electrodes at high frequency, has shown potential in inducing long lasting perceptual improvements when coupled with tasks such as contrast detection. However, its cortical mechanisms and online effects have not been fully understood yet, and it is still unclear whether these long-term improvements are due to early-stage perceptual enhancements of contrast sensitivity or later stage mechanisms such as learning consolidation. Here we tested tRNS effects on multiple spatial frequencies and orientation, showing that tRNS enhances detection of a low contrast Gabor, but only for oblique orientation and high spatial frequency (12 cycles per degree of visual angle). No improvement was observed for low contrast and vertical stimuli. These results indicate that tRNS can enhance contrast sensitivity already after one training session, however this early onset is dependent on characteristics of the stimulus such as spatial frequency and orientation. In particular, the shallow depth of tRNS is likely to affect superficial layers of the visual cortex where neurons have higher preferred spatial frequencies than cells in further layers, while the lack of effect on vertical stimuli might reflect the optimization of the visual system to see cardinally oriented low contrast stimuli, leaving little room for short-term improvement.Taken together, these results suggest that online tRNS effects on visual perception are the result of a complex interaction between stimulus intensity and cortical anatomy, consistent with previous literature on brain stimulation.

Wall patterns influence the perception of interior space

The texture of an object's surface influences its perceived spatial extent. For example, Hermann von Helmholtz reported that a square patch with black and white stripes appears elongated perpendicular to the stripes' orientation. This time-honoured finding stands in contrast with more recent recommendations by interior-design experts who suggest that stripe wall patterns make rooms appear elongated in the direction parallel to the stripes' orientation. In a series of four experiments, we presented stripe wall patterns and varied the orientation of the stripes (horizontal vs. vertical) and their density (number of stripes per degree of visual angle). Subjects estimated the width and height of stereoscopically presented interior spaces. Stripe patterns with higher densities made rooms appear both wider and higher than did stripe patterns with lower densities or plain walls. In contrast to both the predictions from the Helmholtz-square and the design guidelines, this effect was only weakly modulated by pattern orientation, in the sense that rooms appeared elongated in the direction parallel to the stripes' orientation. We conclude that object-based texture effects cannot be generalised to interior space perception. For a room's perceived spatial extent, pattern density is more important than pattern orientation.

A free and simple computerized screening test for visual field defects.

About 30-40% of stroke patients suffer from visual field defects following injury. These can interfere with the standard neuropsychological assessment and complicate the interpretation of tests that use visual materials. However, information about the integrity of a patient's central visual field is often unavailable. We, therefore, designed a screening tool, the computerized visual field test (c-VFT), specifically targeted at providing easily available, but rough, information about patients' central visual field. c-VFT was tested in two samples of stroke patients. Eleven patients were tested on c-VFT and on the Esterman test. Five patients were tested on c-VFT and the Humphrey Visual Field Analyzer (HFA), central 10-2. Criterion validity of the c-VFT was investigated by calculating quadrantwise intraclass correlation for both comparisons. For the HFA comparison, we also calculated point-to-point intraclass correlation, sensitivity, and specificity. Analyses revealed moderately good correspondence between c-VFT and the Esterman test, and between c-VFT and HFA 10-2, respectively. When looking specifically at test points within one degree of visual angle apart in the two tests, intraclass correlation increased. For these points, the sensitivity of c-VFT was 0.89 and specificity was 0.97. While the c-VFT is not designed to be diagnostic nor to replace the detailed visual field analysis, this study shows that it provides a reasonable screening of the central visual field. The test can easily be used and will be made freely available to neuropsychological clinicians and researchers.

The contrast sensitivity function of a small cryptobenthic marine fish.

Spatial resolution is a key property of eyes when it comes to understanding how animals' visual signals are perceived. This property can be robustly estimated by measuring the contrast sensitivity as a function of different spatial frequencies, defined as the number of achromatic vertical bright and dark stripe pairs within one degree of visual angle. This contrast sensitivity function (CSF) has been estimated for different animal groups, but data on fish are limited to two free-swimming, freshwater species (i.e., goldfish and bluegill sunfish). In this study, we describe the CSF of a small marine cryptobenthic fish (Tripterygion delaisi) using an optokinetic reflex approach. Tripterygion delaisi features a contrast sensitivity that is as excellent as other fish species, up to 125 (reciprocal of Michelson contrast) at the optimal spatial frequency of 0.375 c/°. The maximum spatial resolution is instead relatively coarse, around 2.125 c/°. By comparing our results with acuity values derived from anatomical estimates of ganglion cells' density, we conclude that the optokinetic reflex seems to be adapted to process low spatial frequency information from stimuli in the peripheral visual field and show that small marine fish can feature excellent contrast sensitivity at optimal spatial frequency.

Motion-Induced Scotoma.

We investigated artificial scotomas created when a moving object instantaneously crossed a gap, jumping ahead and continuing its otherwise smooth motion. Gaps of up to 5.1 degrees of visual angle, presented at 18° eccentricity, either closed completely or appeared much shorter than when the same gap was crossed by two-point apparent motion, or crossed more slowly, mimicking occlusion. Prolonged exposure to motion trajectories with a gap in most cases led to further shrinking of the gap. The same gap-shrinking effect has previously been observed in touch. In both sensory modalities, it implicates facilitation among codirectional local motion detectors and motion neurons with receptive fields larger than the gap. Unlike stimuli that simply deprive a receptor surface of input, suggesting it is insentient, our motion pattern skips a section in a manner that suggests a portion of the receptor surface has been excised, and the remaining portions stitched back together. This makes it a potentially useful tool in the experimental study of plasticity in sensory maps.

The impact of stimulus size and orientation on individual face coding in monkey face-selective cortex

Face-selective neurons in the monkey temporal cortex discharge at different rates in response to pictures of different individual faces. Here we tested whether this pattern of response across single neurons in the face-selective area ML (located in the middle Superior Temporal Sulcus) tolerates two affine transformations; picture-plane inversion, known to decrease the average response of face-selective neurons and the other, stimulus size. We recorded the response of 57 ML neurons in two awake and fixating monkeys. Face stimuli were presented at two sizes (10 and 5 degrees of visual angle) and two orientations (upright and inverted). Different faces elicited distinct patterns of activity across ML neurons that were reliable (i.e., predictable with a classifier) within a specific size and orientation condition. Despite observing a reduction in the average response magnitude of face-selective neurons to inverted faces, compared to upright faces, classifier performance was above chance for both upright and inverted faces. While decoding was largely preserved across changes in stimulus size, a classifier trained with one orientation condition and tested on the other did not lead to performance above chance level. We conclude that different individual faces can be decoded from patterns of responses in the monkey area ML regardless of orientation or size, but with qualitatively different patterns of responses for upright and inverted faces.

P20. Disentangling the effect of pre-stimulus oscillatory power on visual detection using spatio-spectral decomposition and heterogeneous choice models

Introduction Analyzing the influence of pre-stimulus EEG/MEG-oscillations on stimulus detection is typically performed by calculating the oscillatory power in each individual EEG/MEG sensor and a single frequency band and correlating it to the parameters of standard Signal Detection Theory (SDT). In the visual domain, this classical approach revealed an effect of widely distributed alpha activity on the perception bias (i.e., how conservative or liberal subjects decide) but not on the overall sensitivity (i.e., the ability to discriminate between stimulus presence and absence). However, sensor-space EEG/MEG data is spatially non-specific and highly susceptible to far-field activity and noise sources. Moreover, classical equal-variances SDT analysis might lead to spurious results. Here, we extract sources of alpha and theta activity using spatio-spectral decomposition (SSD) and model their collective effect on visual detection effectivity using heterogeneous choice models. Methods 64-multichannel EEG of 30 subjects was recorded during a visual yes/no detection task with 60% stimulus present trials and 40% stimulus absent trials. The stimuli consisted of low-contrast Gabor patches displayed at 10 degrees of visual angle in the left or right hemifield. The two strongest alpha and theta sources common for all subjects were identified by SSD applied at group-level. The rank-normalized alpha and theta powers at stimulus presentation (t = 0 ms) were used as regressors in a heterogeneous choice model. Results The heterogeneous choice model replicated the finding that oscillatory alpha power influences the perceptual bias. Additionally, strong occipital alpha power was found to increase the variability in the detection model. Furthermore, the power of a bipolar theta source, tentatively attributable to a deeper origin, was found to be negatively correlated to the overall sensitivity of the visual detection task. Discussion and significance SSD effectively extracted collectively strongest sources of alpha and theta activity across subjects, hereby increasing the spatial specificity of the data and the signal-to-noise ratio. The heterogeneous choice model revealed that alpha oscillations not only influence perceptual bias but also the variability in the model and that strong regional theta oscillations are correlated with less effective visual detection, potentially due to drowsiness. However, further studies are needed to characterize the neurophysiological underpinnings of these observations.

Neither Cholinergic Nor Dopaminergic Enhancement Improve Spatial Working Memory Precision in Humans.

Acetylcholine and dopamine are neurotransmitters that play multiple important roles in perception and cognition. Pharmacological cholinergic enhancement reduces excitatory receptive field size of neurons in marmoset primary visual cortex and sharpens the spatial tuning of visual perception and visual cortical fMRI responses in humans. Moreover, previous studies show that manipulation of cholinergic or dopaminergic signaling alters the spatial tuning of macaque prefrontal cortical neurons during the delay period of a spatial working memory (SWM) task and can improve SWM performance in macaque monkeys and human subjects. Here, we investigated the effects of systemic cholinergic and dopaminergic enhancement on the precision of SWM, as measured behaviorally in human subjects. Cholinergic transmission was increased by oral administration of 5 mg of the cholinesterase inhibitor donepezil, and dopaminergic signaling was enhanced with 100 mg levodopa/10 mg carbidopa. Each neurotransmitter system was separately investigated in double-blind placebo-controlled studies. On each trial of the SWM task, a square was presented for 150 ms at a random location along an invisible circle with a radius of 12 degrees of visual angle, followed by a 900 ms delay period with no stimulus shown on the screen. Then, the square was presented at new location, displaced in either a clockwise (CW) or counterclockwise (CCW) direction along the circle. Subjects used their memory of the location of the original square to report the direction of displacement. SWM precision was defined as the amount of displacement corresponding to 75% correct performance. We observed no significant effect on SWM precision for either donepezil or levodopa/carbidopa. There was also no significant effect on performance on the SWM task (percent correct across all trials) for either donepezil or levodopa/carbidopa. Thus, despite evidence that acetylcholine and dopamine regulate spatial tuning of individual neurons and can improve performance of SWM tasks, pharmacological enhancement of signaling of these neurotransmitters does not substantially affect a behavioral measure of the precision of SWM in humans.

Psychophysical Measurements of Luminance Contrast Sensitivity and Color Discrimination with Transparent and Blue-Light Filter Intraocular Lenses.

IntroductionThe purpose of this study was to measure luminance contrast sensitivity and color vision thresholdfs in normal subjects using a blue light filter lens and transparent intraocular lens material.MethodsMonocular luminance grating contrast sensitivity was measured with Psycho for Windows (version 2.36; Cambridge Research Systems) at 3.0, 6.0, 12.0, 20.0, and 30.0 cycles per degree of visual angle (cpd) in 15 normal subjects (eight female), with a mean age of 21.6 years (SD = 3.8 years). Chromatic discrimination was assessed with the Cambridge colour test (CCT) along the protan, deutan, and tritan color confusion axes. Both tests were performed in a darkened room under two situations: with a transparent lens and with blue light filter lens. Subjective impressions were taken by subjects regarding their visual experience under both conditions.ResultsNo difference was found between the luminance contrast sensitivity measured with transparent and blue light filter. However, 13/15 (87%) of the subjects reported more comfortable vision with the blue filter. In the color vision test, tritan thresholds were significantly higher for the blue filter compared with the transparent filter (p = 0.003). For protan and deutan thresholds no differences were found.ConclusionBlue-yellow color vision is impaired with the blue light filter, and no impairment occurs with the transparent filter. No significant differences in thresholds were found in the luminance contrast sensitivity comparing the blue light and transparent filters. The impact of short wavelength light filtering on intrinsically photosensitive retinal ganglion cells is also discussed.

CAN EYETRACKING METRICS RELATE TO PERFORMANCE ON VISUAL COGNITIVE TESTS OF INDIVIDUALS WITH YOUNG-ONSET ALZHEIMER’S DISEASE?

Eyetracking has been used to determine oculomotor impairments in individuals with Alzheimer's disease (AD) and other neurodegenerative conditions (Crawford et al., 2005; Boxer et al., 2012; Yang et al., 2013; Shakespeare et al., 2015). Phenotypic differences in eyetracking metrics have also been reported between typical AD and the clinicoradiological syndrome posterior cortical atrophy (PCA; Shakespeare et al., 2015). In this study, we explored the relationship between eyetracking metrics and standard visual cognitive tests in a phenotypically diverse cohort of patients with young onset dementia AD (YOAD), all with symptom onset before the age of 65 years. Fifty-seven participants were included: 36 individuals with YOAD (10 of whom also met criteria for PCA) and 21 age-matched healthy controls. Participants completed three eyetracking experiments: fixation, pro-saccade and smooth pursuit tasks (see Figure 1). Frequentist summary metrics were used as outcome measures and their relationship with aspects of cognitive function explored by looking at correlations with basic visual (shape discrimination), visuoperceptual (fragmented letters, object decision), visuospatial (dot counting, letter cancellation) and measures of single word recognition included in the cognitive tests. Consistent with previous findings, the small PCA subgroup showed more abnormalities in oculomotor characteristics compared to the remainder of the cohort for 4/7 eyetracking metrics (see Table 1). Across the whole YOAD cohort, the pro-saccade and smooth pursuit tasks showed moderate correlations with visuoperceptual and visuospatial test scores. Conversely, the measure for the fixation task showed no significant relationship to any visual cognitive task. Basic visual shape discrimination performance was only related to pro-saccade performance, whilst the spatial dot counting test alone associated with pro-saccade accuracy. The relatively simple and specific nature of eyetracking opens routes to sensitive, quantitative, repeatable evaluations not only of basic oculomotor function but also visuoperceptual and visuospatial processes and other domains of higher cognitive function in individuals with neurodegenerative disease. The current findings provide further insights into the complex relationship between aspects of visual processing and oculomotor function. Performance of a control and a patient with YOAD (PCA) in the different eyetracking tasks: light blue circles show fixations, orange arrows indicate saccades and red dots/crosses represent target position. Orange circles in the pro- saccade task outline the interest area (an area within two degrees of visual angle from the target position).

Discrimination of curvature from motion during smooth pursuit eye movements and fixation.

Smooth pursuit and motion perception have mainly been investigated with stimuli moving along linear trajectories. Here we studied the quality of pursuit movements to curved motion trajectories in human observers and examined whether the pursuit responses would be sensitive enough to discriminate various degrees of curvature. In a two-interval forced-choice task subjects pursued a Gaussian blob moving along a curved trajectory and then indicated in which interval the curve was flatter. We also measured discrimination thresholds for the same curvatures during fixation. Motion curvature had some specific effects on smooth pursuit properties: trajectories with larger amounts of curvature elicited lower open-loop acceleration, lower pursuit gain, and larger catch-up saccades compared with less curved trajectories. Initially, target motion curvatures were underestimated; however, ∼300 ms after pursuit onset pursuit responses closely matched the actual curved trajectory. We calculated perceptual thresholds for curvature discrimination, which were on the order of 1.5 degrees of visual angle (°) for a 7.9° curvature standard. Oculometric sensitivity to curvature discrimination based on the whole pursuit trajectory was quite similar to perceptual performance. Oculometric thresholds based on smaller time windows were higher. Thus smooth pursuit can quite accurately follow moving targets with curved trajectories, but temporal integration over longer periods is necessary to reach perceptual thresholds for curvature discrimination.NEW & NOTEWORTHY Even though motion trajectories in the real world are frequently curved, most studies of smooth pursuit and motion perception have investigated linear motion. We show that pursuit initially underestimates the curvature of target motion and is able to reproduce the target curvature ∼300 ms after pursuit onset. Temporal integration of target motion over longer periods is necessary for pursuit to reach the level of precision found in perceptual discrimination of curvature.

Macular Pigment Quantification with Multispectral Retinal Image Analysis

Objective: Variations in macular pigment (MP) have been linked with changes in the risk of visual loss secondary to macular pathology. MP density can be modified by diet; however there doesn’t appear to be a direct link between dietary intake of MP components and MP density in the retina and clinicians therefore need a reliable and objective method for MP measurement to establish if any intervention is yielding the required results. The objective of this study was to investigate whether multispectral retinal image analysis (MRIA), a new technique for mapping retinal pigments, is useful for measuring levels and distribution of MP to find differences between individuals with no clinical evidence of macular pathology and those diagnosed with Age-related Macular Degeneration (AMD). Methods: The study involved 90 volunteers from three subject groups: aged under 50 without AMD, aged 50 and over without AMD and aged 50 and over with AMD. The experiments yielded 607 usable data sets that were used for analysis. Multispectral image data was acquired at six selected wavelengths using a modified fundus camera. MRIA maps of MP were computed from 3 × 3 mm regions of interest (approximately 10 degrees of visual angle) centred at the fovea. Indices characterising MP distribution were computed both for individuals and for the three subject groups. For comparison Macular Pigment Optical Density (MPOD) measurements were acquired from the Macular Pigment Screener 9000 (MPS) based on the heterochromatic flicker photometry (HFP). Correlations for MP quantities measured with the two methods were computed between MP quantity and age, MP quantity and AMD diagnosis, and between the two methods. Results: MP maps obtained from MRIA were consistent with known histology and in agreement with expectations based on previous studies. Pooled results from the three groups suggest that the overall levels of MP across both the fovea and the parafovea are on average higher in healthy under-50 individuals than that over-50 with or without AMD. MP distribution might be more irregular in the over-50 groups than in the younger group. The correlation between age and MP levels was weak as measured individually by both techniques. The MRIA indices were not correlated with HFP-MPOD measurements for individuals, but high correlation was found between mean HFP-MPOD and mean MRIA peak value for pooled results. Conclusion: MRIA has potential to offer an objective, fast and reliable method of measuring MP throughout the posterior pole.

Decoding the Locus of Covert Visuospatial Attention from EEG Signals.

Visuospatial attention can be deployed to different locations in space independently of ocular fixation, and studies have shown that event-related potential (ERP) components can effectively index whether such covert visuospatial attention is deployed to the left or right visual field. However, it is not clear whether we may obtain a more precise spatial localization of the focus of attention based on the EEG signals during central fixation. In this study, we used a modified Posner cueing task with an endogenous cue to determine the degree to which information in the EEG signal can be used to track visual spatial attention in presentation sequences lasting 200 ms. We used a machine learning classification method to evaluate how well EEG signals discriminate between four different locations of the focus of attention. We then used a multi-class support vector machine (SVM) and a leave-one-out cross-validation framework to evaluate the decoding accuracy (DA). We found that ERP-based features from occipital and parietal regions showed a statistically significant valid prediction of the location of the focus of visuospatial attention (DA = 57%, p < .001, chance-level 25%). The mean distance between the predicted and the true focus of attention was 0.62 letter positions, which represented a mean error of 0.55 degrees of visual angle. In addition, ERP responses also successfully predicted whether spatial attention was allocated or not to a given location with an accuracy of 79% (p < .001). These findings are discussed in terms of their implications for visuospatial attention decoding and future paths for research are proposed.

Performance of a simple remote video-based eye tracker with GPU acceleration

Eye tracking is a well-established tool that is often utilised in research. There are currently many different types of eye trackers available, but they are either expensive, or provide a relatively low sampling frequency. The eye tracker presented in this paper was developed in an effort to address the lack of low-cost high-speed eye trackers. It utilises the Graphical Processing Unit (GPU) in an attempt to parallelise aspects of the process to localize feature points in eye images to attain higher sampling frequencies. Moreover, the proposed implementation allows for the system to be used on a variety of different GPUs. The developed solution is capable of sampling at frequencies of 200 Hz and higher, while allowing for head movements within an area of 10×6×10 cm and an average accuracy of one degree of visual angle. The entire system can be built for less than 700 euros, and will run on a mid-range laptop.

Effects of chronic alcoholism in the sensitivity to luminance contrast in vertical sinusoidal gratings

The aim of this study was to measure visual contrast sensitivity (CS) of luminance using vertical sinusoidal gratings with spatial frequencies of 0.6, 2.5, 5.0 and 20.0 cycles per degree of visual angle in chronic alcoholics in abstinence period. The participants were 20 volunteers (26–59 years of age) divided into two groups: the study group (SG) consisted of 10 volunteers with a clinical history of chronic alcoholism abstinence and the control group (CG) consisted of 10 healthy volunteers. Each group had five female and five male participants. All participants had normal or corrected visual acuity and were free of identifiable diseases. The psychophysical method of forced choice between two temporal alternatives (2AFC) was used to measure visual CS of luminance of 41.2 cd/m2. The results showed significant differences between groups for all spatial frequencies tested (p < 0.001). These results suggest alterations in the visual perception related to chronic alcohol consumption even after years of abstinence.

Influência do Cronotipo e do Horário da Medida na Sensibilidade ao Contraste Visual

Este estudo teve como objetivo avaliar a curva de sensibilidade ao contraste para estimulos de grades senoidais verticais de frequencias espaciais de 0,2; 0,6; 1; 3,1; 6,1; 8,8; 13,2 e 15,6 ciclos por grau de ângulo visual; em um ritmo de 24 horas de adultos saudaveis de diferentes cronotipos. Participaram desta pesquisa 18 voluntarios do sexo masculino com idade de 19 a 31 anos (M=23,8; DP=3,29). Os participantes foram divididos em grupos de acordo com o cronotipo: Matutino (n=5), Intermediario (n=9) e Moderadamente vespertino (n=4). Utilizaram-se como instrumentos medidas neuropsicologicas e o software Metropsis para avaliar a Funcao de Sensibilidade ao Contraste (FSC). As medidas da FSC foram realizadas com o metodo psicofisico da escolha forcada entre duas alternativas espaciais. Os resultados apresentaram alteracoes nas dimensoes relacionadas ao processamento cognitivo e na FSC de sujeitos Matutinos [F (14; 76303) = 38,92; p<0,05], Intermediarios [F(14; 505) = 2,79; p<0,05] e Moderadamente vespertinos [F(14; 279) = 2,75; p<0,05] de acordo com os horarios de medida. Concluiu-se que a sensibilidade ao contraste e um aspecto importante para o estudo da percepcao visual circadiana, sugerindo que os mecanismos sensorios que processam o contraste visual estao diretamente relacionados a hora e o padrao do cronotipo.

Visual distraction in a patient with abnormal occipital gyration – an eye-tracking study

Maintaining focus and resisting distraction is critical for many visual tasks. We investigated these abilities in a patient with congenital abnormal bilateral occipital and parietal gyration, caused by a single gene (LAMC3) mutation. Because of the location and nature of these structural idiosyncrasies as well as compromised performance in several attention-related screening tasks we set out to measure the patient's visual attention in detail. Here, we assessed the patient's ability to maintain focus in the presence of several types of visual distractors. The patient's performance was compared to that of five sex- and education-matched healthy controls. Participants performed four eye-tracking conditions in randomized order in one experimental session: Fixation-only, Fixation with rapid sequential visual perception (RSVP), Fixation with relevant peripheral distractor (RPD), and Fixation with irrelevant peripheral distractors (IPD). Participants were required to fixate at the center and to respond when a target was detected (in RSVP, RPD, IPD). In the RPD condition relevant cues were presented for 100 ms at 8.71 degrees visual angle eccentricity in one of four possible directions, followed by a 200 ms target in the same location. In the IPD condition no cues preceded targets, everything else was as in the RPD condition. Experiments were done in MATLAB using Psychtoolbox and with an ASL Eye-Trac6 D6 Desk Mounted Optics. We measured percent correct in RSVP, RDP and IDP tasks and mean deviation from fixation in horizontal and vertical directions in all experimental conditions. Patient and control group performed comparably in behavioral tasks. Fixation results, however, indicated a decreased fixation quality of the patient in the presence of visual distractors, with deviations from fixation more than doubling (compared to controls) in RDP and IDP conditions. We discuss these results in light of the patient's structural connectivity and morphometry. Meeting abstract presented at VSS 2015. Language: en

Adaptive-optics SLO imaging combined with widefield OCT and SLO enables precise 3D localization of fluorescent cells in the mouse retina.

Adaptive optics scanning laser ophthalmoscopy (AO-SLO) has recently been used to achieve exquisite subcellular resolution imaging of the mouse retina. Wavefront sensing-based AO typically restricts the field of view to a few degrees of visual angle. As a consequence the relationship between AO-SLO data and larger scale retinal structures and cellular patterns can be difficult to assess. The retinal vasculature affords a large-scale 3D map on which cells and structures can be located during in vivo imaging. Phase-variance OCT (pv-OCT) can efficiently image the vasculature with near-infrared light in a label-free manner, allowing 3D vascular reconstruction with high precision. We combined widefield pv-OCT and SLO imaging with AO-SLO reflection and fluorescence imaging to localize two types of fluorescent cells within the retinal layers: GFP-expressing microglia, the resident macrophages of the retina, and GFP-expressing cone photoreceptor cells. We describe in detail a reflective afocal AO-SLO retinal imaging system designed for high resolution retinal imaging in mice. The optical performance of this instrument is compared to other state-of-the-art AO-based mouse retinal imaging systems. The spatial and temporal resolution of the new AO instrumentation was characterized with angiography of retinal capillaries, including blood-flow velocity analysis. Depth-resolved AO-SLO fluorescent images of microglia and cone photoreceptors are visualized in parallel with 469 nm and 663 nm reflectance images of the microvasculature and other structures. Additional applications of the new instrumentation are discussed.

EXTENDING THE LIMITS FOR GAZE POINTING THROUGH THE USE OF SPEECH

Eye trackers have been used as pointing devices for a number of years. Due to the inherent limitations in the accuracy of eye gaze, however, interaction has been limited to targets that are at least one degree of visual angle in size. Consequently, targets in today’s gaze-based interfaces have sizes and layouts quite distant from what is perceived as “natural settings”. To cope with the accuracy constraints, we developed a multimodal pointing technique combining eye gaze and speech inputs. The technique was tested in a user study on pointing at multiple targets. Results suggest that pointing accuracy is 93% for targets subtending 0.85 degrees and 0.3-degree gaps between them. User perfor-mance is thus shown to approach the limit of practical pointing. Effectively, developing a user interface that supports the hands-free style of interaction and has a design similar to that of today’s common interfaces seems a feasible task.

Pupil size dynamics during fixation impact the accuracy and precision of video-based gaze estimation

Video-based eye tracking relies on locating pupil center to measure gaze positions. Although widely used, the technique is known to generate spurious gaze position shifts up to several degrees in visual angle because pupil centration can change without eye movement during pupil constriction or dilation. Since pupil size can fluctuate markedly from moment to moment, reflecting arousal state and cognitive processing during human behavioral and neuroimaging experiments, the pupil size artifact is prevalent and thus weakens the quality of the video-based eye tracking measurements reliant on small fixational eye movements. Moreover, the artifact may lead to erroneous conclusions if the spurious signal is taken as an actual eye movement. Here, we measured pupil size and gaze position from 23 human observers performing a fixation task and examined the relationship between these two measures. Results disclosed that the pupils contracted as fixation was prolonged, at both small (<16s) and large (∼4min) time scales, and these pupil contractions were accompanied by systematic errors in gaze position estimation, in both the ellipse and the centroid methods of pupil tracking. When pupil size was regressed out, the accuracy and reliability of gaze position measurements were substantially improved, enabling differentiation of 0.1° difference in eye position. We confirmed the presence of systematic changes in pupil size, again at both small and large scales, and its tight relationship with gaze position estimates when observers were engaged in a demanding visual discrimination task.