Publications

In Press
Bauer C, Heidary G, Koo B, Bex PJ, Merabet L.

Characterization of White Matter Tractography in Cortical Visual Impairment using High Angular Resolution Diffusion Imaging (HARDI)

. Journal of the American Association for Pediatric Ophthalmology and Strabismus. In Press.Abstract
Cortical visual impairment (CVI) is currently the leading cause of pediatric visual impairment in developed countries (Durnian et al., 2010; Good et al., 2001; Hoyt, 2007; Kong et al., 2012) and is characterized by visual impairment of non-ocular cause and its presumed association with damage to central cerebral structures (Hoyt, 2007). The diagnosis of CVI is confirmed by ultrasound and standard MRI; however, these neuroimaging modalities are limited in revealing underlying neurophysiological and morphological changes to visual and other cerebral structures and their relation to observed clinical and functional visual deficits associated with CVI.
Kwon M, Lu ZL, Miller A, Kazlas M, Lesmes LA, Hunter DG, Bex PJ.

Clinical value of binocular interaction assessment in amblyopia

. PLoSOne. In Press.Abstract
Purpose: To measure binocular interaction in amblyopes using a rapid and patient-friendly computer-based method, and to estimate the clinical value of this additional information for detecting and monitoring changes in amblyopic vision. Methods: Binocular interaction was assessed in subjects with strabismic amblyopia (n=7), anisometropic amblyopia (n=6), strabismus without amblyopia (n=15) and normal vision (n=40). Binocular interaction was measured with a dichoptic task in which subjects matched the position of a binocular probe to the cyclopean perceived phase of a dichoptic pair of gratings whose contrast ratios were systematically varied. Testing was performed in an ophthalmology clinic in under 8 mins. We assessed the relationship between binocular interaction, interocular acuity difference and stereoacuity, which are standard clinical measures of binocular visual function. Logistic regression and principal components analyses were used to determine the contribution of binocular interaction in identifying amblyopia. Results: Compared to normally-sighted controls, amblyopes exhibited significantly reduced effective contrast (~20%) of the weak eye suggesting a higher contrast requirement for the amblyopic compared to the strong eye. The effective contrast ratio of binocular vision was closely related to standard clincal measures of binocular vision. Furthermore, knowing the effective contrast of the weak eye contributed significantly to classifying patients as amblyopic vs. non-amblyopic. Conclusions: Our findings demonstrate that abnormal binocular interaction can be reliably captured by measuring the effective contrast of the weak eye and quantitative assessment of binocular interaction is a quick and simple test that can be performed in the clinic. This suggests that reliable and timely assessment of deficits in a binocular interaction measure may improve detection of amblyopia and allow clinicians to more accurately assess individual patients’ outcomes and prognosis in addition to standard visual acuity.
Maiello G, Chessa M, Solari F, Bex P.

Simulated disparity and peripheral blur interact during binocular fusion

. Journal of Vision. In Press.Abstract
We developed a gaze contingent display in which natural images are presented to the observer with dioptric blur and stereoscopic disparity that are dependent on the 3 dimensional structure of natural scenes. Our system simulates a distribution of retinal blur and depth similar to that experienced in real world viewing conditions by emmetropic observers. We implemented the system using light field photographs taken with a plenoptic camera which supports digital refocusing anywhere in the images. We coupled this capability with an eye-tracking system and stereoscopic rendering. With this display, we examine how the time course of binocular fusion depends on depth cues from blur and stereoscopic disparity in natural images. Our results show that disparity and peripheral blur interact to modify eye movement behavior and facilitate binocular fusion and the greatest benefit was gained by observers who struggled most to achieve fusion. Even though plenoptic images do not replicate an individual’s aberrations, the results demonstrate that a naturalistic distribution of depth-dependent blur may improve 3D virtual reality and interruptions of this distribution, for example with multi-focal lenses, which flatten the distribution of retinal blur  may affect binocular fusion. 
Feke GT, Bex PJ, Taylor CP, Rhee DJ, Turalba AV, Chen TC, Wand M, Pasquale LR.

Effect of Brimonidine on Retinal Vascular Autoregulation and Short-Term Visual Function in Normal Tension Glaucoma

. Journal of Ophthalmology. In Press.Abstract
PURPOSE: To assess whether brimonidine 0.15% alters retinal vascular autoregulation (RVA) and short-term visual function in normal tension glaucoma (NTG) patients who demonstrate retinal vascular dysregulation (RVD). DESIGN: Non-randomized clinical trial. METHODS: In this prospective study, 46 NTG patients not previously treated with brimonidine underwent retinal vascular autoregulation (RVA) testing and visual function assessment using frequency doubling technology (FDT) perimetry and equivalent noise motion sensitivity testing. We measured blood flow in a major temporal retinal artery with subjects seated and then while reclined for 30 minutes using a Canon Laser Blood Flowmeter. Patients having a change in retinal blood flow with posture change outside the range previously found in healthy subjects were classified as having RVD. They were treated with brimonidine 0.15% for eight weeks and designated for re-testing. RESULTS: 23 of the 46 patients demonstrated RVD at the initial visit. Logistic regression analysis indicated that younger age (p=0.050) and presence of diabetes (p=0.055) were marginally significant risk factors for RVD. 17 patients with RVD completed the study (5 withdrew due to brimonidine intolerance; one was not re-tested due to technical issues). Fourteen of the 17 showed a return of the posture-induced change in retinal blood flow to levels within the range consistent with normal RVA (p<0.0001) following brimonidine. We found no significant changes in FDT perimetry or in motion detection parameters following treatment (p>0.09 for all tests performed). CONCLUSIONS: Brimonidine significantly improved impaired RVA in NTG patients, but short-term alteration in visual function could not be demonstrated.
2014
Harrison WJ, Bex PJ.

Integrating Retinotopic Features in Spatiotopic Coordinates

. Journal of Neuroscience. 2014;34(21):7351-7360.Abstract
The receptive fields of early visual neurons are anchored in retinotopic coordinates (Hubel and Wiesel, 1962). Eye movements shift these receptive fields and therefore require that different populations of neurons encode an object’s constituent features across saccades. Whether feature groupings are preserved across successive fixations or processing starts anew with each fixation has long been hotly debated (Melcher and Morrone, 2003; Melcher, 2005; Knapen et al., 2009; Cavanagh et al., 2010a; 2010b; Melcher, 2010; Morris et al., 2010). Here we show that feature integration initially occurs within retinotopic coordinates, but is then conserved within a spatiotopic coordinate frame independent of where the features fall on the retinas. With human observers, we first found that the relative timing of visual features plays a critical role in determining the spatial area over which features are grouped. We exploited this temporal dependence of feature integration to show that features co-occurring within 45 ms remain grouped across eye movements. Our results thus challenge purely feed-forward models of feature integration (Pelli, 2008; Freeman and Simoncelli, 2011), that begin de novo after every eye movement, and implicate the involvement of brain areas beyond early visual cortex. The strong temporal dependence we quantify, and its link with trans-saccadic object perception, instead suggest that feature integration depends, at least in part, on feedback from higher brain areas (Mumford, 1992; Rao and Ballard, 1999; Di Lollo et al., 2000; Moore and Armstrong, 2003; Stanford et al., 2010).
Saunders DR, Bex PJ, Rose DJ, Woods RL.

Measuring Information Acquisition from Sensory Input Using Automated Scoring of Natural-Language Descriptions

. PLoSOne [Internet]. 2014;9(4):93251. WebsiteAbstract
Information acquisition, the gathering and interpretation of sensory information, is a basic function of mobile organisms. We describe a new method for measuring this ability in humans, using free-recall responses to sensory stimuli which are scored objectively using a ‘‘wisdom of crowds’’ approach. As an example, we demonstrate this metric using perception of video stimuli. Immediately after viewing a 30 s video clip, subjects responded to a prompt to give a short description of the clip in natural language. These responses were scored automatically by comparison to a dataset of responses to the same clip by normally-sighted viewers (the crowd). In this case, the normative dataset consisted of responses to 200 clips by 60 subjects who were stratified by age (range 22 to 85y) and viewed the clips in the lab, for 2,400 responses, and by 99 crowdsourced participants (age range 20 to 66y) who viewed clips in their Web browser, for 4,000 responses. We compared different algorithms for computing these similarities and found that a simple count of the words in common had the best performance. It correctly matched 75% of the lab-sourced and 95% of crowdsourced responses to their corresponding clips. We validated the measure by showing that when the amount of information in the clip was degraded using defocus lenses, the shared word score decreased across the five predetermined visual-acuity levels, demonstrating a dose-response effect (N = 15). This approach, of scoring open-ended immediate free recall of the stimulus, is applicable not only to video, but also to other situations where a measure of the information that is successfully acquired is desirable. Information acquired will be affected by stimulus quality, sensory ability, and cognitive processes, so our metric can be used to assess each of these components when the others are controlled.
Bogfjellmo LG, Bex PJ, Falkenberg HK.

The development of global motion discrimination in school aged children

. Journal of Vision. 2014;2(19):1-12.Abstract
Purpose: Global motion perception matures during childhood and involves the detection of local directional signals that are then integrated across space. We examine the maturation of local directional selectivity and global motion integration with an equivalent noise paradigm applied to direction discrimination. Methods: 103 observers (6-17 years) identified the global direction of motion in a 2AFC task. The 8 degree central stimuli consisted of 100 dots of 10% Michelson contrast moving 2.8 or 9.8°/s. Local directional selectivity and global sampling efficiency were estimated from direction discrimination thresholds as a function of external directional noise, speed and age. Results: Direction discrimination thresholds improved gradually until the age of 14 years (linear regression, p<0.05) for both speeds. This improvement was associated with a gradual increase in sampling efficiency (linear regression, p< 0.05), with no significant change in internal noise. Direction sensitivity was lower for dots moving at 2.8°/s than 9.8°/s at all ages (paired t-test, p<0.05) and is mainly due to lower sampling efficiency. Conclusion: Global motion perception improves gradually during development and matures by age 14. There was no change in internal noise after the age of 6, suggesting that local direction selectivity is mature by that age. The improvement in global motion perception is underpinned by a steady increase in the efficiency with which direction signals are pooled, suggesting that global motion pooling processes mature for longer and later than local motion processing.
Wallis TSA, Taylor CP, Wallis J, Jackson ML, Bex PJ. Characterisation of field loss based on microperimetry is predictive of face recognition difficulties. Investigative Ophthalmology and Visual Science. 2014;55(1):142-154.Abstract
Purpose: Face recognition is important for daily living, and visual field loss can impair face recognition performance. We aimed to determine how field loss as assessed by microperimetry was correlated with deficits in face recognition by characterising performance as a function of stimulus blur. Methods: Patients with impaired visual sensitivity in the central visual field caused by a variety of retinal pathologies (N = 12, ages 26–70 years) and normally-sighted control subjects (N = 12, ages 20–68 years) performed a face recognition task for blurred and un-blurred faces. For patients we assessed central visual field loss using microperimetry, fixation stability, Pelli Robson contrast sensitivity, and letter acuity. Results: Patients were divided into two groups by microperimetry: an LV group (N = 8) had impaired sensitivity at the anatomical fovea and / or poor fixation stability whereas an LV:F group was characterised by at least some residual foveal sensitivity but marked loss in other retinal regions. Both control subjects and the LV:F group showed worse recognition performance as stimulus blur increased. The LV group instead showed poor performance regardless of blur. Visual acuity and fixation stability were significantly correlated with recognition performance. Conclusions: In the present task, persons diagnosed with eye disease affecting the central visual field can recognise faces as well as persons with no visual disease provided that they have residual sensitivity in the anatomical fovea and show stable fixation patterns. Performance is limited by the poor resolution of non-foveal vision or image blur, whichever is worse.
Kalia A, Lesmes LA, Dorr M, Gandhi T, Chatterjee G, Ganesh S, Bex PJ, Sinha P.

Contrast Sensitivity Development After Extended Congenital Blindness

. Proceedings of the National Academy of Sciences [Internet]. 2014;111(5):2035-2039. downloadAbstract
Visual plasticity peaks during early critical periods of normal visual development. Studies in animals and humans provide converging evidence that gains in visual function are minimal and deficits are most severe when visual deprivation extends beyond the critical period. Here we demonstrate visual development in a unique sample of patients who experienced early onset blindness (beginning before 1 year of age and lasting 8-17 years).  before removal of bilateral cataracts. These patients show surprising improvements in contrast sensitivity, an assay of basic spatial vision. We find that contrast sensitivty development is independent of the age of sight onset once critical periods have passed, and that individual rates of improvement can exceed those of normally-developing infants. These results reveal that the visual system can retain considerable plasticity, even after early and extended blindness.
2013
Elze T, Taylor CP, Bex PJ. Organic Light Emitting Diode Monitors for Medical Applications: Great Timing, but Luminance Artifacts. Medical Physics. 2013;40(9):092701.Abstract
PURPOSE: In contrast to the dominant medical liquid crystal display (LCD) technology, organic light-emitting diode (OLED) monitors control the display luminance via separate light-emitting diodes for each pixel and are therefore supposed to overcome many previously documented temporal artifacts of medical LCDs. We assessed the temporal and luminance characteristics of the only currently available OLED monitor designed for use in the medical treatment field (SONY PVM2551MD) and checked the authors' main findings with another SONY OLED device (PVM2541). METHODS: Temporal properties of the photometric output were measured with an optical transient recorder. Luminances of the three color primaries and white for all 256 digital driving levels (DDLs) were measured with a spectroradiometer. Between the luminances of neighboring DDLs, just noticeable differences were calculated according to a perceptual model developed for medical displays. Luminances of full screen (FS) stimuli were compared to luminances of smaller stimuli with identical DDLs. RESULTS: All measured luminance transition times were below 300 μs. Luminances were independent of the luminance in the preceding frame. However, for the single color primaries, up to 50.5% of the luminances of neighboring DDLs were not perceptually distinguishable. If two color primaries were active simultaneously, between 36.7% and 55.1% of neighboring luminances for increasing DDLs of the third primary were even decreasing. Moreover, luminance saturation effects were observed when too many pixels were active simultaneously. This effect was strongest for white; a small white patch was close to 400 cd/m(2), but in FS the luminance of white saturated at 162 cd/m(2). Due to different saturation levels, the luminance of FS green and FS yellow could exceed the luminance of FS white for identical DDLs. CONCLUSIONS: The OLED temporal characteristics are excellent and superior to those of LCDs. However, the OLEDs revealed severe perceptually relevant artifacts with implications for applicability to medical imaging.
Bogfjellmo LG, Bex PJ, Falkenberg H. Reduction in direction discrimination with age and slow speed is due to both increased internal noise and reduced sampling efficiency. Investigative Ophthalmology and Visual Science. 2013;54(8):5204-5210.Abstract
PURPOSE. Sensitivity to moving structure decreases with age and slow speeds may be selectively impaired. This loss could be caused by elevated internal noise in the responses of motion sensors or a reduction in the efficiency with which motion responses are integrated. We adapt an equivalent noise paradigm to analyze the perception of slow and fast speed motion as a function of normal aging. METHODS. A total of 70 observers (20–89 years) identified the direction of global motion in a two-alternative forced choice task. In a central 88 aperture, 100 dots of 10% Michelson contrast were moving at 1.6 or 5.58/s. The direction of each dot was drawn from a Gaussian distribution whose mean and SD were adaptively changed. Internal noise and sampling efficiency were estimated from direction discrimination thresholds as a function of external direction noise, speed, and age. RESULTS. Direction sensitivity was significantly worse for slow speeds at all ages (paired t-test, P < 0.05) and decreased approximately 2% per year (linear regressions, P < 0.01). This aging deficit was due to significant changes in internal noise (5.58/s) and sampling efficiency (1.68/s) (linear regression, P < 0.05). CONCLUSIONS. There is motion sensitivity loss with age that arises from an increase in internal noise in the responses of directional sensors and a decrease in responses that contribute to the global decision. Differences in the rates of progression at each speed indicate that motion is processed by independent systems tuned to different speeds, and that the channel for slow speed may be more vulnerable to normal age-related changes.
Jackson ML, Bex PJ, Ellison J, Wicks P, Wallis J. Feasibility of a Web-based Survey of Hallucinations and Assessment of Visual Function in Patients with Parkinson’s Disease. Interactive Journal of Medical Research. 2013;2(2):e30.Abstract
Background: Patients with Parkinson’s disease experience hallucinations, which may be related to decreased contrast sensitivity, i.e. the ability to discern shades of grey. Objective: To investigate if an online research platform can be used to survey patients with Parkinson’s disease regarding visual hallucinations and also used to assess visual contrast perception. Methods: Nine hundred and sixty four members of the online patient community, PatientsLikeMe, were invited via email to participate in this study. Participants completed a modified version of the University of Miami Parkinson’s Disease Hallucinations Questionnaire and an online vision test. Results: Results: Two hundred and sixty nine members (28% of invited) completed the study: 153 of this group had Parkinson's disease (57% of completers) and 116 (43%) were non-Parkinson’s controls. Hallucinations were reported by 28 subjects (18%) of the Parkinson’s group (PD). Although 10 subjects (9%) in the control group reported experiencing hallucinations only two of those subjects actually described formed hallucinations. Participants with Parkinson’s disease and the control group showed relatively good contrast perception as measured with the online letter test (F(1, 265) = 3.28, MSE = .414 , etap² = .012, P = .07). People who reported hallucinations showed contrast sensitivity levels that did not differ from levels shown by people without hallucinations (F(1, 265) =1.23, MSE =.155, etap² = .005, P = .27 ) although there was a trend towards lower contrast sensitivity in hallucinators. Two hundred and sixty nine members (28% of invited) completed the study: 153 of this group had Parkinson's disease (57% of completers) and 116 (43%) were non-Parkinson’s controls. Hallucinations were reported by 18% of the Parkinson’s group (PD). Although 9% of the control group reported experiencing hallucinations only 20% of those subjects actually described formed hallucinations. Participants with Parkinson’s disease and the control group showed relatively good contrast perception as measured with the online letter test (F(1, 265) = 3.28, MSE = .414 , etap² = .012, P = .07). People who reported hallucinations showed contrast sensitivity levels that did not differ from levels shown by people without hallucinations (F(1, 265) =1.23, MSE =.155, etap² = .005, P = .27 ) although there was a trend towards lower contrast sensitivity in hallucinators. Conclusions: Although more patients with Parkinson’s disease reported visual hallucinations than the control group, the report of hallucinations by controls may indicate that online questionnaires regarding hallucinations needs to be more specific to accurately detect a true history of formed visual hallucinations. Multiple questions outlining the nature of the hallucinations are required. In a clinical interview the specific nature of the hallucination would be further refined to rule out a vague description that does not indicate a true, formed visual hallucination. Contrary to our hypothesis both groups showed relatively good contrast sensitivity, perhaps representing a ceiling effect or limitations of online testing conditions that are difficult to standardize. Steps can be taken in future trials to further standardize online visual function testing, to refine control group parameters and to take steps to rule out confounding variables such as co-morbid disease that could be associated with hallucinations. Contacting subjects via an online health social network is a novel, cost-effective l method of conducting vision research that allows large numbers of individuals to be contacted quickly, and refinement of questionnaires and visual function testing may allow more robust findings in future research.
Dorr M, Lesmes LA, Lu ZL, Bex PJ. Rapid and precise contrast sensitivity assessment on a tablet device. Investigative Ophthalmology and Visual Science. 2013;54:7266-7273.Abstract
PURPOSE. Letter acuity, the predominant clinical assessment of vision, is relatively insensitive to slow vision loss caused by eye disease. While the contrast sensitivity function (CSF) has demonstrated the potential to monitor the slow progress of blinding eye diseases, current tests of CSF lack the reliability or ease-of-use to capture changes in vision timely. To improve the current state of home testing for vision, we have developed and validated a computerized adaptive test on a commercial tablet device (iPad) that provides an efficient and easy-to-use assessment of the CSF. METHODS. We evaluated the reliability, accuracy, and flexibility of tablet-based CSF assessment. Repeated tablet-based assessments of the spatial CSF, obtained from four normally-sighted observers, which each took 3 to 5 minutes, were compared to measures obtained on CRTbased laboratory equipment; additional tablet-based measures were obtained from six subjects under three different luminance conditions. RESULTS. A Bland-Altman analysis demonstrated that tablet-based assessment was reliable for estimating sensitivities at specific spatial frequencies (coefficient of repeatability 0.14–0.40 log units). The CRT- and tablet-based results demonstrated excellent agreement with absolute mean sensitivity differences <0.05 log units. The tablet-based test also reliably identified changes in contrast sensitivity due to different luminance conditions. CONCLUSIONS. We demonstrate that CSF assessment on a mobile device is indistinguishable from that obtained with specialized laboratory equipment. We also demonstrate better reliability than tests used currently for clinical trials of ophthalmic therapies, drugs, and devices.
Saunders DR, Bex PJ, Woods RL. Crowdsourcing a normative natural language dataset: A comparison of Mechanical Turk and in-lab data collection. Journal of Medical Internet Research. 2013;15(5):e100.Abstract
Background: Crowdsourcing has become a valuable method for collecting medical research data. This approach, recruiting through open calls on the web, is particularly useful for assembling large normative datasets. However, it is not known how natural language datasets collected over the web differ from those collected under controlled laboratory conditions. Objective: To compare the natural language responses obtained through crowdsourcingfrom a crowdsourced sample with responses collected in a conventional laboratory setting from participants recruited according to specific age and gender criteria. Methods: We collected natural language descriptions of 200 half-minute movie clips, from Amazon.com Mechanical Turk workers (crowdsourced) and 60 participants recruited from the community (lab-sourced). Crowdsourced participants responded to as many clips as they wanted, and typed their responses, whereas lab-sourced participants gave spoken responses to 40 clips, and their responses were transcribed. The content of the responses was evaluated using a take-one-out procedure, which compared responses to other responses to the same clip and to other clips, with a comparison of the average number of shared words. Results: In contrast to the 13 months of recruiting that was required to collect normative data from 60 lab-sourced participants (with specific demographic characteristics), only 34 days were needed to collect normative data from 99 crowdsourced participants (contributing a median of 22 responses). The majority of crowdsourced workers were female, and the median age was 35y, lower than the lab-sourced median of 62y but similar to the median age of the {U.S.} population. The responses contributed by the crowdsourced participants were longer on average, 33 words compared to 28 words (P {\textless} .001), and they used a less varied vocabulary. However, there was strong similarity in the words used to describe a particular clip between the two datasets, as a cross-dataset count of shared words showed (P {\textless} .001). Within both datasets, responses contained substantial relevant content, with more words in common with responses to the same clip than to other clips (P {\textless} .001). There was evidence that responses from female and older crowdsourced participants had more shared words (P = .004 and .01 respectively), whereas younger participants had higher numbers of shared words in the lab-sourced population (P = .01). Conclusions: Crowdsourcing is an effective approach to quickly and economically collect a large, reliable dataset of normative natural language responses.
Pelli DG, Bex PJ. Measuring Contrast Sensitivity. Vision Research. 2013;90:10-14.Abstract
Contrast sensitivity defines the threshold between the visible and invisible, which has obvious significance for basic and clinical vision science. Fechner's 1860 review reported that threshold contrast is 1% for a remarkably wide range of targets and conditions. While printed charts are still in use, computer testing is becoming more popular because it offers efficient adaptive measurement of threshold for a wide range of stimuli. Both basic and clinical studies usually want to know fundamental visual capability, regardless of the observer's subjective criterion. Criterion effects are minimized by the use of an objective task: multiple-alternative forced-choice detection or identification. Having many alternatives reduces the guessing rate, which makes each trial more informative, so fewer trials are needed. Finally, populations who may experience crowding or target confusion should be tested with one target at a time.
Pomarjanschi L, Dorr M, Bex PJ, Barth E. Simple gaze-contingent cues guide eye movements in a realistic driving simulator. Proceedings of the SPIE 8651, Human Vision and Electronic Imaging. 2013;XVIII:865110.Abstract
Looking at the right place at the right time is a critical component of driving skill. Therefore, gaze guidance has the potential to become a valuable driving assistance system. In previous work, we have already shown that complex gaze-contingent stimuli can guide attention and reduce the number of accidents in a simple driving simulator. We here set out to investigate whether cues that are simple enough to be implemented in a real car can also capture gaze during a more realistic driving task in a high- delity driving simulator. We used astate-of-the-art, wide- eld-of-view driving simulator with an integrated eye tracker. Gaze-contingent warnings were implemented using two arrays of light-emitting diodes horizontally tted below and above the simulated windshield. Thirteen volunteering subjects drove along predetermined routes in a simulated environment populated with autonomous trac. Warnings were triggered during the approach to half of the intersections, cueing either towards the right or to the left. The remaining intersections were not cued, and served as controls. The analysis of the recorded gaze data revealed that the gaze-contingent cues did indeed have a gaze guiding eff ect, triggering a signifi cant shift in gaze position towards the highlighted direction. This gaze shift was not accompanied by changes in driving behaviour, suggesting that the cues do not interfere with the driving task itself.
Dorr M, Bex PJ. Peri-Saccadic Natural Vision. The Journal of Neuroscience. 2013;33:1211–1217.Abstract
The fundamental role of the visual system is to guide behavior in natural environments. To optimize information transmission, many animals have evolved a non-homogeneous retina and serially sample visual scenes by saccadic eye movements. Such eye movements, however, introduce high-speed retinal motion and decouple external and internal reference frames. Until now, these processes have only been studied with unnatural stimuli, eye movement behavior, and tasks. These experiments confound retinotopic and geotopic coordinate systems and may probe a non-representative functional range. Here we develop a real-time, gaze-contingent display with precise spatiotemporal control over high-definition natural movies. In an active condition, human observers freely watched nature documentaries and indicated the location of periodic narrow-band contrast increments relative to their gaze position. In a passive condition under central fixation, the same retinal input was replayed to each observer by updating the video's screen position. Comparison of visual sensitivity between conditions revealed three mechanisms that the visual system has adapted to compensate for peri-saccadic vision changes. Under natural conditions we show that reduced visual sensitivity during eye movements can be explained simply by the high retinal speed during a saccade without recourse to an extra-retinal mechanism of active suppression; we give evidence for enhanced sensitivity immediately after an eye movement indicative of visual receptive fields remapping in anticipation of forthcoming spatial structure; and we demonstrate that perceptual decisions can be made in world rather than retinal coordinates.
Lesmes LA, Jackson ML, Bex P. Visual function endpoints to enable dry AMD clinical trials. Drug Discovery Today: Therapeutic Strategies. 2013.Abstract
The slow progression of non-exudative age-related macular degeneration (dry AMD) presents challenges for drug discovery. The standard endpoint used for ophthalmic clinical trials, best-corrected visual acuity, is insensitive to the early stages and slow progression of dry AMD. Effective drug discovery for dry AMD treatments will therefore require novel applications of more effective visual function endpoints. This review will present candidates for visual function endpoints for dry AMD clinical trials. The promising visual assessments include contrast sensitivity, reading speed, microperimetry, and dark adaptation. Their adoption as exploratory endpoints in future trials will be critical for determining their accuracy, precision, and applicability, and ultimately determine their value for drug discovery.
2012
Elze T, Tanner TG. Temporal Properties of Liquid Crystal Displays: Implications for Vision Science Experiments. PloS ONE. 2012;7:e44048.Abstract
Liquid crystal displays (LCD) are currently replacing the previously dominant cathode ray tubes (CRT) in most vision science applications. While the properties of the CRT technology are widely known among vision scientists, the photometric and temporal properties of LCDs are unfamiliar to many practitioners. We provide the essential theory, present measurements to assess the temporal properties of different LCD panel types, and identify the main determinants of the photometric output. Our measurements demonstrate that the specifications of the manufacturers are insufficient for proper display selection and control for most purposes. Furthermore, we show how several novel display technologies developed to improve fast transitions or the appearance of moving objects may be accompanied by side-effects in some areas of vision research. Finally, we unveil a number of surprising technical deficiencies. The use of LCDs may cause problems in several areas in vision science. Aside from the well-known issue of motion blur, the main problems are the lack of reliable and precise onsets and offsets of displayed stimuli, several undesirable and uncontrolled components of the photometric output, and input lags which make LCDs problematic for real-time applications. As a result, LCDs require extensive individual measurements prior to applications in vision science.
Poppe S, Benner P, Elze T. A Predictive Approach to Nonparametric Inference for Adaptive Sequential Sampling of Psychophysical Experiments. Journal of Mathematical Psychology. 2012;56:179–195.Abstract
We present a predictive account on adaptive sequential sampling of stimulus-response relations in psychophysical experiments. Our discussion applies to experimental situations with ordinal stimuli when there is only weak structural knowledge available such that parametric modeling is no option. By introducing a certain form of partial exchangeability, we successively develop a hierarchical Bayesian model based on a mixture of Polya urn processes. Suitable utility measures permit us to optimize the overall experimental sampling process. We provide several measures that are either based on simple count statistics or more elaborate information theoretical quantities. The actual computation of information theoretical utilities often turns out to be infeasible. This is not the case with our sampling method, which relies on an efficient algorithm to compute exact solutions of our posterior predictions and utility measures. Finally, we demonstrate the advantages of our framework on a hypothetical sampling problem. (C) 2012 Elsevier Inc. All rights reserved.