Cue combination for 3D location judgements

Cue combination rules have often been applied to the perception of surface shape but not to judgements of object location. Here, we used immersive virtual reality to explore the relationship between different cues to distance. Participants viewed a virtual scene and judged the change in distance of an object presented in two intervals, where the scene changed in size between intervals (by a factor of between 0.25 and 4). We measured thresholds for detecting a change in object distance when there were only 'physical' (stereo and motion parallax) or 'texture-based' cues (independent of the scale of the scene) and used these to predict biases in a distance matching task. Under a range of conditions, in which the viewing distance and position of the tarte relative to other objects was varied, the ration of 'physical' to 'texture-based' thresholds was a good predictor of biases in the distance matching task. The cue combination approach, which successfully accounts for our data, relies on quite different principles from those underlying geometric reconstruction

An automated calibration method for non-see-through head mounted displays

Accurate calibration of a head mounted display (HMD) is essential both for research on the visual system and for realistic interaction with virtual objects. Yet, existing calibration methods are time consuming and depend on human judgements, making them error prone, and are often limited to optical see-through HMDs. Building on our existing approach to HMD calibration Gilson et al. (2008), we show here how it is possible to calibrate a non-see-through HMD. A camera is placed inside a HMD displaying an image of a regular grid, which is captured by the camera. The HMD is then removed and the camera, which remains fixed in position, is used to capture images of a tracked calibration object in multiple positions. The centroids of the markers on the calibration object are recovered and their locations re-expressed in relation to the HMD grid. This allows established camera calibration techniques to be used to recover estimates of the HMD display's intrinsic parameters (width, height, focal length) and extrinsic parameters (optic centre and orientation of the principal ray). We calibrated a HMD in this manner and report the magnitude of the errors between real image features and reprojected features. Our calibration method produces low reprojection errors without the need for error-prone human judgements

Spatial calibration of an optical see-through head-mounted display

We present here a method for calibrating an optical see-through Head Mounted Display (HMD) using techniques usually applied to camera calibration (photogrammetry). Using a camera placed inside the HMD to take pictures simultaneously of a tracked object and features in the HMD display, we could exploit established camera calibration techniques to recover both the intrinsic and extrinsic properties of the~HMD (width, height, focal length, optic centre and principal ray of the display). Our method gives low re-projection errors and, unlike existing methods, involves no time-consuming and error-prone human measurements, nor any prior estimates about the HMD geometry

Changes in chemsex and sexual behaviour over time, among a cohort of MSM in London and Brighton: Findings from the AURAH2 study.

BACKGROUND: Recent evidence has suggested that chemsex (the use of mephedrone, crystal methamphetamine and γ -hydroxybutyrate/ γ -butryolactone (GHB/GBL) to enable, enhance and prolong sexual interactions) has increased among men having sex with men (MSM) attending sexual health clinics in large UK cities. To date there has been no data from the UK or Europe that describes changes in chemsex over time within a cohort of MSM. METHODS: The prospective cohort study, Attitudes to and Understanding Risk of Acquisition of HIV over Time (AURAH2), collected online questionnaire data from HIV negative or undiagnosed MSM (at enrolment) from 2015 to 2018, recruited from sexual health clinics. We aim to investigate changes in chemsex, three individual drugs associated with chemsex, frequency of chemsex sessions and measures of sexual behaviour, among the cohort of MSM over the study's 3 year follow-up period. RESULTS: In total 622 MSM completed at least one online questionnaire for the AURAH2 study, of which 400 (64.3%) were still engaged with the study within the last six months of follow-up. Prevalence of chemsex significantly declined during the follow-up from 31.8% (198/622) at the first online questionnaire, to 11.1% (8/72; p < 0.001) at the 9th. This decline was reflected in the proportion of MSM reporting use of two of the three individual chemsex drugs: mephedrone use had significantly declined from 25.2% at the first online questionnaire to 9.7% (p < 0.001) at the 9th, GHB/GBL use had also declined from 19.9% to 8.3% (p = 0.001). While crystal methamphetamine use declined, but not significantly (11.1%-6.9% [p = 0.289]). Most measures of sexual behaviour (any anal sex, group sex, recent HIV test and bacterial STI) also tended to decline over the follow-up period, with the exception of CLAI with more than one and more than two partners. CONCLUSIONS: Chemsex and use of two individual chemsex drugs (mephedrone and GHB/GBL) significantly declined over time among individuals in the study, alongside most measures of sexual behaviour with the exception of those related to CLAI. Focusing health promotion and HIV prevention, such as awareness of post-exposure prophylaxis (PEP) and access to pre-exposure prophylaxis (PrEP), on MSM that report chemsex, and in particular problematic chemsex, would be highly beneficial, potentially only necessary for a relatively short period of time for individuals, and could have long term benefits for HIV and STI prevention

Changes in recreational drug use, drug use associated with chemsex, and HIV-related behaviours, among HIV-negative men who have sex with men in London and Brighton, 2013-2016.

OBJECTIVES: The objective of this study was to compare the prevalence of polydrug use, use of drugs associated with chemsex, specific drug use, and HIV-related behaviours, between two time periods , using two groups of HIV-negative men who have sex with men (MSM) attending the same sexual health clinics in London and Brighton, in two consecutive periods of time from 2013 to 2016. METHODS: Data from MSM in the cross-sectional Attitudes to and Understanding Risk of Acquisition of HIV (AURAH) study (June 2013 to September 2014) were compared with baseline data from different MSM in the prospective cohort study Attitudes to and Understanding Risk of Acquisition of HIV over Time (AURAH2) (November 2014 to April 2016). Prevalence of polydrug use, drug use associated with chemsex and specific drug use, and 10 measures of HIV-related behaviours including condomless sex, post-exposure prophylaxis (PEP) use, pre-exposure prophylaxis (PrEP) use, and HIV testing, were compared. Prevalence ratios (PRs) for the association of the study (time period) with drug use and HIV-related behaviour measures were estimated using modified Poisson regression analysis, unadjusted and adjusted for sociodemographic factors. RESULTS: In total, 991 MSM were included from AURAH and 1031 MSM from AURAH2. After adjustment for sociodemographic factors, use of drugs associated with chemsex had increased (adjusted PR (aPR) 1.30, 95% CI 1.11 to 1.53) and there were prominent increases in specific drug use; in particular, mephedrone (aPR 1.32, 95% CI 1.10 to 1.57), γ-hydroxybutyric/γ-butryolactone (aPR 1.47, 95% CI 1.15 to 1.87) and methamphetamine (aPR 1.42, 95% CI 1.01 to 2.01). Use of ketamine had decreased (aPR 0.54, 95% CI 0.38 to 0.78). Certain measures of HIV-related behaviours had also increased, most notably PEP use (aPR 1.50, 95% CI 1.21 to 1.88) and number of self-reported bacterial STI diagnoses (aPR 1.24, 95% CI 1.08 to 1.43). CONCLUSIONS: There have been significant increases in drug use associated with chemsex and some measures of HIV-related behaviours among HIV-negative MSM in the last few years. Changing patterns of drug use and associated behaviours should be monitored to enable sexual health services to plan for the increasingly complex needs of some clients

A demonstration of 'broken' visual space

It has long been assumed that there is a distorted mapping between real and ‘perceived’ space, based on demonstrations of systematic errors in judgements of slant, curvature, direction and separation. Here, we have applied a direct test to the notion of a coherent visual space. In an immersive virtual environment, participants judged the relative distance of two squares displayed in separate intervals. On some trials, the virtual scene expanded by a factor of four between intervals although, in line with recent results, participants did not report any noticeable change in the scene. We found that there was no consistent depth ordering of objects that can explain the distance matches participants made in this environment (e.g. A > B > D yet also A < C < D) and hence no single one-to-one mapping between participants’ perceived space and any real 3D environment. Instead, factors that affect pairwise comparisons of distances dictate participants’ performance. These data contradict, more directly than previous experiments, the idea that the visual system builds and uses a coherent 3D internal representation of a scene

Poly drug use, chemsex drug use, and associations with sexual risk behaviour in HIV-negative men who have sex with men attending sexual health clinics

BACKGROUND: Recreational drug use and associated harms continue to be of significant concern in men who have sex with men (MSM) particularly in the context of HIV and STI transmission. METHODS: Data from 1484 HIV-negative or undiagnosed MSM included in the AURAH study, a cross-sectional, self-completed questionnaire study of 2630 individuals from 20 sexual health clinics in the United Kingdom in 2013–2014, was analysed. Two measures of recreational drug use in the previous three months were defined; (i) polydrug use (use of 3 or more recreational drugs) and (ii) chemsex drug use (use of mephedrone, crystal methamphetamine or GHB/GBL). Associations of socio-demographic, health and lifestyle factors with drug use, and associations of drug use with sexual behaviour, were investigated. RESULTS: Of the 1484 MSM, 350 (23.6%) reported polydrug use and 324 (21.8%) reported chemsex drug use in the past three months. Overall 852 (57.5%) men reported condomless sex in the past three months; 430 (29.0%) had CLS with ≥2 partners, 474 (31.9%) had CLS with unknown/HIV+ partner(s); 187 (12.6%) had receptive CLS with an unknown status partner. For polydrug use, prevalence ratios (95% confidence interval) for association with CLS measures, adjusted for socio-demographic factors were: 1.38 (1.26, 1.51) for CLS; 2.11 (1.80, 2.47) for CLS with ≥2 partners; 1.89 (1.63, 2.19) for CLS with unknown/HIV+ partner(s); 1.36 (1.00, 1.83) for receptive CLS with an unknown status partner. Corresponding adjusted prevalence ratios for chemsex drug use were: 1.38 (1.26, 1.52); 2.07 (1.76, 2.43); 1.88 (1.62, 2.19); 1.49 (1.10, 2.02). Polydrug and chemsex drug use were also strongly associated with previous STI, PEP use, group sex and high number of new sexual partners. Associations remained with little attenuation after further adjustment for depressive symptoms and alcohol intake. CONCLUSION: There was a high prevalence of polydrug use and chemsex drug use among HIV negative MSM attending UK sexual health clinics. Drug use was strongly associated with sexual behaviours linked to risk of acquisition of STIs and HIV

Controversies in the Science of Sedentary Behaviour and Health: Insights, Perspectives and Future Directions from the 2018 Queensland Sedentary Behaviour Think Tank

The development in research concerning sedentary behaviour has been rapid over the past two decades. This has led to the development of evidence and views that have become more advanced, diverse and, possibly, contentious. These include the effects of standing, the breaking up of prolonged sitting and the role of moderate-to-vigorous physical activity (MVPA) in the association between sedentary behaviour and health outcomes. The present aim is to report the views of experts (n = 21) brought together (one-day face-to-face meeting in 2018) to consider these issues and provide conclusions and recommendations for future work. Each topic was reviewed and presented by one expert followed by full group discussion, which was recorded, transcribed and analysed. The experts concluded that (a). standing may bring benefits that accrue from postural shifts. Prolonged (mainly static) standing and prolonged sitting are both bad for health; (b). ‘the best posture is the next posture’. Regularly breaking up of sitting with postural shifts and movement is vital; (c). health effects of prolonged sitting are evident even after controlling for MVPA, but high levels of MVPA can attenuate the deleterious effects of prolonged sitting depending on the health outcome of interest. Expert discussion addressed measurement, messaging and future directions