Motion parallax has been shown to be an effective and unambiguous source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movements with respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, and/or (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.

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The horizon-ratio relation was found to be an effective source of information for relative size in pictures under some conditions: when the difference in image size of depicted "same real size" objects was not too great (Experiment 1), and when the horizon line was not too high or too low in the picture (Experiment 2). The latter fmding seemed to be linked to the observers' identification of the horizontal line as the horizon (and not as the edge of a finite surface). In addition, individual patterns of response were remarkably systematic even in the absence of a horizon, or any other pictorial information, (Experiment 3). It is suggested that in this case observers imposed a horizon on the picture on which to base their relative size judgments, possibly based on the observer's own eye level or on the content of the picture. It is concluded that although the horizon-ratio relation provides the same kind of information as that available in the optic arrays from real scenes, pictorial information requires the satisfaction of additional constraints in order to be fully effective.Questions concerning the perception ofspatial layout in pictures have traditionally been framed in terms of"pictorial cues" (e.g., height in the picture, relative size, interposition). Research has tended to focus on the development of sensitivity to these cues in infants and young children and, to a lesser extent, among adults from non-Western cultures (see, e.g., Benson & Yonas, 1973;Granrud, Yonas, & Opland, 1985;Ireson & McGurk, 1985;Jahoda & McGurk, 1974;Olson, 1975;Olson & Boswell, 1976; Wilcox & Teghtsoonian, 1971;Yonas & Hagen, 1973). Less attention has been paid to how adults, within our own culture, successfully perceive depicted spatial relations.New questions were raised, however, by the alternative approach to visual perception proposed by 1. 1. Gibson (1966Gibson ( , 1979. Gibson argued that perception does not require the construction of space from local and punctate cues but rather the direct detection of invariant optical structures that are informative about properties of the environment. Gibson also extended this theory to account for the perception of pictures. He claimed that there is an intrinsic structural relation between pictures and the real scenes they represent; that there are informative structures in pictures; and that these structures are of the same sort as those found in the changing optic array from a real scene (1. 1. Gibson, 1971Gibson, , 1979).Gibson's followers have been keen to maintain the case for the structural equivalence of pictures and scenes (see, e.g., the contributors to Hagen, 1980). So far, however, little has been provided by way ofexample of invariant structure in the changing optic array that might also constitute a possible pictorial "invariant." Little, that is, until SedgCorrespondence may be sent to S. Rogers, Department of Psychology, University of Wisconsin, 1202 W. Johnson St., Madison, WI 53706 (e-mail: sheenar@ macc.wisc.edu).wick (1973) identified the horizon as the basis of a s...

Background Violence toward MSM and female sex workers (FSW) is associated with HIV risk, and its prevention is prioritized in international HIV/AIDS policy. Methods Sociodemographic and behavioural data derived from HIV risk and follow-up cohorts including MSM and FSW in coastal Kenya between 2005 and 2014 was used to estimate the risk of rape, physical assault and verbal abuse, and to assess associations between first occurrence of assault with individual and recent behavioural factors. Results Incidence of first reported rape was similar for MSM [3.9, confidence interval (CI) 3.1–5.0 per 100 person-years (pyrs)] and FSW (4.8 CI 3.5–6.4 per 100 pyrs), P =0.22. Incidence of first reported physical and verbal assault was higher for FSW than MSM (21.1 versus 12.9 per 100 pyrs, P =0.14 and 51.3 versus 30.9 per 100 pyrs, P =0.03 respectively). Recent alcohol use was associated with reporting of all forms of assault by MSM [adjusted odds ratio (AOR) 1.8, CI 0.9–3.5] and FSW (AOR 4.4, CI 1.41–14.0), as was recent sale of sex for MSM (AOR 2.0, CI 1.1–3.8). Exclusive sex with men, active sex work, and group sex were also specifically associated with reporting rape for MSM. Perpetrators of sexual and verbal assault were usually unknown, whilst perpetrators of physical violence toward FSW were usually regular sexual partners. Conclusion MSM and FSW experienced a similarly high incidence of sexual assault in coastal Kenya, in addition to physical and verbal assault. Current national policies focus heavily on gender-based violence against women and young girls, but need to be inclusive of MSM and FSW.