Detection of linear ego-acceleration from optic flow.

Human observers are able to estimate various ego-motion parameters from optic flow, including rotation, translational heading, time-to-collision (TTC), time-to-passage (TTP), etc. The perception of linear ego-acceleration or deceleration, i.e., changes of translational velocity, is less well understood. While time-to-passage experiments indicate that ego-acceleration is neglected, subjects are able to keep their (perceived) speed constant under changing conditions, indicating that some sense of ego-acceleration or velocity change must be present. In this paper, we analyze the relation of ego-acceleration estimates and geometrical parameters of the environment using simulated flights through cylindrical and conic (narrowing or widening) corridors. Theoretical analysis shows that a logarithmic ego-acceleration parameter, called the acceleration rate ρ, can be calculated from retinal acceleration measurements. This parameter is independent of the geometrical layout of the scene; if veridical ego-motion is known at some instant in time, acceleration rate allows updating of ego-motion without further depth-velocity calibration. Results indicate, however, that subjects systematically confuse ego-acceleration with corridor narrowing and ego-deceleration with corridor widening, while veridically judging ego-acceleration in straight corridors. We conclude that judgments of ego-acceleration are based on first-order retinal flow and do not make use of acceleration rate or retinal acceleration.

[Investigation on occupational hazards of ultraviolet light, sunscreen awareness and behaviors in Wuhan city traffic police].

OBJECTIVE: To understand the awareness of occupational hazards to ultraviolet (UV) and sunscreen awareness, protective measures in Wuhan City traffic police on duty outside. METHODS: The investigation included questionnaire survey in Wuhan City 367 traffic police on duty outside, talk with them face to face, fill in the questionnaires, and medical examine skin of exposed parts of body of them and 134 Wuhan City administration staffs. RESULTS: They understand UV harm to the human body and skin well (94.8% of them know that UV harm to skin), did not understand sun skin care and protective measures enough, and did not adopt enough sun skin care and protective measures (only 3.8% of them use sun skin care more than twice); but contrast to older persons, younger traffic police had better understanding of UV radiation damage on the human body and the skin, and sunscreen products and protective measures, paid more attention to sunscreen, and had less chance of sunburn (in the past 5 years, 18.3% of younger traffic police had sunburnt more than 3 times, but for older traffic police, the number is 30.3%). Traffic police had more skin problems than administration staffs in exposed parts of body (Traffic police face appears oily and large pores, facial pigmentation spots, face telangiectasia, deep wrinkles crude rates respectively were 73.7%, 40.4%, 36.5%, 10.4%, but for administration staffs, the numbers respectively were 26.1%, 15.7%, 15.7%, 1.5%). CONCLUSION: UV can induce skin problems in exposed parts of body. The traffic police should be enhanced the publicity and education on UV-related knowledge and occupational hazards, especially for older traffic police.