Visual latency difference was determined directly in normal volunteers, using the rotating Pulfrich technique described by Nickalls [Vision Research, 26, 367-372 (1986)]. Subjects fixated a black vertical rod rotating clockwise on a horizontal turntable turning with constant angular velocity (16.6,33.3 or 44.7 revs/min) with a neutral density filter (OD 0.7 or 1.5) in front of the right eye. For all subjects the latency difference associated with the 1.5 OD filter was significantly greater (P < 0.001) with the rod rotating at 16.6 rev/min than at 33.3 revs/min. The existence of an inverse relationship between latency difference and angular velocity is hypothesized.

No abstract

This article describes a new and general line-and-conic property, leading to a consideration of a ‘six-point’ circle and the related geometry of an interesting visual illusion.

Girls aged between 18 and 25 were eligible for the job, provided they looked the part and had a basic grasp of maths. From The Times, 4 November 1999, gleaned by Frank Tapson who comments 'They had to be good-with figures!'

We tested the validity of Nickalls’ formula for determining visual latency difference by using two rotating Pulfrich techniques: (A) varying viewing distance while keeping target angular velocity constant (33 rev/min) and (B) varying the target angular velocity while keeping the viewing distance constant (180 cm). The formula predicts that the latency differences estimated by the two techniques are equal with a given neutral density filter. Observers were asked to judge whether or not the rotating target (clockwise) appeared to move back-and-forth from side-to-side with a neutral density filter (OD = 0.7, 1.0, 1.3) in front of the right eye. The results with ten observers showed that the mean visual latency differences associated with each technique for a given filter were not significantly different. These results further validate the Nickalls’ formula and, therefore, support the visual-latency hypothesis to account for the Pulfrich phenomenon.