1 Introduction The Freezing Rotation illusion, reported by Du«rsteler (2006), occurs when a centre stimulus rotates in one direction with a constant angular velocity across a background pattern that itself rotates concentrically with an angular velocity that varies sinusoidally (its direction thus alternating between clockwise and counterclockwise): during the phase when the centre and background stimuli move in the same direction the centre stim- ulus is seen as stationary in space, ie as frozen on the screen, but when they move in opposite directions there seems to be no evidence of an illusion. Although at first glance this phenomenon seems to resemble the freezing illusion described earlier by Mesland and Wertheim (1996), it differs fundamentally from it. Mesland and Wertheim found that, when a visual pattern moves horizontally with a fixed velocity across a monitor, eg a laptop, the grating appears to be frozen on the screen when the laptop itself is moved in the opposite direction. This has been explained as a misperception of relative motion (between the pattern and the rims of the laptop's screen), which presumably is caused by an elevation of the threshold for relative motion. When that threshold becomes large enough, relative motion between the grating and the rims of the laptop is no longer perceivable, and consequently the grating appears to be frozen on the screen (see Wertheim and Reymond 2007, for details). The Freezing Rotation illusion, however, concerns the perception of absolute motion: the centre stimulus is not seen as frozen relative to its moving surround, but as sta- tionary relative to external space. Relative motion between centre and background, ie a difference in their velocities, remains very well perceivable. Hence it is a different phenomenon from the one reported by Mesland and Wertheim. However, Du« rsteler (2008) suggested that relative motion between the centre stimulus and its background may nevertheless play a central role in the Freezing Rotation illusion. Centre ^ surround relative motion and the Freezing Rotation illusion Perception, 2009, volume 38, pages 1610 ^ 1620 Alexander H Wertheim, Chris L E Paffen Helmholtz Institute and Department of Experimental Psychology, Utrecht University, Heidelberglaan 2, 3584-CS Utrecht, The Netherlands; e-mail: a.h.wertheim@uu.nl Received 16 February 2009, in revised form 24 March 2009; published online 28 October 2009 Abstract. In the Freezing Rotation illusion a stimulus rotating with a constant velocity is perceived as stationary on the screen, when it is presented in front of a background pattern that moves with a sinusoidal velocity profile, during the phase in which stimulus and background rotate in the same direction. It has been suggested that this illusion is caused by the interfering effect of induced motion resulting from the relative motion between the centre and the surround. Since the magnitude of such an induced motion component presumably relates to the difference between background and centre velocities, the illusion itself should also be related to the amount of relative motion between the centre and the surround, and it should not occur when this differ- ence is zero. We present evidence here that this hypothesis is incorrect. First, we show that the illusion also occurs when the background moves with a constant velocity instead of sinusoidally. Second, we show that the illusion consists of a fixed underestimation of centre velocity when the centre and the surround move in the same direction and a fixed overestimation of centre velocity when they move in opposite directions or when the background remains stationary. The amount of underestimation and overestimation of velocity is not related to the velocity difference between the centre and the surround. Some factors that may be relevant to the explanation of the illusion are discussed. doi:10.1068/p6387