Abstract. A new motion illusion based on dot-trajectory misperception is presented. The illusory effect can not be explained by the aperture problem unlike some previous illusions characterised by misperception of motion direction.We propose an explanation in terms of `perceptual compromise' between an original type of apparent motion and the veridical motion. Several demonstrations are presented in support of that hypothesis. We present here a new motion illusion based on the misperception of dot trajectories. Watching the pattern of figure 1 while the page is scrolled up and down on the com- puter screen, with central fixation, observers report a diagonal expansion motion of the dots when the matrix stimulus is going up and a contraction when it is going down. Although the physical trajectory of each dot is purely vertical, what is perceived is a diagonal outward movement when the pattern is moving up and diagonal inward movement when it is moving down. The phenomenon can be observed also in a movie of the dots in motion [movie 1 (see http://dx.doi.org/10.1068/p6678), where the speed can be changed by pressing the buttons on the right]. The same effect is shown in a simplified pattern in figure 2 and movie 2 (the speed here can also be changed with the buttons on the right). Some previous motion illusions, such as the Barber Pole illusion (Wallach 1935), the Revolving Wheels illusion by Pinna and Brelstaff (2000), and the Rotating-Tilted- Lines illusion by Gori and Hamburger (2006), are characterised by misperception of motion direction. These illusions have been explained by a stimulus analysis based on local receptors (aperture problem), such as small receptive fields in the primary visual cortex (V1) (Bressan and Vezzani 1995; Lide¨n and Mingolla 1998; Gurnsey et al 2002; Gurnsey and Page¨ 2006; Gori and Yazdanbakhsh 2008; Yazdanbakhsh and Gori 2008). LAST BUT NOT LEAST Perceptual compromise between apparent and veridical motion indices: The Unchained-Dots illusion Perception, 2010, volume 39, pages 863 ^ 866 Simone Gori, Enrico Gioraô, D Alan Stubbs½ Department of General Psychology, University of Padua, via Venezia 8, 35131 Padua, Italy; e-mail: simone.gori@unipd.it; ô Department of Psychology, University of Milano-Bicocca, piazza dell'Ateneo Nuovo 1, 20126 Milan, Italy; ½ Department of Psychology, University of Maine, 301 Little Hall, Orono, ME 04469, USA Received 1 March 2010, in revised form 16 April 2010 doi:10.1068/p6678 Figure 1. On scrolling the figure up and down, dot trajectories are misperceived (diagonal instead of vertical).