1 Introduction Image statistics reveal that visual forms are generally characterised by local edge micro-discontinuities (Field and Brady 1997). The visual system bypasses these irregu- larities by getting them under threshold and unifying discontinuities towards successive approximation of the image (Marr 1982). This conveys the perception of regular and stable shapes. However, recent evidence (Pinna 1990; Pinna et al 2001; Pinna and Gregory 2002; Anstis 2005; Anstis and Casco 2006; Anstis et al 2006) shows that static and visi- ble edge irregularities (terminators, first-order discontinuities, or inflections) can affect global motion signals in a way that is not consistent with retinal stimulation. In such cases, the mismatch between the global motion signal and retinal motion results in an apparent loss of rigidity of the shape (Pinna 1990; Pinna et al 2001). Consistent with this evidence, Pinna and Fantoni (2004) found that, when the eyes move back and forth along the elongation axis of a jagged diamond, the retinal motion of the jags generates strong local motion signals along the sides of the diamond. Far from being ignored, these signals affect the global motion of the diamond sides, which appear to move in a direction almost orthogonal to the direction of eye movement and opposite to that expected on the basis of the aperture effect. Figure 1 depicts three displays used by Pinna and Fantoni (2004): diamond shapes with a vertically oriented major axis and acute half-angles y  358 (referred to here as the convergence angles). When the eyes track a pen tip that is moved along the elongation axis of any one of the two leftmost jagged diamonds (with triangular and wavy edge discontinuities, respectively) two apparent-motion effects co-occur: (i) local shift (LS) öthe jags appear to move in the opposite direction to the eye movement in a way consistent with the retinal displacement Apparent motion by edge discontinuities Perception, 2008, volume 37, pages 973 ^ 992 Carlo Fantoni Department of Psychology, University of Trieste, via Sant'Anastasio 12, I 34134 Trieste, Italy, and B.R.A.I.N. Center for Neuroscience; e-mail: fantoni@psico.units.it Baingio Pinna Department of Sciences of Languages, University of Sassari, via Roma 14, I 74100 Sassari, Italy Received 26 February 2007, in revised form 8 March 2008 Abstract. When the eyes move vertically across a jagged diamond, a local shift (LS) of edge discontinuities and a global shape distortion (GD) (ie expansion/contraction opposite to that expected by the aperture effect) are perceived. These phenomena cannot be accounted for by a local motion signals integration rule based either on the intersection of constraint lines or on the velocity vector summation. The threshold for GD perception and the salience of LS and GD (1 to 10 scale) were measured in two experiments by different methods and displays. In experi- ment 1 we induced GD through mimicking LS with a kinetic pattern constituted of a set of circular illusory apertures revealing drifting gratings. The point of subjective equality for com- pression/expansion was reached for gratings the linear extrapolations of which form an angle of 94.48. In experiment 2 observers followed a dot moving along the vertical elongation axis of a static jagged diamond (with 708 or 908 angles), varying in the shape (triangular, wave, square), frequency, and amplitude of edge discontinuities. GD scores were correlated with LS scores that were inversely related to frequency=amplitude ratios of triangular edge discontinuities. Data are partially accounted for by averaging neighbouring local motion-capture vectors. Results prove that there are strong interrelations between phenomena in which visual motion affects visual localisation and phenomena involving apparent deformation of global shape. doi:10.1068/p5782