Published online by Cambridge University Press: 05 October 2010
Accurately perceiving where objects are in one's visual field is important for making decisions and interacting with the environment, but the visual system must contend with a significant delay – on the order of 100 msec (Lennie 1981; Maunsell & Gibson 1992; Schmolesky et al. 1998) – between the time of retinal stimulation and the time of the elicited percept. To deal with this delay, it has been hypothesized that the visual system has been selected to attempt to generate a percept that compensates for it, so as to perceive the present (Ramachandran & Anstis 1990; De Valois and De Valois 1991; Nijhawan 1994, 1997, 2001, 2002; Berry et al. 1999; Schlag et al. 2000; Sheth et al. 2000; Khurana et al. 2000; Changizi 2001, 2003, 2009; Changizi & Widders 2002). One circumstance where perceiving the present is crucial is when an observer is moving forward and approaching objects. It has been proposed that the classical geometrical illusion stimuli are due to fixations during forward motion and that the illusions are an expected consequence of perceiving the present mechanisms; that is, the classical geometrical stimuli are perceived not as they actually project but as they would project in the next moment if the observer were moving forward (Changizi 2001, 2003; Changizi & Widders 2002). This theory has been used to explain geometrical illusions such as the Hering, Orbison (Ehrenstein), Ponzo, Muller-Lyer, and Poggendorf.