Bilateral mirror symmetry is often thought to be particularly salient to human observers. It has been hypothesized that symmetry engages specialized mechanisms that evolved to sense symmetrical objects in nature. However, although symmetry is a commonly encountered stimulus property, studies have shown that sensitivity to mirror symmetry does not serve an alerting function when embedded in noise (Gurnsey et al., Can Soc Brain Behav Cog Sci, 1998b). Further, sensitivity to symmetry decreases
similarly to other common stimuli when targets are presented away from the centre of the visual field (for review: Wagemans, Spat Vis, 1995).
  The three experiments presented in this thesis show that symmetrical targets are vulnerable to the same interference as other stimuli when surrounded by non-target elements. The data shares many of the common characteristics attributable to the crowding phenomenon in current and historical literature (for review: Whitney & Levi,
Trends Cog Sci, 2011). Namely, we find little or no effect of crowding at fixation. The magnitude of the crowding effect increased nonlinearly with eccentricity and at a greater rate than the linear increase of resolution loss (e.g., Gurnsey et al., JoV, 2011; Latham &
Whitaker, Ophthalmic Physiol Opt, 1996). In this case, standard double linear size scaling procedures were unable to characterize the data across the visual field and
produced untenable results that violate assumptions of the crowding phenomenon. Taken together, the results provide evidence that symmetry is unlikely to be processed in
parallel fashion by low-level mechanisms.