Title: Mechanisms in crowding
Description: Visual Crowding--the impairment in the ability to identify a target flanked by distracters--is a ubiquitous phenomenon. An understanding of crowding is therefore crucial for furthering our knowledge of the visual system. Further, a deeper appreciation of the mechanisms that underlie crowding would shed light on several central visual processes such as visual search, eye movements, reading, object recognition, attention and consciousness. In this thesis, I explore the mechanisms that underlie this phenomenon in a series of three studies. Study 1 investigates the temporal properties of polarity advantage --a decrease in crowding strength when the target does not share the polarity of the distracters. We show that polarity advantage in crowding disappears at around 7.5 Hz, as predicted by high-level explanations of crowding such as the attentional hypothesis (Intriligator and Cavanagh, 2001). Further, lateral masking, when tested under conditions similar to that of crowding, cannot explain the results obtained in Experiment 1, and hence is an unsatisfactory explanation of crowding.

Study 2 demonstrates that crowding exhibits a signature of selective attention--a Bilateral Advantage . Under crowded conditions, it is easier to identify two targets presented in opposite hemifields than those presented unilaterally. This bilateral advantage cannot be attributed to a generalized advantage for letter recognition or to an advantage for storing letters in visual short-term memory. These results provide additional support to the attentional account of crowding. Finally, in Study 3 we masked the crowding distracters using three kinds of masks--noise, metacontrast, and object substitution masks. When distracters were rendered unreportable by noise and metacontrast masks crowding was significantly alleviated, suggesting that signal integrity of the distracters is crucial for crowding. In contrast, object substitution masking of the distracters did not affect the strength of crowding indicating that crowding occurs prior to the stage of distracter identity representation. In conclusion, current evidence strongly supports the contention that crowding is due to a limitation in the resolution of attention.