Opening a window on Bálint syndrome : testing a spatial restriction of attention theory (2010)
Bálint syndrome is a disorder of visual attention resulting from bilateral parieto-occipital lesions. Patients experience several visual deficits, including an inability to see more than one object at once, and often an inability to see single objects as wholes. This dissertation examines whether impaired object processing in Bálint syndrome results from a restricted spatial area of visual attention, which creates a “restricted visual window” through which patients view the world. In Study 1, brain activity of healthy individuals is recorded while they view hierarchical stimuli, stimuli that patients tend to see at a local, but not global, level. Activity increased when participants identified global letters, suggesting that patients may have difficulty perceiving these objects due to extra processing demands of the global form. Study 2 uses hierarchical letters to investigate whether patients can employ explicit viewing strategies to compensate for their visual deficits. The patient showed difficulty identifying the global level of the letters, and did not employ a strategy to compensate- she appeared to have little control over what she sees. Study 3 employs a manipulation of healthy vision to model these behaviours to understand what underlies this disorder. Restricting healthy individuals to seeing a small visual area (like “tunnel vision”) leads to object identification patterns similar those of Bálint patients, supporting the idea that a restricted area of visual processing may underlie the disorder. Study 4 used photos depicting social scenes to investigate how patients view complex stimuli. Unlike healthy individuals, patients do not look at the eyes of people in the scenes. In Studies 5-6, healthy individuals view these stimuli through the restricted viewing scenario from Study 3 to determine whether restricted viewing can also model patients’ viewing of complex stimuli. Like patients, healthy individuals made reduced fixations on eyes of people in scenes. Study 7 revisits a patient after some recovery. Her scanning of scenes was approaching normal. Manipulations of the restricted viewing paradigm modeled this recovery in healthy individuals, lending further support to the restricted vision model. The dissertation provides insights into normal and abnormal vision, particularly how the brain creates the objects we see.