Exploring the world with Bálint syndrome: biased bottom-up guidance of gaze by local saliency differences.

Bálint syndrome is a combination of severe deficits affecting spatial attention, visuo-motor control and oculomotor function. While the severe restriction of attention (simultanagnosia) and impairments of visually guided reaching have been extensively studied, oculomotor apraxia has received comparatively little attention. The main explanatory hypothesis of oculomotor apraxia is that it is a direct consequence of the severe restriction of attention. Here, we examined in a patient with Bálint syndrome to what extent local image features such as luminance and contrast predict whether a region will be fixated or not. During the viewing of natural photographs, the patient made saccades of very small amplitude, but showed strongly increased fixation duration. In addition, the horizontal and vertical range of fixations was severely restrained compared to control subjects. When analysing the local feature content at fixation, we found that central fixations of the patient contained less local luminance and contrast than fixations of controls while he made fixations to peripheral image regions with disproportionately high luminance and contrast. These findings suggest that while our patient gazes at central regions irrespective of their local feature content, he only looks to the periphery when his gaze is captured by particularly conspicuous features. We propose that oculomotor apraxia in Bálint syndrome reflects a combination of biased representations within a parietal priority map and increased fixational activity due to biased interactions within the oculomotor network.