We think we figured out the disease mechanism in simultanagnosia (a neuropsychological disease in which the patients see only one object at the time, and are unable to identify large visual scenes). This is a rare but a very invalidating disease: the patients can no longer read or orient themselves in unfamiliar surroundings. I hope my research will help towards better assessment and treatment for these patients. However, I must confess that I did it also for a selfish reason. I find neuropsychological puzzles cool and I think I solved this one.

In brief, simultanagnosia patients cannot see what they are directly looking at. Central and peripheral objects compete for attention and objects in the periphery win this competition. If multiple objects are instead presented in the visual periphery only, the patients are better able to perceive them simultaneously.
An abstract is below:

Abnormal center-periphery gradient in spatial attention in simultanagnosia

Daniela Balslev, Bartholomaeus Odoj, Johannes Rennig, Hans-Otto Karnath

Journal of Cognitive Neuroscience (in press)

Patients suffering from simultanagnosia cannot
perceive more than one object at a time. The underlying mechanism is incompletely
understood. One hypothesis is that simultanagnosia reflects “tunnel vision”, a
constricted attention window around gaze, which precludes the grouping of individual
objects. Although this idea has a long history in neuropsychology, the question
whether the patients indeed have an abnormal attention gradient around the gaze
has so far not been addressed. Here we tested this hypothesis in two simultanagnosia
patients with bilateral parieto-occipital lesions and two control groups, with
or without brain damage. We assessed the
subjects’ ability to discriminate letters presented briefly at fixation with
and without a peripheral distractor or in the visual periphery, with or without
a foveal distractor. A constricted span of attention around gaze would predict
an increased susceptibility to foveated versus peripheral distractors. Contrary
to this prediction and unlike both control groups, the patients’ ability to
discriminate the target decreased more in the presence of peripheral compared
to foveated distractors. Thus the attentional spotlight in simultanagnosia does
not fall on foveated objects as previously assumed, but rather abnormally
highlights the periphery. Furthermore, we found the same center-periphery gradient
in the patients’ ability to recognize multiple objects. They detected multiple,
but not single objects more accurately in the periphery than at fixation. These
results suggest that an abnormal allocation of attention around the gaze can disrupt
the grouping of individual objects into an integrated visual scene