Eye movements and semantic priming analysis for a differential diagnosis between hemianopia and neglect

The difficulty in exploring the surrounding space after a brain damage could be caused by an attentional deficit (neglect) or by a reduction of visual field width (hemianopia). Even though there are different anatomical substrates between neglect and hemianopia, the performances of the patients may provide confounding results at the specific examinations. In particular it is not yet clear if the observation at the neuropsychological tests is a unique expression of neglect nor if the result at the visual-field exam is exclusively dependent by hemianopia. In the last twenty years only few studies tried to disentangle neglect from hemianopia. Besides these contributions, scientific literature provides lot of results concerning an implicit information processing for patients with neglect which is not present in patients with hemianopia. We then hypothesize that a prime-word in the neglected field should determine a semantic activation effect; whilst, if the prime-word occurs in a blind hemifield, the stimulus should not determine any activation effect. Considering this hypothesis, we studied the performance of a patient with a bilateral brain lesion causing left visual neglect and right homonymous hemianopia. The experimental procedure consisted in a fixation point in the centre of the screen for 150 ms; after that, a prime word occurred in six possible positions on the central horizontal line of the screen corresponding to three positions on the left and three on the right. At that point, a target word appeared in the centre of the screen. The patient was required to press the space-bar only when the target word belonged to a living category. Noteworthy, the primed word could be defined as related, unrelated or neutral (namely, an “x” string) with the target word. During the experimental session, eye movements were recorded by means of an infrared-based video tracking that provided further details in describing how patients explored the space. A significant semantic activation was observed when prime-words occurred in neglect hemifield (left) but no activation was found when they appeared in blind hemifield (right). Eye movement analysis showed the presence of saccades towards neglect hemifield when prime occurred in extreme-left positions and fixations when prime occurred in central-left position. We did not observe any saccade nor fixation when prime occurred in hemianopic field. Our results are in line with the hypothesis concerning semantic activation in the space affected by neglect but not in hemianoptic field. In addition, eye-movement analysis supports evidences concerning a spared automatic orientation of attention towards neglect space. In conclusion, even thought our data concern a single case study and further observations are needed, our findings seem to confirm the importance of a comparison between behavioural and visual exploration data to differentiate between neglect and hemianopia.