Can semantic priming paradigm differentiate between neglect and hemianopia? Experimental evidences from patients with acquired brain lesion

Background: One of the most frequent deals in nuero-rehabilitation field is the patient’s difficulty in exploring the surrounding space and neglect whatever encloses him. This phenomenon could be caused by an attentional deficit (i.e. visuo-spatial neglect) or by a reduction of visual field width (hemianopia); most of the time, this exploration impairment could be determined by an interactive effect between these two causes. Even though there are different neuro-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 disantagle neglect from hemianopia. Besides these contributions, scientific literature provides lot of results concerning an effective implicit processing of information for patients with neglect which is not present in patients with hemianopia. Since there is a wide agreement in an implicit information processing for patients with neglect, we hypothesize that a priming word in the neglected field should determine a semantic activation effect even when it is not consciously perceived by the patient; on the contrary if the priming word occurs in a blind hemifield should not determine any activation effect. In this work we will describe the performance of three patients compaired to 15 healthy subjects in a semantic priming task. Materials and methods: The experimental procedure consisted in a fixation point (+) which lasted in the center 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 (from A=extreme left to F=extreme right). The prime lasted 300 ms and, after 150 ms blanc, by the target word. Target words may belong to living or nonliving category. Subjects and patients were required to press the space-bar only when the target word belonged to a living category. Three different conditions were present: related (the same category for prime and target), unrelated (different category for prime and target) and neutral (instead of a prime word, an “x” string appeared). Three patients took part to the experiment: two of them presented a right hemispheric lesion and left neglect; one a bilateral lesion which determines left neglect and right homonymous hemianopia. Results: We analyzed data of healthy subject by means of a repeated measure ANOVA with two dependent variables: prime (related, unrelated, neutral) and position (A,B,C,D,E,F). The participants showed shorter reaction times activation in the condition of semantic relation between prime and target (p<.05); moreover this semantic activation were present in all prime positions (p<.05). We compared the data of the patients with those one of healthy subjects by means of a Crawford analysis: the two patients with right brain lesion and left neglect showed a significant activation effect when in the “related condition” in all portions of space, except when the prime occurred in the extreme left position (named A). On the other hand the patient with left neglect and right hemianopia showed semantic activation in left portions (neglected space) but not in right space (hemianopic field). Conclusions: We aimed to find a task that could be inserted in the differential diagnostic procedure for neglect and hemianopia. Our experimental results seem to show significant semantic priming effect in healthy partecipants: all of them showed lower RTs in case of relation between prime and target for every position in which prime occurred. In addition the data we obtained from patients confirmed our work hypothesis, that is we found a semantic activation in the space which is affected by neglect (failures to neuropsychological tasks) but not in hemianoptic field.