Emotional Faces Modulate Spatial Neglect: Evidence from Line Bisection

Emotional Faces Modulate Spatial Neglect: Evidence from Line Bisection Marco Tamietto (tamietto@psych.unito.it) Luca Latini Corazzini (latini@psych.unito.it) Lorenzo Pia (l_pia@psych.unito.it) Giuliano Geminiani (geminian@psych.unito.it) Department of Psychology and Center for Cognitive Science, Via Po 14, 10123 Torino, Italy Marina Zettin (marinazettin@virgilio.it) Puzzle Center for Neurocognitive Rehabilitation, Via Cimabue 2 Torino, Italy Maurizio Gionco (neurologia@mauriziano.it) Department of Neurology and Rehabilitation, Mauriziano Hospital Umberto I, Largo Turati 62, 10128 Torino, Italy Abstract One patient with left spatial neglect (FM) and four right-brain damaged controls without neglect were tested on a line bisection task with pictures of neutral and emotional faces of the same size as unilateral cues. Our aim was to investigate whether bisection biases induced by cuing can be better explained as the result of a direct perceptual lengthening of the cued part of the line or, alternatively, because cues draw spatial attention thereby increasing the salience of that side. We thus manipulated the attentional salience of the cues (higher for emotional faces and lower for neutral faces) while keeping physical dimensions and perceptual characteristics of the stimuli constant. Our findings showed that left emotional faces were more effective than left neutral faces in reducing bisection errors in patient FM. These data indicate that in the left neglected hemispace cues bias attention rather than simply altering the perceptual point of balance of the line in the horizontal plane. Keywords: emotion; neglect; attention; face processing. line bisection; cueing; Introduction Patients with unilateral spatial neglect tend to ignore (i.e., do not react to and do not search for) stimuli in the contralesional hemispace. A typical clinical test for the diagnosis of neglect is the line bisection task where subjects are asked to mark the midpoint of a horizontal line (Schenkenberg, Bradford, & Ajax, 1980). Generally, the rightward bisection bias shown by neglect patients tends to decrease with unilateral left cues and is enhanced with unilateral right cues (Riddoch & Humphreys, 1983). This effect is usually obtained by placing either numbers or letters at the left or right end of the line and asking subjects to pay attention to them (reading them out) prior to bisection. Despite its empirical consistency, the explanation for the cueing effect is still the subject of debate. Indeed, several authors explained cueing effects in terms of attentional mechanisms, whereas other authors have challenged this view proposing a perceptual account. The former hypothesis posits that a cue draws attention to that side, thereby rendering the cued side more salient. As a result of this increased salience, the cued side is overestimated in length and the mark is placed toward that side (Milner, Brechmann & Pagliarini, 1992; Olk & Harvey, 2002). Conversely, the latter interpretation points out that since cues are placed beyond the true endpoint of the line, they extend the line’s horizontal extent by several millimetres. This misperception of line’s length, induced by the alteration in the perceptual point of balance of the stimulus in the horizontal plane, explains the cueing effect without the need of postulating any attentional shift (Fischer, 1994; Mattingley et al., 1993). Disentangling attentional from perceptual interpretations has been particularly problematic with unilateral cueing as it is impossible to separate the relative contributions of an attentional manipulation from those caused by the alteration of the perceptual point of balance when the same cue is used. One way to empirically contrast attentional and perceptual accounts in a line bisection task with unilateral cues is to vary the attentional salience of the cues while keeping their physical extent constant. Previous studies showed that emotional faces have a special advantage over neutral faces in summoning spatial attention (Hansen & Hansen, 1988; Ohman, Lundqvist & Esteves, 2001; Vuilleumier et al., 2001; Vuilleumier & Schwartz, 2001). This is probably due to the high biological and social and value of emotional stimuli. In the present study we used neutral and emotional faces of the same size as unilateral left or right cues to address the issue of the underlying mechanisms, attentional or perceptual, responsible for the cueing effect in neglect. By considering the foregoing alternative explanations of the cueing effect, we can formulate the following hypotheses: if the cueing effect works through biasing attention we should observe a modulation of the line bisection error as a function of the salience conveyed by the cues. That is; emotional cues