Emotional processing and its impact on unilateral neglect and extinction.

Unilateral spatial neglect is a neurological disorder characterized by impaired orienting of attention to stimuli located in the contralesional space, typically following right-hemisphere damage. Neuropsychological investigations in the past two decades have demonstrated that neglect is caused by deficits affecting a widespread cortico-subcortical fronto-parietal network controlling spatial attention, but usually sparing early sensory pathways. As a consequence, certain residual abilities in sensory processing remain intact and still take place for stimuli in the neglected space, such as the extraction and organization of coherent or meaningful object features. Moreover, these residual abilities can alleviate inattention symptoms when contralesional stimuli are perceptually or biologically salient. Here we review recent studies suggesting that the emotional content of stimuli may also be processed despite impaired attention towards contralesional space, and that such processing may act to enhance attention and partly reduce neglect for these stimuli, relative to similar but emotionally neutral stimuli. For example, faces with emotional expressions, voices with emotional prosody, as well as pictures of scenes or even spiders have been found to be less severely extinguished from awareness in conditions of bilateral stimulations, and/or lead to fewer omissions in search tasks with multiple distracters. Gaze cues and reward learning might also produce similar effects. Altogether, these findings suggest that emotionally significant information is not only extracted from stimuli at neglected locations through spared pathways, but can also induce emotional biases in attention that partly counteract the abnormal spatial biases caused by fronto-parietal damage. We discuss results from neuropsychology and neuroimaging research suggesting that specific mechanisms for emotional attention might exist, centered on the amygdala and other limbic regions, and that these mechanisms can operate partly independent from other circuits controlling spatial and object-based attention. Although we are only beginning to understand these interactive effects of emotion and attention and to identify their neuroanatomical substrates, we believe that a deeper knowledge of such mechanisms and their conditions of optimal operation will help develop or improve therapeutic strategies in neglect patients.