emotional prosody perception, though processed by both hemi- spheres, is relatively lateralized to the right hemisphere [43]. How alexithymia affects the processing of the emotional qualities of speech has only been investigated by two previous studies. In a behavioral study, Swart and coworkers [15] presented high- and low-scorers on the verbalizing subscale of the BVAQ with sentences conveying an emotional content (e.g., sad) spoken in incongruous (e.g., happy) emotional prosody. No statistically significant differences in emotional prosody identification were observed as a function of alexithymia. Nevertheless, it is conceiv- able that alexithymia affects emotional prosody comprehension in a more subtle manner evading detection through behavioral measures. ERPs with their measurement sensitivity in the range of milliseconds are potentially more suited to detect such subtle processing impairments. Following this rationale, we conducted a previous ERP study [44] using emotional prosody, music, and words with emotional connotations in order to test the impact of TAS-20 alexithymia scores on cross-modal affective priming as well as on amplitudes of the N400, an indicator of the perception of mismatches in affective meaning [45]. In line with Swart and colleagues, no behavioral differences were observed. However, alexithymia correlated negatively with N400 amplitudes during affective categorization of happy and sad prosody and music targets, confirming our hypothesis of a reduced sensitivity during the perception of mismatches in the emotional qualities of speech and music with increasing alexithymia scores [44]. The present study was designed to further investigate the impact of alexithymia on the electrophysiological processing of emotional speech, taking its two dimensions into account. In addition to the attended processing of emotional speech (participants detected deviants in emotional prosody), unattended emotional speech processing was tested (participants watched a movie while emotional prosodic stimuli were played in the background). An auditory oddball paradigm was employed in both tasks, in which occasional deviant stimuli (20%) were presented in a sequence of frequent standard stimuli (80%). The relation between alexithymia and abilities to identify emotions conveyed by speech was further tested in a behavioral (off-line) task. Emotional prosody and event-related potentials ERP components of interest during attended emotional prosody processing (deviant detection) are the early components N1 and P2 as well as the late component P3 [46]. The N1 is a negative deflection with a central maximum peaking 100 ms after the onset of a prosodic stimulus. It is generated in bilateral secondary auditory cortex [47] and reflects the extraction of acoustic cues (e.g., stimulus frequency and intensity) during early acoustic processing. The amplitude of the N1 increases with the amount of attention devoted to an acoustic stimulus [48,49]. The P2 is a positive deflection occurring 200 ms after stimulus onset with an anterior maximum. It is thought to reflect the initial detection of emotional salience in auditory material (i.e., early emotional appraisal) independent of whether the stimuli contain semantic emotional information [50,51]. The P3 is a longer-lasting later occurring positivity with a centroparietal maximum starting at 300 ms after the prosodic stimulus. It reflects the cognitive evaluation and classification of task-relevant targets and is therefore related to the decisional, response-related processing stage. The P3 is highly dependent on stimulus context and levels of attention and arousal [52]. Reduced amplitudes and prolonged latencies of the P3 are often used as indicators of cognitive impairment in psychopathology [53], reflecting reduced cognitive resource allocation to task-relevant stimuli and a slowing down of cognitive processes. The Sound of Feelings The ERP component of interest during early unattended processing of emotional prosody (movie watching with prosodic stimuli played in the background) is the Mismatch Negativity (MMN). The MMN, elicited without the participant’s attention, occurs between 100 and 200 ms after the onset of a prosodic stimulus and is generated in secondary auditory cortex and inferior frontal cortex [54]. It reflects the formation of memory traces and the detection of differences between auditory stimuli [55,56], and its amplitude varies with the amount of personal significance assigned to the deviating event. The MMN is thought to reflect higher-order perceptual processes underlying stimulus discrimina- tion rather than only the encoding of simple physical differences between stimuli [57]. Complex stimuli may elicited an MMN with two peaks, with the early peak (eMMN) reflecting the detection of differences based on acoustic stimulus features and the later peak (lMMN), sometimes termed ‘late discriminative negativity’ reflect- ing higher-order integrative processes in auditory perception and a more global, ‘gestalt-based’ processing of auditory stimuli [58,59]. Early components (,300 ms) such as the N1 and the P2 have been suggested before to differ as a function of alexithymia by studies on visual emotional processing [35,38] and by a study on the processing of aversive auditory information [42]. Later components (.300 ms) such as the P3, LPP, and N400 have been found to be reduced during visual [36,38,39] and visual- auditory emotional processing in alexithymia [44]. Though not investigated in relation to alexithymia, the MMN has been reported to differ in individuals with Asperger’s syndrome [59,60], part of the Autism Spectrum with which alexithymia exhibits high comorbidity [3,4]. In light of the existing evidence, we hypothesized a modulation of the early N1 and P2 components by alexithymia during the detection of deviants in emotional prosody, as well as of the MMN during unattended emotional speech processing. We further predicted a reduced sensitivity during overt processing of emotional speech qualities, reflected in reduced amplitudes of the P3 with increasing alexithymia scores. In addition, we hypothesized to find evidence for a left hemisphere preference during the perception of emotional speech with increasing alexithymia scores. Lastly, we wished to determine whether the cognitive and the affective alexithymia dimensions exert a disso- ciable impact on the attended and unattended processing of emotional prosody and on the ability to identify emotions conveyed by speech. A purely female sample of participants was chosen taking into account previously demonstrated gender differences in emotional prosody perception at the behavioral as well as at the electrophysiological level [61–66]. For instance, in an oddball paradigm using emotional and neutral prosody, larger amplitudes of the mismatch negativity (MMN), an event-related potential (ERP) also used in the present study occurred in female, but not in male participants during unattended perception of deviants in emotional prosody [64]. Methods Participants The 20-item Toronto Alexithymia Scale (TAS-20) was used as a brief assessment tool of alexithymia scores in a total sample of 1039 female students from the University of Groningen. From this total sample, twenty students with high TAS-20 scores (57–72, mean 62.9, SD 4.7) and 20 students with low TAS-20 scores (20– 35, mean 28.9, SD 3.6) were selected and invited to the EEG session, along with 20 students with average scores on the TAS-20 PLoS ONE | www.plosone.org 3 May 2012 | Volume 7 | Issue 5 | e36951