اختلالات فکری و روانی و پردازش معنایی: بررسی N400

Accumulating evidence suggests that psychopathy is associated with behavioral and event-related potential (ERP) abnormalities during semantic language tasks. Psychopaths’ ERP abnormalities are most prominent in the 300–500 ms post-stimulus time window. It is unclear whether these ERP differences are related to neurocognitive processes associated with the P300 (i.e., poor attention/orienting/working memory) or N400 (i.e., aberrant semantic processes). To address this issue, the present study employed a canonical semantic sentence processing paradigm known to selectively elicit the N400. Fifty incarcerated participants were divided into psychopathic (n = 25) or nonpsychopathic (n = 25) groups based on scores from the Hare Psychopathy Checklist-Revised. The N400 and P600 components elicited by terminal words of sentences either congruent or incongruent with the previous sentence context were examined. No differences were observed between groups in the behavioral or ERP data. These data do not support the hypothesis that the semantic processes, and underlying neural systems, associated with the generation of the N400 during sentence processing tasks are abnormal in psychopathy.

Clinicians have noted that the behavior of psychopathic individuals is often strikingly inconsistent with their verbalized reports (Cleckley, 1976, Flor-Henry, 1972 and McCord and McCord, 1964). This observation led some clinicians and researchers to speculate that psychopathy may be associated with abnormalities in language processing (Flor-Henry, 1972). Subsequent studies have found that abnormalities in language processes are most prevalent when psychopathic individuals perform tasks that engage semantic processing (Hare, 1979, Hare and Forth, 1985 and Hare and McPherson, 1984). For example, Kiehl, Hare, McDonald, and Brink (1999a) tested the hypothesis that psychopathy is associated with language abnormalities during semantic processing of abstract word stimuli. Consistent with predictions, psychopaths made more errors than did nonpsychopaths when having to classify word stimuli as abstract during a concrete/abstract discrimination task. Event-related potential (ERP) data were also recorded and it was observed that psychopathic individuals failed to show the normal electrocortical differentiation between concrete and abstract words (Tasks 1 and 2 in their study). In noncriminals and in criminal nonpsychopathic individuals, concrete words elicit greater ERP negativity in the 300–800 ms window than do abstract words (Kiehl et al., 1999a, Kounios and Holcomb, 1994 and Paller et al., 1987). Given that the ERP differentiation between concrete and abstract words appears to be most robust 300–500 ms post-stimulus, it has been argued that this effect is due to modulation of the semantic generators believed to contribute to the N400 potential typically observed in semantic word and sentence processing tasks (Kutas and Hillyard, 1980c, Kutas and Hillyard, 1983 and Kutas and Hillyard, 1984). One hypothesis about the functional significance of the N400 suggests that it may reflect processes related to the integration of a word into ongoing cognitive context (Holcomb, 1993). Using this interpretation, Kiehl et al. (1999a) suggested that psychopaths may differ from others in the process responsible for N400 generation. Several additional studies have reported that long-latency ERPs (later than 300 ms) are different in psychopaths than in nonpsychopaths, especially during tasks that present language stimuli in the visual modality. Psychopaths consistently show a large frontally distributed negative wave with a latency of approximately 500 ms to word stimuli. Williamson, Harpur, and Hare (1991) reported that psychopaths exhibited a larger fronto-central N500 to word stimuli during a lexical decision task than did nonpsychopaths. Kiehl et al. (1999a) reported that psychopaths exhibited a large centro-frontal negative wave with latency of about 350 ms during three different language tasks. While both Williamson et al. (1991) and Kiehl et al. (1999a) employed tasks that demanded linguistic processing of different word types, the late negative wave in both studies was elicited for all word types (i.e., positive, negative, and neutral words in Williamson et al., 1991); concrete and abstract words (Tasks 1 and 2) and positive and negative words (Task 3) in Kiehl et al. (1999a), raising the possibility that the negative wave is independent of stimulus content. Based on the similar topography of the psychopathic individuals’ N350 and N500, Kiehl et al. (1999a) suggested two possibilities for the functional significance of these components. The tasks employed by Williamson et al. (1991) and Kiehl et al. (1999a) both involved lexico-semantic processing and required an online or concurrent behavioral response. In the 300–600 ms after a word stimulus is presented both decision-making and semantic processes are engaged and will elicit overlapping ERP components of opposite polarity (i.e., P300 and N400). In general, presentation of a word stimulus in the absence of any online task demands will elicit a large ERP negativity in the 300–500 ms time window (N4 or N400). Thus, one interpretation of the psychopaths’ fronto-central ERP negativities offered by Kiehl et al. (1999a) and Williamson et al. (1991) was that they may be related to an abnormally large N400. The alternative interpretation of the late ERP negativities in psychopaths was that it may be related to an abnormally small positive potential (e.g., P300; Kiehl et al., in press and Kiehl et al., 1999b). However, it is still not clear whether the fronto-central ERP negativities observed in psychopaths can be elicited by visual stimuli that involve semantic processing but do not involve any online behavioral response. One aim of this experiment was to examine the integrity of the neural systems underlying semantic processes in the absence of concurrent task demands to attempt to isolate and characterize the conditions in which large fronto-central negativities are elicited. A second aim was to determine whether the abnormalities observed in psychopaths during semantic language tasks are related to the processes known to elicit the N400 during sentence processing tasks. The literature reviewed above suggests two competing hypotheses. First, given that psychopathy is known to be associated with abnormalities in semantic processes and the interpretation that the N400 indexes semantic processes, psychopaths may show abnormal N400s to the terminal words of sentences that were either congruent or incongruent with the previous sentence context. The alternative hypothesis is that the neurocognitive systems underlying the processes related to the generation of the N400 are not abnormal in psychopathy and that the aberrant late ERP negativities observed in psychopaths are not related to the N400. This hypothesis is supported by aberrant late ERP negativities observed in psychopaths during tasks that place no explicit demands on linguistic processing.