Event-related potentials (ERPs) such as Nd, N2b, and P300 in an attentional task and an auditory oddball task were compared among 54 adult AD/HD patients, 43 schizophrenic patients (SZ), and 40 healthy age-matched volunteers (HC). It is known that Nd, N2b, and P300 reflect selective attention, voluntary attention, and cognitive context updating respectively. The peak amplitude of P300 was significantly lower in the adult AD/HD and SZ groups than in the HC group. The peak latencies of late Nd, N2b, and P300 were significantly longer in the SZ group than in the HC and adult AD/HD groups. Thus, attenuated amplitude and prolonged latency of various ERP components in the SZ group suggest the possibility of impairment of basic mechanisms underlying cognitive processing. Unlike the SZ group, the adult AD/HD group exhibited reduced amplitude of P300 but not prolonged latency. These findings suggest the existence of a different type of cognitive dysfunction in the adult AD/HD group, which might be closely related to attentional function.
A number of studies have recently reported that patients with attention-deficit/hyperactivity disorder (AD/HD) failed to exhibit alleviation of symptoms such as inattentiveness and impulsiveness even when they grew into adulthood (McGough and Barkley, 2004, Barkley and Brown, 2008 and Barkley, 2009). Several electrophysiological findings have been reported regarding the characteristics of event-related potentials (ERPs) in pediatric AD/HD patients (Strandburg et al., 1996, Jonkman et al., 1997a, Jonkman et al., 1997b and Jonkman et al., 2004). Jonkman et al., 1997b and Jonkman et al., 2004 reported that auditory P300, reflecting cognitive context updating (Donchin and Coles, 1988 and Verleger, 1988), was significantly smaller in AD/HD children than in a control group, but also reported that subjects with AD/HD did not differ from normal individuals in N2b, a component of N2 related to voluntary attention (Simson et al., 1977 and Fitzgerald and Picton, 1983). They hypothesized that in AD/HD children, there is a deficit in activation of the P300 process. Strandburg et al. (1996) reported that P300 amplitude during a visual continuous performance test was reduced in AD/HD children, and that its latency was longer than in a normal group. They found that AD/HD children had a diminished late frontal negative component, suggestive of reduced involvement in post-decisional processing. Groom et al. (2008) noted that it would be useful to compare schizophrenia and another neurodevelopmental disorder such as AD/HD. They reported that such a comparison would be supported by extensive evidence of early neurodevelopmental impairment in schizophrenia. They compared ERPs in adolescents with schizophrenia (SZ) with those in AD/HD patients, and reported that the SZ group exhibited reduced P300 amplitude during auditory oddball and visual go/no-go tasks, while the AD/HD group did not. However, both groups exhibited significantly decreased amplitude of N2 during the go/no-go task.
Recently, several studies have reported abnormality of ERPs in adult patients with AD/HD (Prox et al., 2007 and Barry et al., 2009). Prox et al. (2007) reported that N2 amplitude was significantly increased in adults with AD/HD in a visual go/no-go task, compared with healthy subjects. P300 exhibited a tendency toward decreased activity in the AD/HD group. Barry et al. (2009) reported reduced N2 amplitudes to auditory targets, with no differences in target P300, in patients with AD/HD.
Dias et al. (2003) reported that AD/HD and SZ were associated with the impaired continuous performance task associated context, created by a cue stimulus, to guide response to a target. Olincy et al. (2000) insisted that AD/HD and SZ should be both conceptualized as disorders of attention. By examining the inhibition of the P50 auditory event-evoked potential, they found that adult with AD/HD did not show the inhibitory deficit as seen in patients with SZ, suggesting that the mechanism of attentional disturbance in the two illnesses might be fundamentally different. Thus, it must be important to investigate the difference of cognitive mechanism between SZ and AD/HD. It is, however, very difficult to simply compare findings for AD/HD with schizophrenia due to differences in age of onset. Oie et al. (2010) reported the neurocognitive decline in early-onset SZ compared with AD/HD. They argued that few studies comparing SZ and AD/HD have considered the possible effect of age of onset. Groom et al. (2008) compared ERPs in AD/HD group with schizophrenia group. Their data were not age-matched, because the participants in the SZ group were significantly older than those in the healthy controls (HC) and subjects with AD/HD. Since 2001, we have operated a clinic for psychiatric outpatients who are adults with developmental disorders in the Department of Neuropsychiatry of Fukushima Medical University. We have measured various types of ERPs as diagnostic tools in our patients, and compared the ERP data for adult AD/HD patients and schizophrenic patients with those for HC. The purpose of the present study was to examine cognitive function in adult AD/HD, in comparison with age-matched schizophrenia, by measuring ERPs including N2b, P300, and, in addition, Nd as an indicator of selective attention (Näätänen, 1982).
