Clinically, the delay of the P300 latency is a nonspecific change in psychiatric disorder. It can also be found in dementia, schizophrenia, depression, and other organic mental disorders [18
]. The aim of this study is to assess whether the visual ERPs can be a clinically effective diagnostic tool to be used for differentiation of schizophrenic patients or not. Additionally, the ERPs induced by the mental process regardless of the modality of an auditory and a visual input in the same brain structures were also been examined. The paired Student t
test is performed to compare signal processing models, assuming a unique and common mechanism as the locus of action of this effect. It includes visual or auditory ERPs with or without counting process. ERPs recorded in this process, serially presented tones or flashes, which could be in either the auditory or visual modality.
According to the detailed demographic data, there were no differences in sex, age, marital status, or religion, but there were significant differences in educational qualification. In the control group, there were no differences in the latency component of visual ERPs with or without counting, but the early N100 (Fz, Cz, Pz) and delayed P200 (Pz) in auditory ERPs with counting were noted. In the schizophrenic patients, there were no differences in the latency component of visual ERPs with or without counting except delayed P400 (Fz, Cz, Pz) in visual ERPs with counting. However, early N200 (Fz, Cz, Pz) in the auditory ERPs with counting was also noted observed. This finding shows that in either counting or without counting process the latency of visual ERPs This finding shows that in either counting or without counting process the latency of visual ERPs waswas unchangeable and unique in healthy subjects thus. This means that the latency of auditory ERPs was much more influenced by attention than visual ERPs. Otherwise, the decreased N200 (Fz, Cz), decreased P200 (Fz, Cz), increased P300 (Cz, Pz), and increased P400 (Pz) amplitude components of visual ERPs with counting but decreased P200 (Cz, Pz) in auditory ERPs with counting were noted in the control group. It is also observed that, in the case group, there were no differences in the amplitude components of visual ERPs with or without counting, but increased P200 (Fz, Cz, Pz) and P300 (Fz, Cz, Pz) in auditory stimuli the ERPs with counting were noted. This finding shows that the amplitude of visual ERPs was changed in the mental process with counting in healthy subjects but not in schizophrenic patients. This could be the result of a deficiency of signal processing in visual ERPs among schizophrenic patients. In the amplitude of auditory ERPs with counting, the P200 and P300 amplitudes increased in schizophrenic patients, proving that the signal processing enhanced by counting was observed in schizophrenic patients. In other words, the schizophrenic patients could lack abilities, such as attention, required for signal processing.
However, the P400 component exists in visual ERPs. The P400 component was identified as a positive wave at Fz, Cz, and Pz, with a latency of 300 to 500 ms after the start of the stimulus. There were no differences in the latency components in the control group with or without counting process, but there was an increased P400 (Pz) amplitude component in the control group with the counting process. There was also a significant prolonged latency of P400 (Fz, Cz, Pz) in the patient group with the visual counting process. The delayed latency of P400 in the visual counting process was observed in schizophrenic patients, which can be used for differential diagnosis clinically.
In the paired Student t test analysis of case and control groups, the latency components of P300 (Fz, Cz, Pz) in visual ERPs without counting or N200 (Fz, Cz, Pz) and P300 (Fz, Cz) in auditory ERPs with or without counting were significantly different between the case and control groups. This finding indicates that delayed latency of N200 and P300 in the auditory ERPs and P300 in the visual ERPs can be clinically correlated to schizophrenic patients. However, the amplitude components of N100 (Fz, Cz, Pz) and N200 (Fz, Cz) in visual ERPs with counting; N100 (Fz, Cz, Pz) in visual ERPs without counting; N100 (Fz, Cz, Pz), N200 (Fz, Cz, Pz), P200 (Fz), and P300 (Fz, Cz, Pz) in auditory ERPs with counting; or N100 (Cz, Pz), N200 (Cz), P200 (Fz), and P300 (Fz, Cz, Pz) in auditory ERPs without counting were significantly different between case and control groups. This finding implies that decreased amplitude of N100, N200 and P300 in the auditory ERPs and N100 in the visual ERPs can indicate clinical correlation among schizophrenic patients.
Various studies have shown that the amplitude of the P300 component of ERP is reduced in schizophrenic patients [25
]. It is assumed that this P300 abnormality may present a disturbance in information processing required for task performance. Therefore, P300 may be an effective tool used to investigate putative neuro-biological mechanisms underlying schizophrenic symptoms [25
]. Recent studies suggest that ERP measurement of auditory system adaptability characterize the pathophysiological process underlying the cognitive impairment more appropriately in schizophrenia than static measurement of ERP magnitude [26
]. There are also few studies supporting the view that schizophrenia is characterized by fundamental deficits in integrative cortical functions that specifically impair the ability to analyze and represent stimulus context to guide behavior. Moreover, abnormalities of the auditory P3 amplitude in schizophrenia seem to reflect a basic underlying patho-physiological process that is present at illness onset and progresses across the illness course [27
]. Our study showed decreased P300 amplitude and delayed P300 latency in auditory ERPs with or without counting but only delayed P300 latency in visual ERPs without counting between schizophrenic patients and the control group. Other studies have reported the decreased P200 latency for standard stimuli observed in the present study in both schizophrenic subjects and non-schizophrenic college students with high levels of illusory thinking. In our study, there is no significant decrease in P200 latency observed in both auditory and visual ERPs. Several previous studies have found abnormalities of N1 generation in schizophrenia [28
], including both increased and decreased amplitude [29
]. In our study, there is a significant decrease observed in N100 amplitude in schizophrenic subjects related to control and insignificant effect on N100 latency in both auditory and visual ERPs. Differences in findings regarding obligatory ERP components may relate to differences in type of stimulus (tone vs. click), intensity, duration, or interstimulus interval. We observed significantly delayed N200 (Fz, Cz, Pz) and P300 (Fz, Cz) and reduced N100 (Fz, Cz, Pz), N200 (Fz, Cz), and P300 (Fz, Cz, Pz) amplitude to auditory non-target stimuli. Visual N200 to rare target stimuli was assumed to be similar to auditory N200 and to reflect a controlled discriminative processing. In our paradigm, P300 was also thought to reflect an automatic task evaluation processing and controlled cognitive processing. The study reveals the process of cognitive delay existing in schizophrenic patients corresponding to our findings of prolonged N200 latency to auditory stimuli implies that the automatic cognitive processing could be slowed in the disease. However, there are neuropsychological studies that suggest preserved function of automatic cognitive processing in schizophrenia.
According to our study no matter what the auditory stimuli (with or without mental counting) are, the amplitude components of N100, N200, P200, and P300 and the latency components of N200 and P300 were significantly different between the control and the schizophrenic patients. However, the amplitude of N200 (Fz, Cz) induced by the visual stimuli with mental counting was significantly different between the control and the schizophrenic groups. The latency of P300 was not different between the two groups, which mean that some mental processing occurs at the N200 level during visual stimuli but that schizophrenic patients lack this ability. However, when the schizophrenic patients tried to use mental counting in the visual stimuli, the P300 latency was not different between the two groups. This indicates that the time of mental processing is not delayed among schizophrenic patients.
Because of their millisecond-level temporal resolution, ERPs are ideally suited for analysis of the brain activity related to information processing. A major finding of the present study is that the amplitude of N200 and P300 used as an index of cortical processing is delayed in schizophrenia. Mismatched negativity reflects activation of neural structures within primary auditory cortex (Heschl's gyrus) or adjacent supra-temporal auditory regions, as opposed to N200, which primarily reflects activity within auditory association cortex, and P3, which reflects activity in prefrontal, temporo-parietal, and, potentially, other multiple sensory association regions of the cortex. Our findings, therefore, indicate that the neuro-physiological dysfunction in schizophrenia is prevalent and extends even to the level of the sensory cortex [33
An important aspect and contribution of this study is to integrate the auditory and the visual ERPs for patients with schizophrenia. The implementation of such tools may be significantly used for clinical interventions. People with schizophrenia may be followed up with such tools in the longitudinal follow-up study. Since there is no evidence of any published literature along with all meta-analyses, a caution should be taken into account while interpreting the results. Patients with schizophrenia should be considered separately for the study from those with different types if large sample size.