Tadayoshi Nashida

Impairment in activation of a frontal attention-switch mechanism in schizophrenic patients.

The present study addresses the difference in activities of frontal and temporal mismatch negativity (MMN) generators between healthy controls and schizophrenic patients. Auditory MMNs were measured from 13 medicated schizophrenic patients in a post-acute phase and 12 healthy controls. The probabilities of the standard stimuli were, in different experimental blocks, 95, 90, 80 or 70%. The mean amplitude of the MMN recorded at Fz was significantly smaller in schizophrenic patients than healthy controls only in the conditions with high probability of standard stimuli, while that recorded at mastoid sites was not different in any condition. The present study suggested that schizophrenic patients might fail to cause involuntary attention switch to stimulus change reflected in the lowered MMN amplitude recorded at Fz; whereas the patients might index an adequate detection of the deviant event reflected by the similar amplitude of MMN recorded at mastoid sites.

The effect of deviant stimulus probability on the human mismatch process

The present study addresses the separate activities of frontal and temporal MMN generators which might be differentially affected by a change in the probability of standard stimuli. As the probability of standard stimuli was increased, the frontal MMN component significantly increased in amplitude, while the temporal one was not affected. Correspondingly, the scalp current density (SCD) maps showed that the temporal MMN generator was activated even at low probability of standard stimuli, suggesting that even the weak memory trace could start the automatic mismatch process, whereas the frontal MMN generator was activated only with increased probabilities of standard stimuli, suggesting that the stronger the memory trace is, the easier it might trigger the involuntary switching of attention to stimulus change.

The difference in Mismatch negativity between the acute and post-acute phase of schizophrenia

In order to investigate the trait and state aspects of Mismatch negativity (MMN) amplitude reduction in schizophrenia, auditory MMNs were measured from 13 schizophrenic patients on two occasions, initially when they showed acute exacerbation and later when their symptoms improved. Patients exhibited reduced mean amplitude of the MMN recorded at Fz. There were no significant changes in the amplitude of MMN at Fz between the acute patients and the post-acute patients, despite significant improvement in symptomatology. However, the acute patients showed a significant attenuation of MMN recorded at both mastoids as compared with the post-acute patients. Although the findings of the MMN at Fz support the overall longitudinal stability of MMN deficits in schizophrenia, the acute phase patients showed a modestly altered MMN activity compared with the post-acute phase patients, suggesting that there is some state-dependent modulation of these deficits.

Mismatch negativity (mmn) reveals sound grouping in the human brain

To investigate a part of the structure of the memory trace, auditory event-related potentials (ERPs) were recorded from reading subjects while they were presented with two different stimulus-series simultaneously. A clear mismatch negativity (MMN) was obtained from each series, when the stimulus sequence consisted of a high-frequency series and a low-frequency series. Moreover, the MMN showed independent elicitation within each series. However, if the frequency range of one series overlapped with that of the other series, the amplitude of the MMN was prominently reduced, suggesting that the two processing functions indexed by MMN coexisted simultaneously in the preattentive acoustic system and were produced by the respective grouping of high-frequency tones and low-frequency tones.

Early contingent negative variation (CNV) shows a small symmetrical negativity in a somatosensory paradigm.

The influence of sensory modulation on the early stage of information processing was investigated with a somatosensory contingent negative variation (CNV) paradigm. Whether or not even a somatosensory input as well as auditory or visual stimulus to one hemisphere elicits the symmetrical “early CNV” was also examined. Eleven normal individuals (3 males, 8 females) performed a conventional CNV paradigm with a click sound as the warning stimulus (WS) and a red light flash as the imperative stimulus (IS). Nine individuals (5 males, 4 females) did the somatosensory CNV paradigm with paired electrical stimuli as WS and IS. The subjects were instructed to press a button in response to IS as fast as possible. The early CNV amplitude was smaller and P300 latency was longer in somatosensory paradigm than conventional paradigm. In addition, the latency of P100 in a somatosensory paradigm was longer than that of N100 in a conventional paradigm. These findings suggest that the initiation of early detection, reflected by P100, the initiation of cognition, reflected by P300, and orienting response, reflected by early CNV, are delayed in a somatosensory CNV paradigm. Furthermore, all event-related potentials (ERPs) evoked by somatosensory stimuli showed a bilateral symmetry.

Attentional distraction of CNV depending on the spatial focus.

To investigate the distraction of spatial attention to the task-irrelevant visual stimuli, contingent negative variation (CNV) was measured by using a forewarned reaction time task in 20 healthy subjects. The lasting emission of light, irrelevant to the CNV task, at each perimetric angle of 15°, 30° or 45° to the fixated point was presented to the subjects. The amplitude of early CNV was small only under the light-emission at the angle of 30°. Our results indicate that attention is distracted even by the lasting, task-irrelevant stimuli and that distraction is dependent on the focusing function of attention.