Slow event-related potentials (ERP) were examined in healthy
and aphasic subjects in two-stimulus designs comprising a word
comprehension and a rhyming task. Aphasics, though selected
to perform ...above chance level, made significantly more errors
and responded more slowly than controls, although canonical
correlations did not indicate a statistical relationship between
performance measures and ERP amplitudes. A discriminant analysis
of ERP amplitudes distinguished the groups for the slow wave
(SW; 0.5–1.0 s post-S1 onset) in the word comprehension,
for the SW and the initial contingent negative variation (iCNV;
1.0–2.0 s post-S1 onset) in the rhyming task. Similarly
for both tasks, ERP topography showed left-anterior predominance
of the negative SW and iCNV in controls, whereas participants
with aphasia showed smaller anterior and larger left-posterior
amplitudes. The centroparietal terminal CNV (tCNV; 1 s pre-S2)
was smaller in participants with aphasia than in controls, but
similar in topography. Results suggest left-anterior activation
for those language processes that were presumably provoked in
the present tasks, like lexical access, or phonological encoding.
The pattern of participants with aphasia may indicate effects
of language impairment and recovery, but also consequences of
the brain damage.
In a delayed matching-to-sample task, the impact of clear or ambiguous go versus clear no-go signals on the post-imperative negative variation (PINV) was examined in 11 patients with a chronic ...schizophrenic disorder (DSM-III-R) and in a control group of 13 healthy subjects matched to the patient sample by age, sex, and education. Size and spatial position of a visual S2 had to be matched to one of two visual patterns in the S1 presented 4 s earlier. In 96 trials, the S2 was identical in size with one of the two patterns of S1 (clear matching). These trials varied pseudorandomly, with 60 trials in which the S2 was of intermediate size. On a randomly interspersed additional 48 trials, an S2 differing in color and shape signaled no-go. The electroencephalogram was recorded from Fz, Cz, Pz, F3, F4, C3, C4, P3, and P4. Although groups did not differ in contingent negative variation amplitude, the PINV was generally more pronounced in patients than in controls. In both groups, ambiguity of the to-be-matched S2 produced larger PINV amplitudes; the no-go signal elicited only a small PINV. Differential effects of ambiguity and no-go on PINV amplitude and its scalp distribution suggest that "performance" and "action" uncertainty contribute to PINV generation and that thresholds for both effects are reduced in schizophrenics.
Event-related potentials (ERPs) were recorded from 19 aphasic patients and 18 controls in four versions of a feature comparison task, in which the verbal or pictorial representation of a first ...stimulus (S1) had to be compared with the verbal or pictorial representation of a second stimulus (S2) presented 2 s later. These tasks were designed to cover some of the discriminatory variance of the token test (TT) including the analytical isolation, encoding and short-term storage of individual features of objects, independent of auditory verbal comprehension. Aphasics made more errors and had longer response latencies than controls in all four tasks, performance being poorest when verbal stimuli had to be processed. ERP analyses – restricted to subjects performing well above chance and to trials with correct responses – were confined to the slow wave (SW) (250–750 ms post-S1-onset) and the contingend negative variation (CNV) preceding the S2. There was no overall group difference that would have suggested that the patients activated different cortical areas than controls on correct performance. A left-hemispheric predominance of the negative SW was found in all four tasks and in both groups, although it was more pronounced in aphasics, and more pronounced in non-fluent than in fluent aphasics. The CNV was characterized by a left-hemispheric accentuation which was more pronounced in controls than in aphasics, particularly in tasks with a verbal S2. Results indicate that successful feature comparisons in the present tasks activate primarily left-anterior cortical areas. During encoding and short-term storage this activation is more pronounced in aphasics than in controls.
Event-related potentials were recorded during a delayed matching-to-sample design from 17 volunteers (5 f) using high-resolution (65 channels) EEG-recordings. In the two-stimulus paradigm, the 500-ms ...stimulus S1 comprised a visual pattern of two diamonds differing in size, angular rotation and location; in the delay period, Working Memory (WM) load was varied in the following way: a stimulus-free interval of 1 s was followed by a 6-s presentation either of a pattern identical to the S1 (low WM load) or of a pattern differing from S1 (high WM load). The 500-ms stimulus S2 comprised one diamond; the subject's task was to indicate by left- or right-hand (respectively) button press, whether the S2 matched the (a) left- or (b) right-positioned S1-diamond, or (c) did not match at all (NoGo). The topographical distribution of activity in the time intervals (a) following S1-offset, (b) during the WM manipulation interval and (c) prior to S2 were evaluated in the signal (scalp potential) and source (Minimum Norm) space. Following S1-offset the ERP pattern was characterised by negativity over posterior areas, slightly more so over the right hemisphere. In the subsequent 6-s interval high WM load elicited a larger negative slow ERP than low WM load, the negativity increase due to high WM load being larger over frontal than central areas. Source modelling indicated activity in anterior areas under high, and posterior activity under low WM load. Asymmetry of activity, although indicating a shift to left-hemispheric activity under high compared to low WM load, varied considerably between subjects. Results suggest that high-resolution ERP recordings allow to examine cortical activity during WM challenge.
Performance and event-related potentials (ERPs)
were examined in a proactive interference (PI) task with
15 male schizophrenic patients and 15 matched healthy controls.
Within a paired-associate ...task, 30 pairs of semantically
unrelated words (A-B) were presented twice, followed by
cued recall, in which the paired-associate B had to be
named upon cue A. Subsequently, 50% of the A-words were
paired with new words (A-C) and presented in random order
together with 15 novel pairings (D-E). Slower responses
and poorer recall of C- than of E-words in the final recall
indicated PI in both groups. During acquisition, the paired-associates
(C/E) evoked larger P3 and positive slow wave in controls
than in patients. During recall, cues (A/D) evoked a slow
wave with predominating anterior negativity in controls
and posterior positivity in patients. The group-specific
ERP pattern suggests deviant encoding and retrieval processes
in schizophrenic individuals.
Dynamical brain states can be characterized by non-linear measures of EEG. The present study shows that critical transitions, i.e., abrupt changes from one dynamic pattern of neural mass activity to ...another one, may be detected by abrupt variations in local chaoticity. Using an ambulatory device, EEG was recorded from 10 patients with a schizophrenic and two patients with an affective disorder during a series of 25-min interviews. Dynamical aspects, in particular, phase transitions in the EEG-dynamics of the EEG were characterized by means of a measure that continuously estimates the chaoticity of the EEG signal and is thus related to its predictability. Results indicate simpler dynamics of the EEG time series in paranoid–hallucinatory patients, while at the same time these patients tended to exhibit more abrupt transitions/unit of time between different dynamical EEG states. Such sudden phase transitions in brain activity were significantly enhanced prior to expressions of thought disorders that were detected by the interviewer and an observer in the conversation, compared with time periods during the interview without such symptoms.
Surface-negative brain potentials indicate increased excitability of the underlying cortical neural networks. Consequently, deviant patterns of event-related potentials in schizophrenic patients ...reveal an atypical regulation of cortical excitability. Twelve patients with a chronic schizophrenic disorder and 12 matched control subjects were investigated using a probe paradigm: A contingent negative variation (CNV) was evoked in a forewarned reaction time paradigm. Clicks were presented before, during and after elicitation of the CNV. Click-evoked responses allow one to ‘probe’ the current brain state, particularly neuronal excitability, which is also reflected by the slow potentials. During the measurements, subjects pressed one button in response to the offset of the visual warning stimulus and a different button in response to the acoustic probes, the latter button press being a behavioral indication of the brain's exitability. In the forewarned reaction time task, patients developed a CNV with a frontal maximum, while the CNV in control subjects was predominantly centro-parietal. This atypical topographical pattern of the CNV may indicate a different spatio-temporal regulation of cortical preparatory processes in schizophrenics. Motor responses were accelerated during negative potential shifts in both patients and controls, with responses being slower overall in patients. In patients, probe-evoked potentials revealed a smaller N100, but a larger P300, than in controls. The covariation of these brain waves with slow potentials, however, turned out to be similar for both groups.
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