The Waltzing Oddball

We investigated whether task relevance and probability interact to influence P3 amplitude. High and low tones were presented in random order with equal probability. In the control condition (standard oddball), every high tone had to be counted. In the waltz condition, high tones had to be counted only if they were preceded by two other high tones. It was predicted that the P3s evoked by targets in the waltz condition would be larger than the P3s evoked by the same sequence of targets in the oddball condition. That is, the frequency of occurrence of the targets should have an effect on P3, in addition to effects of the frequency of stimulus occurrence and stimulus task relevance (target/nontarget). This prediction was upheld. However, the largest P3s were evoked by nontargets following two high tones in the waltz condition. These P3s had a more anterior topographic maximum than usual. We contend that these anterior P3s reflect the interruption of an ongoing task and cannot be easily fit into the framework of the two concepts of task relevance and probability.     

Biased odds for heads or tails: Outcome‐evoked P3 depends on frequencies of guesses

Events that had to be predicted evoke large P3 components of the event‐related EEG potential. There is conflicting evidence whether these P3s are moderated by participants’ preceding guesses. In the present study, participants made one prediction frequently and the other rarely because one stimulus was presented frequently and the other rarely. Thereby, effects on stimulus‐evoked P3s of both guess frequency and stimulus frequency could be tested. Indeed, P3s were not only larger with rare than frequent stimuli but also larger after rare than frequent guesses. This result pattern may have additionally been affected by expectancies for payoff. In any case, the modification of outcome‐evoked P3 by what had been guessed may reflect that each of the four guess‐stimulus combinations is encoded as a separate event category. In terms of the stimulus‐response link hypothesis of P3b, it is suggested that P3s are evoked by these events because internal responses (right or wrong) are associated to each of these event categories and need to be reactivated with rare guess‐stimulus combinations.     

Sequential effects on P3 in a counting task: a partial replication

High and low tones were presented in random order, and the high tones had to be counted. It was intended to replicate the sequential effects on P3 reported by Squires et al. (1976, 1977) and to test by means of an ANOVA approach how each one of the preceding four tones contributes to these sequential effects. However, results deviated from those studies: (1) P3s were generally somewhat larger with target tones than with nontargets; (2) Squires et al.'s (1976, 1977) sequential effects were replicated for nontargets, but were more or less reversed for targets. The preceding sequence was found also to exert influence on prestimulus baselines, N1, and EOG. It is suggested that target effects constitute the rule rather than the exception and that sequential effects on P3 may be altered by subtle differences of subjects' sets, similarly to sequential effects on choice reaction times.     

Effects of certainty, modality shift and guess outcome on evoked potentials and reaction times in chronic schizophrenics

Evoked potentials and reaction times were obtained from chronic schizophrenics and normal controls to light and sound stimuli presented in random order. In the 'certain' condition subjects were told what the next stimulus would be, in the 'uncertain' condition they were asked to guess. Amplitudes were usually larger for normals than for schizophrenics, for 'uncertain' than for 'certain' conditions, and in cross- than in ipsimodal stimulus-sequences. The effect of certainty was stronger in normals across 4 leads; so was the effect of modality shift at vertex. While these findings replicate earlier results from acute schizophrenics, no condition X group interactions could be found in the reaction time measures. Two additional results were interpreted as showing basically different attitudes with respect to the predictability of events: (1) there was a slow positivity between the verbal information and the following stimuli which was largest for schizophrenics in the conditions of certainty; (2) while normals showed long-term habituation only in N1- but not in P3-amplitudes, the reverse was true for schizophrenics.     

Mechanisms underlying the left visual-field advantage in the dual stream RSVP task: evidence from N2pc, P3, and distractor-evoked VEPs

When two letter streams containing two targets (T1 and T2) are presented left and right, T2 is better identified in the left hemifield. This study on 16 healthy participants used evidence from ERPs to decide whether this advantage is due to better processing in the right hemisphere or to overload of the left one. N2pc and P3 components evoked by T1 and T2 were measured, as well as the VEPs evoked by the stream of distractor stimuli. Already at the onset of the stream, these VEPs peaked earlier at the right than at the left hemisphere. N2pc was evoked earlier and P3 amplitudes were larger with left than with right T2. Previously reported side differences in T1-evoked N2pc were no longer obtained after correcting for constant hemispheric differences. The faster VEP latencies at the right hemisphere from the very beginning of the stimulus series may reflect an advantage in structuring fast sequences, which may cause the left visual-field advantage.     

Precursors of insight in event-related brain potentials

Event-related potentials (ERPs) were investigated to find precursors of insightful behavior. Participants had to process successive pairs in strings of digits to obtain a final response in each trial. Within the sequence of five responses required in each trial, the last two responses mirrored the two preceding ones. This hidden regularity, allowing for shortcutting each trial from five to two responses, was discovered by 6 out of 26 participants. Both groups, solvers and nonsolvers, implicitly learned the regularity, reflected by faster responses to the repeated, predictable responses, but this differential effect was larger in solvers, whereas nonsolvers became unspecifically faster with all responses. Several ERP components were larger in solvers than in nonsolvers from the outset: slow positive wave, frontocentral P3a, anterior N1 to those digits that triggered the critical repeating responses, and P3b to the digit that evoked the immediately repeating response. Being already present in the first block, these effects were early precursors of insightful behavior. This early occurrence suggests that participants who will gain insight may be distinguished beforehand by their individual characteristics.     

Consequences of altered cerebellar input for the cortical regulation of motor coordination, as reflected in EEG potentials

The cerebellum is certainly involved in fine coordination of movements, but has no efferences of its own to the muscles. Thus, it can exert its influence only via other cerebral areas that have those efferences. This study investigated in patients with cerebellar atrophy how cortical motor areas are affected by dysfunction of the cerebellum. The main question was whether the patients’ slow cortical electroencephalogram (EEG) potentials during key-press preparation and execution would be generally altered or would be specifically altered when fine coordination was needed. In the coordination task, right- and left-hand keys had to be pressed simultaneously with different forces, under visual feedback. Control tasks were to press with both hands equally or with one hand only. The patients indeed had a performance deficit in the coordination task. Their cortical EEG potentials were already drastically reduced in the simple tasks, but were enhanced by the same amount as in healthy subjects when more coordination was needed. These results suggest that the cerebellum is not exclusively active in fine coordination, but is generally involved in any kind of preparatory and executive activity, whereas the motor cortex becomes more active with fine coordination. The role of the cerebellum might be to provide the motor cortex with information needed for coordinating movements. In cerebellar atrophy, this altered input may be sufficient for the motor cortex in controlling simple tasks, but not for complex ones. Received: 4 November 1998 / Accepted: 14 April 1999     

The hard oddball: Effects of difficult response selection on stimulus‐related P3 and on response‐related negative potentials

The P3b component of ERPs has been proposed to reflect decisions or, alternatively, activation of stimulus‐response links or, alternatively, to depend on stimulus processing only. These three views make different predictions about how difficulty of response selection will affect the oddball effect. In three experiments, frequent and rare targets were accompanied by ancillary stimuli. Large oddball‐P3bs, evoked by rare targets, were greatly reduced when responses had to be selected by combining information from targets and ancillary stimuli. Difficult response selection with rare targets was also reflected by a large frontocentral negativity, which could be separated from P3b when intervals were varied between targets and ancillary stimuli, and, therefore, did not cause P3b reduction. It is concluded that the usual increase of P3b with rare task‐relevant targets depends on the existence of readily available stimulus‐response links.