Timing of bimanual movements in human and non-human primates in relation to neuronal activity in primary motor cortex and supplementary motor area

It has been established that repeated presentation of a transient target motion stimulus such as a constant-velocity ramp leads to the build up of steady state (SS), anticipatory smooth pursuit eye movements after two or three presentations. Each SS response is then composed of the anticipatory component of nonvisual origin, a visual component associated with the stimulus presentation and another nonvisual component that represents the decay of the response after extinction of the stimulus. Here we investigated the interactions that occur when each motion stimulus was itself a sequence containing more than one ramp component. Ramp components had a velocity of 15 degrees /s or 30 degrees /s to left or right and were separated by gaps of 200 ms duration. In an initial experiment, responses to 2-ramp stimuli were examined and compared with responses to the single-ramp stimuli from which they were constituted. We present evidence that the anticipatory, nonvisual components of the double-ramp response result from the linear summation of the nonvisual components of the responses to the constituent single-ramp components. In a 2nd experiment, we examined responses to a wide variety of 4-ramp sequences and again found evidence that, in the SS, the responses were formed from the linear summation of the constituent single-ramp components. Regression analysis performed on the velocity at onset of each ramp component indicated that this nonvisual part of the response was predictive of the upcoming ramp component. To confirm this, unexpected changes were introduced into single ramp components of the 4-ramp sequence after at least five prior presentations of the sequence had allowed a SS response to be established. Subjects continued to initiate a response to the modified component that was appropriate in velocity and direction for the corresponding part of the previous sequence and inappropriate for the newly modified stimulus. This preprogrammed response persisted unmodified for more than 170 ms after onset of the modified ramp component. In contrast, in the second presentation of the new sequence, the anticipatory component of the response was highly correlated with the SS response of the new sequence, but not with that of the prior sequence, showing that the preprogrammed response had been modified very rapidly. Similar behaviour was observed whichever of the 4-ramp components was modified, indicating that the velocity and direction of the anticipatory response to each component had been preprogrammed. The results suggest that velocity information related to at least four elements of a sequence can be temporarily stored and subsequently released with appropriate temporal order to form an anticipatory response throughout the whole sequence.     

Second Thoughts: Multiple P300s Elicited by a Single Stimulus

In a previous report (Johnson & Donchin, 1978), we manipulated the discriminability of tone pairs that delivered feedback information in a time-estimation paradigm. As in other experiments using feedback stimuli, the event-related potentials elicited by these stimuli did not return to baseline in the 800-ms poststimulus interval. Since we were interested in this “Slow Wave'’activity, the poststimulus interval was lengthened to 1500 ms. Averages revealed that a second positive peak was present for some of the individual subjects. To investigate this activity further, the filtered singletrial waveforms were inspected visually. These data were characterized by the presence of one, and occasionally two, positive peaks, with highly variable latencies, following the P300. These peaks were indistinguishable in frequency and general appearance from the P300s elicited by the feedback stimuli. After latency adjusting the waveforms on the peak of the second positive wave, amplitude and latency were quantified. Whereas P300 amplitude was directly related to stimulus discriminability and positive feedback elicited larger P300s than negative feedback, the amplitude of the second positive wave was constant across levels of discriminability and the same for both types of feedback. In contrast, the latencies of both waves were inversely related to stimulus discriminability and shorter following positive feedback than negative feedback. Evidence is presented to support our contention that these additional positive peaks represent P300 activity. The data are discussed in terms of what these multiple P300s reveal about human information processing.     

On Quantifying Surprise: The Variation of Event-Related Potentials With Subjective Probability

Two factors are known to determine the waveform of event-related potentials (ERP) elicited by task-relevant stimuli: the a priori probability of the stimuli and the sequence of immediately preceding stimuli. The relative contribution of these factors to the ERP waveform was assessed at nine levels of a priori probability (from .10 to .90). Random sequences of high (1500 Hz) and low (1000 Hz) tones were presented to 10 male subjects at each level of probability, both when the events were task-relevant and when the subjects were performing an alternate task to which the tones were irrelevant. The EEG was recorded from five midline electrode sites referred to linked mastoids. The amplitude of the P300 and Slow Wave components was inversely proportional to the a priori probability of task-relevant events. At every level of a priori probability, the magnitude of the P300 complex (N200-P300-Slow Wave) was diminished when the eliciting tone repeated the preceding tone, and was enhanced when it was preceded by the other tone. Thus, a priori probability and sequential structure appear to be independent determinants of the P300 complex.     

Simulation Studies of Latency Measures of Components of the Event-Related Brain Potential

We compared the accuracy of P300 latency estimates obtained with different procedures under several simulated signal and noise conditions. Both preparatory and signal detection techniques were used. Preparatory techniques included frequency filters and spatial filters (single electrode selection and Vector filter). Signal detection techniques included peak-picking, cross-correlation, and Woody filter. Accuracy in the latency estimation increased exponentially as a function of the signal-to-noise ratio. Both Woody filter and cross-correlation provided better estimates than peak-picking, although this advantage was reduced by frequency filtering. For all signal detection techniques, Vector filter provided better estimates than single electrode selection. Large component overlap impaired the accuracy of the estimates obtained with both single electrode selection and Vector filter, but with Vector filter impairment occurred only when the overlapping component had a scalp distribution that was similar to the scalp distribution of the signal component. The effects of varying noise characteristics, P300 duration and latency, and the parameters of Vector filter were also investigated.     

P300 and Stimulus Categorization: Two Plus One is not so Different from One Plus One

Event related brain potentials (ERPs) were recorded from subjects who were instructed to count one of three, equally probable tones presented in a random sequence. In another condition, the subjects had to count one of two stimuli, one of which was presented with a probability of .33. The data support the view that the pattern of variation of P300 amplitude with the sequential structure of the series depends on the category to which events are assigned, rather than on the individual stimuli eliciting the P300. Furthermore, the data support the idea that the amplitude of P300 elicited by task-relevant stimuli is determined by the subjective probability associated with the eliciting event. DESCRIPTIONS: ERP, P300, Subjective probability, Stimulus categorization.     

On how P300 amplitude varies with the utility of the eliciting stimuli.

The bulk of the relevant evidence indicates that stimuli which elicit the P300 component of the human event-related potential (ERP) tend to be unexpected and task relevant. Yet, contradictory evidence has been presented which implied that P300 amplitude is modulated by the intensity difference between two equally relevant, equally probable feedback stimuli (Adams and Benson, 1973). The two experiments reported here were designed to clarify this contradiction. Subjects were instructed to press a button one second after a cue light. A tone at one of two intensities was presented 200 msec later indicating whether the time estimate was 'correct' or 'incorrect'. The magnitude of P300 varied as a function of the difference in intensity between S+ and S-; the smaller the difference, the smaller was P300, regardless of stimulus meaning (S+ or S-). We attribute this effect to the degree to which the subject chooses to utilize the tones in the performance of the time estimation task. In support of this hypothesis, we found that performance of the subjects as time estimators degraded as the difference between S+ and S- decreased. Moreover, a study of choice reaction times taken for the different pairings of S+ and S- showed that the stimulus pairs were not equally discriminable. It appears that as the S+/S- pairs become increasingly difficult to discriminate, their feedback value decreased and this decrease is positively correlated with a decrease in P300 amplitude. This interpretation received support from Exp. 2 in which the stimulus conditions were the same as in Exp. 1, but the subject's task was to count the "s+ stimuli. Here, when subject had no option but to process each individual stimulus, the P300 amplitude remained constant despite variations in S- intensity. Thus, it appears that task relevance must be defined in terms of the subject's behavior rather than by the experimenter's instructions to the subject.     

Epidural morphine does not affect the duration of action of epidural 2-chloro-procaine following Caesarean section

The effect of epidural morphine on the duration of action of epidural 2-chloroprocoine was studied in a double-blind fashion in 30 patients following elective Caesarean section. When compared to epidural saline controls (n = 15), patients (n - 15) who received epidural morphine (4.0-5.0 m g) did not experience a prolongation or reduction in the duration of the somatic or sympathetic nervous system blockades produced by epidural 2-ckloro-procaine.     

A "smoke-free" hospital in Israel--a possible mission

BACKGROUND: A revised antismoking law in Israel (August 2001) called for a complete ban of smoking in hospitals. An evaluation of a hospital process and short-term outcome of implementing such policy may guideline its sustainability. METHODS: Two cross-sectional, random-sample surveys of employees 3 months before policy implementation, and 6-9 months post-implementation. The pre-implementation survey included 368 employees (90.4% response rate) and the post-implementation survey, 364 (92.8% response rate). RESULTS: The 'smoke-free' policy significantly reduced smoking in the unauthorized areas. Before implementation, 63% of staff reported frequently observing smoking in the hospital, compared to 40% 6 months post-policy implementation (P < 0.001). Significantly more smokers reported leaving their workplace to smoke (17% pre- vs. 62% post-implementation, P < 0.0001). Sixty-five percent of smokers, in both surveys, agreed that "a smoke-free policy is unfair to smokers". Fewer nonsmokers subscribed to this belief: 42% pre- and 34% post-implementation. Smoker prevalence remained unchanged (19%). CONCLUSIONS: Implementation of a "smoke-free" policy is an effective way to reduce smoking in the hospital environment. More effort is required, however, to help staff quit smoking altogether.