Three groups of children rated firstly as overactive and distractible, secondly as distractible and thirdly as low on both activity and distractibility were examined in a visual search task with three levels of display load: two, three and four items. The children were tested twice in two conditions of stimulus visibility to examine the encoding stage of the model used here. The experimental results reject the hypothesis that an encoding deficit or data limitation may explain the attentional performance of either hyperactive or somewhat hyperactive subjects. They suggest, rather, that a trade-off in speed and accuracy may be evident in (the) hyperactives.

SUMMARY Overactive and distractible (hyperactives), normoactive and distractible, and normoactive and attentive (controls) children were administered a high‐speed visual search task. The display load was manipulated and all subjects were administered the task in three instruction conditions: speed, ‘normal’ and accuracy. Speed‐accuracy trade‐off curves indicated that the controls and distractibles conformed to the fast guess model, which relates speed and accuracy. Hyperactives partially conformed to this model. It is concluded that a structural process deficit is not indicated by these data in hyperactivity. Rather, the evidence suggests that a resource strategy defect may be characteristic of hyperactivity.

In a memory search task with context recognition, three groups of children were examined: hyperactives, somewhat hyperactives and controls. It was found that the hyperactive groups differed from controls at the intercept of reaction time and not the slope. This was interpreted as evidence against a selective attention hypothesis in hyperactivity and evidence of either an 'encoding' or a response organization deficit. The unexpected slower and less accurate performance of somewhat hyperactives than those rated as hyperactives may have been due to the higher rating of variable task application in the former than in the latter.

In order to investigate the development of movement speed in relation to movement organization, children of 5, 6, 7, 8 and 9 years of age and adults carried out a reciprocal tapping task, in which time pressure and distance were manipulated. The duration, velocity, acceleration and accuracy of the movements were compared between age groups. Age differences appeared mainly in the homing time, not in the duration of the distance covering movement phase. Accuracy and velocity of the distance covering movement phase differed with age. Time pressure affected the homing time, but not the duration of the distance covering phase. Distance manipulation affected mainly the velocity and duration of the distance covering movement phase and the homing time. In the discussion it is contended that age differences in homing time may be related to both the accuracy of the distance covering movement phase and the rate of information processing of the subject.

In a group of 9 healthy newborns, spontaneous activity of different muscles was studied for 6 or 8 hours by means of surface electromyography on the 5th or 6th day of life. According to the local nursing routine the infants were placed in alternating left and right side positions. Averaged EMG activities were analysed with respect to behavioural states and observed postural behaviour. An improved recording technique and the logarithmic presentation of averaged EMG activities allowed the study of characteristics of tonic activity in state 1. The chin muscles showed the most specific EMG pattern in state 1. Chin muscle quiescence was observed during only 15% of that state. Lack of tonic activity was observed more in the beginning than at the end, but never in the middle of state 1. By visual analysis of 41 patterns of averaged EMG activities from the chin muscle it was found that increases of tonic activity only happen stepwise following concomitant phasic activities. Relaxation of tonic activity is either asymptotic or abrupt or by steps. The latter two are especially characteristic for the transition to state 2. Long periods of sustained tonic activity and low motility were characteristic for a long lasting state 1. The interval between the beginning of state 1 and the first increase of tonic activity varied more than the interval between the end of state 1 and the final drop of tonic activity back to the noise level. Other muscles showed less tonic activity in state 1: the neck area (sterncleidomastoid muscle 48% of state 1 time, trapezius m. 32%) was followed by the muscles of the forearm (extensor carpi m. 18%, flexor carpi m. 16%). These muscles also lead the rank order of the modulation of tonic activity. Tonic activity sustained for more than 30 sec. was not observed in state 2, except during transitions. In state 2 as in state 1 the muscles from chin, neck and upper extremities were more active than the muscles from the lower extremities. Durations of activities ranged from 28% (chin) to 13% (ant. tibialis) of state 2 time. No dominance of flexor muscles, which might account for the observed flexed posture, was found. Systematically changed side positions during the recording had no consistent effect on either motility or presence of tonic activity when corresponding muscles from the upper- and the lower-lying body side were compared during state 1. Likewise in state 2 no consistent effect on the total duration of phasic activity was observed. The results are discussed in relation to other known state dependent motor phenomena. It is suggested that sustained tonic activity results from an increased gain of the gamma-loop which may also contribute to the regularity of respiration. Differences between muscles probably reflect maturational and functional differences.