Enhancement of declarative memory performance following a daytime nap is contingent on strength of initial task acquisition

STUDY OBJECTIVES: In this study we examined the benefit of a daytime nap containing only NREM sleep on the performance of three declarative memory tasks: unrelated paired associates, maze learning, and the Rey-Osterrieth complex figure. Additionally, we explored the impact of factors related to task acquisition on sleep-related memory processing. To this end, we examined whether testing of paired associates during training leads to sleep-related enhancement of memory compared to simply learning the word pairs without test. We also examined whether strength of task acquisition modulates sleep-related processing for each of the three tasks. SUBJECTS AND PROCEDURE: Subjects (11 male, 22 female) arrived at 11:30, were trained on each of the declarative memory tasks at 12:15, and at 13:00 either took a nap or remained awake in the sleep lab. After the nap period, all subjects remained in the lab until retest at 16:00. RESULTS: Compared to subjects who stayed awake during the training-retest interval, subjects who took a NREM nap demonstrated enhanced performance for word pairs that were tested during training, but not for untested word pairs. For each of the three declarative memory tasks, we observed a sleep-dependent performance benefit only for subjects that most strongly acquired the tasks during the training session. CONCLUSIONS: NREM sleep obtained during a daytime nap benefits declarative memory performance, with these benefits being intimately tied to how well subjects acquire the tasks and the way in which the information is acquired.     

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     

Physiological arousal and attention during a week of continuous sleep restriction

Waking brain physiology underlying deficits from continuous sleep restriction (CSR) is not well understood. Fourteen good sleepers participated in a 21-day protocol where they slept their usual amount in a baseline week, had their time in bed restricted by 33% in a CSR week, and slept the desired amount in a recovery week. Participants slept at home, completing diaries and wearing activity monitors to verify compliance. Each day participants completed an RT task and mood and sleepiness ratings every 3 h. Laboratory assessment of electrophysiology and performance took place at the end of baseline, three times throughout the CSR week, and at the beginning of recovery. Participants reported less sleep during CSR which was confirmed by activity monitors. Correspondingly, well-being and neurobehavioural performance was impaired. Quantitative EEG analysis revealed significantly reduced arousal between the 1st and 7th days of restriction and linear effects at anterior sites (Fp2, Fz, F8, T8). At posterior sites (P4, P8), reductions occurred only later in the week between the 4th and 7th nights of restriction. Both the immediate linear decline in arousal and precipitous drop later in the week were apparent at central sites (C4, Cz). Thus, frontal regions were affected immediately, while parietal regions showed maintenance of function until restriction was more severe. The P300 ERP component showed evidence of reduced attention by the 7th day of restriction (at Pz, P4). EEG and ERPs deficits were more robust in the right-hemisphere, which may reflect greater vulnerability to sleep loss in the non-dominant hemisphere.     

Afferent stimulation facilitates performance on a novel motor task

Training on a motor task results in performance improvements that are accompanied by increases in motor cortex excitability. Moreover, periods of afferent stimulation result in increased motor cortex excitability. There is increasing evidence to suggest that raised motor cortical excitability may facilitate movement and learning. Here we examined whether a period of electrical stimulation of hand afferents (“associative stimulation”), known to increase motor cortex excitability, facilitated the performance of a complex sensorimotor task. Three groups of nine normal subjects participated in these studies. All subjects were trained on the grooved pegboard test (GPT). Training consisted of three blocks, each of five trials, of placing pegs as quickly as possible. The time to complete each block was recorded. One group of subjects had a 1-h period of associative stimulation prior to training on the GPT. A second group received non-associative stimulation (which does not change cortical excitability) of the same hand afferents while a third group received no stimulation prior to training. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) and abductor digiti minimus (ADM) muscles both prior to and following stimulation and performance of the GPT. In contrast to non-associative stimulation, associative stimulation increased motor cortical excitability, as evidenced by an increase in the amplitude of MEPs evoked in the FDI, one of the stimulated muscles, but not the ADM. Training on the GPT resulted in significant improvements in the time taken to complete the task for all three groups. However, in subjects who had preconditioning associative stimulation, performance on the GPT improved more rapidly. Additionally, there was a strong trend for the improvement in the performance of the stimulated group to be greater than that of the control group. The results of the present study suggest that increased motor cortical excitability, induced by associative stimulation, may facilitate the performance of a novel complex sensorimotor task.     

The instructed context of a motor task modulates covert response preparation and shifts of spatial attention

We investigated how covert response preparation is modulated by the instructed cognitive context of a motor task. Participants prepared left-hand or right-hand movements toward or away from the body midline, as indicated by a response cue (S1) presented prior to a go/no-go stimulus (S2). Different participants were instructed that response cues specified the response hand or movement direction, respectively. This emphasis on effector versus movement direction selection modulated lateralized ERP components triggered during the S1–S2 interval. Attention shifts during movement preparation were assessed by measuring ERPs to irrelevant visual probes. Enhanced N1 components were found for probes near the effector when effector selection was emphasized, but for probes near the movement target location when movement direction selection was emphasized. Results demonstrate strong top-down contextual biases on motor control and on the locus of spatial attention during response preparation.     

Energy expenditure and motor performance relationships in humans learning a motor task

The ability of human subjects to learn minimum energy-demanding variants of biological motion was examined in three adult males trained to walk on hands and feet (creep) on a motor-driven treadmill at constant speed (0.64 m/s) for 16 3-min trials. Two subjects systematically decreased oxygen consumption and heart rate over trials. Following this acquisition phase, subjects completed walking and creeping trials at positive and negative treadmill grades and selected a freely chosen creeping grade that felt "most comfortable." One subject selected a grade that was more efficient than those imposed. Oxygen-consumption curves for walking and creeping converged with increasing positive grade, indicating that increased grade influences the metabolic energy viability of the task (creeping or walking). The acquisition data provide empirical support for the "principle of least effort" and lend support to the concept of a "comfort mode" in the execution of motor tasks.     

Action monitoring in motor control: ERPs following selection and execution errors in a force production task

Action monitoring has been studied in many tasks by means of measuring the error-related negativity (Ne/ERN), but never in a motor control task requiring precise force production. Errors in discrete choice reaction tasks are the result of incorrect selections, but errors in force production can also arise from incorrect executions. ERPs were obtained while participants produced low or high isometric forces with their left or right hand. As expected, incorrect choices of hand elicited an Ne/ERN. Interestingly, Ne/ERNs were also present in the less discrete selection error of an incorrect choice of force, but only when erroneously a low instead of a high force was chosen. In both force ranges, no Ne/ERNs were found after errors in execution. These errors showed a large positivity in feedback ERPs and, similar to correct responses, a prolonged negativity in response ERPs. We propose that, compared to selection errors, the time uncertainty aspects of execution errors and the resulting changing response representations prohibit error detection by the internal monitoring system responsible for generating the Ne/ERN.     

Effects of a short midday nap on habitual nappers’ alertness,mood and mental performance across cognitive domains

This study examines whether the benefits of a short midday nap on habitual nappers’ mental performance depend on the cognitive domain and the task difficulty. Eighteen healthy college students with the long‐term habit of a midday nap (13:00–14:00 hours) participated in a nap‐deprivation study. On two separate days with at least 3 days in between, participants either took a nap or remained awake, and were subsequently tested on a simple sustained attention task (Psychomotor Vigilance Test), two more complex attention tasks (Go/No‐Go and Flanker task) and one working memory task (2‐back). For each task, an easy and a difficult version were administered. The time course of subjective sleepiness and mood were also measured in both napping conditions. The results revealed that short midday nap deprivation significantly impaired participants’ performance on both the easy and difficult versions of the Psychomotor Vigilance Test task, as well as accuracy but not reaction speed in the Go/No‐Go task. Accuracy in the difficult version of the Flanker task and the 2‐back task was also lower in the no‐nap condition, while reaction speed in the 2‐back task but not the Flanker task was reduced without a nap in both the easy and difficult versions. Moreover, subjective sleepiness was significantly increased after nap deprivation, but moods remained unaffected in the no‐nap condition. These findings contribute to current research suggesting that effects of a midday nap on task performance depend on the cognitive domain as well as task difficulty. Our study highlights the importance of considering task characteristics to evaluate the benefits of a regular midday nap in practical working life.     

Effect of ramelteon (TAK-375), a selective MT1/MT2 receptor agonist,on motor performance in mice

Effect of (S)-N-[2-(1,6,7,8-tetrahydro-2H-indeno-[5,4-b]furan-8-yl)ethyl]propionamide (ramelteon, TAK-375), a selective MT₁/MT₂ receptor agonist, on motor coordination was studied using rota-rod performance in mice. Ramelteon did not impair rota-rod performance at doses of 3, 10 and 30 mg/kg, p.o. Melatonin and N-acetyl-5-hydroxytryptamine (N-acetyl-5-HT), a ligand of MT₃ biding site, also had no impairment on the performance, per se. However, in combination with a low dose of diazepam (3 mg/kg, p.o.), treatment with melatonin and N-acetyl-5-HT exacerbated the impairment by diazepam. Ramelteon had no significant effect on the diazepam-induced impairment of motor coordination.