Brief periods of NREM sleep do not promote early offline gains but subsequent on-task performance in motor skill learning

Is the role of sleep in memory consolidation overrated?

Substantial empirical evidence suggests that sleep benefits the consolidation and reorganization of learned information. Consequently, the concept of "sleep-dependent memory consolidation" is now widely accepted by the scientific community, in addition to influencing public perceptions regarding the functions of sleep. There are, however, numerous studies that have presented findings inconsistent with the sleep-memory hypothesis. Here, we challenge the notion of "sleep-dependency" by summarizing evidence for effective memory consolidation independent of sleep. Plasticity mechanisms thought to mediate or facilitate consolidation during sleep (e.g., neuronal replay, reactivation, slow oscillations, neurochemical milieu) also operate during non-sleep states, particularly quiet wakefulness, thus allowing for the stabilization of new memories. We propose that it is not sleep per se, but the engagement of plasticity mechanisms, active during both sleep and (at least some) waking states, that constitutes the critical factor determining memory formation. Thus, rather than playing a "critical" role, sleep falls along a continuum of behavioral states that vary in their effectiveness to support memory consolidation at the neural and behavioral level.

Sleep is more than rest for plasticity in the human cortex

Sleep promotes adaptation of behavior and underlying neural plasticity in comparison to active wakefulness. However, the contribution of its two main characteristics, sleep-specific brain activity and reduced stimulus interference, remains unclear. We tested healthy humans on a texture discrimination task, a proxy for neural plasticity in primary visual cortex, in the morning and retested them in the afternoon after a period of daytime sleep, passive waking with maximally reduced interference, or active waking. Sleep restored performance in direct comparison to both passive and active waking, in which deterioration of performance across repeated within-day testing has been linked to synaptic saturation in the primary visual cortex. No difference between passive and active waking was observed. Control experiments indicated that deterioration across wakefulness was retinotopically specific to the trained visual field and not due to unspecific performance differences. The restorative effect of sleep correlated with time spent in NREM sleep and with electroencephalographic slow wave energy, which is thought to reflect renormalization of synaptic strength. The results indicate that sleep is more than a state of reduced stimulus interference, but that sleep-specific brain activity restores performance by actively refining cortical plasticity.

Performance, Achievement, and Success in Psychological Research: Towards a More Transparent Use of the Still Ambiguous Terminology

The present study focused on the terms performance, achievement, and success that have often been used in an ambiguous manner in psychological research. The way in which the meaning of the domain-specific constructs referred to with the terms was established for measurement purposes was investigated on a sample of 262 articles that stemmed from 35 randomly selected journals covering the full range of psychological research. The operational definitions of the constructs referred to with the terms performance, achievement, and success were analysed and compared in both inter- and intra-domain fashion. Additionally, we assessed the match among the observed operational definitions and general conceptual definitions available in the extant literature. The results revealed terminology-related issues in educational and occupational research. Within these domains, lack of adherence to the multidimensionality of the constructs of academic performance, academic achievement, academic success, job performance, and career success was identified as a general issue. Further, the tendency to measure job performance via indicators based on self-rating was considered inadequate given the objective nature of the term performance. In educational research, the overlap of the academic performance, academic achievement, and academic success constructs was confirmed, resulting from the tendency to use GPA as a universal indicator of academic outcomes. Based on the present findings, we provided several recommendations in order to encourage future research towards a more transparent way of dealing with the particular constructs referred to with the terms performance, achievement, and success. We suppose the present study may help researchers in the full range of psychological disciplines to add clarity to their own research.

Sleep-dependent motor memory consolidation in healthy adults: A meta-analysis

It is widely accepted that sleep better facilitates the consolidation of motor memories than does a corresponding wake interval (King et al., 2017). However, no in-depth analysis of the various motor tasks and their relative sleep gain has been conducted so far. Therefore, the present meta-analysis considered 48 studies with a total of 53 sleep (n = 829) and 53 wake (n = 825) groups. An overall comparison between all sleep and wake groups resulted in a small effect for the relative sleep gain in motor memory consolidation (g = 0.43). While no subgroup differences were identified for differing designs, a small effect for the finger tapping task (g = 0.47) and a medium effect for the mirror tracing task (g = 0.62) were found. In summary, the meta-analysis substantiates that sleep generally benefits the consolidation of motor memories. However, to further our understanding of the mechanisms underlying this effect, examining certain task dimensions and their relative sleep gain would be a promising direction for future research.

Offline Bi-Frontal Anodal Transcranial Direct Current Stimulation Decreases Total Sleep Time Without Disturbing Overnight Memory Consolidation

OBJECTIVES

A proposed replay of memory traces between the hippocampus and frontal cortical brain areas during sleep is of high relevance for overnight memory consolidation. Recently, we demonstrated that bi-frontal anodal transcranial direct current stimulation (tDCS) prior to sleep increases waking EEG gamma power and decreases total sleep time during the night. It is unclear whether this effect on cortical excitability has an influence on overnight memory consolidation. We hypothesized that bi-frontal evening tDCS interferes with overnight memory consolidation with a polarity specific impairment following anodal tDCS.

MATERIALS AND METHODS

Nineteen healthy participants underwent a within-subject, repeated-measures protocol in the sleep laboratory with bi-frontal tDCS applied prior to sleep according to the experimental protocol (anodal, cathodal, sham stimulation). Memory tasks for declarative and procedural memory were assessed prior to tDCS and on the following morning.

RESULTS

No deterioration of overnight memory consolidation following evening offline bi-frontal tDCS could be detected.

CONCLUSION(S)

The application of tDCS can be considered safe regarding overnight memory consolidation and represents a promising treatment approach in conditions of decreased vigilance and arousal.