Slow cortical potentials & Spontaneous Fluctuations

Electrophysiological correlates of the brain's intrinsic large-scale functional architecture
PNAS October 14, 2008 vol. 105 no. 41 16039-16044

Spontaneous fluctuations in the blood-oxygen-level-dependent (BOLD) signals demonstrate consistent temporal correlations within large-scale brain networks associated with different functions. The neurophysiological correlates of this phenomenon remain elusive. Here, we show in humans that the slow cortical potentials recorded by electrocorticography demonstrate a correlation structure similar to that of spontaneous BOLD fluctuations across wakefulness, slow-wave sleep, and rapid-eye-movement sleep. Gamma frequency power also showed a similar correlation structure but only during wakefulness and rapid-eye-movement sleep. Our results provide an important bridge between the large-scale brain networks readily revealed by spontaneous BOLD signals and their underlying neurophysiology.

Influenced by reward?

How does reward expectation influence cognition in the human brain?
J Cogn Neurosci. 2008 Nov;20(11):1980-92.Click here to read

The prospect of reward changes how we think and behave. We investigated how this occurs in the brain using a novel continuous performance task in which fluctuating reward expectations biased cognitive processes between competing spatial and verbal tasks. Critically, effects of reward expectancy could be distinguished from induced changes in task-related networks. Behavioral data confirm specific bias toward a reward-relevant modality. Increased reward expectation improves reaction time and accuracy in the relevant dimension while reducing sensitivity to modulations of stimuli characteristics in the irrelevant dimension. Analysis of functional magnetic resonance imaging data shows that the proximity to reward over successive trials is associated with increased activity of the medial frontal cortex regardless of the modality. However, there are modality-specific changes in brain activity in the lateral frontal, parietal, and temporal cortex. Analysis of effective connectivity suggests that reward expectancy enhances coupling in both early visual pathways and within the prefrontal cortex. These distributed changes in task-related cortical networks arise from subjects' representations of future events and likelihood of reward.

Are you getting enough sleep?

The National Sleep Foundation’s 2008 Sleep in America poll reports the average American spends 6 hours and 55 minutes in bed — with 6 hours and 40 minutes spent actually sleeping.

The standard 8-hour business day is no longer the norm in America. NSF’s 2008 Sleep in America poll reports the average American’s work day is now 9 hours and 28 minutes. The average time spent in bed is 6 hours and 55 minutes - with 6 hours and 40 minutes spent actually sleeping. NSF recommends getting at least 7 to 9 hours of sleep each night.

According to the Sleep Foundation poll results, 42 percent of people chose 7-8 hours, 31 percent chose 8-9 hours, 14 percent chose more than 9 hours, 10 percent chose 5-6 hours and 3 percent chose less than 5 hours. The first thing experts will tell you about sleep is that there is no "magic number." Not only do different age groups need different amounts of sleep, but sleep needs are also individual.

Click here to read more about why you shouldn't skimp on sleep.

Sex Differences in Cognitive Estimation

Sex Differences in Cognitive Estimation During Sleep Deprivation: Effects of Stimulant Countermeasures
Int J Neurosci 2008;118(11):1547-57.

Stimulant medications restore simple alertness during sleep loss, but it is not clear how they affect complex executive functions, particularly in light of sex differences in cerebral organization. The effectiveness of caffeine, modafinil, dextroamphetamine, or placebo for sustaining performance on the Biber Cognitive Estimation Test (BCET) was compared in 29 men and 25 women following 46 hr of sleep deprivation. Stimulants had differential effects on BCET performance as a function of the sex of the subjects. Women receiving placebo or caffeine scored significantly worse than males, while modafinil and dextroamphetamine were effective at sustaining BCET performance of men andwomen.

Circadian Clock Could Help Learning Retention

Hippocampal-dependent learning requires a functional circadian system
PNAS October 7, 2008 vol. 105 no. 40 15593-15598

Decades of studies have shown that eliminating circadian rhythms of mammals does not compromise their health or longevity in the laboratory in any obvious way. These observations have raised questions about the functional significance of the mammalian circadian system, but have been difficult to address for lack of an appropriate animal model. Surgical ablation of the suprachiasmatic nucleus (SCN) and clock gene knockouts eliminate rhythms, but also damage adjacent brain regions or cause developmental effects that may impair cognitive or other physiological functions. We developed a method that avoids these problems and eliminates rhythms by noninvasive means in Siberian hamsters (Phodopus sungorus). The present study evaluated cognitive function in arrhythmic animals by using a hippocampal-dependent learning task. Control hamsters exhibited normal circadian modulation of performance in a delayed novel-object recognition task. By contrast, arrhythmic animals could not discriminate a novel object from a familiar one only 20 or 60 min after training. Memory performance was not related to prior sleep history as sleep manipulations had no effect on performance. The GABA antagonist pentylenetetrazol restored learning without restoring circadian rhythms. We conclude that the circadian system is involved in memory function in a manner that is independent of sleep. Circadian influence on learning may be exerted via cyclic GABA output from the SCN to target sites involved in learning. Arrhythmic hamsters may have failed to perform this task because of chronic inhibitory signaling from the SCN that interfered with the plastic mechanisms that encode learning in the hippocampus.

Spontaneous neural activity during human slow wave sleep

Spontaneous neural activity during human slow wave sleep
Proc Natl Acad Sci U S A 2008;105(39):15160-5.

Slow wave sleep (SWS) is associated with spontaneous brain oscillations that are thought to participate in sleep homeostasis and to support the processing of information related to the experiences of the previous awake period. At the cellular level, during SWS, a slow oscillation (<1 Hz) synchronizes firing patterns in large neuronal populations and is reflected on electroencephalography (EEG) recordings as large-amplitude, low-frequency waves. By using simultaneous EEG and event-related functional magnetic resonance imaging (fMRI), we characterized the transient changes in brain activity consistently associated with slow waves (>140 μV) and delta waves (75–140 μV) during SWS in 14 non-sleep-deprived normal human volunteers. Significant increases in activity were associated with these waves in several cortical areas, including the inferior frontal, medial prefrontal, precuneus, and posterior cingulate areas. Compared with baseline activity, slow waves are associated with significant activity in the parahippocampal gyrus, cerebellum, and brainstem, whereas delta waves are related to frontal responses. No decrease in activity was observed. This study demonstrates that SWS is not a state of brain quiescence, but rather is an active state during which brain activity is consistently synchronized to the slow oscillation in specific cerebral regions. The partial overlap between the response pattern related to SWS waves and the waking default mode network is consistent with the fascinating hypothesis that brain responses synchronized by the slow oscillation restore microwake-like activity patterns that facilitate neuronal interactions.