Summary: Researchers used patterned electrical fields to mimic slow-wave sleep and found they can alter brain activity in ways that may improve memory.
Source: University of Alberta
New Clues on How Slow-Wave Sleep and Electrical Stimulation May Boost Memory
Neuroscientists at the University of Alberta report that the rhythmic brain-wave patterns of deep slow-wave sleep appear to be crucial to strengthening memories. Their new study shows that applying controlled electrical fields can reshape those slow-wave patterns across the cortex, producing novel, repeatable activity that could underlie improved memory consolidation.
“During slow-wave activity, brain cells fire in various coordinated patterns, which we believe reflects the strengthening and reorganization of memory traces during sleep,” said Anastasia Greenberg, who led the research while completing her PhD under Professor Clayton T. Dickson in the Department of Psychology.
To probe how electrical stimulation interacts with slow-wave dynamics, the team simulated slow-wave sleep in laboratory preparations and modulated the tissue with rhythmic sinusoidal electrical fields. Prior behavioral experiments had suggested that externally applied electrical fields can enhance memory, but the neural mechanisms remained difficult to observe directly.
In collaboration with researchers at the University of Lethbridge, the group used voltage-sensitive dye (VSD) imaging to visualize cortical activity across both hemispheres. This imaging approach made it possible to record ongoing patterns of slow oscillations without the electrical recording artifacts that typically complicate studies that apply electrical fields.
The imaging revealed that spontaneous slow oscillations typically propagate in two main, opposing directions across an anterior-lateral to posterior-medial axis, and each individual wave can follow a variety of trajectories. When rhythmic electrical fields were applied to frontal regions, however, the cortex adopted new propagation patterns that were not seen during spontaneous activity. Those stimulus-entrained waves exhibited multiple anterior initiation zones but consistently terminated in the posterior somatosensory cortex.
Importantly, the stimulated patterns showed a higher degree of stereotypy: the same propagation sequences tended to repeat from cycle to cycle, unlike the more variable trajectories observed during spontaneous slow waves. “The stimulation dramatically altered those activity patterns into new, previously unseen ones,” Greenberg said. “This suggests the electrical stimulation may be producing an ‘artificial’ form of coordination that could selectively strengthen particular cortical representations.”
“If you could influence the kind of slow-wave sleep you experience, you might be able to enhance memory,” added Professor Dickson. He and colleagues stress, however, that the findings are an early step toward understanding how brain stimulation could be used therapeutically; additional work is needed before any practical recommendations—such as using field stimulators during naps or overnight—can be made.
The study also highlights a significant methodological challenge: typical techniques for recording brain activity, such as EEG, are prone to interference when external electrical fields are present. By using VSD imaging in a bilateral cortical preparation of anesthetized mice, the team could bypass those limitations and directly observe how electrical fields shape ongoing cortical dynamics in real time.
Funding: This research was conducted in collaboration with the University of Lethbridge and supported in part through a Campus Alberta Neuroscience trainee grant.
Source: Katie Willis, University of Alberta.
Publisher: Organized by NeuroscienceNews.com.
Image credit: University of Alberta.
Original research: “New waves: Rhythmic electrical field stimulation systematically alters spontaneous slow dynamics across mouse neocortex” by Anastasia Greenberg, Javad Karimi Abadchi, Clayton T. Dickson, Majid H. Mohajerani, published in NeuroImage, May 3, 2018.
DOI: 10.1016/j.neuroimage.2018.03.019
New waves: Rhythmic electrical field stimulation systematically alters spontaneous slow dynamics across mouse neocortex
The defining rhythm of slow-wave forebrain activity is the large-amplitude slow oscillation (SO: ~1 Hz), made up of alternating synchronous periods of neuronal activity and silence at both the single-cell and network levels. Each SO wave originates in a specific location and propagates across the neocortex. Previous attempts to manipulate SO activity with electrical fields have shown that cortical networks can be entrained and that such entrainment can improve memory performance, but direct observation of neural activity during stimulation has been limited by recording artifacts. Using voltage-sensitive dye imaging in a bilateral cortical preparation of urethane-anesthetized mice, the authors tracked SO activity and its modulation by sinusoidal electrical field stimulation applied to frontal regions. Under spontaneous conditions, SO waves propagate primarily along an anterior-lateral to posterior-medial axis with diverse trajectories. Rhythmic field stimulation induces novel propagation patterns not observed spontaneously, characterized by varied anterior initiation zones and a consistent posterior somatosensory termination. Stimulus-induced patterns repeat cycle after cycle and show greater stereotypy than spontaneous waves. These results demonstrate that slow electrical field stimulation robustly entrains and reshapes ongoing slow cortical dynamics in sleep-like states, suggesting a potential mechanism for targeting cortical representations to influence memory consolidation.
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