By Victoria Comunale
Sleep and memory-formation share a fickle relationship. We are told to get plenty of rest before a test because sleep plays a crucial role in learning by aiding memory consolidation. Yet, sometimes, after a night of rest, we can completely forget about something that was on our minds the night before. Sleep’s role in memory consolidation and memory loss has been explored by neuroscientists before, but the balance between these two dueling occurrences, as well as the role of the brain waves in this phenomenon, still remain somewhat of a mystery.
Researchers at the University of California San Francisco (UCSF), recently published a study examining the role of different kinds of brain waves in memory consolidation and forgetfulness in rats. In this experiment, the researchers trained rats to control a feeding tube. This control was a skill that was gradually learned by the rat, and memory consolidation played a crucial role in this process. Successfully completing the task required the rat to move the tube from point A to point B within 15 seconds. Since this learning process involved the motor cortex, the researchers studied the brain waves in this area of the brain during non-REM sleep.
There are several stages of non-REM sleep that can be differentiated by distinct patterns of brain wave activity. These stages are arranged from lightest to deepest sleep, and the waves themselves arise from the neural activity of a region of the brain known as the thalamus. The first stage of non-REM sleep, the lightest stage, is characterized by both alpha and theta waves. In stage two, theta waves dominate brain activity but are interrupted by brief bursts of higher frequency brain waves known as sleep spindles. The third and fourth stages of non-REM sleep, the deepest stages, also feature sleep spindles, but are predominantly characterized by delta waves and slow oscillation waves, which researchers were most interested in. In order to study the effects of these waves, they employed a recent technique that has grown in popularity in the field of neuroscience: optogenetics. Utilizing this technique, the researchers were able to directly interfere with the activity of neurons in the brain and interrupt the activity of the targeted brain waves, thus establishing a causal rather than correlational relationship.
The role of slow oscillation waves has long been suspected of playing a role in memory consolidation, yet the function of delta waves, which are more prevalent than slow oscillation waves, is still unknown. When the researchers used optogenetics to interfere with the slow oscillation waves, the success rate of the rats in the tube moving task was much lower than that of the control group. Yet, interfering with delta waves had the opposite effect. The rats were able to complete the task with a higher success rate than the control group. From these remarkable results, the researchers concluded that these dueling brain waves have opposite effects during sleep. This conclusion was unexpected, since these brain waves are found in the same stage. Therefore, memories both strengthened and weakened during non-REM sleep. The difference in memory consolidation between delta waves and slow oscillations is stark and undeniable, contradicting prior speculations that they may have similar functions.
The researchers also focused on specific bursts of activity associated with slow wave oscillations called sleep spindles. These spindles are already known to play an essential role in sensory processing and long term memory consolidation. Because delta waves, which are associated with memory loss, are more prevalent during non-REM sleep, the researchers postulated that these spindles, coupled with the effects of slow oscillating waves, aid in balancing memory consolidation against the memory loss associated with delta waves.
These results can also have implications for phenomena observed in humans, especially aging. It has been experimentally demonstrated that slow-wave activity has lower amplitude in the elderly. If the results from the UCSF group can be translated to the human brain, this means that the elderly are less likely to have consolidation benefits during sleep because of a reduction in slow oscillations and spindles. It is still unclear if this newfound information concerning the dueling brain waves can be applied as a useful tool in strengthening human memories, considering that the weakening of less important memories is necessary for memories of higher importance to be strengthened. Nevertheless, these results help bring us one step closer in understanding the complicated activities of our brains.