Visual perceptual learning (VPL) is defined as a long-term increase in visual performance as a result of visual experiences. The purpose of the current R21 resubmitted proposal is to advance the understanding of the mechanisms underlying sleep-dependent performance improvement (SDPI) in VPL. This may, in turn, lead to interventions to improve the efficiency of visual skills with healthy individuals and of rehabilitation with people having sleep problems. A prevailing view is that the role of sleep in SDPI is to merely enhance plasticity states triggered by training. However, our preliminary data suggest that even without prior training, plasticity states occur in early visual areas specifically during sleep stage N3. In contrast to the prevailing view, this raises the novel possibility that without training, activity during sleep stage N3 causes plasticity states (sleep-based plasticity). To test this possibility, we will specifically examine how delta activity (1-4 Hz spontaneous oscillations) during sleep stage N3 is involved in SDPI of VPL. However, there are 2 potential difficulties. One is to isolate the effect of activity in a certain sleep stage. Commonly used methods such as sleep-stage deprivation and an intake of a medicine may influence subject?s emotional and intellectual qualities as well as non-targeted sleep stages. To overcome this difficulty, we will use the phenomenon called the first night effect (FNE) in which delta activity in early visual areas during sleep stage N3 selectively reduced during the first sleep session in an unfamiliar environment. By comparing plasticity states during the first and second sleep sessions, we could better isolate the effect of activity in an involved brain region during sleep stage N3 on SDPI. The other difficulty is that there has been no effective method to directly specify plasticity states during sleep in the human brain. To examine plasticity states without training that induce performance changes, using magnetic resonance spectroscopy during sleep, we will measure an excitatory-inhibitory (EI) ratio, which is defined by the ratio of glutamate/gamma-aminobutyric acid concentrations in early visual areas. An EI ratio has been shown to be correlated with the degree of plasticity in VPL. Thus, by measuring EI ratios during sleep, we can examine the degree of plasticity states. We will test the following two specific aims.
Aim 1 will address the question as to whether there is sleep-based plasticity that occurs without training- based plasticity by testing the hypothesis. We will first test whether the EI ratio during sleep stage N3 is correlated with delta activity without any preceding visual training. Then, we will examine whether stronger delta activity and a higher EI ratio indeed lead to greater VPL.
Aim 2 will address the question as to how much sleep-based plasticity and training-based plasticity contribute to SDPI by testing the hypothesis: Both of the EI ratio changes for sleep-based plasticity and training-based plasticity significantly contribute to SDPI.
While it is suggested that a functional role of sleep is to stabilize and improve visual learning, its underlying neural mechanism is poorly understood. Using advanced non-invasive neuroimaging techniques and careful behavioral measurements, we aim to better understand the mechanisms. Successful research results may give us insights into how learning can be improved through sleep and how vision declined or damaged due to aging, injury or diseases could be improved.