This study is part of an ongoing research project in collaboration between the Psychological Institue, UiO and Rikshospitalet, OUS. The authors collected all the data used in this paper themselves. Long-term potentiation (LTP) is a model that explains the neural basis for Hebbian learning and synaptic plasticity. Measuring LTP has traditionally demanded invasive techniques, and has therefore, until recently, almost exclusively been studied in animals. The use of high frequency sensory stimulation could open for the possibility to induce and observe LTP-like plasticity non-invasively. Hypotheses: This study is focused on three hypotheses: 1. Replicating previous research showing that high-frequency visual stimulation will yield LTP-like plasticity. 2. There is a positive correlation between level of cortisol and LTP-like plasticity in healthy participants. 3. There is a positive correlation between level of physical activity and LTP-like plasticity. Design: This study uses a vertical sine wave grating stimulus paradigm to measure visual evoked potentials (VEP) in 38 healthy adults, using electroencephalogram to measure cortical electrical activity. Testing involved VEP registration for 48 minutes, during which time the participants observed two baseline blocks at 2 and 8 minutes into the paradigm, one modulation block at 10 minutes into the session, and six post-modulation blocks 2, 8, 12, 18, 22, and 28 minutes subsequent to the modulation block of the sine wave grating. In addition, participants responded to questionnaires regarding mood, level of stress and physical activity. Participants also delivered saliva-samples to measure level of cortisol. Results: This study replicates earlier findings confirming modulation of visual evoked potentials as a valid method for studying LTP-like synaptic plasticity. The results show significant increases of the P1 and P1-N1 peak-to-peak amplitudes of the VEP, signifying underlying LTP-like plasticity, thus supporting hypothesis 1. This study demonstrates a positive correlation between level of cortisol and LTP-like plasticity, thus hypothesis 2 is supported. Level of physical activity and LTP-like plasticity did not demonstrate a positive correlation, thus hypothesis 3 is not supported. Conclusions: We conclude that VEP registration of high frequency visual stimulation can be a valid method for inducing and observing LTP-like plasticity in vivo in humans, thus replicating earlier studies. In addition the positive correlations found between level of cortisol and LTP-like plasticity indicate an underlying inverted U-relationship between these variables. The null-finding of physical activity related to LTP-like plasticity indicates that there either is no correlation between these variables, or that our method for measuring physical activity lacked criterion validity.