Using fast visual rhythmic stimulation to control inter-hemispheric phase offsets in visual areas

Spike timing dependent plasticity (STDP) is believed to be important for neural communication and plasticity in human episodic memory, but causal evidence is lacking due to technical challenges. Rhythmic sensory stimulation that has been used to investigate causal relations between oscillations and cognition may be able to address this question. The challenge, however, is that the frequency corresponding to the critical time window for STDP is gamma (~40 Hz), yet the application of rhythmic sensory stimulation has been limited primarily to lower frequencies (<30 Hz). It remains unknown whether this method can be applied to precisely control the activation time delay between distant groups of neurons at a millisecond scale. To answer this question and examine the role of STDP in human episodic memory, we simulated the STDP function by controlling the activation time delay between the left and right visual cortices during memory encoding. This was achieved by presenting flickering (37.5 Hz) movie pairs in the left and right visual fields with a phase lag of either 0, 90, 180 or 270°. Participants were asked to memorize the two movies within each pair and the association was later tested. Behavioral results revealed no significant difference in memory performance across conditions with different degrees of gamma phase synchrony. Yet importantly, our study showed for the first time, that oscillatory activity can be driven with a precision of 6.67 ms delay between neuronal groups. Our method hereby provides an approach to investigate relations between precise neuronal timing and cognitive functions.