Cortical stimulation consolidates and reactivates visual experience: neural plasticity from magnetic entrainment of visual activity

Abstract

Delivering transcranial magnetic stimulation (TMS) shortly after the end of a visual stimulus can cause a TMS-induced 'replay' or 'visual echo' of the visual percept. In the current study, we find an entrainment effect that after repeated elicitations of TMS-induced replay with the same visual stimulus, the replay can be induced by TMS alone, without the need for the physical visual stimulus. In Experiment 1, we used a subjective rating task to examine the phenomenal aspects of TMS-entrained replays. In Experiment 2, we used an objective masking paradigm to quantitatively validate the phenomenon and to examine the involvement of low-level mechanisms. Results showed that the TMS-entrained replay was not only phenomenally experienced (Exp.1), but also able to hamper letter identification (Exp.2). The findings have implications in several directions: (1) the visual cortical representation and iconic memory, (2) experience-based plasticity in the visual cortex, and (3) their relationship to visual awareness.

Figure 1. Procedure of Experiment 1.

The vertical black lines represent the timing of visual stimulus appearing on the monitor; the red lines represent the timing of the paired-pulse TMS (50 ms between the pulses) to visual cortex. (a) In the perceptual entrainment condition, the visual stimulus was followed by the TMS with 300 ms delay in the learning phase. Trials were separated by 6-second intervals. After 10 trials repetition, only TMS was delivered to examine whether replay or phosphene was perceived. (b) In the control condition, a phase offset between the two types of stimuli was introduced, increasing the delay between visual stimulus and TMS to 3 secs, while preserving the 6-second presentation rate for each stimulus type.

Figure 2. Results of Experiment 1.

Individual (cross symbols) and average (solid lines) rating scores as a function of trial numbers in the perceptual entrainment condition (red) and the control condition (blue). The data were from the participants who saw both TMS-induced replay and TMS-entrained replay (n = 10). For the data including all the participants who saw TMS-induced replay, regardless of whether or not seeing TMS-entrained replay (n = 14), see Supplementary Figure 1.

Figure 3. Stimuli in Experiment 2.

Participants' performance in a letter discrimination task was tracked across learning, test, and baseline trials. (a) Timelines of the trial types and block structure. In learning trials, mask preceded the target by 300 ms, and TMS preceded the target by 50 ms. In test trials, no mask was presented. Trials included only TMS and target. In baseline no-TMS trials, no TMS pulse was delivered. Trials included only mask and target. (b) Illustration of the eight possible targets, the mask, and the two possible locations where the target and mask could appear.

Figure 4. Results of Experiment 2.

Mean accuracy of target letter discrimination as a function of mask conditions. Asterisks indicate statistically significant differences between the conditions (P < .05).