Spatial Attention and Session Day Independently Modulate Human Visual Cortical Plasticity

Stimulus-specific response potentiation (SSRP) is a non-invasive form of cortical neuroplasticity that can be induced through repeated presentation of high-contrast visual stimuli. Classically, SSRP, like long-term potentiation and related plasticity mechanisms, has been hypothesized to demonstrate input specificity, such that potentiation is confined to the neural populations and synapses driven by the induction stimulus. In addition, SSRP is observed to accumulate across days in rodent models. Despite growing interest in this phenomenon, it remains unclear whether factors such as selective attention influence the magnitude or specificity of a potentiated response. This study investigated whether covert endogenous attention could modulate the strength of SSRP in high-density EEG recordings from neurotypical human participants. An established SSRP paradigm was modified to include an attention task during induction. Pre-post response amplitudes were measured with frequency-tagged (6 and 7.5 reversals-per-second) bilateral hemifield contrast-sweep checkerboard steady-state visual-evoked potential stimuli. The 10-minute high-contrast 2 Hz sign-reversing SSRP induction stimulus was presented only in the left hemifield, with the right hemifield serving as a control. On two different experimental days, each participant deployed attention either toward or away from the potentiated hemifield during induction. Response amplitudes increased after SSRP, with increases in response amplitude manifesting in both the potentiated and control non-potentiated hemifields, weighing against the idea that SSRP is input-specific. Interestingly, significantly more response potentiation observed on the second session day compared to the first, independent of attention condition. Finally, we observed that attention enhanced SSRP in naive but not experienced observers (i.e. on Session-Day 1 but not Session-Day 2), identifying a significant interaction between attention condition and session day. Overall our results suggest that (1) SSRP in humans may not in fact be stimulus-specific (potentiation effects are not input- or retinotopically- specific), (2) visual spatial attention has a modest enhancing effect on SSRP in first-session participants, and (3) a second session of SSRP leads to a pronounced plasticity effect compared to the first session that occludes any attentional modulation.