Attentional modulation in early visual cortex: a focused reanalysis of steady-state visual evoked potential studies

Steady-state visual evoked potentials (SSVEPs) are a powerful tool for investigating selective attention. Here, we conducted a combined re-analysis of multiple studies employing this technique in a variety of attentional experiments in order to, first, establish benchmark effect sizes of attention on amplitude and phase of SSVEPs, and second, harness the power of a large dataset to test more specific hypotheses. Data of eight published SSVEP studies were combined, in which human participants (n=135 in total) attended to flickering random dot stimuli based on their defining features (e.g. location, color, luminance, or orientation) or feature-conjunctions. The reanalysis established that in all the studies attention reliably enhanced amplitudes, with colorbased attention providing the strongest effect. In addition, the latency of SSVEPs elicited by attended stimuli was reduced by ~4 ms. Next, we investigated the modulation of SSVEP amplitudes in a subset of studies where two different features were attended concurrently. While most models assume that attentional effects of multiple features are combined additively, our results suggest that neuronal enhancement provided by concurrent attention is better described by multiplicative integration. Finally, we used the combined dataset to demonstrate that the increase in trial-averaged SSVEP amplitudes with attention cannot be explained by increased synchronization of single-trial phases. Contrary to the prediction of the phase locking account, the variance across trials of complex Fourier coefficients increases with attention, which is more consistent with boosting of a largely phase-locked signal embedded in non-phase-locked noise.