Exploring the neural correlates of naturalistic hybrid search tasks

Since their introduction in the 60s, we’ve learned a lot about hybrid search (HS) tasks, which require observers to search for any item of a memory set. One of the main signatures of HS is a robust linear relationship between RT and visual set size (as in other visual search -VS- tasks) and a logarithmic relationship between RT and memory set size. This has been investigated under several scenarios, including word searches and categorical targets, and its behavioral correlates have been used to inform theories of VS. However, little is known about the underlying neural mechanisms in HS (and in general about overt attention VS tasks). One reason is that eye movements produce artifacts in M-EEG signals, which are much larger than the signals of interest. Here, we aim to start uncovering the neurophysiological mechanisms underlying HS. We first ran an online behavioral experiment using a new mapping naturalistic search task, where the memory set changes in each trial. We found that the main signatures of HS (linear increase in RT with visual set size and logarithmic increase with memory set size), remain present in target-present trials even when contextual information is present. In a second experiment, we combined EEG and eye tracking recordings while participants performed the same task. By using a deconvolution analysis approach, we found differences in the fixation-related potentials (brain potentials aligned to fixation onset) depending on the memory set size. In a third experiment, we extended our approach to combine MEG and eye movement recordings. After identifying and characterizing robust markers of neural and saccadic spike artifacts in the signal, we found significant task effects in fixation-related fields and low-frequency oscillations. Altogether, our approach provides a way to inquire about the role of specific neurophysiological signals in eye movements and behavior.