Abstract
Contextual information plays a critical role in visual search strategies but its neural timeline and correlates remain poorly understood. This study investigates the neural mechanisms underlying attentional orientation to contextually cued target locations in natural scenes. The primary focus of interest are the target absent scenes, where attention is primarily driven by contextual information rather than target features. 17 human participants performed a visual search task and perceptually decided presence or absence of targets in natural scenes displayed on screen. The targets varied in shape, size and category. Participants reported their response using a 10-point confidence rating and their neural signals were recorded using a 64 channel EEG recording system. From a separate behavioural experiment performed by 20 subjects on the same set of images, the difficulty of the search tasks were evaluated. We used multivariate pattern classifiers to predict coarse contextual locations (left/right) from single trial EEG signals. Our results demonstrate that contextual locations in natural scenes could be predicted reliably from neural activity when observers were searching for targets. Multivariate pattern analysis (MVPA) failed to predict the expected location using the same stimuli in a separate control EEG study when contextual information was made inconsequential. MVPA analysis revealed that the possible location of a target could be predicted more accurately in the easy than difficult search tasks. Finally we demonstrate that for easy targets, contextual facilitation starts as early as 200 ms post stimulus onset whereas for difficult targets, contextual effect was relevant at a later stage alluding to the role of task difficulty in mediating the neural timeline of contextual guidance. Source localization analysis of the event related potential (ERP) suggests that scene information is passed on from Lateral Occipital Complex (LOC) via Intra Parietal Sulcus (IPS) to the fronto-parietal network.