Unconscious associative memory affects visual processing before 100 ms

Searching for an object in a cluttered environment takes advantage of different cues, explicit attentional cues, such as arrows, and visual cues, such as saliency, but also memory. Behavioral studies manipulating the spatial relationships between context and target in visual search suggest that the memory of context-target associations could be retrieved quickly and act at an early perceptual stage. On the other hand, neural responses are usually influenced by memory at a later, postperceptual stage. At which level of neural processing does the memory of context-target associations influence scene analysis? In our experiment, human subjects learned arbitrary associations between given spatial layouts of distractors and target positions while performing a classical visual search task. Behaviorally, context-target associations speed visual search times, although subjects remain fully unaware of these associations. Magneto-encephalographic responses to visual displays containing or not containing relevant contextual information differ before 100 ms, much earlier than any known effect of recent experience. This effect occurs bilaterally at occipital sensors only, suggesting that context affects activity in the underlying early sensory cortices. Importantly, subjects do not show any sign of explicit knowledge about context-target associations: The earliness of the influence of contextual knowledge may be a hallmark of unconscious memory.