At what levels do expectations manifest?

In the process of perception, bottom-up sensory information and top-down expectations become integrated (Friston, 2005; Lee & Mumford, 2003; Summerfield & de Lange, 2014). Prior knowledge can bias perception, and expecting to see a stimulus may elicit stimulus-related activity before the image is even presented. Previous research shows that this is the case for simple objects such as gratings (Kok et al., 2017b), but we wondered whether such prestimulus activity would also be present when participants expect specific object images. To address this question, we repeatedly showed participants images of colored shapes (cues) followed by objects (targets), allowing for learning of the associations between them. Several days later, we showed participants the same images while recording brain activity with magnetoencephalography (MEG). By applying classification analysis techniques to the data, we were able to examine whether cue images elicited prestimulus object-related activity in participants’ brains. We did not find evidence for such prestimulus template activation in our data, which may be because expecting objects does not activate prestimulus templates in the visual system. Objects are highly complex visual stimuli processed by downstream areas in the visual system, so expecting objects may function differently from expecting low-level visual features (Kok et al., 2017b). Alternatively, our research paradigm may have been suboptimal for addressing this research question, and this may have prevented us from observing prestimulus activation. We discuss possible avenues to improve the research design, so future research can more conclusively determine the presence or absence of prestimulus templates of higher-order categories.