Long-lasting feature integration

In the classic model of vision, processing is local, feedforward and hierarchical. The first stages of the visual system are retinotopic, i.e., neighboring points in the outside world are mapped onto neighboring photoreceptors of the retina and this is preserved in the early stages of cortical processing. However, perception is global and non-retinotopic. Sensory information needs to be integrated over space and time to construct a meaningful percept. We make 2 to 4 rapid eye movements, called saccades, per second. Hence, the image of the outer world is constantly moving on the retina. Nevertheless, we perceive a stable and continuous environment. Here, I use the sequential metacontrast paradigm (SQM) to investigate spatiotemporal feature integration, and as a probe into the temporal structure of perception. In the SQM, features integrate along a motion trajectory. First, I show that features integrate in the SQM for several hundreds of milliseconds. This integration is mandatory, i.e., observers do not have access to the individual elements. Importantly, features integrate in discrete temporal windows. Features do not integrate if they are not part of the same window, regardless of spatiotemporal proximity. Hence, my results suggest that perception is discrete and support a 2-stage model in which, first, features are processed continuously with high spatiotemporal resolution in a long-lasting discrete window of unconscious processing. After the window closes, we consciously perceive the output of the processing. Second, I show that perceptual grouping determines feature integration within a window. Finally, features integrate mandatorily across saccades when object identity is preserved. Thus, features integrate non-retinotopically according to object identity with or without eye movements. These results show that feature integration is precise, specific and follows complex rules of grouping. Overall, long-lasting windows of unconscious processing can be seen as periods of sense making, and feature integration as a deliberate strategy of the brain when elements are grouped into a single object.