Temporal aspects of feature integration in vision

To fully comprehend visual perception, we need to necessarily understand its temporal dimension. Our visual environment is highly dynamic, requiring the processing and integration of temporal signals in order to make sense of it. Many processes, such as the extraction of motion signals, require time by definition. Moreover, even seemingly static tasks can benefit from integrating features over time, considering that the information from single snapshots may be noisy and incomplete. That such mechanisms are indeed at work in the human visual system has previously been demonstrated using the sequential metacontrast paradigm (SQM). In this paradigm, features were shown to mandatorily integrate along a motion trajectory, in discrete temporal windows, even if they are separated by up to 450 milliseconds.

In this thesis, I utilized the SQM to systematically examine spatiotemporal processing in the human visual system. In a first study, I showed that it is not the stimulus evidence but the unconscious processing load that determines the duration of temporal integration. Thus, the window duration can be temporally extended with greater processing load. Furthermore, the extent of integration was found to be best captured by the absolute time and not by the number of elements presented. In a second study, I showed that once unconscious processing windows have started, they are stable and cannot easily be disrupted. Finally, I found that subjective perception significantly influences unconscious feature integration. Specifically, different perceptual groupings are elicited in different participants, and whether features are integrated is based on whether or not they are perceptually grouped together.

Taken together, these results highlight that high-level and perceptually meaningful mechanisms, rather than low-level stimulus features, determine the mechanisms underlying spatiotemporal integration. The complex computations underlying such processes further underline the need for extended periods of processing that precede conscious awareness.