Time inSight : the matter of time in visual perception

Time: it seems a simple concept, a single dimension along which the world evolves. The future becomes the present, and the present becomes the past, continuously and inexorably. Open your eyes, and you see the world. You think that what you see now is what is happening now. In fact, processing sensory information takes time. By the time the brain has processed the incoming visual information such that you become aware of it, it is outdated. So what you see now, has in truth already happened: when you open your eyes, you see the past. Furthermore, different visual properties differ not only in the time they take to process, but also in where in the brain they are processed. Nonetheless, the brain manages to generate a coherent, convincing impression of living in the present, one which we might experience for a lifetime without ever questioning. The asynchronous, distributed architecture of the visual brain creates the significant computational problem of reintegrating visual features that have been divided over both space and time. On the basis of experimental findings presented in this thesis, I argue that the brain solves this problem by representing time at each level of visual processing, alongside the particular feature processed at that stage. The temporal signal is established early in the visual pathway in the lateral geniculate nucleus, and is passed down the visual hierarchy as a network-level temporal population code at each stage of processing. Whenever a visual feature becomes available for awareness, it can then be slotted into the appropriate point on the subjective internal timeline according to its temporal tag. In this way, asynchronously processed features can be brought back into register to create the single coherent stream of visual awareness that we experience in daily life.