Top-down effects on early visual processing in humans: a predictive coding framework

An increasing number of human electroencephalography (EEG) studies examining the earliest component of the visual evoked potential, the so-called C1, have cast doubts on the previously prevalent notion that this component is impermeable to top-down effects. This article reviews the original studies that (i) described the C1, (ii) linked it to primary visual cortex (V1) activity, and (iii) suggested that its electrophysiological characteristics are exclusively determined by low-level stimulus attributes, particularly the spatial position of the stimulus within the visual field. We then describe conflicting evidence from animal studies and human neuroimaging experiments and provide an overview of recent EEG and magnetoencephalography (MEG) work showing that initial V1 activity in humans may be strongly modulated by higher-level cognitive factors. Finally, we formulate a theoretical framework for understanding top-down effects on early visual processing in terms of predictive coding

Cognitive modulations of early visual cortex activity in humans

The experiments conducted for this thesis examined the earliest stages of processing in human visual cortex. Using electroencephalographic recordings, it was observed that the first component of the visual evoked potential, the so-called C1, is affected by learning and attention. This contrasts with earlier studies which overwhelmingly reported no effects of higher cognitive processes on early visual cortex activity. It is suggested that methodological improvements such as the adaptation of stimulus parameters to the physiological characteristics of lower visual areas partly explain the new findings. Together with other recent studies, these results show that previously prevalent notions of primary visual cortex (V1) as an inflexible module for the treatment of low-level stimulus characteristics may need to be revised. The idea that V1 can act as an adaptive and flexible processor, previously based on animal studies, is thus starting to gain support from the human literature