Influences of Cognition on Early Visual Processing.

Whether higher-level cognitive processes can influence processing in early visual cortex remains a long-standing issue in cognitive psychology. The aim of the experiments reported in the current thesis was to explore this issue, specifically investigating whether attention could modulate spatial frequency (SF) processing at early stages of visual analysis. Early visual areas, including VI, are known to strongly encode SF information and therefore SF filtered stimuli provided an ideal tool to examine attentional influences on low-level visual processing. Previous studies have shown that observers can selectively use SF information depending on task requirements such as categorisation and prior sensitisation, and work using top-down cues indicates that attention may be responsible for this flexible scale use. However, the locus of such attentional modulation has not been examined in detail. Using methods from experimental psychology and functional Magnetic Resonance Imaging (fMRI), the work reported in the current thesis provides converging evidence that selective scale perception results from attentional modulation of SF processing in early visual areas. Applying the logic used in perceptual learning studies to infer the locus of learning, flexible SF processing was found to be specific to retinotopic location and driven by top-down attention. This was confirmed by an event-related fMRI experiment, demonstrating directly that cue-directed attention to SF influenced neural activity in visual areas as early as VI. Furthermore, this thesis reports an fMRI method that was developed to identify SF channels in V1. It is concluded that SF processing in early visual cortex can be modulated by attention: an influence of cognition on early visual processing