Electronic Thesis and Dissertation Repository


Master of Science




Dr. J. Bruce Morton


During selective attention, visual stimuli compete for processing capacity. Increased activation is found in extrastriate regions that represent the attended stimulus. However, little research has been done looking at activation in extrastriate regions when attention is shifted between stimulus features. To address this, participants completed a switching task during fMRI scanning. They attended to the colour or motion of bivalent stimuli on different trials. It was hypothesized that attentional modulation would be seen in colour area V4 and motion area V5 and that this modulation would help explain switch costs, a term used to describe why we are slower and more error prone on switch trials. Attentional modulation was found in V4, with greater activity when colour was attended. No modulation was observed in V5. The level of competition between these regions did not differ across switch and repeat trials, suggesting that such competition does not explain switch costs.