Behavioral and neural investigation on the effect of spatial attention on surround suppression
When a visual stimulus is presented together with other stimuli surrounding it, behavioral sensitivity and neural responses may change, often reduce, compared to when the same stimulus is presented alone. This is commonly referred to as center-surround interaction or surround suppression, and it is one of the most fundamental mechanisms in biological vision. It is well documented that in motion perception, center-surround interaction is affected by the size and contrast of the stimulus. As the size of a drifting grating increases, motion direction discrimination performance, as well as neural activity in one of the main cortical motion processing areas, medial temporal complex (MT+), decreases if the grating has high contrast (surround suppression). Whereas, when the size increases within certain limits, both the discrimination performance and the neural activity in MT+ may increase if the grating has low contrast (surround facilitation). On the other hand, spatial attention is known to modulate surround suppression both in humans and non-human animals with static stimuli. No previous study, how-ever, has directly and systematically investigated the effect of the spatial extent of attention on surround suppression in human motion perception. The studies presented in this dissertation aim to investigate the effect of the extent of spatial attention on center-surround interaction in visual motion processing. In our experiments, we used two attention conditions and a novel stimulus design, where a ‘center’ and a ‘surround’ drifting grating were presented to the participants. Under one of the attention conditions, which we call the ‘narrow-attention’ condition, participants performed a task that limited their attention to the central part of the stimulus. Under the other attention condition, which we call the ‘wide-attention’ condition, participants performed tasks that required them to extend their attention to both the center and surround gratings. Using this experimental paradigm, we measured motion direction discrimination thresholds behaviorally and cortical activity with fMRI. Behaviorally, we found increased thresholds, that is, stronger surround suppression, under the wide attention condition. In the hu-man homolog of MT+ (hMT+), we found that increasing the spatial extent of attention leads to reduced cortical responses, that is, to stronger neural suppression. This was not the case for the activity in the primary visual cortex (V1). Finally, we show that a parsimonious computational model that incorporates spatial attention and response normalization can successfully predict the response patterns in hMT+ and V1. Furthermore, the model could provide a link between cortical responses and behavioral thresholds. Overall, our findings and analyses showed that the behavioral effect can be successfully predicted by hMT+ activity. These results reveal the critical role of spatial attention on surround suppression, namely that surround suppression in motion perception becomes stronger with a wider attention field, and reveal possible cortical mechanisms underpinning the effect.