Spatiotemporal properties of the visual system and the effect of attention:fMRI and psychophysical studies

The present work investigates the visual system's response to pattern information from both bottom-up and top-down perspectives. Three different experiments were performed. Experiment one is an investigation of the temporal dynamics of the visual system's sensitivity control. We tested whether high spatial frequency channels adjust their sensitivity slower than low spatial frequency channels following orientation-specific pattern adaptation. The results demonstrate that it takes longer for subject's pattern sensitivity to recover after adapting to high spatial frequency patterns as compared to low frequency patterns. This indicates that the magnocellular (low frequency-sensitivity channels) and parvocellular (high frequency-sensitivity channels) systems differ in their sensitivity adjustment mechanisms. Experiment two uses functional magnetic resonance imaging (fMRI) to look at pattern sensitivity in early visual areas. In addition to obtaining visual cortex sensitivity measures, another goal of this study is to present a novel method that relates fMRI blood oxygen level-dependent (BOLD) activity levels to a common and comparable space in which behavioral results can be directly compared to fMRI measurements. The fMRI results provide evidence that the neural contrast sensitivity declines at a slower rate than behavior sensitivity. The study also discusses how the method of using BOLD to bridge between two variables can be applied to other studies comparing fMRI and behavioral data. The third study examines attentional influences on the visual system's response profile to patterns varying in contrast. Many studies have investigated the effects of selective attention, but few have directly addressed the underlying neural mechanisms. The goal of this study is to use fMRI to examine through which mechanisms spatial attention influences the contrast response function in individual visual cortical areas by examining response to contrast under attended and ignored conditions. The main findings were that a significant portion of the attentional effect could be explained by an additive effect of attention plus a contrast gain effect. This indicates that attention increases a visual area's sensitivity to contrast, and attention also cause an overall increase in the BOLD response.