Optimization of visual tasks for detecting visual cortex activity in fMRI studies

Abstract

The aim of this study was to optimize visual stimuli in various forms of visual tasks for acquiring significant and robust signals. The effects of physical parameters of visual stimuli for detecting visual cortical activity were evaluated by functional magnetic resonance imaging. These parameters were temporal frequency (TF), spatial frequency (SF), the different patterns of activation including: square wave and sine wave grating, and two different states of rest including black and white screen. Functional Magnetic Resonance Imaging (fMRI) was performed by 1.5 Tesla General Electric system in 14 volunteers (9 males and 5 females, range 19–26 years). The activation map was created using the data obtained from the block designed fMRI study. Pixels whose Z value was above a threshold of 2.3, in significant level P=0.05 were considered activated. The average percentage BOLD (blood oxygenation level dependent) signal change for all activated pixels within the occipital lobe, multiplied by the total number of activated pixels within the occipital lobe, was used as a measure for the strength of the fMRI signal at each state of TF& SF. The results demonstrated that the strength of the fMRI signal was maximum in the TF of 8 Hz with low spatial frequency of 0.50 cycle per degree (cpd) and using the black screen in the rest state. However there isn’t significant difference between square-wave and sine-wave grating in producing visual activation in cortex. Physical parameters of visual tasks are effective in detecting visual cortical activity, and it is necessary to consider them for taking significant and robust signal.