Introduction: functional magnetic resonance imaging is a useful non-invasive technique for the evaluation and mapping of human brain, especially the visual cortex. One of the most important subjects in this background is optimizing visual stimuli in various forms of visual tasks for acquiring significant and or-bust signals. Materials and methods: The effects of physical pa-ammeters of visual stimuli on 14 healthy volunteers for detecting visual cortical activity were evaluated by functional magnetic resonance imaging. These pa-ammeters were temporal frequency (TF), different pat-terns of activation including, square wave and sine wave grating, and two different states of rest includ-ing black and white screens. Results: The results showed that BOLD signal will be maximally in the TF of 8 Hz, and use the black screen in the rest state. However there was no significant difference between square-Awake and sine-wave gratings in producing visual activation in the cortex. Conclusion: Physical parameters of visual tasks are effective in detecting visual cortical activity, and it is necessary to pay attention to them in order to get sig-indicant and robust signal. Visual tasks with TF of 8 Hz and one pattern of square-wave or sine-wave in activation state, and black screen in rest state are op-timely 

Background: The brain response to temporal frequencies (TF) has been already reported, but with no study reported for different TF with respect to various spatial frequencies (SF). Materials and Methods: fMRI was performed by 1.5T GE-system in 14 volunteers during checkerboard, with TFs of 4, 6, 8 and 10Hz in low and high SFs of 0.5 and 8cpd. Results: Average percentage BOLD signal change demonstrated the amplitude of the fMRI response to different TFs was maximal in 6Hz for high SF of 8cpd, while, it was maximal at TF of 8Hz for low SF of 0.5cpd. Conclusion: The results are useful for vision therapy (such as the treatment of Amblyopia) and visual task selecting in fMRI studies.    

Introduction As functional magnetic resonance imaging (fMRI) is too expensive and time consuming, its frequent implementation is difficult. The aim of this study is to evaluate repeatability of detecting visual cortex activity in fMRI.Materials and Methods In this study, 15 normal volunteers (10 female, 5 male; Mean age±SD: 24.7±3.8 years) attended. Functional magnetic resonance images were obtained during a visual task of sine-wave with spatial frequency of 1.84 cpd and temporal frequency of 8 Hz in three scan runs. Two runs of functional images were provided consecutively in a session, and the third run was provided 1-6 weeks later. The activation map was created using the data obtained from the block-designed fMRI study. Voxels whose Z value was above a threshold of 2.3, at a significance level p=0.05, were considered activated. After image processing, the blood oxygen level dependent (BOLD) signal changes and the number of activated voxels in response to visual stimuli were compared in different runs.Results The results of this study demonstrate no significant difference between the number of activated voxels and BOLD signal in first and second runs in one session (Paired t-test, p>0.05). Moreover, there is a considerable correlation between first and second scan runs (rsignal=0.74, p=0.006 and rvoxel=0.62, p=0.03), while the correlation between the runs in separate sessions is weak (rsignal=0.28, p=0.38 and rvoxel=0.32, p=0.31).Conclusion Since the repeatability of BOLD signal and number of activated voxels in one session is considerably better than that in the separate sessions, it is suggested that in fMRI visual studies that need repeated scanning, scans should be acquired during a single session.