Spatial frequency dependence of the human visual cortex response on temporal frequency modulation studied by fMRI

ABSTRACT

The brain response to temporal frequencies [TF] has been already reported. However, there is no study on different TF with respect to various spatial frequencies [SF]. Functional magnetic resonance imaging [fMRI] was done by a 1.5 T General Electric system for 14 volunteers [9 males and 5 females, aged 19'26 years] during square-wave reversal checkerboard visual stimulation with different temporal frequencies of 4, 6, 8 and 10 Hz in 2 states of low SF of 0.4 and high SF of 8 cycles/degree [cpd]. All subjects had normal visual acuity of 20/20 based on Snellen's fraction in each eye with good binocular vision and normal visual field based on confrontation test. The mean luminance of the entire checkerboard was 161.4 cd/m2 and the black and white check contrast was 96%. The activation map was created using the data obtained from the block designed fMRI study. Pixels with a Z score above a threshold of 2.3, at a statistical significance level of 0.05, were considered activated. The average percentage blood oxygenation level dependent [BOLD] 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 an index for the magnitude of the fMRI signal at each state of TF and SF. The magnitude of the fMRI signal in response to different TF's was maximum at 6 Hz for a high SF value of 8 cpd; it was however, maximum at a TF of 8 Hz for a low SF of 0.4 cpd. The results of this study agree with those of animal invasive neurophysiologic studies showing SF and TF selectivity of neurons in visual cortex. These results can be useful for vision therapy and selecting visual tasks in fMRI studies