The Effects of Stimulus Spatial Frequency, Temporal Frequency and Contrast on the fMRI BOLD Response in Visual Areas of Normal Human Subjects

Abstract

Purpose: : To examine the effects of changes in the spatial frequency, temporal frequency and contrast of a stimulus on the BOLD (Blood Oxygen Level Dependant) response in visual cortical areas of normal subjects.

Methods: : Seven normal subjects underwent fMRI scanning. The stimuli were a 60' black and white chequerboard reversing at 10 Hz, a 10' black and white chequerboard, reversing at 10 Hz and a 10' black and white chequerboard, reversing at 2 Hz (all 50% duty cycle). The fMRI paradigm was an on/off design with a 21 second ON and 21 second OFF period. The viewing distance was 2 metres, and the visual field was 13 by 7 degrees. Stimuli were presented at 5, 10, 20, 40 and 80% contrast. They were viewed monocularly using a mirror mounted on the head coil of the magnet. Gradient echo echoplanar (EPI) images were acquired on a GE Signa 1.5T Neuro-optimized MR system (General Electric, Milwaukee WI, USA) at the Maudsley Hospital, London and analysed using the XBAM software developed at the IoP (c.f. http://brainmap.it for details and references).

Results: : The 60' check produced strong activation of both striate and extra-striate visual areas. The BOLD response from striate cortex increased progressively with increasing stimulus contrast, whereas that from the extra-striate areas did not, so that the overall distribution of activation changed with stimulus contrast. The 10' check at 2 Hz only produced a detectable BOLD response in striate cortex at 80% contrast even though the subjects could see the stimulus clearly at the lower contrast levels. There was however clear activation of extra-striate areas at these lower contrast levels.

Conclusions: : The BOLD response in primary visual cortex increases with contrast whereas that in extra-striate areas does not, so that the distribution of the response can be altered by changes in stimulus contrast. A high spatial frequency stimulus is much less effective than one of lower spatial frequency in producing a BOLD response in striate cortex, but can produce a BOLD response in extra-striate areas without one in striate cortex, suggesting that the coupling between the stimulus, neuronal activity and the BOLD response may be different in these different areas.

Keywords: visual cortex • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)