BOLD fMRI of the visual cortex: Quantitative responses measured with a graded stimulus at 1.5 Tesla

Abstract

Purpose: To measure and quantitatively characterize an activity generated by the neurons of the visual cortex (VC) in response to graded luminous intensity contrast stimuli using a 1.5 Tesla scanner. Materials and Methods:Functional magnetic resonance imaging (fMRI) of the vc with the intrinsic blood oxygenation level dependent (BOLD) mechanism was performed by using a paradigm with a 5 ϫ 5 flashing checkerboard pattern flickering eight times per second at eight luminance contrasts presented in a randomized order. The changes of the luminance contrast were obtained by varying the luminance intensity of the white checkerboard squares. Each of eight trials, corresponding to eight luminance contrasts, consisted of six "rest" and six "activation" epochs, repeated five times, amounting to 60 measurement periods per trial. During each epoch, 10 contiguous oblique axial-to-coronal slices covering the calcarine fissure region and parallel to a line through the anterior-posterior commissure (AC-PC) markers were acquired using a gradient-recalled echo planar imaging (GRE-EPI) sequence. Results:The measurements showed changes in the activation extent in the VC following the stimulus' rising luminance intensity contrast. In addition, the fMRI signal in those activated areas present throughout all eight trials, referred to as "common" voxels in this report, showed an increasing trend as a function of the rising luminance intensity contrast. Conclusion:These results suggest that the processes of the neuronal recruitment that affects the extent and number of activated neurons, and the neuronal enhancement that defines the magnitude of the neuronal activation are dependent on the luminance intensity contrast. These changes can be visualized and quantified using BOLD fMRI at 1.5 Tesla.

Luminance contrast of a visual stimulus modulates the BOLD response more than the cerebral blood flow response in the human brain

“…Our general finding of a graded BOLD response to increasing stimulus contrast is in agreement with earlier studies using BOLD measurements alone (Boynton et al, 1996; Goodyear and Menon, 1998; Mohamed et al, 2002). Two previous studies have measured both CBF and BOLD responses in human visual cortex to varying stimulus contrast, although these studies were not specifically analyzed to test whether n varies with contrast level.…”

Heterogeneity of stimulus-specific response modification—an fMRI study on neuroplasticity

“…Contrary to expectations, activations related to pattern flashes, and also to the visual presentation of letters and to button presses were weaker after HFS. The typical main effects for pattern flashes serve as a visual localizer (Mohamed et al, 2002; Nelles et al, 2002) and together with the responses to the catch trials found in the current study indicate the technical validity. The accurate button presses in responses to catch trials show that subjects were attentive during the task.…”

The coupling of cerebral blood flow and oxygen metabolism with brain activation is similar for simple and complex stimuli in human primary visual cortex

“…There is extensive literature examining the neurophysiologic response to flickering checkerboards at different frequencies, luminance and with different colors (Hoge, Atkinson et al 1999, Vafaee and Gjedde 2000, Mohamed, Pinus et al 2002, Lin, Fox et al 2008), but the literature is sparser on how flow and metabolism in the visual cortex change in response to a more natural and complex stimulus. One study using a James Bond movie found maintained segregation and specialization of functional areas such that this complex movie stimulus and a simple block design stimulus both identified similar visual areas of the brain (Bartels and Zeki 2004).…”