Retinotopic mapping

The neurons in primary visual cortex are organized "retinotopically". This means that different parts of the visual field (what you can see) are laid out in an organized manner in the visual cortex, and this retinotopic map is repeated several times in the early visual areas. By stimulating neighboring parts of the visual field, we can map out these visual areas using fMRI.

One of the classic ways of doing this is to show the subject two kinds of "travelling wave" stimuli (a wedge that rotates around a central fixation point, and an annulus that expands out from the central fixation point), and then look at the activation in the visual cortex. The fourier transform of the activation will show a lot of power at the frequency of the traveling wave, and the phase will signify the relative location of the stimulus.

Stimulus details

• outer radius: 12 degrees
• period (for a full rotation or center-to-outer edge): 24 s
• black and white checkerboards at 100% contrast with drifting phase or phase reversal
• fixation task is useful to aid in proper fixation
• wedge width can be 22.5 or 45 degrees
• ring width can be 12.5 or 25% of the total eccentricity

I have written a wrapper toolbox that uses Psychopy to generate the stimuli. My wrapper code presents a GUI to set size, timing, contrast and other parameters. Documentation is available here; please contact me if you would like access to the repository (gran0260@umn.edu).

Analysis links coming soon!

References