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1 Psychology, Princeton University, Princeton, New Jersey, USA; CSBMB, Princeton University, Princeton, New Jersey, USA
* To whom correspondence should be addressed. E-mail: skastner{at}princeton.edu.
In the human brain, little is known about the functional anatomy and response properties of subcortical nuclei containing visual maps such as the lateral geniculate nucleus (LGN) and the pulvinar. Using functional magnetic resonance imaging (fMRI) at 3 Tesla, collective responses of neural populations in the LGN were measured as a function of stimulus contrast and flicker reversal rate and compared to those obtained in visual cortex. Flickering checkerboard stimuli presented in alternation to the right and left hemifield reliably activated the LGN. The peak of the LGN activation was found to be on average within ±/- 2 mm of the anatomical location of the LGN, as identified on high-resolution structural images. In all visual areas except MT, fMRI responses increased monotonically with stimulus contrast. In the LGN, the dynamic response range of the contrast function was larger and contrast gain was lower than in cortex. Contrast sensitivity was lowest in the LGN and V1 and increased gradually in extrastriate cortex. In area MT, responses were saturated at 4% contrast. Response modulation by changes in flicker rate was similar in the LGN and V1 and occurred mainly in the frequency range between 0.5 and 7.5 Hz; in contrast, in extrastriate areas V4, V3A, and MT, responses were modulated mainly in the frequency range between 7.5 and 20 Hz. In the human pulvinar, no activations were obtained with the experimental designs used to probe response properties of the LGN. However, regions in the medio-dorsal right and left pulvinar were found to be consistently activated by bilaterally presented flickering checkerboard stimuli, when subjects attended to the stimuli. Taken together, our results demonstrate that fMRI at 3 Tesla can be used effectively to study thalamo-cortical circuits in the human brain.
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