Cortical Activation during Visual
Memory-guided and Hand-guided Saccades: an MEG study
L Ren, EK Cressman, G Blohm, DO
Cheyne, JD Crawford
Our previous studies (Ren et al. J. Neurophysiol. 2006, Ren and
Crawford Exp. Brain Res. 2007) examined behavioral aspects of saccades
to hand-held targets. Here, we use Magnetoencephalography (MEG) to
compare the spatial-temporal patterns of cortical activation during
visual memory-guided and hand-guided saccades. MEG measures the
magnetic fields generated by the brain, from which the source of
activity can be mathematically modeled. Electro-oculography (EOG) and
Electromyography (EMG) were used to record eye movement and hand
movement onset respectively. Eight subjects performed three paradigms
in a dark room: 1) visual memory-guided saccades: subjects saccaded
toward a briefly flashed target after the central fixation LED was
extinguished; 2) hand-guided saccades: a handheld target was flashed
and then actively moved (in the dark) to the opposite visual field
while subjects fixated the central LED. The fixation LED was then
extinguished, and subjects saccaded toward the handheld target; 3)
combined task: same as #2, but the handheld target flashed briefly at
its final location before the fixation LED was extinguished. With the
fine time resolution of MEG, we found that in paradigm 1 (in which only
visual information was available), there was a shift of cortical
activation from cuneus to precuneus during the memory delay interval
from the visual stimulus offset to saccade onset. In paradigm 2,
primary sensory-motor cortex activation was observed 100ms after the
onset of the hand movement, followed by frontal cortex activation
(frontal eye fields according to its function and talairach
coordinates) at saccade onset. In paradigm 3, the cortical activation
observed included the areas seen in both paradigms 1 and 2. However,
paradigm 3 showed stronger parietal activation and weaker cuneus
activation than paradigm 1, but stronger cuneus activation and weaker
frontal activation than paradigm 2. Subtraction of the ‘vision’ data or
the ‘hand’ data from the ‘combined’ data left similar, but not
identical patterns of activation compared to the other hand/vision data
set, suggesting that in this task sensory integration is more than the
sum of its parts. In conclusion, the current MEG data trace the
spatialtemporal sequence of brain activation during visual
memory-guided and hand-guided saccades, and illustrate modality-
specific pathways for human saccades.