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Abstract:
The event-related optical signal (EROS) is a new non-invasive
technique for studying the time course of neural activity in
localized cortical areas. It involves the measurement of the time
delay of near-infrared photons migrating between
superficially-located sources and detectors. Previous studies
suggest that the method has good spatial (<1 cm) and temporal
resolution (<20 ms). However, these studies have only used
surface maps. Here we investigate the use of different
source-detector distances to separate responses with different
depths within the brain. Nine subjects were presented with flashing
bilateral vertical grids varying in eccentricity (1, 2, 4, and 8
degrees) in different blocks. Functional MRI data indicated that
more eccentric stimuli activated areas approximately1 cm deeper
than more medial stimuli. EROS was recorded from locations over the
left medial occipital cortex using three source-detector distances
(3, 4, and 5 cm). The EROS response was quantified as the relative
photon delay at a latency interval between 60 and 100 ms after each
flash. The EROS response was larger at short source-detector
distances for medial stimuli and at long source-detector distances
for eccentric stimuli. This indicates that different
source-detector distances can be used to differentiate EROS
responses determined by activity with different depth within the
brain even when the variation in depth is <1 cm.
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