Multiple reference frames in cortical oscillatory activity during tactile remapping for saccades.

Single-unit recordings have shown that the brain uses multiple reference frames in spatial processing. The brain could use this neural architecture to implicitly create multiple modes of representation at the population level, with each reference frame weighted as a function of task demands. Using magnetoencephalography, we tested this hypothesis by studying the reference frames in rhythmic neuronal synchronization--a population measure--during tactile remapping for saccades. Human subjects fixated either to the left or right of the body midline, while a tactile stimulus was applied to an invisible fingertip, located either left or right of fixation. After a variable delay, they looked at the remembered stimulus location. Results show a transient body-centered, stimulus-induced gamma-band response (70-90 Hz) in somatosensory areas, contralateral to the stimulated hand. Concurrently, a gamma-band response occurred in posterior parietal cortex (PPC), contralateral to the gaze-centered location of the stimulus, even though the stimulus was not seen. The temporal overlap of these early representations suggests that there is a fast bottom-up sensory-induced remapping in PPC, taking into account the relative positions of eyes and hand. The gaze-centered representation in PPC was sustained in a high gamma range (85-115 Hz) and increased in power closer to the initiation of the saccade. Lower-frequency rhythms (alpha, beta) showed body-centered power modulations in somatosensory areas in anticipation of the stimulus and a mixture of reference frames in PPC after stimulus presentation. We conclude that oscillatory activity reflects the time-varying coding of information in body- and gaze-centered reference frames during tactile remapping for saccades.