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Recent blood oxygenation level-dependent (BOLD) imaging work has suggested flexible coding frames for reach targets in human posterior parietal cortex, with a gaze-centered reference frame for visually guided reaches and a body-centered frame for proprioceptive reaches. However, BOLD activity, which reflects overall population activity, is insensitive to heterogeneous responses at the neuronal level and temporal dynamics between neurons. Neurons could synchronize in different frequency bands to form assemblies operating in different reference frames. Here we assessed the reference frames of oscillatory activity in parietal cortex during reach planning to nonvisible tactile stimuli. Under continuous recording of magneto-encephalographic data, subjects fixated either to the left or right of the body midline, while a tactile stimulus was presented to a nonvisible fingertip, located either to the left or right of gaze. After a delay, they had to reach toward the remembered stimulus location with the other hand. Our results show body-centered and gaze-centered reference frames underlying the power modulations in specific frequency bands. Whereas beta-band activity (18 -30 Hz) in parietal regions showed body-centered spatial selectivity, the high gamma band (Ͼ60 Hz) demonstrated a transient remapping into gaze-centered coordinates in parietal and extrastriate visual areas. This gaze-centered coding was sustained in the low gamma (Ͻ60 Hz) and alpha (ϳ10 Hz) bands. Our results show that oscillating subpopulations encode remembered tactile targets for reaches relative to gaze, even though neither the sensory nor the motor output processes operate in this frame. We discuss these findings in the light of flexible control mechanisms across modalities and effectors.

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 gammaband 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.

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