Spatial updating in human parietal cortex

Taken together, our results suggest that power in the gamma band is instantly reorganized to encode task-relevant visuomotor space in a gaze-centered reference frame, while power in the alpha band reflects a regulatory mechanism actively facilitating the gating of the saccade target and inhibiting the original stimulus representation.

The posterior parietal cortex (the portion of parietal neocortex posterior to the primary somatosensory cortex) plays an important role in planned movements, spatial reasoning, and attention.

We review human functional neuroimaging studies that have explicitly investigated the reference frames used in different cortical regions for representing spatial locations of objects.

Beyond the general distinction between “egocentric” and “allocentric” reference frames, we provide evidence for the selective involvement of the posterior parietal cortex and associated frontal regions in the specific process of egocentric localization of visual and somatosensory stimuli with respect to relevant body parts (“body referencing”).

In one study, single cell recordings showed activity in parietal reach region while non-human primates decided whether to reach or make a saccade to a target, and activity persisted during the chosen movement if and only if the monkey chose to make a reaching movement.

However, cells in area 5d were only active after the decision was made to reach with the arm.

Damage to the posterior parietal cortex can produce a variety of sensorimotor deficits, including deficits in the perception and memory of spatial relationships, inaccurate reaching and grasping, in the control of eye movement, and inattention.

The superior parietal lobule and inferior parietal lobule are the primary areas of body or spacial awareness.

Saccadic eye movements cause sudden and global shifts in the retinal image. This “spatial updating” mechanism ensures that spatial codes for perception and action are not compromised by eye movements.

Rather than causing confusion, however, eye movements expand our sense of space and detail. Spatial updating occurs in the lateral intraparietal area (LIP) as well as several other cortical and subcortical regions of the monkey brain, including the frontal eye fields (FEF) (12), the parietal reach region (8, 13), extrastriate cortex (14), and the superior colliculus (15).

By varying the onset time of stimulation, we show that the representation of space in IPSp is updated immediately after the first-saccade. In the current study, we stimulated the human IPS of the right hemisphere with TMS and measured effects on spatial updating using a variant of the “double-step saccade” task ().

In contrast, stimulation of an adjacent IPS site had no such effects on second-saccades. This behavioral paradigm (23–25) has been used extensively to study spatial updating in monkeys and humans and requires subjects to perform a sequence of two saccades to sequentially flashed targets (23–25).


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