Human subjects had to remember the location of a target, briefly flashed left or right of central fixation.
Next, they refixated and then, after a further memory delay, made a saccade to the memorized target location.
The posterior parietal cortex receives input from the three sensory systems that play roles in the localization of the body and external objects in space: the visual system, the auditory system, and the somatosensory system.
In turn, much of the output of the posterior parietal cortex goes to areas of frontal motor cortex: the dorsolateral prefrontal cortex, various areas of the secondary motor cortex, and the frontal eye field.
In macaques, a stable representation of space is embodied by neural populations in intraparietal cortex that redistribute activity with each saccade to compensate for eye displacement, but little is known about equivalent updating mechanisms in humans. Functional MRI studies of the human intraparietal sulcus (IPS) have revealed changes in hemispheric activation that are consistent with the spatial updating mechanism identified in monkey PPC (16, 17).
We combined noninvasive cortical stimulation with a double-step saccade task to examine the contribution of two human intraparietal areas to transsaccadic spatial updating. Because the blood oxygenation level-dependent signal is a correlational measure, however, it is not possible for these studies to distinguish activation that directly contributes to spatial updating from that which merely reflects its outcome, such as activation subserving subsequent directed attention or planned action (18–21).
The major sensory inputs from the skin (touch, temperature, and pain receptors), relay through the thalamus to the parietal lobe.
Several areas of the parietal lobe are important in language processing.
Similarly, parahippocampal and retrosplenial regions, together with specific parietal subregions such as the precuneus, are selectively involved in a specific form of allocentric representation in which object locations are encoded relative to enduring spatial features of a familiar environment (“environmental referencing”).
We also present a novel functional magnetic resonance imaging study showing that these regions are selectively activated, whenever a purely perceptual spatial task involves an object which maintains a stable location in space during the whole experiment, irrespective of its perceptual features and its orienting value as a landmark.
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).
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.