TMS in the parietal cortex: Updating representations for attention and action
Section snippets
Visuospatial attention: the case of visual search
In the visual search task subjects are briefly shown an array of stimuli and they are asked if it contains a particular target stimulus. The task is thought to tax attention because the relevant target must be filtered from among the irrelevant distracter stimuli in the array. The task is especially difficult when no single basic visual feature (such as colour or orientation) distinguishes the target from the distracters. In such situations it is the presence of a conjunction of features that
The timing of parietal involvement in visual search
While it might be possible to debate the significance of the timing of posterior IPS/IPL involvement in visual search it is clear that the critical time period occurs relatively late in comparison with other areas. Ashbridge et al. (1997) and Walsh et al., 1998, Walsh et al., 1999 applied single pulse TMS to the posterior IPS/IPL at 20 different SOAs between 0 ms and 200 ms. Trials on which conjunction targets were present were affected by TMS at 100 ms and target absent trials were affected by
The timing of parietal involvement in other attentional processes
A number of studies have been directly concerned with the movement of the focus of covert attention using variations of a paradigm popularized by Posner et al. (1984). In one version of the task subjects indicated detection of a target stimulus in one of two possible locations by pressing a button. On some trials a warning cue instructed subjects where the stimulus was likely to appear but on a few trials the warning was invalid. Subjects respond more slowly on such invalid trials but the
The timing of neurophysiological events in the human parietal cortex
TMS can be used to examine the progress of sensorimotor and cognitive processes in the parietal cortex on a time scale of the order of tens of milliseconds but it can also be used to investigate neurophysiological processes at an even finer time scale of the order of single milliseconds. The impact of a pulse of TMS over the primary motor cortex on muscle activity can be modulated by applying a preceding sub-threshold pulse called a “conditioning” pulse. The conditioning pulse does not in
Localization within the parietal cortex
The much vaunted temporal specificity of TMS has been exploited to gain novel insights into the timing of attentional processes in the parietal cortex. TMS only affects neural activity in a spatially limited region of tissue (Valero-Cabre, Payne, Rushmore, Lomber, & Pascual-Leone, 2005; Walsh & Cowey, 2000; Fig. 4) but it has produced fewer insights into localization of function within the parietal cortex. In contrast fMRI has shown that regional differences in activation within the parietal
Spatial reference frames in the parietal cortex
The disruptive effects of parietal TMS on visuospatial task performance are restricted to particular reference frames. Bjoertomt, Cowey, and Walsh (2002) gave normal subjects a line bisection task often used to detect visuospatial neglect. Subjects had to judge whether long horizontal lines (about 36° of visual angle) were transected by a short vertical line at the midpoint, or whether the horizontal line was shifted slightly (1°) to the left or right. The line was presented at a distance of
Parietal cortex and eye movements
The same regions of parietal cortex that are involved in covert attentional processes are also active when overt eye movements are made (Corbetta et al., 1998; Nobre, Gitelman, Dias, & Mesulam, 2000) so it is not surprising that parietal TMS also affects eye movements (Elkington, Kerr, & Stein, 1992; Kapoula, Isotalo, Muri, Bucci, & Rivaud-Pechoux, 2001; Terao, Fukuda, et al., 1998; Zangemeister, Canavan, & Hoemberg, 1995). Just as TMS over the posterior IPS/IPL region disrupts updating of the
Parietal cortex and the control of limb movement: updating reaching movements
While a number of parietal regions are concerned with visuospatial and attentional processes and overt eye movements there are a number of other regions that are more closely identified with limb movements. In the macaque the anterior intraparietal (AIP) area, like LIP, is in the IPL but situated more anteriorly (Sakata et al., 1999). A similar region is also found in the anterior part of the human IPL (Binkofski et al., 1998; Grefkes & Fink, 2005; Grefkes, Weiss, Zilles, & Fink, 2002). A
A general role for the parietal cortex in the updating of movements?
Despite their influence the reliability and interpretation of the findings have been called into question. Johnson and Haggard (2005) have reported difficulty in finding impaired reaching adjustments when TMS is applied over the parietal cortex. Roy, Stefanini, Pavesi, & Gentilucci (2004) have reported that the problems of an optic ataxic patient occur early during a movement and were not limited to the final adjustments before reaching a target.
On the other hand there appears to be an emerging
Re-directing motor intentions
Theories of motor control based on ideas of feedforward control and the use of internal models emphasize that it is the expected feedback that should be associated with an intended position of the limb that is compared with actual feedback or a target position. It is therefore not surprising that TMS does not just impair movements when the course of an ongoing movement needs to be changed but it also impairs movements when changes need to be made to an intended movement (Rushworth, Ellison, et
Eye-hand coordination and the parietal cortex
Disruption of on-line correction during limb movement has been reported after TMS of both the SPL (Della-Maggiore et al., 2004, Glover et al., 2005) and the IPL (Desmurget et al., 1999, Tunik et al., 2005). It is not clear if the results can be interpreted in terms of an MIP-like system in the SPL and an AIP-like system in the IPL that may be concerned with proximal and distal movements, respectively. Hand aperture has been reported to be disrupted after TMS to both the IPL and the SPL (Glover
Conclusion
TMS is probably the most recent technique to have been employed in investigations of the human parietal cortex. There has been an emphasis on visuospatial and oculomotor function in the most posterior parietal regions but a coherent pattern to the results of TMS investigations of arm movement control is also emerging in other regions. A consistent theme that runs through several of the studies is that TMS particularly disrupts performance when eye or limb movements occur, or even if the
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