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Review
. 2008 Apr;18(2):120-6.
doi: 10.1016/j.conb.2008.08.004. Epub 2008 Aug 29.

The 'when' parietal pathway explored by lesion studies

Lorella Battelli  1 , Vincent Walsh, Alvaro Pascual-Leone, Patrick Cavanagh
Affiliations
Review

The 'when' parietal pathway explored by lesion studies

Lorella Battelli et al. Curr Opin Neurobiol. 2008 Apr.

Abstract

The perception of events in space and time is at the root of our interactions with the environment. The precision with which we perceive visual events in time enables us to act upon objects with great accuracy and the loss of such functions due to brain lesions can be catastrophic. We outline a visual timing mechanism that deals with the trajectory of an object's existence across time, a crucial function when keeping track of multiple objects that temporally overlap or occur sequentially. Recent evidence suggests these functions are served by an extended network of areas, which we call the 'when' pathway. Here we show that the when pathway is distinct from and interacts with the well-established 'where' and 'what' pathways.

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Figures

Figure 1
Figure 1. EEG correlates of the wagon wheel illusion (WWI)
a) Observers were asked to report the perceived direction of a continuous rotating wheel moving at different rates. b) 32-channel EEG recordings were made and the power spectra between the illusory and real motion were compared. A significant difference was found only at 13 Hz over right parietal electrode, independently of the rate of stimulus rotation. The difference must depend on internal processing mechanisms as the stimulus on the retina is the same during both real and illusory motion. (Reproduced with permission from [31]).
Figure 2
Figure 2. TMS disrupts apparent motion perception bilaterally
a) The rate of alternation of the two frames on the left side of the panel was varied across trials and observers perceived either motion (bottom right) or flickering (top right, see [2] for a detailed description of the task). A comparison stimulus with four flickering dots was used and subjects were asked to report whether they saw motion or flickering. b) After 10 minutes of 1 Hz TMS delivered over the right IPL subjects’ reaction times were significantly slower on correctly reported motion trials both in the left (LVF) and right (RVF) visual field (average of 7 subjects). While TMS had no effect on correctly reported flickering trials (data not reported, Battelli L., Cavanagh P., Walsh V. and Pascual-Leone A., unpublished observation).
See this image and copyright information in PMC

References

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    1. Battelli L, Pascual-Leone A, Cavanagh P. The ‘when’ pathway of the right parietal lobe. Trends Cogn Sci. 2007;11:204–210. - PMC - PubMed
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    1. Walsh V. A theory of magnitude: common cortical metrics of time, space and quantity. Trends Cogn Sci. 2003;7:483–488. - PubMed
    1. Burr D, Tozzi A, Morrone MC. Neural mechanisms for timing visual events are spatially selective in real-world coordinates. Nat Neurosci. 2007;10:423–425. The authors report psychophysical data using an adaptation paradigm in which subjects were asked to adapt to a drifting grating and were subsequently tested on retinotopic or spatiotopic locations relative to the adapting stimulus location. They provide clear evidence that the timing of visual events in the milliseconds range is likely performed by neural mechanisms localized in spatiotopic coordinates. - PubMed

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