The 'when' parietal pathway explored by lesion studies

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.

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. 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on correctly reported flickering trials (data not reported, Battelli L., Cavanagh P., Walsh V. and Pascual-Leone A., unpublished observation).