Suppressive lateral interactions in the lateral intraparietal area (LIP) of the monkey may have a role in the “line-motion” illusion

Abstract

When a static line appears immediately after a brief cue at one end, observers sense motion propagating from the cued end towards the uncued end (the “line-motion” illusion, Hikosaka et al. 1993). Since endogenous attention and cross-modal stimuli can also induce the line-motion illusion (Shimojo et al. 1997), both attentional factors and low-level stimulus summation are believed to play a role. We recorded responses from single cells in area LIP of the fixating macaque to stimuli which induce the line-motion effect in humans. We found that a spot flashed well outside the classical excitatory receptive-field (RF) strongly suppresses the onset-response to a line terminating at the center of the cell's RF. Maximal suppression was observed at spot-line onset asynchronies between 100 and 200 ms. Furthermore, a line with one end terminating at the RF center evoked a much larger response when preceded by a spot cue at the RF center than it did when preceded by a spot cue at the end away from the RF center; this was true even if the line-end away from the RF center lay entirely outside the excitatory RF. These results indicate that the activity evoked in the LIP neuronal population encoding the cued end of the line is larger than that in the LIP neuronal population encoding the uncued end of the line, thereby potentially leading to greater attentional allocation to the cued end of the line. This difference in attentional allocation may contribute to the attentional component of the line-motion illusion.

Footnotes

 This work was supported by the National Science Foundation, National Eye Institute, the James S. MacDonnell Foundation, the W. M. Keck Foundation, and the Whitehall Foundation.