Neural correlates of illusory motion perception: The influence of apparent motion on plaid motion aftereffects

Abstract

It has been proposed that enhanced activity in the human motion complex (hMT⁺/V5) underlies the perception of illusory motion. Recent studies, however, have argued that in the case of motion aftereffect (MAE) this increase is due to visual selective attention rather than to the perception of MAE itself. It has also been reported that no net increase of activity occurs in hMT⁺/V5 upon MAE-eliciting motion adaptation if subjects report on a motion property of the test stimulus itself. These findings raised the question whether it is possible to disentangle MAE-related activity in area hMT⁺/V5 from motion specific selective attention. We have analyzed blood oxygen level dependent (BOLD) responses in visual area hMT⁺/V5 to stationary test patterns under adapting and non-adapting (variable directions) plaid motion conditions. Visual attention was controlled for using a superimposed apparent motion (AM) stimulus: participants had to report speed changes of the AM signal. Superimposed AM yielded a motion signal that had much lower amplitude activation in hMT⁺/V5 than plaid motion. Our participants reported strong MAEs upon adapting and no MAEs upon non-adapting conditions even when there was an overlaid AM-task during the plaid motion period. The observed MAEs were only represented in terms of hMT⁺/V5 activation when there was no motion specific attention task during the test phase. In summary, our results show that focusing attention to an AM stimulus during plaid adaptation does not have a profound effect on MAE generation; however the same attention task during the test phase minimizes differences in hMT⁺/V5 activation levels comparing adapting and non-adapting conditions. These findings extend previous knowledge on how selective attention modulates activity in hMT⁺/V5, suggesting that the amplitude of attentional modulation is in the magnitude of illusory motion elicited activations.

Footnotes

 FCT SFRH/BD/13344/2003, POCTI/NSE/46438/2002, BIAL15/02, Gulbenkian Progr. Stim. Invest.