Abstract

In the literature about the effect of attention on motion processing, it is clear that attention improves the processing of motion. However, it is not clear what happens to the processing of a motion in the same visual field as another actively attended motion. Is the unattended motion unprocessed, processed to a lesser degree or suppressed? This is the question that I investigated in this thesis. The effect of active visual selection during adaptation to components of a plaid on the motion aftereffect (MAE) duration was investigated using a dynamic test stimulus oriented either like the attended component or like the unattended component. The plaids were composed of two spatially superimposed, but temporally alternating square-wave or sine-wave gratings differing by 140 degrees in motion direction. The results show that active suppression occurs in the MAE duration for a non-attended moving component of a plaid when attention is actively directed to another moving component in the same visual field during adaptation. This is true whether the adaptation plaid is made up of either: (1) two first-order gratings, (2) two second-order gratings, or (3) a mixture of first-order and second-order gratings. I also demonstrated that adapting to a single grating produced longer MAE than grating when the same grating was a component of a plaid. Therefore, the presence of an unattended moving component in the same visual field as an attended moving component reduces the strength of the MAE for the attended component of a plaid The results of this thesis suggest that: (1) Attention is decreased when many moving stimuli are present in the visual field. (2) Attention acts on motion processing in the same manner when first-order motion and second-order motion are processed. (3) The suppression of the unattended moving stimuli is not perfect. During adaptation, attention involuntarily switched to the unattended moving component, which could be an adaptive mechanism in order to be able to react and avoid collision with unattended moving stimuli heading toward us.