Previous studies have shown that the visual system integrates local motions and perceives the average direction (Watamaniuk & Duchon, 1992 Vision Research 32 931–941). We investigated whether the surface of the blind spot is filled in with the average direction of the surrounding local motions. To test this, we varied the direction of a random-dot kinematogram (RDK) both in adaptation and test. Motion aftereffects (MAE) were defined as the difference of motion coherence thresholds between with and without adaptation. The participants were initially adapted to an annular RDK surrounding the blind spot for 30 s in their dominant eyes. The direction of each dot in this RDK was selected equally and randomly from either a normal distribution with the mean of 15° clockwise from vertical, 15° counterclockwise from vertical, or from the mixture of them. Immediately after the adaptation, a disk-shaped test RDK was presented for 1 s to the corresponding blind-spot location in the opposite eye. This RDK moved either 15° clockwise, 15° counterclockwise, or vertically (the average of the two directions). The participants' task was to discriminate the direction of the test RDK across different coherence levels. We found significant MAE when the test RDK had the same directions as the adaptor. More importantly, equally strong MAE was observed even when the direction of the test RDK was vertical, which was not physically present during adaptation. The result demonstrates that the visual system uses the average direction of the local surrounding motions to fill in the blind spot.
