We used an obliquely oriented moving luminance grating within a square aperture as a stimulus. We probed how the aperture problem (determining direction of a contour within an aperture) is solved following onset. It is perceived to move in horizontally (H), diagonally (D) or vertically (V), shifting perception during extended presentation. The initial percept (D) leads to two competing "stable" solutions H and V from the orthogonal 2D cues around the edges. During brief stimulus presentations of 200-500ms, participants reported perceived direction (H, D or V) in a 3-alternative forced choice task while eye movements were recorded. As expected when solving the aperture problem, integration took time: reported direction is predominantly 1D (D) at 200ms, shifting to 2D (H/V) by 500ms. Eye direction traces converge to an average direction (H, D or V) that corresponds to participant decisions. The latency of this separation of averaged traces depends on input signal strength parameters like contrast. The relationship between input signal, distributions of perceived direction and forced choice decision thresholds are well described by a neural fields model in our companion abstract (II). The onset dynamics of multistable direction representation are demonstrated to be well studied by ocular following eye movements.