Direct Evidence That “Speedlines” Influence Motion Mechanisms

Determining the direction of visual motion poses a serious problem for any visual system, given the inherent ambiguities. Geisler (1999)has suggested that motion streaks left in the wake of a moving target provide a rich source of potential information that could aid in resolving direction ambiguities. Here we provide strong experimental evidence that the human visual system does in fact exploit motion streaks in direction discrimination. Masks comprising oriented random noise impeded direction discrimination of moving dots when the masks were oriented parallel to the direction of motion but had very little effect when oriented orthogonal to the direction of motion. The masking effect decreased systematically with increasing bandwidth for the parallel masks and increased with bandwidth for the orthogonal masks. Importantly, these masks had little effect on neither contrast sensitivity for detecting the moving stimuli nor for speed discrimination. Experiments with “Glass patterns” (moirépatterns constructed from random dot pairs) confirmed that misleading pattern information can impede motion detection. The results show that the oriented streaks left by moving stimuli provide fundamental information about the direction of visual motion; removing these streaks or augmenting them with erroneous streaks severely confounds motion direction discrimination.