Spatial Spread of Visual Attention while Tracking a Moving Object

We conducted three experiments to investigate the spatial spread of visual attention. In Experiment 1, we measured the contrast sensitivities at various locations (spatial sensitivity function) relative to the moving target that the observer attended to track in an attentive tracking display. A probe was presented at a distance from the target at a location randomly chosen from within a certain range. The range of probe presentation location varied to examine whether the observer changes the area of attention to cope with this range. The results show that the probe range influenced the shape of spatial sensitivity function. The change in shape of this function suggests that the observer covers a wider area with attention for large probe ranges than small probe ranges. In the following experiments, we investigated the effect of the distance between the tracking target and a probe at a fixed location relative to the target (Experiment 2), or between the target and the center of a probe range of fixed size (Experiment 3). Since the relative probe location in a session was fixed in the experiments, the observer would pay attention to the target and probe locations independently of the relative distance if he/she could focus attention at multiple locations. Spatial sensitivity functions obtained in Experiments 2 and 3 showed that this was not the case. In both experiments the sensitivity to the probe decreased with increase in the relative distance as in Experiment 1, where the probe was presented at a location randomly chosen within each range. This indicates that attention cannot be divided among multiple locations, at least under the present experimental conditions. We will discuss a possible interpretation of the present results with a limited attentional resource and its spatial distribution.