Abstract
Judging the speed of objects moving in three dimensions is important in our everyday lives because we interact with objects in a three-dimensional world. However, speed perception has been seldom studied for motion in depth, particularly when using monocular cues such as looming. Here, we compared speed discrimination, and speed change discrimination, for looming stimuli, in order to better understand what visual information is used for these tasks. For the speed discrimination task, we manipulated the distance and duration information available, in order to investigate if participants were specifically using speed information. For speed change discrimination, total distance and duration were held constant; hence, they could not be used to successfully perform that task. For the speed change discrimination task, our data were consistent with observers not responding specifically to speed changes within an interval. Instead, they may have used alternative, arguably less optimal, strategies to complete the task. Evidence suggested that participants used a variety of cues to complete the speed discrimination task, not always solely relying on speed. Further, our data suggested that participants may have switched between cues on a trial to trial basis. We conclude that speed changes in looming stimuli were not used in a speed change discrimination task, and that naïve participants may not always exclusively use speed for speed discrimination.
Highlights
Perceiving the speed of objects moving towards us in the world is important in our daily lives, for example when safely crossing a road
We investigated speed and speed change discrimination for looming stimuli, and explored the strategies that naïve participants may be using for speed discrimination
When the Speed change condition was analysed in terms of the speed change within the test interval, speed change discrimination was found to be more difficult than speed discrimination when duration or distance information was available (Figure 5)
Summary
Perceiving the speed of objects moving towards us in the world is important in our daily lives, for example when safely crossing a road. Of particular importance is the ability to judge the speed, and speed changes, of objects approaching in three dimensions There are both monocular and binocular sources of visual information we can use to judge these movements. Speed discrimination using binocular cues to motion in depth has been well studied [1,2,3,4,5,6,7]. Looming is usually defined as the change in retinal size that occurs when an object moves towards or away from an observer (e.g., [8]). This is the definition we use here. The first evidence for the existence of mechanisms sensitive to such change in size, which could be used for the perception of motion-in-depth, came from motion adaptation studies demonstrating that adaptation to size-change was separable to that for lateral motion [9,10]
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