Distortions in predicted motion: Pitch and direction influence imagined speed for a visual object during occlusion
Authors:   , Hogan Adele Cherise 
Published: November 2021

<p>Visual motion prediction is essential for making key judgements about objects in the environment. These judgements are typically investigated using a time-to-contact (TTC) task, in which an object travels along a straight trajectory and disappears behind an occluder. Participants make a response coinciding with the moment the object would have contacted a visual landmark. The assumption is that the motion continues behind the occluder. This task is used to measure how we perceive and predict the arrival-time of objects. The addition of sound to TTC tasks generally enhances visual judgements. One characteristic which may affect how sound influences visual motion judgements is pitch. A rising pitch is associated with speeded motion and a falling pitch with slowed motion. Pitch change could therefore lead to biases in visual motion judgements; however, this has not yet been investigated. Furthermore, TTC tasks can utilise horizontal or vertical motion. In vertical motion, an additional variable that may be critical for TTC estimations is gravity. It is postulated that humans possess an internal model of gravity that allows us to make accurate predictions for downward motion. This model assumes faster downward than upward motion. However, this model can be wrongfully applied in constant speed tasks, producing faster speed estimations for downward stimuli when there is no acceleration. Therefore, vertical motion could lead to additional biases in visual motion judgements.  This thesis investigated whether pitch and gravity could affect the imagined speed of an object under occlusion. Specifically, a rising pitch was hypothesised to produce speeded predicted motion and falling pitch, slowed predicted motion. I investigated the influence of pitch change in vertical and horizontal planes. I also investigated two different aspects of pitch change, since dynamic pitch is a novel addition to TTC paradigms. Experiment 1A explored gradual pitch change and Experiment 1B used sudden pitch change. The hypothesised pitch effects were observed for a gradual, but not a sudden pitch change. However, a gravity effect was observed across both Experiments 1A and 1B, suggesting the presence of sound does not moderate this effect.  I also examined the cortical substrates of the audio-visual TTC task components by using transcranial magnetic stimulation (TMS) in Experiment 2. The superior temporal sulcus (STS) was targeted in this experiment, as it has been implicated in audio-visual integration. TMS causes neuronal inhibition, and as such, can be used to determine whether an area is involved in a task. If the STS is responsible for audio-visual integration in a TTC task, then TMS to the STS should disrupt the pitch effects evidenced in Experiment 1A. That is, a change in pitch should have no effect on TTC judgements compared to a constant tone. This result was evident only for rising tones, suggesting the involvement of the STS in the generating speeded predicted motion. The pitch effects observed in Experiment 1A and Experiment 2 implicate pitch in the production of biases in motion imagery for visual motion judgements, particularly for visual stimuli under occlusion.</p>