The sensory properties of the fast visuomotor system
Abstract
The brain has a remarkable capacity to rapidly transform vision into action, which allows us to initiate reaches towards targets within fractions of a second. Despite being fundamental to our interaction with a dynamic environment, these fast visuomotor transformations and their underlying neural substrates are poorly understood. This gap in the literature is further exacerbated by the unreliable presence of rapid visuomotor responses on the upper limb, likely due to the use of less optimal stimuli and paradigms. My thesis explores the stimulus properties which best evoke short latency reaction times and electromyographic responses during visually guided reaching, their application in clinical populations, and the implicated neural substrates.
Our first study (chapter 2) examined visually-guided reaching towards Gabor patches composed of varying spatial frequencies. Our results indicate that stimuli composed of low rather that high spatial frequencies are effective at evoking fast visuomotor responses. Furthermore, we demonstrate the presence of fast visuomotor transformations on the upper limb, irrespective of whether the reach was initiated from stable starting position, or when the reach was corrected mid-flight. Chapter 3 details a new behavioural paradigm for evoking fast visuomotor responses, requiring participants to reach to moving targets which emerge from behind an occluder. This method has been shown to be highly effective at evoking fast visuomotor responses, doing so in almost every participant. In chapter 4, we use this paradigm to show that higher contrast, faster moving targets evoke the largest, fastest, and most prevalent fast visuomotor responses. In chapter 5, we apply findings from chapters 2, 3, and 4, and use such targets to investigate patients with a common movement disorder, Parkinson’s disease (PD). Our results suggest that multiple descending motor pathways contribute to visually guided reaching, the most rapid of which are preserved in PD, and are likely mediated by separate neural substrates than those involved in typical volitional reaching.
The work here builds on a stream of findings that emphasize the importance of visual stimulus attributes on rapid motor responses in both health and disease. These findings provide insight into the underlying neural processes which mediate rapid visuomotor transformation.