Hemispheric Lateralisation for Motion Perception

Project Outline

The classical understanding of the visual brain suggests that each half (hemifield) of space is processed in the opposite side (hemisphere) of the brain. For example, if you are reading this text, anything to the left of your screen is currently being processed in your right hemisphere and vice versa. However, this raises a problem because we do not perceive two half-worlds but a single integrated scene. Indeed, visual perception operates seamlessly even when a dynamic stimulus moves from one hemi-field to the other, or with optic flow, where we move through the environment and ‘global motion’ signals derive from (multi-) directional information across a large proportion of the visual field. Computationally, it makes sense for a single cortical area to take command of these signals before they contribute directly to our perceptions and action, suggesting a role for hemispheric lateralisation. This requires the transmission of motion signals from one hemisphere to the other to achieve a unified visual perception of global motion.

Previous work[1] investigated the function of motion-sensitive brain area V5/MT+, a complex containing two sub-divisions, MT/TO-1 and MST/TO-2[2-3], and provided preliminary evidence to support the proposal above. For example, alongside the typical disruption of contralateral stimuli produced by delivery of TMS, both MT/TO-1 and MST/TO-2 in the right hemisphere also appear to process translational direction information from the same (ipsilateral) side of visual space but, crucially, the corresponding areas in the left hemisphere do not exhibit the same effect. This suggests that lateralisation may occur within the right hemisphere, within one or more sub-divisions of V5/MT+.

This prompts two key research questions: first how do MT/TO-1 and MST/TO-2 in the right hemisphere receive the signal from ipsilateral stimuli, and second, do these areas possess lateralised ability to combine the visual motion signals?

Hypothesis and Objectives

We predict that the signal transmission path from the left to right hemisphere is through the splenium of the corpus callosum. This makes three predictions for right V5/MT+: (1) ipsilateral processing should take longer than contralateral processing because the signal has further to travel, (2) the latency of whole-field processing will correlate better with the time-signature of the ipsilateral signal than the contralateral signal, and (3) lateralised cortical disruption should interfere with visual motion perceptions that require global processing.

To investigate this, the project has three objectives:

O1: To Identify Inter- and Intra-hemispheric Connections and Localisation of Areas

This first step involves using structural and functional brain imaging techniques to investigate connections between MT/TO-1 and MST/TO-2 both within and between hemispheres. This requires the use of magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) and resting state MRI.

O2: Quantify the Difference in Activation for Contralateral, Ipsilateral, and Whole-field Motion in Right and Left MT/TO-1 and MST/TO-2

This part of the project will use functional MRI (fMRI) to investigate differences in blood-oxygen-level dependent (BOLD) signal in MT/TO-1 and MST/TO-2 across both hemispheres in response to contralateral, ipsilateral, and whole-field motion stimuli.

O3: Determine the Role of Right MT/TO-1 and MST/TO-2 in Processing Whole-Field Motion

This part of the project will collect causal evidence of hemispheric lateralisation by using transcranial magnetic stimulation (TMS) to interfere with the function of MT/TO-1 and MST/TO-2 while participants perform behavioural whole-field motion tasks in a virtual environment (Aston Laboratory for Immersive Virtual Environments; ALIVE).

References

[1] Strong et al. (2019). Behav Brain Res, 372:1120670.

[2] Dukelow et al. (2001). J Neurophysiol 86:1991-2000.

[3] Amano et al. (2009). J Neurophysiol 102:2704-2718.

Techniques

• Magnetic resonance imaging (MRI), including:
• Functional MRI (fMRI)
• Diffusion Tensor Imaging (DTI)
• Resting State fMRI (rs-fMRI)
• Transcranial Magnetic Stimulation (TMS)
• Psychophysics (behavioural measures of perception)