re: Thesis Overview

TASK RELATED NEUROMAGNETIC ACTIVITY UNDERLYING THE VISUAL PERCEPTION OF VELOCITY CHANGE: A MEG STUDY

Supervisor: Dr. JFX DeSouza (link)
MEG Analyses Mentor: Dr. P Ferrari


Research questions

Part 1: Motion perception (fast & slow moving dots)

• Is there different cortical representation for fast & slow motion?
• Are there different temporal dynamics for fast & slow motion?

Part 2: Decision-making

• Subjects need to detect a change in the motion velocity and respond with button press
• When and where in the brain is a visual perception (velocity change) transformed into the neural signals for action?

Experiment

• Prior to the imaging sessions, I conducted a perceptual thresholds test (N=22) to acquire the minimal detectable increase and decrease in motion velocity for the subjects
• I set up the experimental environment and collected MEG & MRI data (N=12) with the helps from S. Bells and M. Lalancette at Toronto Sickkids hospital.
• Velocity change in the experiment was set according to each subject's perceptual thresholds, so the correct and incorrect responses could be compared
• A delayed motor response paradigm was used to separate decision-making signals from motor related signals.

Analyses

Results

Part 1 - Motion Perception

No difference in the MT+ locations (Talairach) for fast and slow motion was found (Hotelling's T2 for two multivariate independent samples)

Fig - MT+ locations
Fig - comparing the MT+ locations with previous studies

No temporal dynamics for source peak amplitude and latency found

• Velocity and visual display had no effects (3-way ANOVA)
• The amplitude and latency btw the three ROIs were different (P<.05)
• Multiple-comparison showed cuneus had higher amplitude & earlier latency than V3A & MT+

 

Part 2 - Decision-making

Comparing the grand average event-related beamformer images: 

• In the correct responses, the frontoparietal sources were observed at various time points after the velocity change onset (left column, perm-test, P<.05)
• No significant frontoparietal activations were observed in the incorrect responses (right column)

Comparing the time-frequency plots for correct and incorrect responses from IPL and SMC sources:

• IPL source showed a beta power difference between correct and incorrect responses from 200 - 400 ms (area encircled in dotted line)
• The initiation of IPL beta ERS was aligned with the high beta ERD increase in SMC after 400 ms (dotted line)

[Thesis Download]