EEG Oscillations as a Neural Correlate for Brightness Enhancement of Flickering Stimuli

  • Author / Creator
    Bertrand, Jennifer K
  • Perception is to sensation what illusion is to the actual state of the world. How we sense information varies greatly from how we ultimately perceive and understand that information - a result of top-down and bottom-up processes at work. Work from over 100 years ago (Brucke, 1864) found an exaggerated display of this discrepancy when the rate at which a light flickered affected its perceived brightness. While the true luminance, or sensation, of the flickering light remained constant across frequencies, the perceived brightness, or perception, of an 8 to 10 Hz flickering light varied up to double the brightness of a constant light. Here, we sought to replicate and extend this finding: first, to make comparisons between two flickering stimuli and second, to explore if there were neural signatures of brightness enhancement observable through electroencephalography (EEG). We hypothesized that this frequency-dependent brightness enhancement should result from the dynamics of entraining neural oscillations in the brain at different frequencies. Specifically, given the historical results, we postulated that this divergence of sensation and perception would lie in the phasic properties of 8 to 10 Hz alpha oscillations, similar to the link between the phase of oscillatory alpha and visual detection by Mathewson et al., 2009. To test these ideas, we carried out two experiments. Experiment 1 (E1) collected behavioural reports from participants (n = 29) making brightness judgements about all possible pairs of 5 frequencies: 0 Hz (no flicker), 4 Hz, 9 Hz, 13 Hz, and 17 Hz. Experiment 2 (E2) (n = 23) included EEG and used a reduced set of frequencies (4 Hz, 9 Hz and 13 Hz) which had the greatest behavioural effects in E1. Both E1 and E2 provided strong behavioural evidence of the greatest brightness enhancement occurring from a 4 Hz stimulus, where 4 Hz looked brighter than all other frequency stimuli upwards of 80% of the time. Critically, all stimuli had the same physical luminance, meaning this brightness enhancement was entirely generated from perception and not sensation. Our EEG analysis showed that this 4 Hz brightness enhancement occurred together with an increase in the amount of 4 Hz inter trial phase coherence (ITC), or phase consistency. We further tested the link between brightness enhancement and ITC in the 4 Hz band by binning the trials by the brightness judgment, rather than experimental condition (as was done with the other ITC analyses). Specifically, we examined trials where there were two of the same frequency stimuli on the screen (e.g. 4 Hz versus 4 Hz) and found a significant difference between the amount of 4 Hz ITC in the hemisphere contralateral to the 4 Hz stimulus chosen as brighter as opposed to the 4 Hz stimulus not chosen as brighter. The same pattern was not observed for the 9 Hz and 13 Hz stimuli, which had shown only slight phase locking in the previous analysis. This demonstrates that 4 Hz ITC could predict which stimulus was chosen as brighter even when it was identical in both frequency and luminance. We suggest that this brightness enhancement effect is a result of optimal transfer of the brightness information to higher order areas via an information transferring rhythm that resides around 4 Hz. Specifically, we posit that the theta rhythm, around 4 Hz, is the necessary oscillation for broadcasting visual information to areas responsible for brightness discrimination. In this experiment, we provide an exogenous entraining rhythm at or close to that of the endogenous theta rhythm, aligning the natural rhythm in a way that boosts the information transfer to higher areas responsible for decision making. This process results in brightness enhancement. We speculate that our original hypotheses regarding brightness enhancement residing in the alpha band would have been correct had this been a detection rather than a discrimination task. Thus, future experiments will see if the frequency of maximum brightness enhancement shifts to the alpha range if a simpler task is required. Further, we also hope to test if this entrainment of theta will enhance other percepts relying on higher-order processing, like motion coherence or visual search.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.