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Motion Coherence and Luminance Effects on Visual Evoked Potentials
The electroencephalogram (EEG) as described by David Eagleman and Jonathan Downar (2016), is a device designed to measure and record brain wave patterns, and was originally designed for evaluating brain activity of epileptic and brain damaged patients (p. 312). The current study is a replication of Kubova, Kuba, Spekreijse, and Blakemore’s 1995 study utilizing the EEG to measure the effects of coherence and luminance on motion-onset visual evoked potentials (VEPs). The two independent variables looked at in this study were motion coherence (100% coherent and 1% coherent), and luminance (dim vs bright). The hypotheses were (a.) coherence should affect the amplitude of the N2 peak; specifically, that low coherence should result in a higher peak amplitude, and (b.) luminance (brightness) should affect the latency; specifically, that low luminance should delay the peak.
The study participants were Kennesaw State University (KSU) student volunteers from three sections of PSYC 4410. Participants were offered course extra credit of 25 points to take part in the study. Student participants served as both subjects and research assistants. There was no selection criterion for the subjects. The participants serving as research assistants administered the electroencephalogram (EEG) on the subjects (n = 16) in Dr. Tim Martin’s lab at KSU, under direct supervision of Dr. Martin.
Recording and Procedure
VEPs were recorded with a 40-channel NuAmps amplifier with the filter set at 0.03 – 200 Hz, the sampling rate set at 500 Hz, and with linked earlobe reference. Participant subjects and research assistants met for scheduled sessions in Dr. Martin’s lab. Research assistants applied the EEG cap, gel, and electrodes to the subjects. Subjects were given instructions to look at computer generated stimuli: 400 trials (100 per condition) of 500 moving dots (each 0.02° of visual angle in size). Stimuli were presented in viewing aperture 5° across. Duration was 500 ms, inter-trial interval 500-2000 ms; randomly varied, using uniform distribution. The subjects were asked to perform small mechanical manipulations on the computer keyboard in response to the visual stimuli. The EEG data was processed by using a low pass filter at 45 Hz. The bad epochs were removed from the data, and all blink artifacts were removed with independent components analysis.
Results successfully replicated earlier findings (Bach, M., & Ullrich, D., 1997; Kubova et al., 1995) showing a clear effect of coherence for dim stimuli on amplitude, but there was no apparent effect of coherence for the bright stimuli. See Figure 1.
Analysis of VEPs serves as a tool in detecting various abnormalities that may involve the visual pathways, such as epilepsy and other brain dysfunctions. Implications of the results of this study support previous research providing evidence that there are various neural mechanisms involved in motion-related VEPs that expand beyond the primary visual cortex (Bach, M., & Ullrich, D., 1997; Braddick, O.J., O’Brien, J.M.D., Wattam-Bell, J., Atkinson, J., Hartley, T., & Turner, R., 2001; Kubova et al., 1995). The hypotheses that low coherence would result in a higher N2 peak amplitude was confirmed. Potential confounders to the study would be research assistant inexperience, as well as the inability to control for both internal and external noise, which could degrade the recordings (Jackson, A.F., & Bolger, D.J., 2014).
Bach, M., & Ullrich, D. (1997). Contrast dependency of motion-onset and pattern-reversal VEPs: Interaction of stimulus type, recording site, and response component. Vision Research, 37, 1845-1849.
Braddick, O.J., O’Brien, J.M.D., Wattam-Bell, J., Atkinson, J., Hartley, T., & Turner, R. (2001). Brain areas sensitive to coherent visual motion. Perception, 30, 61-72.
Eagleman, D., & Downar, J. (2015). Brain and Behavior: A Cognitive Neuroscience Perspective. Oxford University Press.
Jackson, A.F., & Bolger, D.J. (2014). The neurophysiological bases of EEG and EEG measurement: A review for the rest of us. Psychophysiology, 51, 1061-1071.
Kubova, Z., Kuba, M., Spekreijse, H., & Blakemore, C. (1995). Contrast dependence of motion-onset and patter-reversal evoked potentials. Vision Research, 35, 197-205.
Figure 1. Coherence for dim and bright stimulus on amplitude.
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