Abstract
Frequency-dependent brightness enhancement, where a flickering light can appear twice as bright as an equiluminant constant light, has been reported to exist within the alpha (8–12 Hz) band. Could oscillatory neural activity be driving this perceptual effect? Here, in two experiments, human subjects reported which of two flickering stimuli were brighter. Strikingly, 4 Hz stimuli were reported as brighter more than 80% of the time when compared to all other tested frequencies, even though all stimuli were equiluminant and of equal temporal length. Electroencephalography recordings showed that inter-trial phase coherence (ITC) of theta (4 Hz) was: (1) Significantly greater than alpha, contralateral to the flickering stimulus; (2) Enhanced by the presence of a second ipsilateral 4 Hz flickering stimulus; and (3) Uniquely lateralized, unlike the alpha band. Importantly, on trials with two identical stimuli (i.e. 4 Hz vs 4 Hz), the brightness discrimination judgment could be predicted by the hemispheric balance in the amount of 4 Hz ITC. We speculate that the theta rhythm plays a distinct information transfer role, where its ability to share information between hemispheres via entrainment promotes a better processing of visual information to inform a discrimination decision.
Highlights
When asked to judge the brightness of a stimulus, decisions regard the objective amount of light entering the eye but are based on our subjective experience of this luminance
Flicker at theta frequency produces the greatest behavioural brightness-enhancement effect. In both Experiment 1 (E1) and Experiment 2 (E2) participants were asked to judge which of two stimuli on the screen was brighter
We presented a range of frequencies spaced across the originally reported range of enhancement[2] in E1 (0, 4.4, 9.2, 13.3, 17.1 Hz) and a restricted subset in E2 (4.4, 9.2, 13.3 Hz) in all possible pairs (Fig. 1) and recorded the side and frequency of the stimulus participants selected as brighter on each trial
Summary
When asked to judge the brightness of a stimulus, decisions regard the objective amount of light entering the eye (luminance) but are based on our subjective experience of this luminance (brightness). A strong example of this disconnect is the reported Brücke effect, where a stimulus flickering around 10 Hz requires half the luminance of a constant stimulus for the two stimuli to appear at equal subjective brightness[2] This frequency-dependent brightness enhancement[2] occurs over a range of slower frequencies (between 1–17 Hz). Of particular relevance to the current study, recent work from Han & VanRullen[16,17] shows that the phase of theta oscillations plays a role in the apparent brightness enhancement of a meaningful image as opposed to a random line image They suggest the appearance of the fully connected and interpretable line object as brighter than the random line image is due to theta oscillations carrying top-down signals from higher areas when making luminance judgments. In line with the results of Han & VanRullen[16,17], a mask “slightly faster than the perceptual cycle” governed by theta optimally disrupted judgments in this experiment[18]
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