EEG-based characterization of flicker perception

Steady-State Visual Evoked Potential (SSVEP) is an oscillatory electrical response appearing in the electroencephalogram (EEG) in response to flicker stimulation. The SSVEP manifests more prominently in electrodes located near the visual cortex and has oscillatory components at the stimulation frequency and/or harmonics. The phase and amplitude of the SSVEP are sensitive to stimulus parameters such as frequency, modu-lation depth, and spatial frequency. Research related to SSVEP and the human visual system has mainly focused on brain computer interfaces (BCI) applications, cognitive and memory performance, pathophysiology of diseases. Some other research has been focus inthe influence of light properties (i.e. colour and size of stimuli)on brain activity. Sensitivity to flicker can be studied from a perception viewpoint. By presenting flickering light to an observer itis possible to find the frequency and modulation depth that is required to detect that the light is flickering rather than steady. Thesensitivity varies between people and change due to factors like age, concentration and fatigue, while for the stimulus; colour, background an intensity of the light have an influence on the perception. Also, we can find various studies on the response of human visual per-ception to flicker. However, very little research has been investigated about the SSVEP sensitivity base on the influence of the frequency and modulation depth of flicker. In this work, we present ZEERunit, a criterion to measure the intensity of SSVEP. This unit allows us to quantify the oscillatory response of the brain to flicker. With the ZEER unit we are able to make the estimation of the SSVEP sensitivity curve according to the frequency and modulation depth of flicker. We implemented an experiment to acquire the visual perception and SSVEP response to the flicker stimulation with goal of characterizing the link between visual perception of flicker and the corresponding SSVEP response. An experiment was conducted where 25 flicker stimuli with different properties were presented to 12 voluntary participants. The flicker stimuli result from the combina-tion of five different frequencies and five different Modulation Depths (MDs). The MDs were selected around the values defined by a perception curve which defines the relation between perception and MD for a given flicker frequency. In the study, the EEG and the visual perception data from each participant were collected. The EEG data was pre-processing by peak-filtering, subsampling, artifact correction and averaging the result. Then we applied statistical analysis on the distribution of the samples before and during stimulation. The results were spatially analysed all over the scalp and we used 2 different methodsfor the estimation of SSVEP sensitivity and flicker perception curve: absolute modulation depth and psychometric method. The results of the estimated curves indicate that visual perception of flicker and the SSVEP sensitivity are not aligned. We can start to see entrainment at stimuli frequencies below ~35 Hz only for flicker that are perceived by the observer. On the contrary, for higher stimuli frequencies it is possible to elicit oscillatory responses below the visual perception of flicker.