CFF threshold measurement
We measured critical flicker fusion thresholds using a novel measuring device and custom methods, the accuracy and robustness of which were validated in a separate, parallel study (Haarlem et al., in press). The protocol for measuring CFF thresholds outlined below are reproduced from that study’s methods section. All CFF thresholds measured in the current study were also included in the dataset used in Haarlem et al.
The device consisted of an Arduino Uno R3 microprocessor and a 4500K white LED light, housed in a black container. An opaque, black viewing tube was mounted on top of the container, to standardize viewing distance at ~16 cm and to eliminate almost all ambient light. Flicker frequency of the LED could be adjusted with a rotary encoder dial that either increased or decreased flicker rate in 1 Hz increments. We measured luminance and colour temperature with a ColorCal Mk II. Mean luminance of the LED was ~255 lux. The device was powered by a personal laptop, using a USBC cable. We measured the power input with a Tektronix TDS 210 oscilloscope to ensure stability. The precision and stability of the LED flash timings were measured using a photometer, both before the start of, and after completion of the study.
The protocol consisted of a measurement of 3 different quantifications of a participant’s ability to perceive flashes and was based on a combination of two commonly used psychophysical experimental techniques: the method of limits and the method of constant stimuli. The protocol started with the method of limits: participants observed a constantly lit LED light through the viewing tube and were then instructed to turn the rotary encoder clockwise, to make the light source start flashing. Participants were then asked to keep rotating the dial to increase the flash frequency in 1 Hz increments until flashes could no longer be perceived and the light appeared steady. The frequency at which this occurred was recorded. The LED light source was then set to a flash frequency of 65 Hz, which is above the threshold at which humans are able to perceive light flashes at the described intensity. For the second measurement, participants were instructed to observe the light source again, but this time they were asked to turn the dial counter-clockwise to decrease flash frequency, until they first observed flashing. The frequency at which this happened was again recorded.
The third step consisted of the method of constant stimuli: for each participant, we took the highest value measured with the method of limits, and created a series of 10 flash frequencies in 1 Hertz steps: 5 frequencies below the recorded threshold, and 4 frequencies above it. Each of the frequencies in the series was then multiplied by 5 to produce a set of 50 stimuli. We then randomized the entire set and presented it to the participant. Participants were then instructed to cycle through all of the 50 frequencies in the series, one by one, using the rotary encoder dial to instantiate the next stimulus in the set. Participants were asked to inform the experimenter for each stimulus whether they observed a steady light or not. The final critical flicker fusion threshold for the individual was determined by calculating the highest frequency in the series at which the user was able to perceive flashing 80% of the time.
Participants completed the CFF task roughly 30 minutes prior to the first eyes-closed EEG recording.