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.