Figure 1.
Three accounts describing the influence of objective flicker frequency, subjective flicker perception, and the neural response to flicker on subjective duration.
We measured participants' flicker fusion threshold (the highest frequency perceived as flicker) and their SSVEP threshold (the highest frequency still evoking a significant frequency-specific SSVEP), and tested predictions from three accounts regarding how subjective duration is affected by flicker frequencies below, between and above these two thresholds. Left panel: The rate-of-change-account predicts that subjective duration increases monotonically with the objective rate of change (the flicker frequency) up to the flicker fusion threshold or the SSVEP threshold. Middle panel: The change-saliency account predicts that the effect of flicker frequency on perceived duration is maximal when the flicker is subjectively perceived as most salient. This should result in an inverted u-shaped relationship between flicker frequency and perceived duration, with a maximal effect at frequencies between 8–15 Hz. Invisible flicker (faster than the flicker fusion threshold) should not affect subjective duration, even if it evokes a frequency-specific neural response. Right panel: The neural-energy-account predicts that subjective duration depends on the neural energy expended in processing a stimulus. Hence, subjective duration should be longest for frequencies that evoke the largest neural responses (typically at 12–15 Hz). Note that in contrast to the change-saliency account, frequencies above the flicker-fusion threshold that still evoke a neural response should affect perceived duration.
Figure 2.
Steady State Visual Evoked Potentials.
A: Grand-average frequency spectra for 30 participants as recorded during phase I, based on 30 s stimulation intervals and averaged across all channels. Gray shades indicate a ±2 Hz range around stimulation frequencies (7.7 Hz to 165.7 Hz). Note that spectral peaks indicating steady state evoked potentials were found even at the highest stimulation frequencies that were never perceived as flickering. B: Data of an exemplary participant. Frequency spectra evoked by four stimulation frequencies: 7.7 Hz, 31.3 Hz, 71.1 Hz, 165.7 Hz (dark gray). The light gray area indicates the 99.9%-percentile of the resampled data, as used to determine statistical significance of peaks at the stimulation frequency. Blue stars indicate significant amplitude peaks at the stimulation frequency (p<0.001). The topographies show the scalp distribution of amplitudes at the peak frequency.
Figure 3.
Subjective Flicker Perception.
Psychometric function as determined in phase II, describing the relation between stimulation frequency and flicker perception (for the same participant as in Figure 2). The dashed vertical line shows the 90% threshold of the curve, at which the stimulus was reported to be “steady” in 90% of trials. This frequency was taken as the flicker fusion threshold.
Figure 4.
A: Histogram of individual flicker fusion thresholds (green) and SSVEP thresholds (blue) as determined in phases I and II of the experiment. B: Average perceived duration for all stimulation frequencies (positive values on the y-axis indicate over-estimation, negative values indicate under-estimation). The dark gray area indicates 95% confidence intervals. The dashed horizontal line shows the threshold estimated for the reference frequency (165.7 Hz) against which all other frequencies were compared (the light gray shade indicates 95% confidence interval). Frequencies up to 41.7 Hz were significantly perceived as longer than the standard. Vertical lines show the flicker fusion threshold (cyan) and the SSVEP threshold (blue) averaged across participants. C: Same data as in B, shown separately for stimuli perceived as flicker, not perceived as flicker but evoking an SSVEP, and no SSVEP. The duration of stimuli perceived as flicker (red) was overestimated compared to the reference frequency, for frequencies up to 31.4 Hz (all p<0.05), resulting in a linear relationship between flicker frequency and perceived duration. Flicker frequencies that were not perceived as flicker but evoked a frequency-specific SSVEP (green), and frequencies that were neither perceived nor evoked a SSVEP (blue) were not perceived as longer than the reference frequency. The numbers next to the data points indicate the number of participants contributing to this data point (if they do not add up to 30, this is because of excluded outliers).
Table 1.
Subjective duration (i.e. difference from the 2 s standard stimulus) estimated by the Quest algorithm for each stimulation frequency (mean of all 30 participants).
Figure 5.
A: Correlations between proportion of trials seen as flicker, and perceived duration. Although very high and low frequencies did not show great variation in perception of flicker, overall, conscious perception of flicker led to longer perceived duration. (r = 0.47; p<0.001). B After a z-transform, which removed all variance between frequencies while keeping the variance within frequencies, a significant correlation remained between flicker perception and perceived duration (r = 0.14; p = 0.02). This result indicates that participants who perceived a given frequency as flicker perceived stimulus duration as longer than those who did not perceive this frequency as flicker. C: SSVEP amplitudes evoked by each flicker frequency significantly correlated with perceived duration (r = 0.30; p<0.001). D: A z-transform of SSVEP amplitudes, which removes the between-frequency effect, removed the correlation between SSVEP and subjective duration (r = −0.06; p = 0.30), indicating that participants with stronger SSVEP at a given frequency did not perceive stimuli of that frequency as longer.