Afterimage duration depends on how deeply invisible stimuli were suppressed

Abstract

Quantifying visual responses to stimuli that are outside of awareness is a critical task for the study of visual consciousness. The current study psychophysically investigated whether afterimages reflect visual responses to stimuli that are not consciously visible throughout adaptation due to interocular suppression. A Gabor adaptor was presented to one eye of the observer, and a counterphase-flickering Gabor suppressor was presented to the other eye, thereby rendering the adaptor invisible during adaptation. To manipulate the depth of the suppression of the invisible adaptor, we varied the orientation difference between the adaptor and suppressor. We found that, even though the adaptor was not visible during adaptation, the afterimage duration varied depending on the orientation selectivity of interocular suppression. The duration was the shortest when the orientations of the adaptor and suppressor were identical and lengthened when the orientation differences increased. This finding could not be explained by confounding factors such as potential changes in contrast sensitivity that were caused by the suppressor. Our findings suggest that the magnitude of visual responses to stimuli suppressed below the threshold of awareness can be measured using the afterimage duration. Afterimages could be an effective tool for quantifying visual responses, irrespective of observers' conscious awareness of a presented stimulus.

Figure 1.

Schematic diagram of the stimulus sequence in Experiments 1a and 1b. A suppressor was presented to the observer's dominant eye (DE, left column), and an adaptor was presented to the non-dominant eye (NDE, middle column). A negative afterimage of the adaptor was seen on the blank display after the offset of the dichoptic stimuli. A typical percept is illustrated in the right column of the figure. A random noise mask was presented after the observer's response only in Experiment 1b.

Figure 2.

Results of Experiments 1a and 1b. Two experiments were conducted with a basically identical procedure but different sets of equipment. The afterimage duration was plotted as a function of the orientation difference between the suppressor and adaptor (open symbols). The filled symbols represent the results in the control (no-suppression) condition. The black symbols and lines show the mean afterimage duration averaged across Experiments 1a and 1b. The error bars indicate ±1 SEM across different observers. The asterisks denote the conditions in which the afterimage duration was significantly longer than that in the 0° condition.

Figure 3.

Schematic diagram of the stimulus sequence in Experiment 2. Two dynamic stimuli were presented to the dominant eye (DE, left column), and the target was presented to the non-dominant eye (NDE, middle column). A typical percept is illustrated in the right column of the figure.

Figure 4.

Results of Experiment 2. The mean contrast threshold in the log unit is plotted for the suppression (open squares) and control (filled square) conditions. The thresholds for individual observers are also shown using gray circles. Error bars denote ±1 SEM across different observers.

Figure 5.

Schematic diagram of the stimulus sequence in Experiment 3. A suppressor was presented to the observer's dominant eye (DE, left column), and two identical adaptors were presented to the non-dominant eye (NDE, middle column). Although one of the two adaptors was rendered invisible by the suppressor during the adaptation, two afterimages were perceived after the stimulus offset. A dynamic mask was presented following the observer's response. A typical percept is illustrated in the right column of the figure.

Figure 6.

Results of Experiment 3. The percentage of trials in which the visible adaptor produced a longer afterimage than the invisible one is shown. Different symbols represent different spatial frequencies of the adaptor, as shown in the legend. The error bars denote 95% confidence intervals. The dashed line represents the 50% level, which indicates that the two afterimages were comparable in duration.