Subjective and objective measures of visual awareness converge

Abstract

Within consciousness research, the most appropriate assessment of visual awareness is matter of a controversial debate: Subjective measures rely on introspections of the observer related to perceptual experiences, whereas objective measures are based on performance of the observer to accurately detect or discriminate the stimulus in question across a series of trials. In the present study, we compared subjective and objective awareness measurements across different stimulus feature and contrast levels using a temporal two-alternative forced choice task. This task has the advantage to provide an objective psychophysical performance measurement, while minimizing biases from unconscious processing. Thresholds based on subjective ratings with the Perceptual Awareness Scale (PAS) and on performance accuracy were determined for detection (stimulus presence) and discrimination (letter case) tasks at high and low stimulus contrast. We found a comparable pattern of thresholds across tasks and contrasts for objective and subjective measurements of awareness. These findings suggest that objective performance measures based on accuracy and subjective ratings of the visual experience can provide similar information on the feature-content of a percept. The observed similarity of thresholds validates psychophysical and subjective approaches to awareness as providing converging and thus most likely veridical measures of awareness.

Fig 1. Time course of the tasks.

Each of the two tasks consists of two target-mask sequences with a target (one frame) followed by a mask formed by a string of false font letters (200 ms). Stimulus onset asynchrony (SOA) between target and mask was similar in both sequences, adaptively varied in a range between 6.7 ms and 340 ms in order to measure objective and subjective perception thresholds. The interval between the two sequences was 900 ms. After the second sequence two questions were asked subsequently: (1) In what interval was the target flashed? (indicated by a question mark); (2) What was the visibility like? (PAS-scale, 1–4). In the detection task in one of the two sequences a word was flashed, the other sequence consisted of a mask only. The discrimination task consisted of two target words, one written in capital letters, the other in small letters.

Fig 2. Analysis of raw data.

Results from one subject are shown. In the left column the time course of the experiment is depicted. Three staircases randomly interleaved started at different SOAs (red 20 ms, blue 40 ms, and purple 73 ms). Trial number was fixed at n = 76. Sigmoidal fits over the answers merged from the 3 staircases (circles) are shown in the middle column (objective responses, temporal 2-AFC) and right column (PAS ratings). Number of observations per SOA is depicted by the radius of the circles. Each of the four rows displays a task condition as indicated in the left column. The range of objective responses is between 0.5 (guessing rate) and 1 (no errors), the PAS ratings vary between 1 and 4. Please notice that subjective threshold definition differs between detection task (1.5) and discrimination task (2.5). For sake of clarity fits are only depicted between 0 and 100 ms SOA.

Fig 3. Scatter-icon histograms for responses from all subjects.

The four graphs depict the variation of task (detection and discrimination) as well as variation of contrast (low and high). The abscissa shows the SOA in ms, truncated at 200 ms. The ordinate represents the four levels of the PAS scale, the subjective responses. The area of each dot represents the number of presentations per SOA with a response at the given PAS level. The pie chart in each dot depicts the ratio of correct (blue) and wrong (red) answers in the objective response (2-AFC).

Fig 4. Thresholds in ms for objective (blue) and subjective (red) responses.

Error bars depict 95% confidence intervals (within design). On the abscissa, the different conditions are shown.

Fig 5. Width of estimated psychometric functions (range 0.05–0.95, in ms) for objective (blue) and subjective (red) responses.

Error bars depict 95% confidence intervals (within design). On the abscissa, the different conditions are shown.