The Emergence of Visual Awareness: Temporal Dynamics in Relation to Task and Mask Type

Abstract

One aspect of consciousness phenomena, the temporal emergence of visual awareness, has been subject of a controversial debate. How can visual awareness, that is the experiential quality of visual stimuli, be characterized best? Is there a sharp discontinuous or dichotomous transition between unaware and fully aware states, or does awareness emerge gradually encompassing intermediate states? Previous studies yielded conflicting results and supported both dichotomous and gradual views. It is well conceivable that these conflicting results are more than noise, but reflect the dynamic nature of the temporal emergence of visual awareness. Using a psychophysical approach, the present research tested whether the emergence of visual awareness is context-dependent with a temporal two-alternative forced choice task. During backward masking of word targets, it was assessed whether the relative temporal sequence of stimulus thresholds is modulated by the task (stimulus presence, letter case, lexical decision, and semantic category) and by mask type. Four masks with different similarity to the target features were created. Psychophysical functions were then fitted to the accuracy data in the different task conditions as a function of the stimulus mask SOA in order to determine the inflection point (conscious threshold of each feature) and slope of the psychophysical function (transition from unaware to aware within each feature). Depending on feature-mask similarity, thresholds in the different tasks were highly dispersed suggesting a graded transition from unawareness to awareness or had less differentiated thresholds indicating that clusters of features probed by the tasks quite simultaneously contribute to the percept. The latter observation, although not compatible with the notion of a sharp all-or-none transition between unaware and aware states, suggests a less gradual or more discontinuous emergence of awareness. Analyses of slopes of the fitted psychophysical functions also indicated that the emergence of awareness of single features is variable and might be influenced by the continuity of the feature dimensions. The present work thus suggests that the emergence of awareness is neither purely gradual nor dichotomous, but highly dynamic depending on the task and mask type.

Keywords: awareness; consciousness; dichotomous; graded; psychophysics; vision; visual masking.

FIGURE 1

Schematic drawing of the tasks. Each of the four tasks consists of two target-mask sequences with target pairs (targets 1 and 2) representing the respective task features: word and blank screen, word in upper and lower cases, word and pseudoword, natural object and artifact. Duration of target presentation was one frame on the CRT screen. Stimulus onset asynchrony (SOA) between target and mask was identical in both target-mask sequences. It was adaptively varied in order to measure discrimination thresholds for the given task between 6.7 ms (no blank frame between target and mask) and 340 ms (50 blank frames between target and mask). Mask duration was 200 ms as depicted in the semantic condition, interval between the two target-mask sequences was 900 ms. Subjects had to indicate by button press in which of the two intervals the feature of the given task was seen (response). The four masks depicted as inset in interval 1 for the absent/present task (pattern, false font, random strings, and abstract words) were not varied within subjects but were administered to four different groups of subjects. The mask in interval 2 was always different to the mask in interval 1. The hash displayed at the begin of each interval and the question mark displayed following mask 2 are not depicted. Notice that text sizes and mask sizes are enlarged in relation to the screen for sake of clarity. The fifth task (visibility) was identical with the absent/present task, using only the shortest SOA of 6.7 ms.

FIGURE 2

Examples of psychometric functions fitted on data of two subjects in different masking conditions. Top: participant in the pattern mask condition, bottom: participant in the word mask condition. The four different tasks are depicted in different symbols and different colors. The size of the symbols codes the number of presentations per SOA which differs due to the staircase procedures. In the examples the range of number of presentations is n = 1 (lower graph, red square at 17 ms) to n = 17 (lower graph, blue triangle at 73 ms). The largest SOA of 173 ms is not included into the graph for sake of clarity (performance 1.0 in each task). Slope values in the order of the data labels: upper 1.33, 0.0499, 0.0729, 0.139; lower 1.33, 0.0862, 0.227, 0.110.

FIGURE 3

Thresholds for each task (SOA in ms) in each of the four mask groups. Depicted are mean values and the ±95% confidence interval. Please notice that dotted lines connecting the given task over the four different masks connect results from the four different groups.

FIGURE 4

Slopes of the psychometric functions, grouped by factor TASK. Depicted are mean values ± 95% confidence interval.