Time Slices: What Is the Duration of a Percept?

Abstract

We experience the world as a seamless stream of percepts. However, intriguing illusions and recent experiments suggest that the world is not continuously translated into conscious perception. Instead, perception seems to operate in a discrete manner, just like movies appear continuous although they consist of discrete images. To explain how the temporal resolution of human vision can be fast compared to sluggish conscious perception, we propose a novel conceptual framework in which features of objects, such as their color, are quasi-continuously and unconsciously analyzed with high temporal resolution. Like other features, temporal features, such as duration, are coded as quantitative labels. When unconscious processing is "completed," all features are simultaneously rendered conscious at discrete moments in time, sometimes even hundreds of milliseconds after stimuli were presented.

Fig 1. The color phi phenomenon.

If two differently colored disks are shown at different locations in rapid succession, observers experience just one disk moving from the first to the second location, changing color abruptly midway on the illusory path. How can the observer know in advance what color the second disk will be and where it will appear? The conscious percept cannot have been formed in a time-ordered fashion, but must have been constructed retrospectively.

Fig 2. Long-lasting visual feature integration.

Feature fusion: A vernier, which is a pair of vertical bars that are spatially offset either to the left or right, is presented in rapid succession with a vernier of opposite offset direction. Interestingly, the two verniers are not perceived individually, one after the other, but as one fused vernier. The perceived spatial offset of the fused vernier is a combination of the offsets of both verniers, i.e., the offsets integrate unconsciously. TMS applied over the occipital cortex at different times after the presentation of the verniers selectively rendered the offset direction of either the first or the second vernier more dominant, even when applied up to 400 ms after the presentation of the verniers. Hence, the integration of the vernier offsets is not completed beforehand, and a conscious percept is only elicited a few hundred milliseconds after stimulus presentation. Vernier presentations are indicated by the small depictions in the graph. Before TMS application, vernier offsets were adjusted such that performance was at 50%, i.e., on average observers reported equally often the offset direction of the first and the second vernier (dashed line). A first vernier dominance of more than 50% indicates that the offset direction of the first vernier was reported more often. Conversely, a first vernier dominance of less than 50% indicates dominance of the second vernier. Adapted from [29].

Fig 3. Two-stage model of visual perception.

A stimulus, for example, a cross, is presented on a computer screen. Sensory information about the cross is quasi-continuously and unconsciously analyzed by feature detectors. Just like color and orientation, temporal features are also coded as quantitative labels. When unconscious feature integration is completed, all features are simultaneously rendered conscious at one discrete point in time, sometimes even hundreds of milliseconds after the object was presented [29]. The conscious representation can be seen as a feature vector, which contains a value that represents the stimulus best for each feature dimension, for example, a black cross with a 50 ms duration. Only the feature labels can be reported and measured in experiments, but the actual percept during feature integration is not experimentally determinable. It might be that the previously generated conscious percept stays constant until the next percept is elicited, or that the percept may be rendered conscious just for a moment of time and there is no perception until the next percept emerges (as shown here).