Academic journal article Perception and Psychophysics

Unconscious, Stimulus-Dependent Priming and Conscious, Percept-Dependent Priming with Chromatic Stimuli

Academic journal article Perception and Psychophysics

Unconscious, Stimulus-Dependent Priming and Conscious, Percept-Dependent Priming with Chromatic Stimuli

Article excerpt

Using metacontrast masking to suppress the conscious registration of a prime stimulus, Breitmeyer, Ro, and Singhal (2004) showed that color priming produced by a masked prime disk occurs at unconscious stimulus-dependent rather than at percept-dependent levels of visual processing. The current set of experiments compares this type of unconscious stimulus-dependent priming to conscious priming produced by a prime that, in two separate ways, is rendered visible and thus activates percept-dependent visual processes. The results indicate that while the masked prime again acts at a stimulus-dependent level of processing, the unmasked, visible primes additionally act at a later percept-dependent level of processing.

During the past decade, the distinction between unconscious and conscious information processing and the types and levels of brain activity corresponding to them have been major topics of interest among cognitive psychologists and neuroscientists (Crick & Koch, 1998; de Gelder, de Haan, & Heywood, 2001; Kihlstrom, 1996; Kreiman, Fried, & Koch, 2002; Leopold & Logothetis, 1996; Metzinger, 2000; Milner & Goodale, 1995; Weiskrantz, 1997; Zeki, 1997). An effective way to investigate differences between the two types of processing is to use stimuli whose attributes are suppressed from conscious registration by visual masks, without, however, suppressing information that can be made available to systems that rely on unconsciously processed information (Ansorge, Klotz, & Neumann, 1998; Dolan, 2002; Esteves, Parra, Dimberg, & Ohman, 1994; Klotz & Wolff, 1995; Morris & Dolan, 2001, Neumann & Klotz, 1994). For example, when a masked prime and a following mask have congruent colors, such as a green prime followed by a green mask, choice reaction times (CRTs) to the mask color are shorter than when they have incongruent colors, such as a green prime followed by a red mask (Schmidt, 2000, 2002) or a blue mask (Breitmeyer, Ögtnen, & Chen, 2004; Breitmeyer, Ro, & Singhal, 2004). We call such unconscious processing of the prime stimulus-dependent, since it depends only on the presence (but not the perceptual state: invisible or visible) of the prime stimulus. On the other hand, a processing level that is activated only if a prime stimulus is perceived can be characterized as percept-dependent.

Regarding chromatic stimuli, we take their wavelength composition to be an objectively specifiable physical property, regardless of whether they are perceived or not. On the other hand we take color to refer to one of the several consciously perceived attributes of the chromatic stimuli. Using the CRT paradigm with metacontrast masking to render a prime stimulus invisible, Breitmeyer, Ro, and Singhal (2004) found that the unconscious chromatic priming produced by a masked prime was consistent with an early, wavelength- or stimulus-dependent level of visual processing rather than a later, color- or percept-dependent one. In that study, white as well as desaturated blue or green prime disks were followed at an optimal metacontrast stimulus onset asynchrony (SOA) of 42 msec by either a desaturated blue or green mask ring surrounding the primes. CRTs to a blue mask following a white prime were significantly slower than CRTs to a blue mask following a blue prime. In contrast, CRTs to a green mask following a white prime were not significantly different from CRTs to a green mask following a green prime. That is, the white prime had effects similar to those of the green prime but different from those of the blue prime. Moreover, control experiments, in which the prime disks (presented without the mask) were clearly visible, indicated that the white disk perceptually resembled the blue one more than the green one. The similarity of the priming effect of the suppressed white disk to that of the suppressed green prime cannot, therefore, be based on the perceptual color properties of the disks. …

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