Academic journal article Attention, Perception and Psychophysics

Response Priming Driven by Local Contrast, Not Subjective Brightness

Academic journal article Attention, Perception and Psychophysics

Response Priming Driven by Local Contrast, Not Subjective Brightness

Article excerpt

We demonstrate qualitative dissociations of brightness processing in visuomotor priming and conscious vision. Speeded keypress responses to the brighter of two luminance targets were performed in the presence of preceding dark and bright primes (clearly visible and flanking the targets) whose apparent brightness values were enhanced or attenuated by a visual illusion. Response times to the targets were greatly affected by consistent versus inconsistent arrangements of the primes, relative to the targets (response priming). Priming effects could systematically contradict subjective brightness matches, such that one prime could appear brighter than the other but could prime as if it were darker. Systematic variation of the illusion showed that response-priming effects depended only on local flanker-background contrast, not on the subjective appearance of the flankers. Our findings suggest that speeded motor responses, as opposed to conscious perceptual judgments, access an early phase of lightness and brightness processing prior to full lightness constancy.

The subjective perception of lightness and brightness is the final outcome of a complex processing system that tries to infer the physical properties of surfaces in the environment from the distribution of light on the retina. Both classic (Land & McCann, 1971) and current (Adelson, 2000; Gilchrist et al., 1999) models of lightness perception stress that this is a multistage process: A 2-D pattern of raw local luminance or contrast values on the retina must be interpreted to disentangle 3-D surface properties from a bewildering array of lighting effects, such as illumination gradients, shadows, transparency, and gloss. This system's most basic task is to achieve lightness constancy by telling differences in reflectance (the percentage of light a surface reflects into the observer's eye) from differences in illumination (the amount of light incident on that surface; Adelson, 1993; Li & Gilchrist, 1999). The term lightness is defined as the subjectively perceived reflectance of a surface, whereas the term brightness refers to its perceived luminance. For example, if you take a book from a sunny to a shadowy place, the coloring of the paper does not seem to change from white to gray, even though its luminance is vastly decreased and the pages appear less bright. Instead, the coloring of the pages remains largely invariant-that is, constant in lightness-corresponding more closely to its reflectance than to its luminance value (see Gilchrist, 2006, for further examples of lightness constancy).

Psychophysical judgments of lightness and brightness are typically based on the final outcome of the processing system-presumably, a highly integrated representation of surfaces and illuminants in visual awareness. But is it possible to access earlier, more preliminary steps in the process? In this article, we utilize the fact that visual stimuli are able to directly trigger assigned motor responses without mediation by visual awareness (Neumann, 1990). In the response priming paradigm (Neumann & Klotz, 1994; Vorberg, Mattler, Heinecke, Schmidt, & Schwarzbach, 2003), participants have to perform a speeded response to a target stimulus that is preceded by a prime stimulus triggering either the same response as the target (consistent prime) or the opposite response (inconsistent prime). Consistent primes speed responses to the target, whereas inconsistent primes slow them, and this priming effect increases with the stimulus onset asynchrony (SOA) between the prime and target (Vorberg et al., 2003). Strikingly, this typical time course of response priming is independent of the visibility of the prime: For SOAs up to 100 msec, it remains invariant under various conditions of visual masking, no matter whether the prime is identified perfectly or not at all, and no matter whether prime identification performance increases or decreases with SOA. In particular, priming effects can become larger with SOA even though the prime is becoming more difficult to discriminate (Mattler, 2003b; Schmidt & Vorberg, 2006; Vorberg et al. …

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