Academic journal article Attention, Perception and Psychophysics

Effects of Contrast Polarity in Paracontrast Masking

Academic journal article Attention, Perception and Psychophysics

Effects of Contrast Polarity in Paracontrast Masking

Article excerpt

The visibility of a target stimulus can be suppressed (inhibition) or increased (facilitation) during paracontrast masking. Three processes have been proposed to be involved in paracontrast masking: brief inhibition, facilitation, and prolonged inhibition (Breitmeyer et al., 2006). Brief inhibition is observed when the mask precedes the target at short stimulus onset asynchronies (SOAs) ranging from -10 to -30 msec, whereas prolonged inhibition is effective up to very large SOAs of -450 msec. Facilitation, enhancement in target visibility, can be observed at SOA values between -20 and -110 msec. We further investigated these processes by changing target-mask spatial separation and the contrast polarity of the mask. Our results show that (1) facilitation weakens when spatial separation between the target and mask is increased or when they have opposite contrast polarity, and (2) brief inhibition turns into facilitation for the opposite-polarity mask, whereas prolonged inhibition does not change significantly. These results suggest a fast inhibition mechanism realized in the contrast-specific center-surround antagonism of classical receptive fields for brief inhibition and a slower, higher level cortical processing that is indifferent to contrast polarity for prolonged inhibition.

Visual masking refers to the reduction in the visibility of a stimulus, called the target, caused by another visual stimulus, called the mask (Breitmeyer & Ögmen, 2006). Visual masking is a phenomenon deemed worthy of study in its own right but is also a powerful tool for investigating the dynamics of vision, including the interactions between different levels and streams of visual processing (Bachmann, 1994; Breitmeyer & Ögmen, 2000; Breitmeyer, Ögmen, & Chen, 2004; Breitmeyer, Ögmen, Ramon, & Chen, 2005; Ögmen, Breitmeyer, & Melvin, 2003). When the mask stimulus follows the target stimulus, backward masking prevails, and when it is followed by the target, forward masking prevails. Paracontrast and metacontrast are specific types of forward and backward masking, respectively. In paracontrast and metacontrast masking, target and mask stimuli, although not overlapping in space, are typically close to each other. The plot of target visibility as a function of the stimulus onset asynchrony (SOA) between the target and the mask is called the masking function. By convention, in paracontrast, the SOA is given in negative values, to indicate that the mask occurs before the target.

Although many masking studies have assessed the underlying mechanisms of metacontrast (e.g., Breitmeyer & Ögmen, 2000, 2006; Francis, 1997; Hermens, Luksys, Gerstner, Herzog, & Ernst, 2008), only a few studies have addressed paracontrast masking. Recently, Breitmeyer et al. (2006) investigated not only meta- but also paracontrast masking, using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. Paracontrast masking functions for both tasks are shown in Figure 1 (upper panel). Both brightness match and contour identification tasks tended to yield paracontrast functions with somewhat complicated nonmonotonicities. The brightness judgment task showed enhancement in target visibility at SOAs of about -50 to -100 msec, whereas a weaker suppression effect was evident at longer SOAs. The contour judgment task yielded large suppressive effects, which were maximal at an SOA of -10 msec and lasted up to SOA values beyond -350 msec. These results have been interpreted in terms of three processes (Breitmeyer et al., 2006): brief inhibition, facilitation, and prolonged inhibition (Figure 1, lower panel). These three processes interact with different magnitudes and produce distinct paracontrast masking functions for surface brightness and contour identification tasks. In other words, a strong (weak) facilitatory effect interacts with very weak (strong) brief and prolonged inhibition processes for surface brightness (contour identification) task and yields the net paracontrast masking function, as shown in Figure 1 (upper panel). …

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