Visual Attention and Consciousness by Friedenberg Jay;

Author:Friedenberg, Jay;
Language: eng
Format: epub
Publisher: Taylor & Francis Group

MASKING

In backward masking experiments a target stimulus is presented and then followed by a mask (Breitmeyer & Ogmen, 2000; Enns & Di Lollo, 2000). In forward masking the mask precedes the target. Metacontrast masking occurs when the contours of the target and mask overlap or are in close spatial proximity. If this contour relationship is present and the mask precedes the target it is known as paracontrast masking. In typical backward masking experiments, both the target and mask are presented for a very short duration, usually less than 50 ms. In these studies, the target stimulus is typically chosen such that it would be fairly easy to identify if it were presented in isolation, with no mask present. Yet, when a mask is introduced only 100 ms after target offset, the ability to process it markedly reduced, with many observers unable to see the target at all. Backward masking has been studied in its own right but is also commonly used in many perceptual experiments to limit how long a particular stimulus is processed.

The variable most often manipulated in backward masking experiments is stimulus onset asynchrony (SOA). This is the time between the target onset and the mask onset. Performance plotted as a function of SOA is called a masking function. There are two primary types of masking function found in the literature (Francis & Cho, 2006). The first is a monotonic increase in performance with increased SOA. The second is a U-shaped function with performance better at very short SOAs, a dip at intermediate values and then a subsequent rise. Figure 9.2 shows examples of both.

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