Academic journal article Perception and Psychophysics

The Effect of Saccades on Number Processing

Academic journal article Perception and Psychophysics

The Effect of Saccades on Number Processing

Article excerpt

Recent research has shown that saccadic eye movements interfere with dorsal-stream tasks such as judgments of object orientation, but not with ventral-stream tasks such as object recognition. Because saccade programming and execution also rely on the dorsal stream, it has been hypothesized that cognitive saccadic suppression occurs as a result of dual-task interference within the dorsal stream. Judging whether one number is larger or smaller than another (magnitude comparison) is a dorsal-stream task that relies especially on the right parietal cortex. In contrast, judging whether a number is odd or even (parity judgment) does not involve the dorsal stream. In the present study, one group of subjects judged whether two-digit numbers were greater than or less than 65, whereas another group judged whether two-digit numbers were odd or even. Subjects in both groups made these judgments while making no, short, or long saccades. Saccade distance had no effect on parity judgments, but reaction times to make magnitude comparison judgments increased with saccade distance when the eyes moved from right to left. Because the right parietal cortex is instrumental in generating leftward saccades, these results provide further evidence for the hypothesis that cognitive suppression during saccades occurs as a result of dual-task interference within the dorsal stream.

People inspect the world by means of rapid eye movements called saccades. Saccades are separated by fixations during which the eyes are relatively still. Saccades occur about 3 times each second, or approximately 172,800 times during the course of the average day. Saccade duration depends on saccade distance, with the average duration during reading and picture viewing being approximately 30-50 msec (Rayner, 1978, 1998). Thus, the eyes are in motion about 90-150 mm each day.

It is well known that visual sensitivity is reduced during saccades, with the result that people do not ordinarily notice visual information sweeping across their retinas during saccades (Matin, 1974; Volkmann, 1986; Zuber & Stark, 1966). Not all visual information is suppressed during saccades, however; rather, suppression appears to be restricted to the magnocellular visual pathway, which carries information about luminance, contrast, and motion, while the parvocellular visual pathway (which carries information about color and form) remains largely unaffected (Burr, Morrone, & Ross, 1994).

Perhaps surprisingly, recent evidence has shown that some cognitive processes are also suppressed during saccades (see Irwin, 2003, for a review); these include stimulus encoding (Sanders & Houtmans, 1985), the resolution of degraded stimuli (Sanders & Rath, 1991), memory scanning (van Duren, 1993), counting (Matin, Shao, & BofF, 1993), mental rotation (Irwin & Brockmole, 2000; Irwin & Carlson-Radvansky, 1996), and changes in attentional scale from local to global levels of a form (Brockmole, Carlson, & Irwin, 2002). Other cognitive processes are not suppressed during saccades, however, and these include response selection (van Duren & Sanders, 1995), semantic priming (Henderson, 1992; Henderson, Pollatsek, & Rayner, 1987), identity priming (Irwin, Carlson-Radvansky, & Andrews, 1995), and word recognition and word identification (Irwin, 1998).

The distinction between the processes that are suppressed during saccades and those that are not corresponds reasonably well to the distinction between dorsal and ventral processing streams in vision. To review, Mishkin, Ungerleider, and Macko (1983) proposed that two functionally separable streams of visual processing exist in the brain. The ventral stream projects to areas of inferotemporal cortex and has been hypothesized to be responsible for perceptual recognition and cognitive representations of objects, whereas the dorsal stream projects to the posterior parietal cortex and has been hypothesized to be responsible for visuospatial transformations and for representing an object's location in space (but see Goodale & Milner, 1992, and Merigan & Maunsell, 1993, for alternative views). …

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