Academic journal article Psychonomic Bulletin & Review

Why Eye Movements and Perceptual Factors Have to Be Controlled in Studies on "Representational momentum"/Computational Theory and Cognition in Representational Momentum and Related Types of Displacement: A Reply to Kerzel

Academic journal article Psychonomic Bulletin & Review

Why Eye Movements and Perceptual Factors Have to Be Controlled in Studies on "Representational momentum"/Computational Theory and Cognition in Representational Momentum and Related Types of Displacement: A Reply to Kerzel

Article excerpt

DIRK KERZEL

University of Geneva, Geneva, Switzerland

In order to study memory of the final position of a smoothly moving target, Hubbard (e.g., Hubbard & Bharucha, 1988) presented smooth stimulus motion and used motor responses. In contrast, Freyd (e.g., Freyd & Finke, 1984) presented implied stimulus motion and used the method of constant stimuli. The same forward error was observed in both paradigms. However, the processes underlying the error may be very different. When smooth stimulus motion is followed by smooth pursuit eye movements, the forward error is associated with asynchronous processing of retinal and extraretinal information. In the absence of eye movements, no forward displacement is observed with smooth motion. In contrast, implied motion produces a forward error even without eye movements, suggesting that observers extrapolate the next target step when successive target presentations are far apart. Finally, motor responses produce errors that are not observed with perceptual judgments, indicating that the motor system may compensate for neuronal latencies.

In his review of the literature on representational momentum, Hubbard (2005) presents two basic paradigms that have been used to investigate the localization of the final position of a moving target and concludes that

Even though Freyd and Finke (1984) and Hubbard and Bharucha [1988] used different methods of stimulus presentation and response collection, the results from these studies converged on the idea that memory for the final position of a moving target was displaced forward in the direction of target motion. (p. 824)

Contrary to Hubbard's (2005) conclusions, I will show in this comment that the observed convergence is the result of an artifact related to the poor control of eye movements in Hubbard and Bharucha (1988). Furthermore, I will present an alternative view suggesting that forward displacement arises partly from low-level processes, that more than a single process of extrapolation exists, and that each extrapolation process is specific to certain types of motion and responses. To begin with, I will briefly recapitulate the major difference between the work of Jennifer Freyd in the 1980s (e.g., Freyd & Finke, 1984, 1985; Freyd & Johnson, 1987) and that of Timothy Hubbard in the 1990s (e.g., Hubbard, 1995, 1996; Hubbard & Bharucha, 1988).

In Freyd's work (e.g., Freyd & Finke, 1984), observers were shown a succession of rectangles, and each rectangle was shown for about 250 msec at the same position; after a blank interval of 250 msec, it appeared at a new orientation that implied the rotation of the rectangle during the blank interval. After seeing three such stimuli, a fourth stimulus, the probe, was presented at an orientation that differed only slightly from the third, and the observers were asked to report whether the third and the fourth orientations of the stimuli were the same or not. The observers were more prone to accept probe stimuli that had been rotated slightly further as being in the same orientation, indicating a forward error. The stimuli created by Freyd may be referred to as implied motion stimuli, and she used the method of constant stimuli that involved a symbolic response, with an arbitrary mapping between response and perceptual content (e.g., pressing one of two buttons). Most of the subsequent work on representational momentum has used Freyd's methodology, even if the stimulus type and the target's trajectory have been changed (for a sample of recent research, see Thornton & Hubbard, 2002).

In Hubbard's work (e.g., Hubbard & Bharucha, 1988), observers were shown a smoothly moving target that resembled real motion. The target moved on a linear trajectory and disappeared suddenly. The observers' task was to adjust a mouse cursor so that it corresponded to the remembered final position, which involved a nonarbitrary relationship between the motor response and the perceptual content. …

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