Academic journal article Perception and Psychophysics

Factors Affecting Curved versus Straight Path Heading Perception

Academic journal article Perception and Psychophysics

Factors Affecting Curved versus Straight Path Heading Perception

Article excerpt

Displays commonly used for testing heading judgments in the presence of rotations are ambiguous to observers. They can be interpreted equally well as motion in a straight line while rotating the eyes or as motion on a curved path. This has led to conflicting results from studies that use these displays. In this study, we tested several factors that might influence which of these two interpretations observers see. These factors included the size of the field of view, the duration of the stimulus, textured scenes versus random-dot displays, and whether or not observers were given a description of their path. The only factor that had a significant effect on path perception was whether or not observers were given instructions describing their path of motion. Under all conditions without instructions, we found that observers responded in a way that was consistent with the perception of motion on a curved path.

When an observer moves through the world, he or she must be able to judge his or her path of motion when moving in a straight line while making an eye rotation or when moving on a curved path. Previous research has yielded conflicting results about how this may be accomplished, with some results suggesting that extraretinal information is required for computing heading in the presence of eye movements (Banks, Ehrlich, Backus, & Crowell, 1996; Ehrlich, Beck, Crowell, Freeman, & Banks, 1998; Royden, Banks, & Crowell, 1992; Royden, Crowell, & Banks, 1994) and other results suggesting that this computation can be accomplished from the retinal information alone (Cutting, Springer, Braren, & Johnson, 1992; Stone & Perrone, 1997; van den Berg, 1992, 1993, 1996; Warren & Hannon, 1988, 1990). Here, we examine some of the features of the stimuli commonly used in these experiments to help determine the reasons for these discrepancies.

When an observer moves in a straight line through a stationary scene, the image motion on the retina, known as the optic flow field, forms a radial pattern (Figure 1A) (Gibson, 1950,1966; Longuet-Higgins & Prazdny, 1980). The center of this pattern, known as the focus of expansion, corresponds to the observer's direction of motion. If the observer is undergoing a rotation while he or she is translating, as might occur if he or she were making an eye movement or were moving on a curved path, then the flow field is more complex (Figure 1B). Numerous models have been proposed to explain how the human visual system might recover the observer's motion parameters, both translation and rotation, from the flow field shown in Figure 1B (Beintema & van den Berg, 1998; Hatsopoulos & Warren, 1991; Heeger & Jepson, 1992; Hildreth, 1992; Lappe & Rauschecker, 1993; Perrone, 1992; Perrone & Stone, 1994; Rieger & Lawton, 1985; Royden, 1997). These models clearly show that it is possible to recover both translation (straight path motion) and rotation parameters from the optic flow field, but there has been considerable controversy in the literature about whether the human visual system can actually recover the translational component of motion separate from rotational motion using visual information alone. Some psychophysical results suggest that extraretinal information about eye movements is necessary for human observers to judge their translation direction accurately in the presence of eye movements (Banks et al., 1996; Ehrlich et al., 1998; Royden et al., 1992; Royden et al., 1994), whereas other results seem to suggest that visual information alone is sufficient (van den Berg, 1992, 1993, 1996; Warren & Hannon, 1988, 1990).

Many of the experiments that measured these judgments used a design in which subjects observed two different types of displays: the "real" and "simulated" eye-movement conditions (Banks et al., 1996; Ehrlich et al., 1998; Regan & Beverley, 1982; Rieger & Toet, 1985; Royden et al., 1992; Royden et al., 1994; Warren & Hannon, 1988, 1990). …

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