Visual Bias for Gravitationally Downward Motion

Michael K. McBeath, Mary K. Kaiser

ABSTRACT

When an occluded row of shapes is shifted, apparent motion can be experienced in either direction along the row. Two experiments using a staircase procedure with such stimuli provide evidence for both egocentrically downward and exocentrically (gravitationally) downward motion biases. The first experiment demonstrated that seated observers with their heads oriented upwards experience a downward motion bias independent of location of fixation. The second experiment demonstrated that observers lying with their heads oriented sideways experience both egocentrically downward and exocentrically downward motion biases. Both studies also replicated findings for an egocentrically-oriented leftward motion bias, shown by Morikawa and McBeath (1991) to be associated with exposure to left-to-right reading. The results provide evidence that both physiological and conceptual factors contribute to a downward motion bias.

	<<<Figure 2. Experimental Technique. Subjects were presented two-frame apparent motion using rows of circles at four diffent orientations.
	Figure 5. Experiment 2. .>>> Subjects viewed the same stimuli as in experiment 1, but while lying on their left and right sides, respectively. Results indicated significant downward biases both egocentrically and exocentrically (gravitationally) as well as an egocentrically lefward bias

Figure 4b. Directional Results Experiment 1. Results pooled over fixation conditions yielded significant motion biases both horizontally leftward and vertically downward.

Fig. 4c. Fixation Results Experiment 1. The influence of location of fixation did not exhibit a systematic pattern across row orientation. Shown are the leftward biases with the horizontal stimulus row for the three fixation conditions.