Abstract
We investigated the effects of a tilted reference frame (i.e., allocentric visual context) on the perception of the gravitational vertical and saccadic eye movements along a planned egocentric vertical path. Participants (n = 5) in a darkened room fixated a point in the center of a circle on an LCD display and decided which of two sequentially presented dots was closer to the unmarked ‘6 o’clock’ position on that circle (i.e., straight down toward their feet). The slope of their perceptual psychometric functions showed that participants were able to locate which dot was nearer the vertical with a precision of 1°–2°. For three of the participants, a square frame centered at fixation and tilted (in the roll direction) 5.6° from the vertical caused a strong perceptual bias, manifest as a shift in the psychometric function, in the direction of the traditional ‘rod-and-frame’ effect, without affecting precision. The other two participants showed negligible or no equivalent biases. The same subjects participated in the saccade version of the task, in which they were instructed to shift their gaze to the 6 o’clock position as soon as the central fixation point disappeared. The participants who showed perceptual biases showed biases of similar magnitude in their saccadic endpoints, with a strong correlation between perceptual and saccadic biases across all subjects. Tilting of the head 5.6° reduced both perceptual and saccadic biases in all but one observer, who developed a strong saccadic bias. Otherwise, the overall pattern and significant correlations between results remained the same. We conclude that our observers’ saccades-to-vertical were dominated by perceptual input, which outweighed any gravitational or head-centered input.
Highlights
In the classic rod-and-frame paradigm introduced by Witkin and Asch (1948), a vertical rod inside a frame tilted in the fronto-parallel plane is seen as tilted in the opposite direction to the frame, implying that the subjective vertical has shifted in the same direction as the frame
Tilting the frame by 5.6° in the anti-clockwise or clockwise directions resulted in participants MM, SG and JS always judging the standard to be more vertical than the target when it was tilted in the same direction as the frame
Their perception of gravitational vertical was determined by the frame tilt (‘field dependent’), whereas perceptions of the vertical for participants DM and EP were independent of the frame orientation (Fig. 4)
Summary
In the classic rod-and-frame paradigm introduced by Witkin and Asch (1948), a vertical rod inside a frame tilted in the fronto-parallel plane (roll direction) is seen as tilted in the opposite direction to the frame, implying that the subjective vertical has shifted in the same direction as the frame. We wished to determine whether the control mechanism for saccades made in egocentric coordinates is affected by tilted frames of reference in the same way that apparent spatial relationships are To examine this question, we used a task in which participants were required to make a saccade to a virtual target, rather than explicit target. We measured perceptual accuracy and context effects on the location of the apparent 6 o’clock position using a 2AFC method with a roving pedestal and saccade biases using ‘saccade-to-vertical.’ In both cases, three frames were used: (1) a control where the frame had zero tilt and two experimental conditions, in which the frame was tilted by (2) 5.6° in an anti-clockwise direction or (3) 5.6° in a clockwise direction. We investigated the effect of a head tilt (5.6° anti-clockwise) under clockwise and anti-clockwise frame tilts, both for perceptual and for saccade tasks, with the hypothesis that tilting the participant’s head would make them more likely to use perceptual cues such as the frame and less likely to use gravity, thereby enhancing the frame effect
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