Abstract
Antisaccades entail decoupling the spatial relations between stimulus and response and executing a saccade to a target’s mirror-symmetrical location. The indirect spatial relations require that a relative target percept supports antisaccade sensorimotor transformations. Here, we sought to identify whether the percept supporting antisaccades results in a respective over- and undershooting bias for the near and far targets within a stimulus-set (i.e., oculomotor range effect hypothesis) or renders an eccentricity-specific bias based on a statistical summary of the individual target percepts in a stimulus-set (i.e., perceptual averaging hypothesis). Antisaccades (and complementary prosaccades) were completed in separate blocks (i.e., proximal and distal) that contained an equal number of target eccentricities, but differed with respect to their magnitudes. The proximal block included eccentricities of 3.0°, 5.5°, 8.0°, 10.5°, and 13.0°, whereas the distal block included eccentricities of 10.5°, 13.0°, 15.5°, 18.0°, and 20.5°. The proximal block showed that antisaccade amplitudes to the central target (8.0°) did not elicit a reliable bias, whereas the block’s ‘near’ (3.0° and 5.5°) and ‘far’ (10.5° and 13.0°) targets produced an over- and undershooting bias, respectively. Notably, however, the distal block showed a reliable—and large magnitude—undershooting bias for the central target (i.e., 15.5°): a bias that generalized to each target within the block. Taken together, results for the proximal and distal blocks are incompatible with the range effect hypothesis. Instead, results indicate that the visual percept supporting antisaccades is based on the statistical summary of the range of target eccentricities within a stimulus-set (i.e., perceptual averaging). Moreover, perceptual averaging represents a parsimonious basis by which the oculomotor system can specify sensorimotor transformations via non-veridical (i.e., relative) visual information.
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Notes
We recognize that Kapoula’s (1985) work reported that prosaccades elicit a range effect. Importantly, however, Gillen et al’s (2013) re-examination of Kapoula’s data provides no evidence of a reliable range effect. As well, Gillen et al’s empirical findings provide direct evidence that prosaccades reliably undershot veridical target location independent of the magnitude and range of target eccentricities from within the block in which they were performed. In other words, Gillen et al’s findings support extensive evidence that prosaccades are governed by an invariant oculomotor strategy that minimizes saccade flight time (Harris 1995) and/or the energy requirements of the task (Becker 1989).
Visual space (left, right) was included as an initial variable in our initial ANOVA model; however, it did not elicit a significant effect or interaction Fs < 1.72 (see also Gillen et al. 2013; Heath et al. 2011; Weiler and Heath 2014). As such, visual space was included as a collapsed factor in our main analyses.
We used Loftus and Masson’s (1994) method for computing confidence intervals in a within-participant design.
Within-participant confidence intervals were computed separately for proximal and distal blocks via the mean squared error term for task (Loftus and Masson 1994). For that reason, the confidence intervals for proximal block pro- and antisaccades are the same as the confidence intervals for distal block pro- and antisaccades.
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Supported by a Grant from the Natural Sciences and Engineering Research Council of Canada and Major Academic Development Fund and Faculty Scholar Awards from the University of Western Ontario.
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Gillen, C., Heath, M. Perceptual averaging governs antisaccade endpoint bias. Exp Brain Res 232, 3201–3210 (2014). https://doi.org/10.1007/s00221-014-4010-1
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DOI: https://doi.org/10.1007/s00221-014-4010-1