Abstract
It has been suggested that the evidence used to support a decision to move our eyes and the confidence we have in that decision are derived from a common source. Alternatively, confidence may be based on further post-decisional processes. In three experiments, we examined this. In Experiment 1, participants chose between two targets on the basis of varying levels of evidence (i.e., the direction of motion coherence in a random dot kinematogram). They indicated this choice by making a saccade to one of two targets and then indicated their confidence. Saccade trajectory deviation was taken as a measure of the inhibition of the non-selected target. We found that as evidence increased so did confidence and deviations of saccade trajectory away from the non-selected target. However, a correlational analysis suggested they were not related. In Experiment 2, an option to opt-out of the choice was offered on some trials if choice proved too difficult. In this way, we isolated trials on which confidence in target selection was high (i.e., when the option to opt-out was available but not taken). Again saccade trajectory deviations were found not to differ in relation to confidence. In Experiment 3, we directly manipulated confidence, such that participants had high or low task confidence. They showed no differences in saccade trajectory deviations. These results support post-decisional accounts of confidence: evidence supporting the decision to move the eyes is reflected in saccade control, but the confidence that we have in that choice is subject to further post-decisional processes.
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Notes
In contrast to McSorley and McCloy (2009), we did not find an effect of choice and level of evidence on saccade landing position deviation. They showed similar displays to those employed here and used the same task but found, similarly to saccade trajectory deviation, that landing positions were deviated away from the non-selected target and that this deviation increased as the amount of evidence supporting target choice also increased. There are several possible reasons why this may have been case, the most likely being that the targets are relatively far apart (45°) and saccades are generally only consistently affected by competing stimuli within about 30° of the target (Findlay and Brown 2006; McSorley and Findlay 2003; McSorley et al. 2009; Ottes et al. 1985; Walker et al. 1997). This is especially so at the relatively longer latencies elicited in these choice tasks. In contrast, saccade trajectories continue to be affected by distracting stimuli at distances of up 120 deg from the target (McSorley et al. 2009). We would suggest from this that the online control of saccade trajectory is much more sensitive to the presence of the other competing stimuli than is the final saccade landing position.
Unlike in Experiment 1, and McSorley and McCloy (2009), latency was not found to decrease as motion coherence increased. We suggest this to be as a result of the confidence manipulation and possibly due to the fact that both of these previous experiments included a higher motion coherence level (64 %) that was not present in this experiment. Removing this level, which gives the most evidence for one of the options, thereby making the decision easier, may have made the usual decrease in saccade latency less evident.
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McSorley, E., Lyne, C. & McCloy, R. Dissociation between the impact of evidence on eye movement target choice and confidence judgements. Exp Brain Res 232, 1927–1940 (2014). https://doi.org/10.1007/s00221-014-3884-2
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DOI: https://doi.org/10.1007/s00221-014-3884-2