This experiment focused on the effect of the eye movement strategy manipulation on tracking accuracy. Accuracy was measured as the percentage of trials in which all the targets were correctly selected at the end of the trial, a result with a chance level of 0.45% (1 correct out of 220 possibilities of choosing 3 of 12 dots). Average performance for the first session, in which participants were free to use any eye movement strategy they chose, was 80.4%. The accuracy of free-looking trials in the second session was 82.5%, which did not differ significantly from the first session data (
F(1, 24) = 2.0,
ns). Performance was lower when participants were instructed to pursue a specific eye movement strategy, regardless of whether this was target-looking (57.0%) or center–target switching (76.7%;
Figure 4A). An ANOVA on the second session accuracy data confirmed that there was a significant main effect of instructions (
F(2, 48) = 52.6,
p < 0.01). Adding the constraint of a specific eye movement strategy did reduce accuracy for both center–target switching (
t(24) = 2.1,
p < 0.05) and target-looking (
t(24) = 10.0,
p < 0.01). However, there was a benefit from engaging in center–target switching over the target-looking strategy (
t(24) = 7.6,
p < 0.01). This result clearly supports the notion that center-looking is valuable to the tracking process.
As in the previous experiments, we conducted the region of interest analysis to calculate the percentages of gaze overlap for each region. Similar to those results, the data from session 1 showed a significant effect of region (
F(2, 48) = 181.9,
p < 0.01), with a higher percentage of time that gaze overlapped the center (40.2%) than the targets (12.7%) and distractors (3.8%) on average. Data from the second session showed a significant effect of region (
F(2, 48) = 377.7,
p < 0.01) and instructions (
F(2, 48) = 21.9,
p < 0.01), as well as a significant interaction between the two (
F(4, 96) = 95.6,
p < 0.01). Simple effects analysis for each region showed a significant effect of instructions for each one (Center:
F(2, 48) = 74.1,
p < 0.01; Targets:
F(2, 48) = 108.5,
p < 0.01; Distractors:
F(2, 48) = 12.3,
p < 0.01). These differences across instructions can be seen in
Figure 4B. Consistent with instructions, participants looked at the center less when given target-looking instructions than when given center–target switching or no instructions. Complementarily, participants looked at targets more when given target-looking instructions than the other conditions.
It is possible that participants' compliance with the instructed strategies varied across trials, potentially interacting with tracking accuracy. To quantify compliance with instructions, we calculated the number of trials in which participants modified their eye movement behavior consistent with instructions. For a trial to be counted as a compliant target-looking trial, a participant had to have looked at the targets more than he or she did, on average, in session 1, and looked at the center less than in session 1. For a trial to be considered compliant with center–target switching, a participant had to have increased the time of center-looking and decreased the time of target-looking from session 1. Participants were significantly more compliant with target-looking instructions (71.6%) than center–target switching instructions (36.8%; F(1, 24) = 45.7, p < 0.01). This is most likely due to the fact that participants were spending a lot more time center-looking when given free-looking instructions, so the criterion for compliance was harder to reach. However, both compliant and incompliant trials show higher accuracy for center–target switching relative to target-looking instructions (76.0% vs. 53.3% for compliance and 80.3% vs. 61.0% for incompliance). An ANOVA of these data shows that the main effect of instructions was significant ( F(1, 24) = 34.7, p < 0.01), and the effect of compliance was marginal ( F(1, 24) = 3.1, p = 0.09), but the interaction was not significant ( F(1, 24) < 1). Thus, even when the data were reduced to only those trials that exhibit signs of following the strategy indicated, there was still a clear benefit seen in tracking performance due to center-looking.