The fit of the linear model and the measured mean performance are shown in
Figure 2A. The model shown includes fixed effects for Task and Working Memory Load and a random intercept per participant. The average response precision on the tasks as predicted by the model is lower in the dual-task (
M = 6.37, 95% CI = 5.23–7.48) compared to the VWM only block (
M = 7.96,
95% CI = 6.86–9.11). Participants also got worse as the number of items increased (one item:
M = 8.92, 95% CI = 7.69–10.19, two items:
M = 6.98, 95% CI = 5.77 to 8.21, three items:
M = 5.60, 95% CI = 4.32–6.85).
Figure 2A visually indicates a slight overlap of the 95% CI between the two tasks and the working memory load, due to uncertainty in the estimate of the mean score that the other scores are centered around. Therefore, we also plotted the joint posterior distributions of Task and Working Memory Load in
Figure 2B. Negative values for Task show that the dual-task block was performed less accurately than the VWM only block. Similarly, negative values for VWM precision show the decrease of precision per added item. Two separate point clouds are plotted, one for the difference in precision between one and two items and the other for the difference in precision between two and three items. A Bayesian hypothesis test supports that the difference between dual task and VWM task is different from 0, BF
10 = 23.93, 95% CI = −2.72 to −0.53. Furthermore, the difference between one and two items retained as well as the difference between two and three items retained is different from 0, BF
10 = 23.43, 95% CI = −3.26 to −0.63 and BF
10 = 3.11, 95% CI = −2.69 to −0.08. Lastly, we found marginal evidence for no difference between one and two items retained versus two and three items retained, BF
01 = 1.49. Importantly, we hypothesized that the cost of making a saccade may be close to the cost of remembering an additional item, as saccade targets have been shown to be mandatorily encoded into VWM prior to a saccade. A Bayesian hypothesis test indicates that performing the saccade task elicits a similar load on working memory as remembering an additional item (BF
01 = 3.74, when comparing the dual-task versus the VWM task to one versus two items, BF
01 = 3.85 when comparing single task versus dual-task to two versus three items). These results agree with results from a frequentist paired
t test,
t(12) = −0.40,
p = 0.35. Thus, participants were significantly less precise in reporting the items when they were instructed to perform a concurrent saccade task. The loss of precision of performing a dual-task is comparable to remembering an extra item.