The Backward Crosstalk Effect Does Not Depend on the Degree of a Preceding Response Conflict

Variable, sometimes absent, but never negative: Applying multilevel models of variability to the backward crosstalk effect to find theoretical constraints

When performing two tasks at the same time, the congruency of the second task's features influences performance in the first task. This is called the backward crosstalk effect (BCE), a phenomenon that influences both theories of binding and of dual-task capacity limitations. The question of whether the BCE is found in all participants at all times is relevant for understanding the basis of the effect. For example, if the BCE is based on strategic choices, it can be variable, but if it is automatic and involuntary, it should never vary in whether it is present or not. Variability in observed BCE sizes was already documented and discussed when the group average effect was first reported (Hommel, 1998). Yet the theories discussed at the time did not motivate a more direct analysis of this variability, nor did the readily available statistical tools permit it. Some statistical approaches recently applied in cognitive psychology allow such a variability-focused analysis and some more recent theoretical debates would benefit from this as well. We assessed the variability of the BCE as well as a BCE-like free-choice congruency effect by applying a Bayesian multilevel modeling approach to the data from a dual-tasking experiment. Trials consisted of a two- and a four-response task. We manipulated which task was presented first and whether the response to the four-choice task was free or forced choice. RT data were best predicted by a model in which the BCE is zero in part of the population and drawn from a normal distribution truncated at zero (and thus always positive) in the rest of the population. Choice congruency bias data were best predicted by a model assuming this effect to be drawn from a normal distribution truncated at zero (but, in contrast to the RT data, without the subset of the population where it is zero). The BCE is not an inflexible and universal phenomenon that is directly linked to an inherent structural trait of human cognition. We discuss theoretical constraints implied by these results with a focus on what we can infer about the traits of the factors that influence BCE size. We suggest that future research might add further major constraints by using multi-session experiments to distinguish between-person and within-person variability. Our results show that the BCE is variable. The next step is understanding along which axes it is variable and why it varies.

Opposing influences of global and local stimulus-hand proximity on crosstalk interference in dual tasks

In contrast to traditional dualistic views of cognition, visual stimulus processing appears not independent of bodily factors such as hand positioning. For example, reduced crosstalk between two temporally overlapping tasks has been observed when the hands are moved into the attentional window alongside their respective stimuli (i.e., establishing global stimulus-hand proximity). This result indicates that hand-specific attentional processing enhancements support a more serial rather than parallel processing of the two tasks. To further elucidate the nature of these processing modulations and their effect on multitasking performance, the present study consisted of three interrelated crosstalk experiments. Experiment 1 manipulated global stimulus-hand proximity and stimulus-effect proximity orthogonally, with results demonstrating that hand proximity rather than effect proximity drives the crosstalk reduction. Experiment 2 manipulated the physical distance between both hands (i.e., varying local stimulus-hand proximity), with results showing weak evidence of increased crosstalk when both hands are close to each other. Experiment 3 tested opposing influences of global and local stimulus-hand proximity as observed in Experiment 1 and 2 rigorously within one experiment, by employing an orthogonal manipulation of these two proximity measures. Again, we observed slightly increased crosstalk for hands close to each other (replicating Experiment 2); however, in contrast to Experiment 1, the effect of global stimulus-hand proximity on the observed crosstalk was not significant this time. Taken together, the experiments support the notion of hand-specific modulations of perception-action coupling, which can either lead to more or less interference in multitasking, depending on the exact arrangement of hands and stimuli.

Temporal aspects of two types of backward crosstalk in dual-tasks: An analysis of continuous mouse-tracking data

A common explanation for processing limitations in dual-tasking is the existence of a bottleneck during response selection, meaning that the selection of responses can only occur serially for different tasks. However, a large body of data shows that features of a (secondary) Task 2 can already influence the processing of a (primary) Task 1. Such effects are referred to as backward crosstalk effects (BCEs). In the present study, two types of such BCEs were investigated: the compatibility-based BCE, which depends on the dimensional (often spatial) overlap between task features, and a BCE based on a go/no-go task in Task 2 (no-go BCE). Joining a line of research that suggests different mechanisms for these two types of BCEs, we investigated them using a mouse-tracking setup. Time continuous analyses revealed that the compatibility-based BCE triggered a spatial activation of the Task 2 response early during Task 1 processing, whereas the no-go BCE triggered an inhibitory effect in the case of a no-go Task 2, which spills over to Task 1 execution. This occurred, however, earlier in the time course than expected. The results are discussed with regard to recent models of dual-task processing.

No reduction of between-task interference in a dual-task with a repeating sequence of SOAs

A frequent observation in dual-tasking is that spatially or conceptually (in)compatible Task 2 response features can interfere with responses in Task 1 (backward crosstalk effect; BCE). Such between-task interference is, at least to some degree, under strategic control. It has been shown that the size of the BCE can be modulated by instructions, contextual regularities, recent experience of conflict, and motivational factors. Especially large temporal task overlap (i.e., short stimulus onset asynchrony, SOA) represents a condition of potentially high levels of between-task interference. Accordingly, Fischer and Dreisbach (2015) showed that specific stimuli, associated with mostly short SOAs, were able to reduce the size of the BCE. In the present study, we investigated whether a regular sequence of SOAs can also be used for contextual regulation of the BCE. In a dual-task with spatially (in)compatible hand- and foot-responses, we implemented a repeating sequence of three SOAs. If participants learned this sequence and used it for task shielding, the BCE should decrease over time in the sequence blocks, but should increase in a subsequent random block. However, this prediction was not supported in two experiments (N = 32 each). Instead, the size of the BCE was constant across all blocks (BFs₁₀ < 1 for the respective interactions). This is an important result, as it points at the necessity to discover the appropriate conditions allowing implicit SOA sequence learning and to further investigate whether or how the resulting implicit sequence knowledge can serve shielding against between-task interference.

Two types of between-task conflict trigger respective processing adjustments within one dual-task

In dual tasking, two different kinds of between-task conflict occur. Because in both cases, Task 2 characteristics affect Task 1 performance, they are commonly referred to as backward crosstalk effects (BCE): One with a conflict at the response selection stage when Task 1 and Task 2 have dimensional overlap (the compatibility-based BCE) and one with a conflict at the motor execution stage when response inhibition resulting from a Task 2 no-go-trial interferes with simultaneous response execution in Task 1 (the no-go BCE). Recent research suggests that these BCEs differ not only in their underlying cognitive processes, but also in how cognitive control is regulated. Here, we investigated whether both can be produced in a single dual-task set up and whether they trigger their respective processing adjustments (i.e., a sequential modulation). In three experiments, participants categorized numbers as smaller or larger than 5 in Task 1. In Experiments 1 and 2, numbers were responded to irrespective of numerical size (go-response) as Task 2. Further, dimensional overlap was provided by (non)corresponding size information in both stimuli, which was strengthened in Experiment 2 by presenting S1 and S2 in the same/different color in compatible/incompatible trials, respectively. In Experiment 3, participants were required to perform a number-size categorization also in Task 2, establishing strong dimensional overlap by activating the same or different response categories in both tasks. In all three experiments, the number 5 served as the no-go stimulus in Task 2 to induce a no-go BCE on Task 1. By and large, our results show that both types of between-task conflicts not only occur within the same type of BCE, but they also trigger their respective sequential modulation.