Parallel and serial task processing in the PRP paradigm: a drift-diffusion model approach

Can frequent long stimulus onset ansynchronies (SOAs) foster the representation of two separated task-sets in dual-tasking?

Recent findings suggest that in dual-tasking the elements of the two tasks are associated across tasks and are stored in a conjoint memory episode, meaning that the tasks are not represented as isolated task-sets. In the current study, we tested whether frequent long stimulus onset ansynchronies (SOAs) can foster the representation of two separated task-sets thereby reducing or even hindering participants to generate conjoint memory episodes-compared to an integrated task-set representation induced by frequent short SOAs. Alternatively, it is conceivable that conjoint memory episodes are an inevitable consequence of presenting two tasks within a single trial. In two dual-task experiments, we tested between consecutive trials whether repeating the stimulus-response bindings of both tasks would lead to faster responses than repeating only one of the two tasks' stimulus-response bindings. The dual-task consisted of a visual-manual search task (VST) and an auditory-manual discrimination task (ADT). Overall, the results suggest that, after processing two tasks within a single trial, generating a conjoint memory episode seems to be a default process, regardless of SOA frequency. However, the respective SOA frequency affected the participants' strategy to group the processing of the two tasks or not, thereby modulating the impact of the reactivated memory episode on task performance.

Response-code conflict in dual-task interference and its modulation by age

Difficulties in performing two tasks at once can arise from several sources and usually increase in advanced age. Tasks with concurrent bimodal (e.g., manual and oculomotor) responding to single stimuli consistently revealed crosstalk between conflicting response codes as a relevant source. However, how this finding translates to unimodal (i.e., manual only) response settings and how it is affected by age remains open. To address this issue, we had young and older adults respond to high- or low-pitched tones with one (single task) or both hands concurrently (dual task). Responses were either compatible or incompatible with the pitch. When responses with the same level of compatibility were combined in dual-task conditions, their response codes were congruent to each other, whereas combining a compatible and an incompatible response created mutually incongruent (i.e., conflicting) response codes, potentially inducing detrimental crosstalk. Across age groups, dual-task costs indeed were overall highest with response-code incongruency. In these trials, compatible responses exhibited higher costs than incompatible ones, even after removing trials with strongly synchronized responses. This underadditive cost asymmetry argues against mutual crosstalk as the sole source of interference and corroborates notions of strategic prioritization of limited processing capacity based on mapping-selection difficulty. As expected, the effects of incongruent response codes were found to be especially deleterious in older adults, supporting assumptions of age-related deficits in multiple-action control at the level of task-shielding. Overall, our results suggest that aging is linked to higher response confusability and less efficient flexibility for capacity sharing in dual-task settings.

Limited Pairings of Electrical Micro-stimulation of the Ventral Tegmental Area and a Visual Stimulus Enhance Visual Cortical Responses

Previous studies demonstrated that pairing a visual stimulus and electrical micro-stimulation of the ventral tegmental area (VTA-EM) for multiple days is sufficient to induce visual cortical plasticity and changes perception. However, a brief epoch of VTA-EM-stimulus pairing within a single day has been shown to result in a behavioral preference for the paired stimulus. Here, we investigated whether a brief single-day session of VTA-EM-stimulus pairings is sufficient to induce changes in visual cortical responses. We examined macaque posterior inferior temporal (PIT) cortex because previous studies demonstrated response changes after VTA-EM stimulus pairing in that area. Multi-unit recordings in PIT were interleaved with VTA-EM-stimulus pairing epochs. During the short VTA-EM-stimulus pairing epochs (60 pairings), one image (fractal) was paired with VTA-EM (STIM) whereas another, unpaired fractal was presented as control. Two other fractals (dummies) were presented only during the recordings. The difference in response between the STIM and control fractals already increased after the first VTA-EM-stimulus pairing epoch, reflecting a relative increase of the response to the STIM fractal. However, the response to the STIM fractal did not increase further with more VTA-EM-stimulus pairing epochs. The relative increase in firing rate for the paired fractal was present early in the response, in line with a local/ bottom-up origin. These effects were absent when comparing the responses to the dummies pre- and post-VTA-EM. This study shows that pairing a visual image and VTA-EM in a brief single-day session is sufficient to increase the response for the paired image in macaque PIT.

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.

Effects of task probability on prioritized processing: Modulating the efficiency of parallel response selection

Four experiments investigated the extent to which a limited pool of resources can be shared between different tasks performed simultaneously when it is efficient to do so. The experiments used a prioritized processing paradigm, in which stimuli for both a primary task and a background task were presented in each trial. If the primary-task stimulus required a response in a trial, participants made only that response. If the primary-task stimulus did not require a response, participants responded to the background task. The main manipulation was the relative probability that a response would be required to the primary versus background task. In some blocks, the majority of trials required responses to the primary task (Experiments 1 and 2: 80%; Experiments 3 and 4: 60%), whereas in other blocks the majority required responses to the background task. Background-task responses were substantially faster in blocks where they were more likely to be required, consistent with the idea that more capacity was allocated to them in these blocks. Backward compatibility effects on primary-task responses and stimulus-onset asynchrony effects on background-task responses provided further evidence of greater capacity allocation to the background task when there was a higher probability of responding to it. The results support the view that two tasks can be processed in parallel, with resources divided between them, when it is efficient to do so.