Divide And Conquer: The Effect Of Neural Functional Segregation On Task-switching Performance. - Info and Reading Options

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It is a very robust finding that certain cognitive functions are strongly lateralized in the human brain, eliciting predominantly right hemispheric or left hemispheric activation. The most extensively studied (typically left) lateralized function is language (Knecht et al., 2000; Willemin et al., 2016). Some typical right lateralized functions include (holistic) face processing (Calvo & Beltrán, 2014) and spatial attention (Fink et al., 2000; Jansen et al., 2006). In this study we are testing a prominent hypothesis on the putative advantage of functional cerebral asymmetries (FCA's), namely the enhancement of parallel processing (Levy, 1969; Rogers, 2000). The exact origins of functional segregation in the human brain remain a debated topic, the theory on lateralization and parallel processing argues that by predominantly engaging one hemisphere for a certain set of functions, and the other for another set of functions, the workload can be divided over the two cerebral hemispheres in order to allow humans to more efficiently engage in multitasking. One would expect that parallel processing is better when the two tasks can be executed by different hemispheres, as the workload can be divided. This study aims to answer the question: ‘Are humans better at multitasking (task switching) when both tasks are predominantly executed by opposing hemispheres?’ As we will use task switching in this experiment, we will refer to the situation where both tasks are processed by the same hemisphere as ‘ipsilateral task switching’ or ‘ipsilateral switches’. We will refer to the situation in which the two tasks are processed by opposing hemispheres as ‘contralateral task switching’ or ‘contralateral switches’. In addition to differences between ipsilateral and contralateral switches, we will investigate a potential difference between contralateral homotopic switches (switching from a task that elicits activity in one hemisphere to a task that elicits activity in the same area but in the opposite hemisphere) and contralateral heterotopic switches (switching from a task that elicits activity in one hemisphere to a task that activates a different area in the opposite hemisphere) In order to answer these research questions we will use a task switching paradigm that includes three tasks: 1. A lexical decision (LD) task, which in right handed participants, predominantly elicits neural activity in the fusiform area of the left hemisphere - visual word form area VWFA. (Cohen et al., 2000; Cai et al., 2008) 2. A face processing (FP) task, which in right handed participants, predominantly elicits neural activity in the fusiform area of the right hemisphere - fusiform face area FFA, contralateral homologue of VWFA. (Tong et al., 2000) 3. A landmark (LM) task, which in right handed participants, predominantly elicits neural activity in the posterior parietal cortex of the right hemisphere (Seydell-Greenwald et al., 2019). With these three tasks we will measure performance in 6 conditions: 1. Pure LD condition: this condition includes LD task trials only (no other tasks or task switching) 2. Pure FP condition: this condition includes face recognition trials only (no other tasks or task switching) 3. Pure LM condition: this condition includes landmark task trials only (no other tasks or task switching) 4. LD_LM condition: This condition includes both lexical decision trials as landmark task trials. Participants have to switch between both tasks at unexpected moments. 5. LD_FP condition: This condition includes both lexical decision trials as face recognition trials. Participants have to switch between both tasks at unexpected moments. 6. LM_FP condition: This condition includes both landmark task trials as face recognition trials. Participants have to switch between both tasks at unexpected moments. To provide an answer to our first research question (Is there less interference when both tasks preferentially engage a different hemisphere?) we will compare interference due to task switching in the face recognition task of the LD_FP condition with interference in the face recognition task in the LM_FP condition. Since face recognition is a typically right lateralized function, as well as spatial attention that is required for the landmark task, the LM_FP condition will provide us with a measure of interference that results from ipsilateral task switching. The lexical decision task on the other hand typically elicits strong left lateralized activity, therefore the LD_FP condition will provide us with a measure of interference that results from contralateral task switching. To answer our second research question (is there a difference between contralateral homotopic and contralateral heterotopic switches?) we will compare interference in the lexical decision task in the LD_LM condition with interference in the lexical decision task in the LD_FP condition. Since face recognition primarily activates the FFA, the contralateral homologue of the VWFA (as activated by the lexical decision task), this condition will provide us with a measure of interference that results from contralateral homotopic switches. As the landmark task primarily activates right parietal areas, this condition will provide us with a measure of interference that results from contralateral heterotopic switches. Should we observe the hypothesised differences, then we should rule out the possible explanation of pre-existing differences in task difficulty. For example, greater interference due to task switching in the LM_FP condition compared to the LD_FP condition might be interpreted as evidence that parallel processing is facilitated when both tasks are preferentially processed by a different hemisphere. An alternative explanation might be that the landmark task is simply more challenging than the lexical decision task and hence causes more interference. To address this potential problem, we will verify that the three tasks are similar in difficulty by comparing performance in the three ‘pure’ conditions.

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