The Effects of Extensive Single Task and Dual Task Training on the Neural Bases of Visual Object Categorization: Escaping the Frontal Bottleneck

Abstract

Perceptual learning of visual categorization has been described as a two-stage model, wherein the tuning of representations in visual areas is sharpened to the physical attributes of the stimuli and dorsolateral prefrontal areas develop task-dependent category tuning. Studies have also shown that the lateral prefrontal cortex is the source of a decisional bottleneck, limited to serial task processing, which implies a serious limitation to multitasking. However, some studies have shown the ability to multitask despite this bottleneck. Here we tested the hypothesis that extensive experience with a categorization task in single task (Study 1) and dual task (Study 2) scenarios leads to a shift in the task circuitry out of the frontal bottleneck to more posterior brain regions, accompanied by a concomitant decrease in dual task interference. We used a mobile app to attain >30,000 training trials per participant. We then assessed training related changes in the underlying neural circuitry using both fMRI and EEG rapid adaptation (RA) techniques, and changes in task automaticity using traditional lab-based behavioral testing. In Study 1, EEG-RA showed that extensive single task categorization training led to a categorical signal arising ~100ms earlier compared to initial training and MRI-RA showed numerous categorical clusters outside of the frontal bottleneck, including in left posterior parietal cortex (PPC) and ventral occipitotemporal cortex. As predicted, this posterior shift in the underlying task circuitry was accompanied by a decreased dual task interference effect. In Study 2, after extensive dual task categorization training MRI-RA revealed categorical clusters in bilateral PPC alone. The EEG-RA results were in good agreement, revealing only one posterior categorical cluster at around 250ms. A comparison of the single task and dual task trained participants revealed a behavioral double dissociation whereby the single task trained subjects where more efficient at processing the category of single stimuli, but the dual task trained subjects showed less dual task interference on categorization. Overall, extensive training in both single and dual task conditions lead to posterior shifts in the underlying categorization circuitry and, with that, improved dual tasking abilities.