Brain Dynamics Related to Distracted Effect

5.1.1 Distracted Effect in Frontal Area

Frontal lobes are positioned in front of (anterior to) the parietal lobes (as showed in Fig. 5-1). The frontal lobes have been found in response to impulse control, judgment, language production, working memory, motor function, problem solving.

The reports of study were showed divided attention in frontal lobes [46] [47].

Parietal Lobe Parietal Lobe

Fig. 5-1: Picture showed the principle fissures and lobes cerebrum [55]. The blue part is the frontal lobe and the white area is the location of parietal lobe.

Fig. 4-23 showed the EEG response (ERSP) in the frontal component under 5 conditions. Compared with the ERSP of single task (case-4 and -5), there were higher total power in theta (5 ~7.8 Hz) band of dual tasks (case-1, -2, and -3). The phenomenon suggested that the dual tasks induced more event-related EEG activity in theta band, that was, subjects needed to consume more brain source to accomplish dual tasks at the same time. In a verbal n-back working memory paradigm, evoked theta activity (4-8 Hz) phase-locked to the visual stimulus was evidence in the parieto-occipital and frontal regions in all tasks. It is suggested that theta activity of EEG in the frontal area appears during concentrated performance of mental tasks in normal subjects and reflects attention processing [15]. During mental work load, the EEG process producing 5-7 Hz frontal midline theta activity. The process accounting for the EEG theta increase in midline frontal area during mental work load was separated from channel data into independent brain sources by ICA [45]. Therefore, the theta band increase together with the raising workload is associated with numerous processes such as mental work load, solving problem, encoding, or self

monitoring.

According to the evidences presented in previous studies, we could first prove that the subjects were distracted under dual-task conditions in the experiment.

Furthermore, we would like to compare the differences in specific bands among the three kinds of dual-task conditions, as shown in Fig. 4-24.

Compared with total power intensity among dual-task conditions, the total power of math-before-400ms was higher than that of the other dual-task cases. It was suggested that the maximal energy will be induced when the second task present following the other task is current under processing. In addition, since human visual sensory needs about at 300 ms to perceive stimulus (P300 activity [48]), 400 ms is sufficient for subject to process the first task. There was a processing task in brain first and subjects needed more brain source to manage the second task presented after the first task at 400 ms. Therefore, the total power in theta (5~7.8 Hz) band of math-400ms-deviation case was higher than that of the other dual-task cases. In the case of deviation-400ms-math, subjects just dealt with the simple deviation task first, and then processed the task of math. The theta power in deviaiton-400ms-math case was not higher than that of math-400ms-deviaiton case. The other fact was the onset of significant theta power presented, as shown in Fig. 4-24. It was clearly that the theta power increase presented most early in math-400ms-deviation than that in the other cases. The early phyasic theta band response in frontal regions primarily reflected the activation of neural networks involved in allocation of attention related to target stimulus [49].

We also found power increase in beta band (12.2 ~ 17 Hz) in all cases (as showed in Fig. 4-22). From the ERSP images, the patterns were time-locked to the onsets of the math. It is suggested that EEG changed due to a specific component of mental calculation. Significant differences were obtained in delta and theta band in

right posterior areas and in the beta band in frontal areas [50].

The presented evidence proved that when human faced a difficult task first and then the other tasks presented, it would not only be induced the faster attention-related activation, but also led to the maximal distraction effect in the experiment. The theta activity of EEG in the frontal area could be used as the index of distracted effect and distracted extent.

5.1.2 Distracted Effect in Mu Area

Mu rhythm (μ rhythm) is an EEG rhythm recorded usually from the motor cortex of the dominant hemisphere. It is also called aciform rhythm given the shape of the waveforms. It is a variant of normality, and it can be suppressed by a simple motor activity such as clenching the fist of the contra lateral side, or passively moved [51]

[52] [53]. Mu suppression is believed to be the electrical output of the synchronization of large portions of pyramidal neurons of the motor cortex which control the hand and arm movement when it is inactive.

According to the ERSP of single deviation and single math in Fig. 4-25, respectively, the mu suppression was caused mostly by subjects steering the wheel and pressing the bottoms (answer mathematical questions). It was obviously that the mu suppression caused by wheel steering is almost time-locked to the response onset.

The mu suppression caused by bottom press was present before the math reply.

Suggest that it involves motor planning to prepare to answer the math question [54].

As for in the dual-task cases, the mu suppression was mixed by the two main reasons, wheel steering and bottom press, and it was weaker in dual -task cases than that in single-task. Due to the more activation in dual-task cases in the frontal lobe, it may be

reasonable to infer that the math processing occupy more brain source in frontal lobe so that the less activation was induced in the motor area. However, it was difficult to find an index of distraction effect and distraction extent in motor area since the mixed/

undistinguishable activity.

5.1.3 Distracted Effect in Occipital Area

The occipital lobe is the visual processing center of the mammalian brain, containing most of the anatomical region of the visual cortex. The region specialized for different visual tasks, such as visuospatial processing, color discrimination and motion perception [55].

In our experiment, we also investigated the pattern inducing by visual stimulus.

In Fig. 4-28, power increase in low frequency accompanied the onset of math. From the ERP (Fig. 4-29) activity, we also found the pattern of P300 that involves the visual induced activity [56]. According to ERSP of the single math case, the alpha increase was time-locked to the response of math reply. The phenomenon is known as alpha rebound after a mental task being finished [57]. Compared with other cases with math, the alpha rebound power was maximal in the single math case. It is suggested that the subjects were able to concentrate on solving math task without other distraction in the single-math case. It is also suggested that perceptual switching by the button press showed characteristic occipital alpha and frontal theta band activity prior to a switch [59]. The alpha activity was specific to switch, the theta activity was generic to perceptual processing conditions. These results suggest that the ability to concentrate attentional effort on the task is responsible for the differences in perceptual switching rates.