Somatosensory P2 reflects resource allocation in a game task: Assessment with an irrelevant probe technique using electrical probe stimuli to shoulders Fumie Sugimoto ⁎, Jun'ichi Katayama Department of Psychological Science, Kwansei Gakuin University, Nishinomiya 662-8501, Japan abstract article info Article history: Received 19 July 2012 Received in revised form 13 January 2013 Accepted 16 January 2013 Available online 23 January 2013 Keywords: Resource Attention Mental workload ERP Probe technique The present study investigated whether event-related brain potentials (ERPs) elicited by task irrelevant somatosensory stimuli to the shoulders reflect the amount of processing resources allocated to a game task. In the experiment, electrical stimuli were presented to the right (or left) shoulder with a high probability (80%) and to the other shoulder with a low probability (20%) while participants were performing a driving simulation game. The deviant low-probability stimuli elicited somatosensory P2, and this P2 amplitude decreased when the task was difficult. The results show that the ERPs for somatosensory stimuli to the shoulders can reflect the amount of resources deployed even when the stimuli are ignored. This is a useful method for the evaluation of mental workloads in complex circumstances because it does not interfere with inputs of auditory or visual information or operations using the limbs. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Many psychophysiological studies have demonstrated that event- related brain potentials (ERPs) reflect the deployment of processing resources. This psychophysiological index has been utilized in studies of mental workloads to assess the relative quantity of processing, or attentional, resources that are required to perform a task. A dual task paradigm is one common method for this assessment. It is based on the established idea that processing resources are limited in quantity (Kahneman, 1973; Norman and Bobrow, 1975). Because of this limitation, when more resources are allocated to a primary task, there is lower task performance as well as smaller responses of ERPs in a secondary task. Previous studies have shown that ERPs elicited in a secondary task provide an indicator of the amount of resources deployed in the primary task (e.g., Wickens et al., 1983). In that study, for example, auditory stimuli (e.g., tones) were presented while participants were working on the main task. Participants were required to re- spond to the tones at the same time. Under such a dual task condition, performance on the secondary task and responses of ERPs for the auditory stimuli were reduced when the task difficulty was high com- pared to when it was low. However, although the dual task paradigm has been employed widely, there has been criticism that performing a secondary task could interfere with participants' concentration in the primary task. To resolve this concern, an irrelevant probe technique was proposed as a method that would not cause interference. The irrelevant probe technique evaluates the amount of resources allocated to a task by assessing the response to a task irrelevant probe stimuli (see Papanicolaou and Johnstone, 1984, for review). In this method, people perform a task while ignoring probe stimuli presented at the same time. It has been shown that several ERP components elic- ited by the ignored stimuli reflect the amount of resources utilized. For example, Allison and Polich (2008) presented auditory probe stimuli while their participants were performing a computer shooting game. Pure tones were presented in a single-stimulus paradigm in which the inter-stimulus interval of the tones randomly changed. The results showed that even when no response was required for the auditory stimuli (i.e., the ignore condition), the amplitude of most ERP compo- nents elicited by the tones decreased with an increase in task difficulty. Allison and Polich's study indicated that the ERPs produced by task- irrelevant auditory stimuli reflect the amount of resources deployed, which vary with task difficulty. The study demonstrated the utility of the method with auditory probe stimuli. However, there are still issues about which modality would be most reasonable to use for the probe stimuli in various circumstances. Al- though many other studies have employed auditory stimuli as probe stimuli (e.g., Kramer et al., 1995; Sirevaag et al., 1993), auditory is one of the most utilized modalities in real-world activities. This raises the possibility that auditory probe stimuli could interfere with task- relevant sensory information. For example, in situation where people are engaged in the detection of subtle changes in a sound, auditory stimuli overlapping with the sound could prevent them from getting the proper signal. An equivalent situation exists for visual modalities. For this reason, another modality could be more appropriate for probe stimuli in various circumstances. We consider that the somatosensory modality could be appropriate generally since it does not interfere International Journal of Psychophysiology 87 (2013) 200–204 ⁎ Corresponding author. Tel./fax: +81 798 54 6209. E-mail addresses: fsugimoto@kwansei.ac.jp (F. Sugimoto), jkatayama@kwansei.ac.jp (J. Katayama). 0167-8760/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijpsycho.2013.01.007 Contents lists available at SciVerse ScienceDirect International Journal of Psychophysiology journal homepage: www.elsevier.com/locate/ijpsycho