Neural codes for somatosensory two-point discrimination in inferior parietal lobule: An fMRI study Kosuke Akatsuka, a,b,c, ⁎ Yasuki Noguchi, a Tokiko Harada, d Norihiro Sadato, b,d and Ryusuke Kakigi a,b,e a Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki, 444-8585, Japan b Department of Physiological Sciences, School of life Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa, Japan c Japan Society for the Promotion of Science, Tokyo, Japan d Department of Cerebral Research, National Institute for Physiological Sciences, Okazaki, Japan e RISTEX, Japan Science and Technology Agency, Tokyo, Japan Received 2 July 2007; revised 27 November 2007; accepted 5 December 2007 Available online 23 December 2007 This is the first functional magnetic resonance imaging (fMRI) study to investigate the hemodynamic response related to somatosensory spatial discrimination, so-called two-point discrimination. During scanning, we examined two discrimination tasks using four types of electrical stimuli applied to one or two points with strong or weak intensity on the right and left forearm, respectively. In the two-point discrimination task (TPD), subjects reported whether they thought the stimulus was applied to one point or two. In the intensity discrimination task (ID), subjects were required to judge whether the stimulus was strong or weak. In each task, they pressed a button to report their choice. Comparing TPD with the control, we found activated regions in the inferior parietal lobule (IPL) around the supramarginal gyrus (SMG) (Brodmann’s area 40) and anterior cingulate cortex (ACC). These areas were significantly activated irrespective of the forearm stimu- lated. Comparing ID with the control, there were no significantly activated regions. By comparing the TPD and ID, we identified that the left IPL was significantly activated, specifically in TPD, irrespective of the forearm stimulated. In contrast, there were no significantly activated regions in the ID task. Therefore, the left IPL is considered to play an important role in two-point discrimination. © 2007 Elsevier Inc. All rights reserved. Introduction The discrimination of stimuli is necessary in daily life and involves the peripheral and central nervous system. Recent fMRI studies involved different brain systems, ranging from unimodal somatosensory to higher order cognitive brain areas, and evolved with different time windows (Stoeckel et al., 2003; Kaas et al., 2007; Pleger et al., 2006) using various discrimination tasks, such as frequency (Pleger et al., 2006), grating orientation (Kitada et al., 2006; Van Boven et al., 2005), or Braille tactile (Harada et al., 2004) in humans. However, the regions activated are different with each task. For example, Li Hegner et al. (2007) found that blood- oxygen-level-dependent (BOLD) adaptation is initiated in the contralateral primary somatosensory cortex (SI) and superior temporal gyrus (STG) using a tactile frequency discrimination task. On the other hand, Zhang et al. (2005) reported that a tactile grating orientation task activated regions around the postcentral sulcus and intraparietal sulcus (IPS). However, to our knowledge, there have been no fMRI studies focusing on the somatosensory two-point discrimination (TPD) task that required the discrimina- tion of stimuli whether applied to one or two points. Therefore, the TPD task would help us to reveal which cortical regions are activated by discriminating simultaneous stimuli at different locations. TPD is an important and frequently used clinical test of the higher function of somatosensory perception. TPD is based on the slowly adapting type I afferent fiber system, one of four afferent fiber systems in the skin. Our group investigated the cortical cognitive processes during TPD in a reaction time task (Tamura et al., 2003, 2004) and suggested the presence of a cortical cognitive process in TPD. Therefore, TPD is considered to reflect cognitive functions taking place in the central nervous system, but its underlying mechanisms have still not been clarified. One major problem with this test is that it is very subjective, being dependent on the examiners’ skills and subjects’ reactions. To solve these problems, we recently reported automatic detection systems for somatosensory spatial and temporal dis- crimination using electroencephalography (EEG) and magnetoen- www.elsevier.com/locate/ynimg NeuroImage 40 (2008) 852 – 858 ⁎ Corresponding author. Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki, 444-8585, Japan. Fax: +81 564 52 7913. E-mail address: akatuka@nips.ac.jp (K. Akatsuka). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2007.12.013