168 Learning and memory Subsequent memory-dependent EEG h correlates to parahippocampal blood oxygenation level-dependent response Naoyuki Sato a,b , Takashi J. Ozaki b , Yoshiaki Someya c,d , Kimitaka Anami c,e , Seiji Ogawa c,f , Hiroaki Mizuhara b,g and Yoko Yamaguchi b The 4–12 Hz (h rhythm)-dependent neural dynamics play a fundamental role in the memory formation of the rat hippocampus. Although the power of human scalp electroencephalography h (EEG h) is known to be associated with a hippocampus-dependent memory encoding, it remains unclear whether the human hippocampus uses h rhythm. In this study, we aim to identify the scalp EEG h-related neural regions during memory encoding by using a simultaneous EEG–functional magnetic resonance imaging recording. We showed that the parahippocampal and the medial frontal and posterior regions were significantly correlated to subsequent memory-dependent EEG h power. This evidence suggests that the human parahippocampal region and associated structures use h rhythm during hippocampal memory encoding as in rodents. NeuroReport 21:168–172  c 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins. NeuroReport 2010, 21:168–172 Keywords: object–place associative memory, subsequent memory effect, simultaneous EEG–fMRI recording, theta phase coding a Department of Complex Systems, School of Systems Information Science, Future University–Hakodate, Hakodate, Hokkaido, b Laboratory for Dynamics of Emergent Intelligence, RIKEN Brain Science Institute, Wako-shi, Saitama, c Ogawa Laboratory for Brain Function Res, Hamano Life Science Research Foundation, Tokyo, d Global COE Program Center for Advanced Research on Logic and Science, Keio University, e Musashino Mental Hospital, Saitama, f Kansei Fukushi Research Center, Touhoku Fukushi University, Sendai and g Department of Intelligence Science and Technology, Graduate School of Informatics, Kyoto University, Kyoto, Japan Correspondence to Naoyuki Sato, PhD, Department of Complex Systems, School of Systems Information Science, Future University–Hakodate, 116-2 Kamedanakano, Hakodate, Hokkaido 041-8655, Japan Tel: + 81 138 34 6235; fax: + 81 138 34 6301; e-mail: satonao@fun.ac.jp Naoyuki Sato and Takashi J. Ozaki contributed equally to this study Received 27 July 2009 accepted 18 August 2009 Introduction The human hippocampus is known to be involved in episodic memory formation as shown in functional neuroimaging [1,2] and by lesion studies [3,4], although its neural dynamics remain an open question. The power of human scalp electroencephalography y (EEG y) (4–8 Hz in humans) is known to increase during the memory encoding of subsequently recalled words [5], words–color association [6], and hippocampus-related object–place associations [7]. It is, however, unclear whether the human hippocampus uses EEG y-dependent neural dynamics during memory encoding. In the rat hippocampus and parahippocampal regions, 4–12 Hz (y–band) neuronal dynamics (‘theta phase precession’) are observed and are considered to contribute to the spatial memory formation [8]. More importantly, a computational study showed the necessity of the y rhythm-dependent neural dynamics in human memory encoding of object–place associations in the hippocampus [9]. According to the experimental and theoretical evidences presented above, the human hippocampus is strongly expected to be associated with EEG y dynamics during memory encoding. In this study, we aim to clarify EEG y-correlated neural regions during hippocampus-dependent memory encod- ing by using a simultaneous EEG–functional magnetic resonance imaging (fMRI) recording [10,11]. We first explored a subsequent memory-dependent EEG index in y power index and then localized neural regions indicat- ing correlations between the blood oxygenation level- dependent (BOLD) responses and EEG index. Methods Participants Eleven right-handed volunteers (10 males; mean aged 27.9 ± 10.9 years) participated in this study after giving informed consent. The RIKEN Ethics Committee and the Institutional Review Board of Ogawa Laboratories for Brain Function Research approved the study protocol. Procedure Visual stimuli were projected onto a screen viewed through a mirror. Responses were collected by using a magnet-compatible track ball. Nine simple, two-dimen- sional shapes were used as the objects in the experiment and the objects were presented in the training session before the experiment so that the participants became familiar with all objects. The experiment consisted of resting (10 s), encoding (10 s), retention with random saccade (18 s), and recall phases (19 s) (Fig. 1). During resting, a small cross appeared at the center of the screen and the participants were asked to fixate on it. During encoding, the participants were asked to memorize all four object–place associations. The four objects used in Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Website (www.neuroreport.com). 0959-4965  c 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins DOI: 10.1097/WNR.0b013e328332072a