UNCORRECTED PROOF BRESC40073 Cognitive Brain Research 1 (2001) 000–000 4 www.elsevier.com / locate / bres 10 Research report 11 Theta synchronization during episodic retrieval: neural correlates of 12 conscious awareness a, a b b b a * ¨ 13 W. Klimesch , M. Doppelmayr , A. Yonelinas , N.E.A. Kroll , M. Lazzara , D. Rohm , a 14 W. Gruber a 15 Department of Physiological Psychology, Institute of Psychology, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria b 16 Department of Psychology, University of California, Davis, USA 17 Accepted 30 January 2001 18 19 Abstract 20 The neural correlates of conscious awareness during successful memory retrieval were examined. In a recognition test, subjects 21 indicated whether they consciously recollected the event in which a word was earlier presented (Remembering), or whether they 22 recognized it on the basis that it was familiar in the absence of recollection (Knowing). An early EEG synchronization in the theta band 23 predicted knowing, and a later remembering. Moreover, early and late event-related potentials were also found to predict knowing and 24 remembering, respectively. The results indicate that the temporal dynamics of theta synchronization are related to the particular conscious 25 experiences associated with memory retrieval.  2001 Published by Elsevier Science B.V. 26 Keywords: Theta; Memory; Consciousness; ERP; ERD 27 28 1. Introduction the hippocampal region increase during exploratory be- 48 havior and orienting responses [16], but are virtually 49 29 The search for neural correlates of conscious experience absent during automatic behaviors, such as eating or 50 30 represents one of the most challenging questions in drinking. Furthermore, theta oscillations can be observed 51 31 cognitive neuroscience. One common approach is to use during REM-sleep where dreaming episodes occur [21]. 52 32 neuroimaging methods like positron emission tomography These findings provide preliminary evidence for the hy- 53 33 to identify the brain regions that are activated when pothesis that an increase in theta activity is related to states 54 34 subjects report specific conscious experiences such as of increased awareness or to an altered state of awareness 55 35 detecting an object or recalling a previous event [6]. These as indicated by increased theta during dreaming episodes. 56 36 methods have been useful in identifying the brain regions Moreover, there is convincing evidence that an increase in 57 37 that are associated with consciousness. However, because hippocampal theta is related to the encoding and retrieval 58 38 of limitations in temporal sensitivity, these techniques have of new episodic information [8]. In human subjects, lesions 59 39 provided very little information about the temporal dy- of the medial temporal lobe, including the hippocampus, 60 40 namics of the relevant cortical activation. In the current are known to cause a loss of the capacity for conscious 61 41 study, we take an alternative approach and measure scalp recollection of new events [4,15,23]. These findings sug- 62 42 EEGs in order to examine the temporal dynamics of gest that hippocampal theta is a promising neural correlate 63 43 widespread neural synchronization in the theta bandwidth of conscious awareness in memory. Although it has been 64 44 that is associated with the conscious experience of memory questioned for a long time, there is meanwhile converging 65 45 retrieval. evidence that theta oscillations can also be observed in 66 46 Animal research has provided good evidence that the humans, This evidence comes from studies with patients 67 47 amplitudes (power) of theta oscillations (e.g., 4–6 Hz) in using depth [2] and subdural [10] electrodes as well as 68 from studies with normal subjects using scalp electrodes 69 [7] and the MEG [19]. 70 5 *Corresponding author. Tel.: 143-662-8044-5120 or 5100; fax: 143- Results from our laboratory have shown that in human 71 6 662-8044-5126. 7 E-mail address: wolfgang.klimesch@sbg.ac.at (W. Klimesch). scalp EEG, theta responds selectively (with an increase in 72 1 0926-6410 / 01 / $ – see front matter  2001 Published by Elsevier Science B.V. 2 PII: S0926-6410(01)00024-6