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NeuroImage 235 (2021) 117971 Contents lists available at ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/neuroimage EEG cross-frequency phase synchronization as an index of memory matching in visual search Anna Lena Biel a , c , Tamas Minarik a , b , Paul Sauseng a , c , ∗ a Department of Psychology, Ludwig-Maximilians-Universität München, Leopoldstr. 13, 80802 Munich, Germany b School of Psychology, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom c Graduate School of Systemic Neurosciences, Ludwig-Maximilians-Universität München, Großhaderner Str. 2, 82152 Planegg-Martinsried, Germany a r t i c l e i n f o Keywords: Cross-frequency phase coupling Gamma oscillations Memory matching Theta oscillations Visual attention Working memory a b s t r a c t Visual perception is inf l uenced by our expectancies about incoming sensory information. It is assumed that mental templates of expected sensory input are created and compared to actual input, which can be matching or not. When such mental templates are held in working memory, cross-frequency phase synchronization (CFS) between theta and gamma band activity has been proposed to serve matching processes between prediction and sensation. We investigated how this is af f ected by the number of activated templates that could be matched by comparing conditions where participants had to keep either one or multiple templates in mind for successful visual search. We found a transient CFS between EEG theta and gamma activity in an early time window around 150 ms after search display presentation, in right hemispheric parietal cortex. Our results suggest that for single template conditions, stronger transient theta-gamma CFS at posterior sites contralateral to target presentation can be observed than for multiple templates. This can be interpreted as evidence to the idea of sequential attentional templates. But mainly, it is understood in line with previous theoretical accounts strongly arguing for transient synchronization between posterior theta and gamma phase as a neural correlate of matching incoming sensory information with contents from working memory and as evidence for limitations in memory matching during multiple template search. 1. Introduction Working memory and selective attention interact in many situations of our everyday life, inf l uencing how we perceive the world. Image your- self looking for your car keys that you must have left somewhere in the kitchen. During your search, you will scan a rich visual environ- ment for something that matches the representation of keys that you have in mind. Such situations are commonly described as visual search. Brought to a cognitive psychology laboratory, participants in a visual search paradigm are usually asked to search for a target object among a number of distractor objects presented on a computer screen. Cur- rent theories of attention hold that when we are searching for a tar- get, then keeping a template representation of the target in working memory –a so called attentional template –leads to a bias in the com- petition for neuronal resources in favor of template-matching stimuli ( Bundesen, 1990 ; Bundesen et al., 2005 ; Desimone and Duncan, 1995 ; Duncan and Humphreys, 1989 ). Insight into the neural mechanisms un- derlying the activation of such mental templates and their comparison with sensory input comes from studies in healthy humans ( Gayet et al., 2017 ; Soto et al., 2007 ; Spaak et al., 2016 ) patients with frontal le- ∗ Corresponding author. E-mail address: paul.sauseng@lmu.de (P. Sauseng). sions ( Soto et al., 2006 ; Yago et al., 2004 ), lesion studies in primates ( Everling et al., 2006 ; Lba and Sawaguchi, 2003 ; Rossi et al., 2007 ), as well as from formal theoretical models ( Friston, 2005 ) . Thereof, espe- cially prefrontal brain regions are known to be involved in visual search and the top-down control of visual perception from working memory by impacting on lower visual cortex (for review, see Soto et al., 2008 ). Interactions between higher and lower brain areas, as assumed to be involved in visual search, can be well investigated by analysis of oscillatory brain activity. Interaction within or between brain areas is implemented by synchronous neural activity, as ref l ected by rhythmi- cal oscillations of the fi eld potential which can be recorded using scalp electroencephalography (EEG). Oscillatory EEG activity is commonly re- ported to play a functional role for perceptual and cognitive processes ( Buzsáki and Draguhn, 2004 ; Fell and Axmacher, 2011 ; Fries, 2005 ). Two brain areas are assumed to be functionally coupled when their ac- tivity is more synchronous than what would be expected from random fl uctuations. It has been suggested that the complexity of the neural network(s) involved will determine the frequency range of the dynam- ics in a given interaction ( Buschman and Miller, 2007 ; Fell and Ax- macher, 2011 ; Fries, 2005 ), such that long-range interactions during top-down processes draw on lower frequencies in the theta band (~6 Hz) https://doi.org/10.1016/j.neuroimage.2021.117971 . Received 29 September 2020; Received in revised form 2 March 2021; Accepted 5 March 2021 Available online 8 April 2021. 1053-8119/© 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )