Neuropsychologia 45 (2007) 2294–2306 Auditory cortex and anterior cingulate cortex sources of the early evoked gamma-band response: Relationship to task difficulty and mental effort Christoph Mulert ∗ , Gregor Leicht, Oliver Pogarell, Roland Mergl, Susanne Karch, Georg Juckel, Hans-J¨ urgen M¨ oller, Ulrich Hegerl Department of Psychiatry, LMU, Nußbaumstraße 7, D-80336 Munich, Germany Received 21 September 2006; received in revised form 29 January 2007; accepted 21 February 2007 Available online 4 March 2007 Abstract High frequency oscillations in the 40 Hz (gamma-band)-range are involved in the synchronization of brain regions, e.g., in cognitive functions. It has been suggested that the auditory evoked gamma-band response (GBR) is affected by attention and apart from auditory cortex activity a frontal or anterior cingulate cortex (ACC) generator could be involved. It was the aim of the present study to address three questions: (1) is there a neural generator of the early evoked GBR in the dorsal (d)ACC? (2) Are there different activation patterns in the dACC and the auditory cortex areas in response to task difficulty? (3) Is it possible to detect an influence of early ACC-gamma-band activity (GBR timeframe) to later auditory information processing (N1 timeframe)? In the present EEG/ERP-study we have investigated 30 healthy subjects using six auditory reaction tasks with increasing difficulty and mental effort demands. In the MANOVA analysis we found a significant main effect of task difficulty on both the GBR amplitude (F = 7.75; p < 0.001) and the auditory evoked N1 potential (F = 7.00; p < 0.001) with higher amplitudes in the more difficult tasks. In the LORETA region of interest (ROI) analysis, this effect was only due to increased dACC-activity during the GBR-timeframe. For the ROI analysis during the N1 timeframe, in addition to a strong effect of task difficulty in the dACC a similar main effect was found in the auditory association area 22. These findings are in line with a top-down influence of dACC-activity to the auditory association area 22 during the early evoked GBR. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mental effort; Event-related potential; ERP; LORETA; ACC 1. Introduction In the last years neuroimaging techniques like Positron Emis- sion Tomography (PET) or functional Magnetic Resonance Imaging (fMRI) have allowed us to investigate brain activity with high spatial resolution. While these techniques are indi- rect measurements of brain activity, loosing most or all of the temporal resolution of neuronal activity, some conclusions are possible about how brain regions interact (Friston, 2002). How- ever, detailed questions about how information is processed by the brain and how brain regions cooperate, for example in terms of “bottom-up” or “top-down” information processing, can only sufficiently be answered if the real temporal dynamics of brain activity is considered (Engel, Fries, & Singer, 2001). In this context much effort has been undertaken to explore the ∗ Corresponding author. Tel.: +49 89 5160 5550; fax: +49 89 5160 5542. E-mail address: cmulert@med.uni-muenchen.de (C. Mulert). role of high frequency oscillations in the 40 Hz (gamma-band)- range and recently a model has been suggested to explain early gamma-band responses in terms of the match between bottom- up and top-down information (Herrmann, Munk, & Engel, 2004). Interestingly, the functional role of gamma-activity seems to be different for “induced” and “evoked” gamma-band activity. Induced gamma-activity is related to an external event, but with variations in latency or phase. Evoked gamma-activity typically appears earlier (before 150 ms) and is both in terms of phase and latency strictly triggered to stimulus onset. For evoked gamma- activity, several authors have described its role in the context of attentional processes (Fell, Fernandez, Klaver, Elger, & Fries, 2003; Gurtubay, Alegre, Labarga, Malanda, & Artieda, 2004; Senkowski, Talsma, Grigutsch, Herrmann, & Woldorff, 2007). Concerning induced gamma-activity its contribution to feature binding, sensory awareness and consciousness (Engel, Fries, Konig, Brecht, & Singer, 1999; Engel & Singer, 2001) has been discussed. 0028-3932/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2007.02.020