Transient Neural Activation in Human Amygdala Involved in Aversive Conditioning of Face and Voice Tetsuya Iidaka 1 , Daisuke N. Saito 2 , Hidetsugu Komeda 2 , Yoko Mano 2 , Noriaki Kanayama 1 , Takahiro Osumi 1 , Norio Ozaki 1 , and Norihiro Sadato 2 Abstract ■ Elucidating the neural mechanisms involved in aversive conditioning helps find effective treatments for psychiatric dis- orders such as anxiety disorder and phobia. Previous studies using fMRI and human subjects have reported that the amyg- dala plays a role in this phenomenon. However, the noxious stimuli that were used as unconditioned stimuli in previous studies (e.g., electric shock) might have been ecologically in- valid because we seldom encounter such stimuli in daily life. Therefore, we investigated whether a face stimulus could be conditioned by using a voice that had negative emotional va- lence and was collected from a real-life environment. A skin conductance response showed that healthy subjects were con- ditioned by using these stimuli. In an fMRI study, there was greater amygdala activation in response to the faces that had been paired with the voice than to those that had not. The right amygdala showed transient activity in the early stage of acquisi- tion. A psychophysiological interaction analysis indicated that the subcortical pathway from the medial geniculate body to the amygdala played a role in conditioning. Modulation of the subcortical pathway by voice stimuli preceded the transient ac- tivity in the amygdala. The finding that an ecologically valid stim- ulus elicited the conditioning and amygdala response suggests that our brain is automatically processing unpleasant stimuli in daily life. ■ INTRODUCTION Significant involvement of neural responses in the amygdala and related structures has been observed in the classical aversive conditioning paradigm in animals, which involves learning by association between neutral and noxious stim- uli (Sotres-Bayon, Bush, & LeDoux, 2004; Quirk, Likhtik, Pelletier, & Pare, 2003; Quirk, Armony, & LeDoux, 1997). A previously neutral stimulus (conditioned stimulus or CS) elicits behavioral and autonomic responses after effective pairing with an unconditioned stimulus (US) that has un- pleasant features. This phenomenon is a potential model for psychiatric disorders such as anxiety disorder (Milad, Rauch, Pitman, & Quirk, 2006), phobia, and other stress- related disorders (Ollendick & Hirshfeld-Becker, 2002) in human subjects. Investigation of the precise mechanisms of aversive conditioning helps clarify the pathogenesis of the disorders and helps us find effective treatment. However, exposing human subjects to a US such as an electric shock or a loud tone might be ecologically invalid because we seldom encounter such intense and physical stimuli in daily life. In a study using US such as an unpleas- ant noise collected from the environment, the subjects were effectively conditioned to the stimuli and autonomic responses were elicited (Neumann & Waters, 2006). In the case of social anxiety disorder which exhibits elevated fear in social situations, it is hypothesized that the symptoms may be acquired through aversive conditioning in social circumstances (Ollendick & Hirshfeld-Becker, 2002). Mild emotional stress induced by simulated social situations re- sulted in significant changes in the plasma cortisol levels in patients with major depressive disorder (Belmaker & Agam, 2008). Thus, in addition to the degree of the stimulus inten- sity, we considered it important to ensure that the experi- mental paradigm would simulate the real-life situations in which the subjects were exposed to emotional stress. Neural correlates involved in aversive conditioning were investigated by using fMRI and human subjects (Phelps, Delgado, Nearing, & LeDoux, 2004; Buchel, Morris, Dolan, & Friston, 1998; LaBar, Gatenby, Gore, LeDoux, & Phelps, 1998). In these studies, in line with animal experi- ments, the amygdala was a key structure in the conditioning process and mediated neuronal input from several percep- tual modalities to elicit autonomic responses. In particular, the amygdala response was greater in the first half than in the second half of the acquisition phase (Morris, Buchel, & Dolan, 2001; Buchel, Dolan, Armony, & Friston, 1999; Buchel et al., 1998; LaBar et al., 1998) and it showed rapid habituation. However, the temporal dynamic of the amyg- dala response in the conditioning paradigm has hitherto not been investigated. Although it is not possible to track 1 Nagoya University, Nagoya, Japan, 2 National Institute for Physio- logical Sciences, Okazaki, Japan © 2009 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 22:9, pp. 2074–2085