Clarifying the role of neural networks in complex hallucinatory phenomena.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Mégevand et al. Visual hallucinations and related phenomena , such as déjà vu, are reminders that conscious perception does not always accurately reflect external reality. We might be startled by a garden hose that at first glance looks like a snake, or feel an eerie sense of familiarity in new surroundings. For the most part these are transitory and intriguing perceptual glitches, yet in many neuropsychiatric disorders they are recurrent and troubling symptoms. Currently there is no unified brain– behavior framework to conceptualize these phenomena. The work by Mégevand and colleagues (2014), which uses intracranial EEG to discern the neural correlates of topographic visual hallucinations and déjà vu in the parahippocampal place area, is timely and informative, hinting at a mechanism for these phenomena involving interactions between large-scale neural networks. Visual hallucinations range from the simple to the complex. Simple hallucinations are typically abstract shapes or patterns , and they can be induced via electrical brain stimulation to the primary and secondary visual cortices (Selimbeyo-glu and Parvizi, 2010). In contrast, complex hallucinations comprise recognizable objects, such as people, animals, or topo-graphical scenery, which are often imbued with personal meaning. The elaborate quality of complex hallucinations suggests involvement of regions outside the primary visual areas, and indeed, they are widely reported during electrical stimulation of medial temporal lobe sites, including the hippocampus and amygdala, fusiform and parahippocampal gyri, and entorhinal and perirhinal cortices (Selim-beyoglu and Parvizi, 2010). Similarly, déjà experiences exist on a continuum, with déjà vu representing the attenuated form of familiarity in a novel context, and déjà vécu the more encompassing experience of having previously lived through a certain situation (Illman et al., 2012). Déjà phenomena are also strongly associated with medial temporal lobe disturbances, most commonly manifesting experimentally with electrical stimulation of the amygdala, hippocampus, and parahip-pocampal cortex (Vignal et al., 2007). Despite the broad implication of the parahippocampal cortex in complex hal-lucinatory phenomena, its posterior region , the parahippocampal place area (PPA), specialized for processing visual scenes, had not previously been linked to topographic hallucinations. The PPA is historically defined as the posterior subre-gion of …