The authors respond
===================

* Iris E.C. Sommer
* André Aleman
* René S. Kahn

We would like to thank Sommer and colleagues for their comments regarding our article and for their meta-analysis of research into the lateralization of verbal hallucinations. 1 However, we believe that it is difficult on the basis of a meta-analysis alone to settle, once and for all, the issue of the lateralization of hallucinations.

First, 2 types of experimental paradigms have proved instructive regarding cerebral functions affected by schizophrenic hallucinations: (1) those where cerebral activity in the presence of a specific hallucination is compared with the activity that occurs in its absence; and (2) those where the cerebral activity that occurs during the performance of a specific task involving lexical or auditory data processing is compared with the activity that occurs when the participant is in a defined controlled state that clearly differs from the state under study. In each of these paradigms, if we wish to arrive at a sound interpretation of the data, measures must be taken to control the numerous sources of fluctuations in cerebral activity that could easily skew matters. Consequently, the studies to date have necessarily been heterogeneous, thereby complicating any meta-analysis.

Second, reviewing the conclusions drawn in functional imaging studies, we notice that anomalies have been observed in several regions of the brain, but there is no evidence of the worsening of a function located consistently in one or other of these. The problem appears to stem rather from an abnormal coordination of brain activation in various sites. In other words, we should be focusing more on connectivity than on lateralization.

Third, it seems conceivable that certain functional connectivity anomalies in the cortical regions could serve as relatively stable features capable of characterizing the disease, whereas others are transitory phenomena characteristic of a particular state. In this regard, certain observations suggest that during the acute stage of the illness, a generalized deficit occurs in the functioning of the 2 hemispheres, but particularly in the right. After remission, however, disturbances persist only in the left.2 Any meta-analysis must take this into account.

Finally, it is useful to refer to a theory of language such as access to the mental lexicon (contribution of the left hemisphere), which calls for experimental paradigms such as those used for lexical decisions. The 2 hypotheses currently most studied to explain the genesis of auditory hallucinations are (1) an internal discourse not recognized as his own by the hallucinating subject and (2) aberrant activity accessing the primary auditory area. In most cases, the verbal content of auditory hallucinations remains the same from 1 psychotic episode to the next, even when the 2 episodes are separated by a long period of remission. This has led us to hypothesize that the verbal content of auditory hallucinations persists at a subclinical level even during periods of full remission and is quickly reactivated when the subject is exposed to stressful events. We also suggest that these 2 hypotheses apply above all to this specific verbal content.

To test these assumptions, we gation of auditory verbal imagery and language perception in relation to hallucinations.

## Footnotes

*   **Competing interests:** None declared.

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