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The periaqueductal gray (PAG) is less referred in relationship with emotions than other parts of the brain (e.g. cortex, thalamus, amygdala), most probably because of the difficulty to reach and manipulate this small and deeply lying structure. After defining how to evaluate emotions, we have reviewed the literature and summarized data of the PAG contribution to the feeling of emotions focusing on the behavioral and neurochemical considerations. In humans, emotions can be characterized by three main domains: the physiological changes, the communicative expressions, and the subjective experiences. In animals, the physiological changes can mainly be studied. Indeed, early studies have considered the PAG as an important center of the emotions-related autonomic and motoric processes. However, in vivo imaging have changed our view by highlighting the PAG as a significant player in emotions-related cognitive processes. The PAG lies on the crossroad of networks important in the regulation of emotions and therefore it should not be neglected. In vivo imaging represents a good tool for studying this structure in living organism and may reveal new information about its role beyond its importance in the neurovegetative regulation.

Sensory substitution refers to a process whereby an agent, by means of a removable specialized instrumentation, becomes capable of exploiting an available sensory modality in order to perceive properties of the environment which are normally accessible by means of a different modality. We describe a situation of visual-auditory sensory substitution in the rat. Rats were placed in complete darkness, and trained to follow a virtual path whose position was signalled by a sound activated by a video-tracking device. Our hypothesis was that the rats would be able to succeed in this task of spatial navigation, following the sound contour by means of sensory-motor coupling based on seeking the sound (all-or-none) and mastering the relation between their own actions and the expected sensory feedback. Our results confirm this hypothesis and show the progressive structuring of meaningful exploratory activity, leading from the appearance of stopping behaviour when the sound is lost or acceleration when the sound is discovered, up to a veritable sensory-motor strategy which maximizes the possibilities for discovering and following the sound path. Thus, the animals seem to have developed a new form of perception which translates in particular into motor behaviour adapted to the search for sound., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The periaqueductal gray (PAG) is a mesencephalic brain structure involved in the expression of numerous behaviours such as maternal, sexual and emotional. Histological approaches showed the PAG is composed by subdivisions with specific cell organisation, neurochemical composition and connections with the rest of the brain. The comparison of studies performed in rodents and cats as the most often examined species, suggests that PAG organisation differs between mammals. However, we should also consider the plurality of the methods used in these studies that makes difficult the comparison of the PAG organisation between species. Therefore, to study the PAG in all mammals including human, the most relevant in vivo imaging method seems to be the magnetic resonance imaging (MRI). The purpose of this review was to summarize the knowledge of the anatomical organisation of the PAG in mammals and highlights the benefits of MRI methods to extend this knowledge. Results obtained by MRI so far support the conclusions of ex vivo studies, especially to describe the subdivisions and the connections of the PAG. In these latter, diffusion-weighted MRI and functional connectivity seem the most appropriate methods. In conclusion firstly, the MRI seems to be the best judicious method to compare species and improve the comprehension of the role of the PAG. Secondly, MRI is an in vivo method aimed to manage repeated measures in the same cohort of subjects allowing to study the impact of aging and the development on the anatomical organisation of the PAG., (Copyright © 2016 Elsevier B.V. All rights reserved.)