Auditory and visual systems organization in Brodmann Area 8 for gaze-shift control: where we do not see, we can hear

Hearing is especially important for most primate species as they live in habitats of dense vegetation that limits vision. Stebbins (1980) summed up the evolution of the auditory system by assuming that earliest mammals exploited nocturnal niches since they were relatively free of many of the large, diurnal, predacious reptiles. Therefore, hearing and smell were more useful at night than vision. Our vision is limited not only in the dark but also outside the visual field. In fact, if we observe the behavior of a predator like a feline, oriented toward its prey, and at the same time a sound occurs behind, we might note three principal different behaviors: the predator could maintain its gaze and ears on the prey neglecting the sound source; the predator could maintain its gaze on the prey rotating ears and then shifting its auditory attention toward the sound source; finally the predator could break its attention and orient gaze and ears toward the sound source. A similar behavior is seen in human beings during social interaction with two or more interlocutors. In humans, orienting movements are carried out by the eyes, head, and/or body operating alone or in various combinations depending on the behavioral situation. However, in non-human primates, such as macaque monkeys, head orienting movements and, more generally, gaze-shift are accompanied by ear orienting movements, which allow the shifting of auditory attention toward a sound of interest (Bon and Lucchetti, 1994, 2006; Lucchetti et al., 2008; Lanzilotto et al., 2013; Yin, 2013). Considering all these assumptions, the auditory system could have an important role to detect information even from regions of the space that the visual system cannot explore without orienting movements. In other words, where we cannot see, we can hear. Through this opinion article, we argue that Brodmann Area 8 receives information from both auditory and visual systems and organizes a transformation of these sensory signals into gaze-shift motor commands. Our hypothesis is that this sensory-motor transformation is spatially organized, from both anatomical and functional points of view. Anatomical and functional properties of the Brodmann Area 8 (consisting in Area 8A plus Area 8B) support a medio-lateral organization for both auditory and visual systems. In particular, the lateral portion, corresponding to Area 8A or Frontal Eye Field (FEF), could play a role in receiving visual and auditory information from a central part of the visual field and then in organizing gaze-shift motor commands toward it. Otherwise, the medial portion, corresponding to Area 8B or Premotor Ear-Eye Field (PEEF), could play a role in receiving principally auditory information from a peripheral region of the space and then in organizing gaze-shift motor commands toward it.