Eye movements: smooth pursuit
Another natural start point in the search for links between the organism and its world are the senses, at least the classical ones (vision, touch, smell, hearing, taste). The senses can be considered as the neural systems providing the mechanistic link between body and world. They perform the function of keeping the body connected to changes in its surroundings. Let us describe in more detail an example of this instance. For primates, a central sense is vision. Animals use their eyes to detect changes and features of their surroundings and also to guide their movements. A visual phenomenon that beautifully illustrates the body-world coupling is what is called (ocular) smooth pursuit. This is a movement of the eyes that allows the continuous visual tracking of an external object (see Figure 2). As Barnes defines it, ”Ocular pursuit is a mechanism that enables tracking of moving objects in extra-personal space with the eyes alone (emphasis added)” \cite{Barnes_2008}. Neuroscientists have uncovered the interplay of several brain areas that are involved in the motor control of the ocular muscles during pursuit processes. The brain circuit usually involves signals that start in the retina and travel through the thalamus and primary visual cortex, reaching centers such as the middle temporal area and the middle superior temporal area. The latter is connected with key areas such as the frontal eye field and pontine nuclei, which in turn send activity, via the cerebellum,  to motor nuclei such as the fastigial and vestibular, which directly activate oculomotor nuclei. The specific role of each area and the exact set of areas involved are currently active topics of research \cite{Fukushima_2013,Krauzlis_2004,Ono_2015}. The different proposed mechanisms for implementation of the pursuit always involve some sort of feedback mechanism, in which the motion of the retinal image is minimized. The vast majority of proposed mechanisms focus completely on the set of brain areas and their roles (See Figure 2), and not on the outside object. This is somewhat intriguing, given that the ability to perform smooth pursuit degrades in the absence of an external visual stimulus \cite{Berryhill_2006}. It requires an actual external visual object moving. Authors describe pursuit as constituted by three periods: initiation, pursuit, and termination \cite{Missal_2017}. Of these, initiation and termination refer to the moments at which the system engages and disengages from the external visual object, whereas the pursuit is the period where the world and the eye are co-evolving. By manipulating the external object, we are able to causally  manipulate the internal mechanisms that produce the movement.