## 3 The Human Role in Teleoperation
### Depth Perception  
The majority of human interactions depend on space and motion perceptions. This ability enables navigation in an environment and handling objects. _Depth perception_ is a filtering process that enables us to perceive the world's tridimensionality. Humans can identify information in images and correlate it with depth in the scene, using both psychological and physiological cues [89]. For example, if an object X partially covers another one Y, then object X is inferred as the nearest to us (i.e., occlusion). These different depth cues can be classified into three groups according to the sensory information: _oculomotor cues_, _monocular cues_, and _binocular cues_.  
_Oculomotor cues_ for depth/distance are based on:  
* _vergence_, which results from the sense of inward movement that occurs when the two eyeballs rotate to fixate near or distant objects, and
* _acommodation_, the change in the eye lens's shape required to focus on the object at different distances. In fact, accommodation is a monocular cue, based on the kinesthetic sensation of stretching the eye's lens and sensing the tension of the eye's muscles [30].  
_Monocular cues_ provide distance/depth information by viewing the scene with one eye only. Besides accommodation, they include:pictorial cues_, which identify depth information in a single two-dimensional image, and  
_movement-based cues_, which extract depth from the perceived movement.  
Pictorial cues include (a) occlusion, (b) relative height, (c) relative size, (d) perspective convergence, (e) familiar size, (f) atmospheric perspective, (h) texture gradient, and (i) shadows. The mentioned cues consider a stationary observer; however, new depth cues arise when a person rotates his/her head or starts walking. _Motion-produced cues_ include (1) _motion parallax_ and (2) _accretion and deletion_. _Motion parallax_ results from the fact that when we move, nearby objects seem to move faster than the more distant ones. As we move and due to perspective projection, the displacement of the projection of the objects in the retinal image depends on the distance. _Accretion_ and _deletion_ result from sideways movements of the observer, leading some object parts to become hidden, and others visible. These cues, based on both occlusion and motion parallax, arise when superimposing surfaces seem to move in relation to one another. Two surfaces, at different depths, can be detected using these cues.  
Binocular cues use information from both eyes to provide important distance/depth information. _Binocular disparity_ (_binocular parallax_), i.e., the difference of corresponding image points in both eyes, combined with the geometry of the eyes (convergence) enable localizing a tridimensional point (triangulation). _Stereopsis_ is the feeling of depth that results from information provided by binocular disparity.  
In teleoperation, by changing the point of view of remote monocular or supporting stereo vision allows operators to get important cues for depth perception, crucial for navigation and telemanipulation. In telesurgery or laparoscopic tasks, the observation through an indirect method affects the surgeon's depth perception and diminishes his/her eye-hand coordination ability. Depth perception problems result from accommodation and convergence inconsistencies, the lack of shadows in endoscopic video images, and the lack of movement parallax and stereovision. Eye-hand coordination problems are due to the distance location of the monitor and because the video images of the doctor's hand movements appear mirrored, rotated, and amplified [90]. To overcome these contingencies, surgeons must have intensive, long, and specialized training. Technological solutions [91; 92] include flexible and motorized endoscopes that compensate misorientation and provide movement parallax through probe positioning controlled by the surgeon's head movements.