# Robot Bionic Vision Technologies: A Review
## 2 Literature Review
### Human Visual System
Movement of the eyeball in all directions requires fine coordination of the external eye muscles, and no eye muscle activity is isolated. Regardless of the direction of the eye movement, 12 extraocular muscles are involved. When a certain extraocular muscle contracts, its synergistic, antagonistic, and partner muscles must act simultaneously and equally to ensure that the image is accurately projected onto the corresponding parts of the retinas of the two eyes [65; 66]. The extraocular muscle is driven and controlled by the cranial nerve, which directs the muscle to contract or relax. Eye movements occur when one part of the extraocular muscle contracts and the other relaxes. The motion features of the human eyes mainly include conjugate movement, vergence or disjunctive movement, vestibulo-ocular reflex (VOR), and optokinetic reflex (OKR) [63]. Specifically, the human eye also exhibits the following  
Figure 3: A region at the junction of the prefrontal and premotor cortex in **(a)** rhesus monkeys and primates, (b) human brain. Known as the anterior field of vision (FEF), it is primarily involved in controlling eye-movement behavior and spatial attention. Humans have at least two types of forwarding fields of vision. After comparing the letters in the visual nerve regions of the human brain and monkey brain, it was found that the monkey visual cortex corresponds to the human visual cortex; and the principles of spatial attention, dynamic vision control, and attention orientation control are very similar [63]. The letters A, B, C, D, and E in the figure represent the visual cortex regions of the human and monkey brains. The image has been licensed by ELSEVIER. motion characteristics [67; 68; 69; 70; 71; 72; 73]. The saccade, the most common feature, is a movement occurring rapidly and in parallel from one fixation point to another during which the human eye freely observes its surroundings, lasting between 10 and 80 ms. Mainly used by the human eye to actively adjust the direction of gaze, large saccades require head and neck movement. Smooth pursuit is the movement of the eye tracking a moving object, which is characterized by smooth tracking of the target. The angular velocity of the eye movement corresponds to the velocity of the moving object when the eye is observing a low-speed object. The adult eye can follow objects with 100% smoothness. When the target angular velocity is greater than 150\({}^{\circ}\)/s, the human eye cannot track the target. When the target object appears in the visual field of the human eye and begins to move at a high speed, the human eye tracks the target. The eyes have rhythmic horizontal nystagmus and maintain the same direction as the movement of the object. When the direction and speed of the human eye are consistent with those of the object, the human eye can clearly see the object. For example, one can sit in a fast-moving car and see the scenery outside the window. When the head moves, the eyes move in the opposite direction; thus, the image maintains steady reflective movement on the retina. For example, when the head moves to the right, the eyes move to the left, and vice versa. The primary function of OVR is motion stabilization, which is characterized by the coordination of head movements to obtain a more stable image. OVR has been used in camera anti-shake mechanisms. Conjugate motion is the simultaneous movement of the eyes in the same direction so that diplopia does not occur. Its characteristics can be divided into saccades and smooth tracking motions. When a target approaches, the visual axes of the two eyes intersect inward, presenting a convergent motion state, whereas when the target moves far away, the visual axis of the two eyes diverges outward, presenting a state of divergent motion. The hold time of the convergent motion is approximately 180 ms, and that of the divergent motion is approximately 200 ms. The process of keeping one's eyes on the target for a long time is called fixation.