Sanjukta Jana, B.Optom Student
Student, NSHM Knowledge Campus – Kolkata, India
Lord Shiva, the supreme being in Hindu mythology is also known as ‘Tryambaka Deva’ meaning the three-eyed lord. The third eye mainly refers to sensory eyes which have no physical existence. The main purpose of this eye is to perceive symbolically metaphysical stimulation.
Unlike the supreme deity, mortals own two eyes. Let me tell you, we all possess an invisible third eye through which we perceive visual stimuli, although, we think we see through two eyes. If it is true, then why don’t we see two images of every object? The best possible answer to this can be explained in terms of Cyclopean Eye.
What is the Cyclopean eye?
According to Hering’s law of visual direction, the images will be perceived as if the observer is viewing the object from a single point. (1) This point is the Cyclopean visual system, which is named after the mythical creature Cyclops which has one single eye on its forehead. (2) The third eye sees in visual space the fused images of two individual eyes.
Figure 1: The efficacy of the cyclopean eye. The visual direction of FL and FR proceed from the fovea OS imaginary cyclopean eye. A and B represent the visual directions of belonging to A and B.
Location of the Cyclopean eye
Hering stated that the Cyclopean eye is specifically located medially on the Vieth-Muller circle. (2) (The theoretical circle joining the corresponding retinal points.) Ono deduced that the Cyclopean eye has diverse functional necessities in judging the exact location of objects. (2)
Figure 2: The Vieth Muller circle of location of points in space that will produce corresponding retinal points.
Efficacy of the Cyclopean Eye
Perception of spatial order is a mental phenomenon. Each eye sees the world from a different viewpoint. When an object is viewed, its image is formed on the foveola. In the case of monocular fixation, the visual direction of the object can be represented by a line joining the object to the centre of the foveola. The oculo-centric frame of reference i.e., the monocular visual axis is different for two eyes. However, in binocular vision, a single frame of visual direction is needed whose frame of reference is related to the head rather than two eyes. Retinal points in two eyes are corresponding to each other. Corresponding retinal points, therefore, have a common visual direction and permit a single visual impression. Images falling on corresponding points give rise to a single mental impression to a new visual direction which we call ‘sensory fusion.’ (Hering’s law of identical visual direction). (1)
Hering described it with an experiment. An observer standing a half metre from a window while closing his right eye and locating his left eye on an object on his right (say a tree) made a black spot on the windowpane in line with the tree. Then he closed his left eye and opened right eye and directed towards another object, say a hut. Then with both eyes open, directed at the spot, the latter will appear to cover parts of the tree and hut, which will be seen simultaneously. (1)
Figure 3: Illustrating Hering’s experiment (1)
Cyclopean eye in simple terms
A third eye which helps to obtain a single image on the retina by accessing functions of right and left eye inducing depth perception.
Clinical Aspects
- The existence of the cyclopean eye confirms the anatomic distribution of retinal elements and physiological distribution of spatial values do not coincide.
- Helps in diagnosing binocular cooperation.
- Clinically significant in orthoptic treatment of comitant strabismus.
Discrepancy in theory
The theories supporting the existence of the third eye have given rise to a long-time unnoticed paradox. The Cyclopean eye is structurally two-dimensional. The third eye can only accommodate monocularly visible details. (3)
Several studies are still going on the validation of this concept hence creating a platform for vision science experts to participate in this debating topic.
References
- Khurana AK. (2001) Anatomy and physiology of Eye. (3rd ed.) Elsevier.
- Wikipedia contributors. (2023). Cyclopean image. Wikipedia. https://en.wikipedia.org/wiki/Cyclopean_image
- Khokhotva M, Ono H, Mapp AP. The cyclopean eye is relevant for predicting visual direction. Vision research. 2005 Aug 1;45(18):2339-45. Image courtesy:
Picture courtesy-
Figure 1: Kalloniatis M, Luu C. The Perception of Space. 2005 May 1 [Updated 2007 Jun 6]. In: Kolb H, Fernandez E, Nelson R, editors. Webvision: The Organization of the Retina and Visual System [Internet]. Salt Lake City (UT): University of Utah Health Sciences Center; 1995- (Available from: https://www.ncbi.nlm.nih.gov/books/NBK11545/figure/ch29space_perceptio.F4)
Figure 2: Vieth-Muller Circle. (n.d.). (https://www.appstate.edu/~steelekm/classes/psy3215/Depth/viethmuller.html)
Figure 3: Khurana AK. (2001) Anatomy and physiology of Eye. (3rd ed.) Elsevier.
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