Ibn al-Haytham was the major figure in the study of optics and vision in the Middle Ages and his influence was pervasive for over 500 years. In this article, Professor Charles G. Gross, a renowned neurophysiologist of vision, outlines his original theory of vision and describes aspects which are less well known, namely Ibn al-Haytham's insights into visual physiology and visual perception. Professor Gross concludes that, although Ibn al-Haytham's unique synthesis of physics, mathematics and physiology into a new theory of vision and its historical importance have been recognized, his insights into the psychology of perception and their influence remains an important and potentially fertile area of research.
Professor Charles G. Gross*
Note of the editor
This article had been published as Charles G. Gross: Ibn al-Haytham on Eye and Brain, Vision and Perception, Bulletin of Islamic Medicine (Kuwait Ministry of Public Health [and] National Council of Culture, Arts and Letters), vol. 3- 1981, pp. 309-312 (this journal is indexed here). We thank Professor Charles G. Gross for the permission he granted us to republish the article on www.MuslimHeritage.com.
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Ibn al-Haytham was the major figure in the study of optics and vision in the Middle Ages and his influence was pervasive for over 500 years. His original theory of vision is briefly outlined. Less well known are his insights into visual physiology and visual perception, some of which are described.
Figure 1: Two views of the frontispice of the first edition of the Latin translation of Ibn al-Haytham's Book of Optics: Opticae Thesaurus...Libri Septem, nunc primùm editi. Eiusdem liber De Crepusculis & Nubium ascensionibus. Item Vitellonis...Libri X. Omnes instaurati, figuris illustrati & aucti, adjectis etiam in Alhazenum commentariis, a Federico Risnero. Basel: Episcopius, 1572. (Source 1 - Source 2).
Ibn al-Haytham was the major figure in the science of optics and the study of vision between classical civilization and Renaissance. He was born in Basra in 965 and died in Cairo in 1040 and was later known in Europe as Alhazen . His work represents the first major advance in optics after Euclid and Ptolemy of Alexandria and in visual physiology after Galen. We must wait until Kepler and Newton in the 17th and 18th centuries for further fundamental understanding of the nature of light and until at least Helmholtz in the 19th century for further advances in understanding visual perception. For further progress on many of the perceptual and psychological questions considered by Ibn al-Haytham, we still have to wait. Like Leonardo, Ibn al-Haytham was a polymath, contributing to astronomy, mathematics philosophy as well as a variety of other subjects. Unlike Leonardo, who had little or no impact on successive generations of scientists, Ibn al-Haytham's influence was pervasive and usually recognized well into the 18th and 19th centuries. As a practicing neurophysiologist of vision, I will concentrate on summarizing some of Ibn Al-Haytham's major contributions to the physiology and psychology of vision. Before doing so, however, let me briefly describe the principal approaches to light and vision of the ancient world that provides the background for Ibn al-Haytham's work.
Euclid and Ptolemy held an extromission view of vision: vision was due to a stream of rays issuing from the eye and somehow apprehending objects.
Galen had provided a very detailed description of the eye and the optic pathways that was hardly surpassed before Vesalius . Particularly crucial for Ibn al-Haytham's theory of vision, as we shall see, was Galen's view that the crystalline humor (our lens) was the sensitive or photoreceptive portion of the eye, Galen, however, had little interest in optics and accepted an intromission view.
In his great work Kitab al-Manazir or Optics, Ibn al-Haytham carefully examined the extromission theories of his predecessors and systematically demolished each of them.
Against the extromission theory he writes: "The act of vision is not accomplished by means of rays emitted from the visual organ"; rather, "vision is accomplished by rays coming from external objects and entering the visual organ" . Essentially Ibn al-Haytham took a new view of light, combined it with Ptolemaic optics, Galenic anatomy and the results of his own extensive experiments and produced a plausible intromission view that lasted until Kepler.
As he put it, "from each point of every coloured body, illumination by any light, issue light and colour along straight lines that can be drawn from that point" . Following Galen, Ibn al-Haytham believed that the crystalline humor was the sensitive surface whose receipt of light was the first step of the visual processes . But, if from every point of every object, light travelled to the crystalline humor, then those light rays would intermix and total confusion would result . How could a point-to-point correspondence between the visual field and the crystalline humor, essential to Ibn al-Haytham theory of vision, be maintained? Ibn Al-haytham postulated that only light rays orthogonal to the surface of the crystalline humor passed through it. The others were refracted and refracted rays were weaker and not perceived . Thus, a topographically ordered point-to-point representation of the visual world entered the crystalline humor.
Figure 2: Front cover of Brain, Vision, Memory: Tales in the History of Neuroscience by Charles G. Gross (Bradford Book, 1999).
Ibn al-Haytham had used a camera obscura in his extensive optical experiments  and compared it to the eye . (He seems to have been the first to do either). Thus, he realized that if the light rays orthogonal to the curved surface of the crystalline lens continued, they would project an inverted image on the back of the eye.
Since the notion of an inverted image was unacceptable, he postulated precisely the appropriate refraction at the interface between the crystalline humor and the vitreous humor so that the rays leaving the latter would be parallel. Thus, they would provide a right-side-up topographic representation of the visual world to the back of the eye (retina) which he viewed as an extension of the optic nerves . Even after Kepler finally elucidated the formation of the retinal image, the problem of how an inverted image could yield vertical perception perplexed him and was not satisfactory handled until Molyneux and Berkeley at the beginning of the 18th century .
Beyond the receipt of light by a sensitive surface (the crystalline humor for him), Ibn al-Haytham realized that strictly optical considerations were no longer required. He did stress, and correctly so, that the point-to-point representation had to be maintained and conveyed to the ultimum sensus in the anterior part of the brain .
The importance of Ibn al-Haytham's idea of a point-to-point projection of the visual world into the brain cannot be over-emphasized. Indeed, it forms one of the bases of modern visual physiology .
Although the importance of Ibn al-Haytham's theory of vision and its pervasive influence for over half a millenium have been well recognized, there has been relatively little close examination of his original views on psychological processes in perception and their influence. Let me indicate some of his ideas in this area that deserve much more study and attention than they have yet received.
Figure 3: Front cover of Al-Hasan Ibn Al-Haytham: His Optical Researches and Discoveries by Mustapha Nadhif (Beirut, 2008, in Arabic; History of Sciences in the Arab World Series, 8, 999 pp.
He was the first to recognize the crucial importance of eye movement for perception. It is only in recent years that it has been recognized that there is indeed no perception without eye movements and that eye movements are crucial to building up our consciousness of the visual world .
Ibn al-Haytham realized that the reception of light by the eye is only the very first step in perception. Beyond this passive process, active processes such as comparison and memory are required before conscious visual experience occurs .
Particularly startling is his realization that a series of logical inferences must occur before sensation can be transformed by the brain into perception. He stressed that the speed of perception demands that these inferences themselves be imperceptible; that is, unconscious to the observer.
This is a clear adumbration of Helmholtz's theory of unconscious inference that played so major a role in the 19th century and continues to pervade the modern study of vision . It would be valuable to explore to what extent Helmholtz was aware of Ibn al-Haytham's ideas on the role of unconscious inference in perception. Helmholtz does cite Alhazen in other contexts, such as when reviewing previous explanations of the moon illusion . Furthermore, Ibn al-Haytham's use of the time required for a perception has, in the last decade, become one of the principal methods for analyzing the "unconscious inferences" that underlie perception .
Among Ibn al-Haytham's other perceptual insights was his understanding of the crucial role of visual contrast. For example, he realized the color of an object depends on the color of the surroundings  and that contrast of brightness provided an explanation of the invisibility of stars in the day time .
My main conclusion is that this remarkable man deserves much further study. Although Ibn al-Haytham's unique synthesis of physics, mathematics and physiology into a new theory of vision and its historical importance have been recognized, his insights into the psychology of perception and their influence remains an important and potentially fertile area of research.
Further resources on Ibn al-Haytham and the history of optics
Nader El-Bizri: Ibn al-Haytham: An Introduction
FSTC Research Team: Camera of Ibn Al-Haytham
FSTC Research Team: How to Make a Pinhole Camera
Craig Aaen-Stockdale: Ibn al-Haytham and Psychophysics
Charles Savage: Reflections on the Optics of Time
Hüseyin Gazi Topdemir: Taqi al-Din ibn Ma‘ruf and the Science of Optics: The Nature of Light and the Mechanism of Vision
Hüseyin Gazi Topdemir: Kamal Al-Din Al-Farisi's Explanation of the Rainbow
Natasha Sopieva: Ibn Al-Haitham, the Muslim Physicist
Ruveyda Ozturk: Book review: Ibn al-Haytham and the New Optics
Ibrahim Shaikh: Eye Specialists in Islam
Saira Malik: The Influence of Ibn al-Haytham on Kamal al-Din al-Farisi
Saira Malik: Kamal al-Din Abu al-Hasan (or al-Hasan) al-Farisi
 For biographical information, see A. I. Sabra in Dictionary of Scientific Biography, ed. C.C. Gillispie (New York, 1972), vol. 6, p.189.
 (Chicago 1941) and The Vertebrate Visual System (Chicago 1957).
 Alhazen, Opticae thesaurus, ed. S. Risnero (Basel, 1672), book 1, chap. 5, sections 14 and 23; translated in Polyak, op. cit. (1957), p. 17.
 Alhazen, op. cit., book 1, chap. 5, section 19; translated in D. C. Lindbeg (ed.), Alhazen, Opticae Thesaurus Alhazeni Arabis libri septem, New-York/London: Johnson Reprint, 1972, p. 73.
 Ibid, book 1, chap. 4, sections 16 and 23; translated in Polyak, op.cit. (1941), p. 122 and translations from the Arabic MS Fatih 3212 by A. I. Sabra in Studies in Perception: Interrelations in the History of Philosophy and Science, edited by P. K. Machamer and R. G. Tumbull (Columbus 1978), p. 167.
 Alhazen, op. cit., book 1, chap. 5, section 14; translated in Lindberg, op. cit., p. 73.
 Ibid, section 18, translated in Lindberg, op. cit., p. 74.
 Russel, op. cit., S. B. Omar, Ibn al-Haytham's Optics: A Study of the Origins of Experimental Science (Minneapolis 1977), ch. 2.
 Alhazen, op. cit., book 1, chap. 5 section 29; translated in Polyak, op. cit., p. 133.
 Alhazen, op. cit., book 2, chap. 1, sections 2-6; cited in Lindberg, op. cit., p. 81.
 Boring, op. cit., p. 222.
 Alhzen, op. cit., book 1, chap. 6, section 27; translated from MS Fatih 3212 by op. cit., p. 168.
 C. G. Gross, C. J. Bruce, R. Desimone, J. Flemings and R. Gatass, in Multiple Cortical Somatic Sensory-Motor, Visual and Auditory Areas and their Connectivities, edited by C. N. Woolsey (New York 1980).
 E. g.: A. L. Yarbus, Eye Movements and Vision, translated from Russian by B. Haigh (New York, 1967).
 Sabra, op. cit., p. 169; S. B. Omar, Ibn al-Haytham's Optics, op. cit, chap. 2.
 H.V. Helmholtz, Handbuch der physiologischen optic (Hamburg 1909-11); E. G. Boring, A History of Experimental Psychology, 2nd ed. (New York 1950), p. 308; Brett, op.cit., p. 255.
 Helmholtz, op. cit., English translation (1925), vol. 3, p. 360.
 E.g: See M. Posner and R. F. Mitchell, Psych. Rev. 74, 342 (1967).
 Alhazen, op. cit., book 1, chap. 3, cited in Ronchi, op. cit., 48; Boring, op. cit., p. 165.
 Ibid, chap. 1, section 2; cited.
* Princeton University, USA.