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Romancing the Chiasm: Vision, Vocalization, and Virtuosity

2008; Lippincott Williams & Wilkins; Volume: 28; Issue: 2 Linguagem: Inglês

10.1097/wno.0b013e31817a7b5f

1536-5166

Joel S. Glaser,

Hemispheric Asymmetry in Neuroscience

Figure: William Fletcher Hoyt, MD, professor emeritus of Ophthalmology, Neurology, and Neurosurgery, University of California, San Francisco, was born and raised in Berkeley, California. He took his undergraduate education at the University of California, Berkeley and his medical education at the University of California, San Francisco (UCSF). After a year's study at the Wilmer Institute, Johns Hopkins University, under the mentorship of Frank B. Walsh, MD, he returned to UCSF in 1958 to found the neuro-ophthalmology service. During a 36-year academic career-all of it at UCSF-he authored 266 journal articles, co-authored (with Frank B. Walsh, MD) the biblical third edition of Clinical Neuro-Ophthalmology, and trained 71 neuro-ophthalmology fellows. In 1983, he received the title of Honorary Doctor of Medicine from the Karolinska Institute. He is widely acknowledged as one of the titans of twentieth century neuro-ophthalmology. In recognition of his contributions, the North American Neuro-Ophthalmology Society (NANOS), in conjunction with the American Academy of Ophthalmology, in 2001 initiated the Hoyt Lecture to be delivered each year at the Annual Meeting of the American Academy of Ophthalmology.Figure: Joel S. Glaser, MD was born in Brooklyn, New York, and was raised in Orlando, Florida. He obtained undergraduate and medical degrees at Duke University and ophthalmology training at the Bascom Palmer Eye Institute, University of Miami. After a year's fellowship in neuro-ophthalmology with William F. Hoyt, MD at the University of California-San Francisco, he returned to Bascom Palmer. In a 38-year career on the faculty there, he established himself as a world-renowned neuro-ophthalmologist, having trained many of the leaders in the field. He is the author of the respected single-volume textbook Neuro-Ophthalmology, now in its third edition.I would like to thank the North American Neuro-Ophthalmology Society (NANOS) committee for the invitation to speak in honor of William Hoyt, who has been for me, as for so many of us, an exacting teacher, a generous colleague, and a warm friend. While still a medical student, I was introduced to the optic chiasm during a Fight for Sight student fellowship at the University of California, San Francisco. Bill Hoyt assigned me the task of cutting out plastic templates of coronal sections of the optic chiasm, then pasting on the locations of crossed and uncrossed fiber pathways, and stacking the sections like slices of salami to form a three-dimensional model (Fig. 1). This labor was performed by hand, with a No. 15 Bard-Parker scalpel blade and normal clotting time.FIG. 1: Models of the optic chiasm constructed by Glaser, with representation of crossed and uncrossed visual fiber pathways. A. Anterior view. B. Posterior view. M, macular projection; IN, inferior nasal fiber pathway; SN, superior nasal fiber pathways.The previous Hoyt Lectures have featured timely topical reviews presented by skilled clinical scientists. I invite you instead on a journey, an adventure that includes a remarkable cast of historical figures-a tale of double-crossings and choosing sides, of black magic and alchemy, of misplaced anatomy and stunning intellectual insights, of why the motor system is crossed, and, especially, why the left brain is master of the right hand. And finally, to consider the synergy of vocalization with the virtuosic skill of the right hand. Considered the most mystical of the senses, the mechanism of vision has been for ages an intriguing philosophical question. In the ancient world, adolescent science was “natural philosophy,” and in ancient Mesopotamia, the healing arts were the equivalent of magic practiced in religious temples (Table 1). The concept of the eye throughout the classic Greek period (400-300 before the common era, BCE) was characterized by a preoccupation with the flow of “humors” and the emission of “rays” originating from the eye (extramission). In 450 BCE, Empedocles declared that “…the eyes project rays that perceive objects and return” (1, p. 35). Even in the infancy of human intelligence, of course, it had been observed around campfires at night that animals' eyes glowed, suggesting that the sense of vision originated in the eye itself. This concept remained mostly unchallenged for a long number of centuries. Indeed, the majority of ancient physicians and philosophers believed in the idea of the proactive role of the eye, including Plato who, in the 4th century BCE, wrote that light emanated from the eye, seizing visual objects with its rays. More metaphorically, Theophrastus, a disciple of Aristotle, stated that the eye had “fire within,” contradicting his teacher. Aristotle was among the first to reject the extramission theory of vision: “In general, it is unreasonable to suppose that seeing occurs by something issuing from the eye.” Furthermore, if vision indeed originated within the eye, Aristotle reasoned, “Why should the eye not be able to see in the dark?” Aristotle advocated the theory of intramission, by which the eye received rays rather than actively directing them outward (1, p. 41).Table 1: Ancient world natural philosophy and scienceBoth Hippocrates and Aristotle considered that the eyes were attached to the brain by hollow tubes that permitted the “principle of vision” to flow away from and toward the brain. Humors would course from the brain down the hollow optic nerves into the eye, pass into space and apprehend the object of regard, return to the eye, be sensed by the lens, and conduct the image centrally where it would be recognized. An overflow of humors, a cataract, would cloud the eye. Although Galen is credited with the initial description of the optic chiasm in the 1st century CE (or common era), our journey starts 2200 years before Hippocrates, during the third dynasty of the pharaoh Geozia (or Djoser, circa 2630-2611 BCE), where we find an extraordinary genius, Imhotep (2), in many ways the spiritual father of the great physician Galen (and of Leonardo da Vinci). Imhotep was the right-hand man of the pharaoh, serving as grand vizier, banker-treasurer, architect and scribe, physician, and also chief priest of Ra at the Temple of Heliopolis. In this role, Imhotep acted as royal embalmer and practiced the art of mummification, becoming familiar with the anatomic contents of body cavities including the thoracic and abdominal viscera. Pulled out through the nose, the stuff of the brain was not likely to provide any neuroanatomic details. An ancient polymath arguably equal to a Leonardo, Imhotep designed and built the layered step pyramid in the burial complex of the pharaoh at Saqqara, the first functional building of which we are archeologically aware. Imhotep's architectural theories formed the basis for all later pyramid constructions. While the Druids had built Stonehenge (circa 2900 BCE), a Neolithic circle of quarried stones erected on Salisbury Plain, this structure was intended for seasonal calendar calculations, possibly a religious erection, but it definitely does not qualify as a functional building as does Imhotep's step pyramid. According to no less a personage than Sir William Osler, Imhotep was the original “Father of Medicine, the first figure of a physician to stand out clearly from the mists of antiquity” (3). Imhotep authored a lengthy medical treatise, the Edwin Smith Papyrus, remarkable for being devoid of magical thinking and containing anatomical observations and details of ailments and cures. Imhotep's career was so remarkable that a series of schools of healing were established in his name. Some centuries after his passing, Imhotep was deified, worshipped as the divine son of the principal Egyptian god Ptah, creator of the universe. At ancient Memphis, a cult center, Asklepion, was founded, and herein lies a major connection with Galen almost three millennia later. The evidence afforded by Egyptian and Greek texts supports the view that Imhotep's works and reputation were widely respected and admired by the Greek natural philosophers, including Pythagoras and Hippocrates. His prestige increasing with the passage of centuries, Imhotep's temples became, in ancient Greece, the centers of medical healing and teaching. Greek proto-scientists were intrigued by the possibilities of uncovering by careful observation and thoughtful experimentation the fundamental systems operative in nature, to find the “finger-prints” of the gods to provide evidence that confirmed an ordered universe. There is the improbable myth of Pythagoras, wondering at the spectrum of tones when hammers of different weights resounded on anvils. By experimenting with a monochord consisting of a single stretched gut string supported by a moveable bridge (the bridge effectively lengthens or shortens the vibrating segment, even as a musician's fingers press on strings to shorten the vibrating portion), Pythagoras discovered that the pitch of a note depends on the length of the string and that concordant intervals in a musical scale are produced by simple numerical ratios. With variable lengths of gut string, Pythagoras elaborated a series of whole number ratios (for example, 1:2, 2:3, and 3:4, constituting the “celestial harmonies of Pythagoras”). The essence of nature was mathematical, elegantly established by the disclosure of the presence of a whole number series of ear-pleasing harmonic ratios, which not incidentally comprised the first musical scale that ultimately became the seven-tone dominant scale of Western music. This discovery was a stunning breakthrough, a corner of science rounded. Harmonic ratios were found not just in music but embedded in the structure of an ordered cosmos; quality became quantity. Do not think that Pythagoras is a minor figure in science. With music, “the harmony of the spheres,” the mathematization of human experience and true quantitative science began at the same point, and the contributions of Pythagoras were fundamental. Nor was he forgotten by Kepler and Newton. Pythagoras considered the earth a sphere and proposed a heliocentric system of planets. As eloquently expressed in Koestler's book The Sleepwalkers (4): “…harmony emerges from chaos. The maestro is Pythagoras of Samos, whose influence on the ideas, and thereby, on the destiny of the human race was probably greater than that of any single man before or after him.” Recall that the step pyramid of Imhotep was based on a seven-layer structure, and that Plato's cosmology consisted of seven cosmic rings (synonymous with the seven Pythagorean spheres, and equivalent to the number of named notes in the musical octave, let alone the number of days of creation, the seven wandering stars-the planets-observed by the Babylonians, and the seven Gates to Paradise). Once it took root in Plato's philosophy (in Greek, “love of knowledge,” a word invented by Pythagoras), the concept of the universe as a musically and numerically tuned system rapidly became the standard throughout the cultured Mediterranean world. Alexander's general and successor, Ptolemy the First, who like Alexander was a student of Aristotle, ruled Egypt and established the university and library at Alexandria (circa 320 BCE), where science flourished. At the illustrious Alexandrian university, Euclid, the genius of optics and mathematics and best known as a geometrist, described spatial vision (the visual field) as a cone thousands of years before Traquair (5)! Also at Alexandria was Herophilus, who taught and practiced medicine and addressed the nature of nerves by human dissections, at times performed in public. In the Roman period we also recognize the name of Celsus (14 BCE-37 CE), whose medical encyclopedia De Mediciná surveyed medicine, surgery, and dentistry. It was in this cultural and scientific milieu that young Galen would come to study. In ancient Asia Minor in the 2nd century CE, Pergamon (Bergama in modern Turkey) was a major center of learning especially dedicated to healing practices and the site of the second largest library of antiquity, the largest being in Alexandria. When papyrus imports from Egypt dried up, the Pergamenes substituted parchment (pergamenum), a local invention. The library accumulated an estimated quarter million volumes, but unfortunately the love-smitten Mark Antony made a gift of these to Cleopatra VII, the last Greek ruler in Egypt, and this Asiatic treasure along with the Alexandrian collection of Greek philosophy and literature was eventually burned or scattered to Constantinople, Damascus, and Baghdad. Born in the sophisticated and intellectual city of Pergamon, Galen (130-201 CE) was home-schooled by his wealthy father Nicon, the king's architect, and so was well versed in mathematics, logic, astronomy, natural philosophy, art, and architecture. At age 17, Galen became a student-attendant (therapeutes in Greek) in the healing temple at Pergamon. The word Asklepion, originally used for the school or cult of Imhotep at Memphis, had eventually been changed to Askelep, or Askelepius, and became the name of the mythologic Greek god of healing (5). Thus, Galen as a teenager was the true inheritor of the ancient art of medicine, training at the Temple of Pergamon dedicated to Askelepius, the Hellenized name of the “first physician,” Imhotep. The works of Galen reached 22 volumes, dealing with philosophy, anatomy, and medicine, including commentaries on Hippocratic texts. His writings on anatomy, although eventually proved to be scientifically stagnant, were preserved as the core curriculum of medieval physicians. Galen's primary theory was that four body humors (blood, yellow bile, black bile, and phlegm) were each related to the elements of matter (fire-hot, air-dry, earth-cold, and water-wet) and that illness occurred when a humor became excessive or deficient. Galen also perpetuated the extramission theory of vision that supported his concept of sight as an optical pneuma (gas, vapor, breath, or animal spirit) that flowed from the brain to the eyes through hollow optic nerves. Based on the works of Rufus of Ephesus, one of the anatomists who dissected in Alexandria, Galen described the retina, cornea, iris, uvea, tear ducts, and eyelids, as well as two fluids, the vitreous and aqueous humors. According to Galen, “…a round lens in the middle of the eye…is the principal instrument of vision, a fact clearly proved by what physicians call cataracts, which lie between the crystalline humor and the cornea and interfere with vision until they are couched.” Indeed, Galen himself performed cataract couching (6). Galen intuited that the retina and optic nerve were part of the brain, stating that “Its chief function, that for which it was sent down by the brain, is to perceive the alterations that occur in the crystalline lens and to communicate them to the brain” (1, p.45). Galen's direct observations of the optic nerves bear quoting (7): “…originating in different places, the optic nerves unite with each other but afterward diverge from each other again…for nature has not interchanged them…the shape of these nerves is very much like the letter chi. If anyone should dissect them rather carelessly, he would perhaps believe that they interchanged…but this is not true; for when they met within the cranium, they united their courses and then again separated, indicating clearly that they came together for no other reason than they might unite their courses…” [My italics]. This is an extraordinary conclusion because Galen was pragmatic, a diligent student of anatomy for 12 years, who published On the Usefulness of the Parts of the Body. Galen insisted that “…nature does nothing without a reason.” There was an answer for all questions, but Galen contradicts himself with regard to his observation on the optic chiasm, concluding that the optic nerves “…came together for no other reason than they might unite their courses...and then again separated.” Exceedingly successful as a physician, Galen at age 22 served the household of Emperor Marcus Aurelius and the stable of gladiators in Rome. As the team physician to the gladiators, he gained valuable practical experience in trauma and, shall we say, sports medicine. So great was Galen's influence that his anatomic dogma went practically unchallenged for 1400 years. As it turns out, Galen's observations tended to have little to do with human anatomy, as he had dissected principally cats, dogs, Barbary apes, and especially the pig, which he declared “…the animal most similar to man.” (For centuries human dissection was forbidden by the Church but also no doubt was considerably retarded by the absence of proper preservation or refrigeration). The most important physician of the Roman Empire, and arguably the most influential physician in medical history, Galen wrote entirely in Greek, and his medical writings preserved today are voluminous. Most were translated into Arabic in the 9th century in Baghdad, and through those translations Galen became the most important formative influence on medieval Islamic medicine. The Ummayad conquest of Spain and Sicily (661-750 CE) led to the revival of learning in medieval Western civilization, with a transfusion of Indian, Persian, Hebrew, and Greek knowledge, including astronomy, mathematics, philosophy, religion, and medicine. To al-Andalus (Spain), the Arabs brought linen paper from Samarkand and Baghdad, the abacus, algebra (al-jabr), chess, soft pillows (almuhada in Spanish) and the concept of the university based on older institutions in Fez and Cairo that offered a variety of academic degrees. During the centuries of the Christian reconquista (900-1400 CE), Arab and European scholars translated into Spanish and Latin the writings of Plato, Aristotle, Euclid, Galen, and other great works of antiquity, again stimulating a renaissance of Western intellectual life (8). With the waning of classical Greek and Roman civilization and an atrophy of accumulated knowledge, medieval Europe had been intellectually asleep for a thousand years. “Natural philosophy” passed into the hands and minds of Arab and Persian scholars, among whom vision and optics were the subjects of extraordinary interest. But human dissection was taboo, an almost insurmountable barrier to progress in anatomy and physiology. Galenic anatomy and physiology were universally accepted, but deviating opinions were expressed. Scholars such as Al-Rhazes (864-930 CE), who alone contributed more than 200 scientific treatises using neuroanatomical details for precise clinical localization, described (after Galen) seven pairs of cranial nerves, but refuted the Galenic doctrine that visual rays emanated from the eye (9). Ibn Sina (Avicenna, 980-1037 CE), author of the five volumes of The Canon of Medicine, also overturned the idea that the eye emits rays that bounce off viewed objects, stating that “…it is not the ray that leaves the eye and meets the object that gives rise to vision, but rather the form of the perceived object passes into the eye and is transmuted by the transparent body, the lens” (10). Al-Rhazes and Ibn Sina hinted at the possibility of total decussation of the optic nerves at the chiasm (11). In the Book of Optics (1021 CE) of Abu Ali Ibn Al Haytham (Alhacen, 965-1039 CE), a scientist and physician born in Basra, we find the oldest known pictorial representation of the chiasm (Fig. 2), actually redrawn after Greek concepts. Al Haytham hinted at the functional role of the chiasm, suggesting that only after the superimposition and fusion of the two monocular images here can a single visual experience occur at a higher center, the ultimum sensus (12, p. 83). With a rather simple experiment, Al Haytham contributed to the reversal of the ancient extramission concept that rays project from the eye to the objects of regard. Al Haytham looked briefly at the sun and noted that “…anyone who looks at a very strong source of light will experience both pain and damage to his eyesight…” and that the after-images persisted after closing one's eyes and looking away. He concluded that “…there is no vision unless something comes from the visual object to the eye…” (1, p. 78). Al Haytham argued that objects are seen because they reflect sunlight. He elaborated the laws of refraction, postulating point-to-point projection of “perpendicular lines” from objects to the surface of the eye. He rather amazingly considered the phenomenon of light to be streams of minute particles (10,13).FIG. 2: Diagram of visual system acknowledging Galen, from Al Haytham's Book of Optics (1083 CE). Note hollow optic nerves merging in common cavity (asterisk) at chiasm (From Ref. 12, p. 84.).Among those scholars who profited from the transfer to Europe of Islamic intellectual baggage was Albertus Magnus (Albert the Great; circa 1197-1280 CE), a Dominican friar and follower of the Aristotelian school of method. Albert was familiar with the theories of both Galen and Al Haytham, as well as the medical works of such remarkable polymaths as Ibn Sina, Ibn Rushd (Averroes), and Moshe ben Maimon (Maimonides). In Paris, Albert contested scientific concepts with Roger Bacon, the mathematician and astronomer. Indeed, it was Bacon who first recognized and discussed with Albert the visible color spectrum as discerned in a glass of water four centuries before Isaac Newton (1642-1727 CE) discovered that prisms could disassemble and reassemble white light. Thomas Aquinas was Albert's sometime student and Albert is mentioned in Dante's La Divina Commedia. On the practice of medicine, Albert commented that it was “…a suitable occupation for manually adept members of the lower classes” (14). Albert described deformation and saccadic phosphenes, nystagmus in alcoholics and in sailors after long sea voyages. He commented on color theory and, as with Empedocles years before Traquair, stated that “…all vision takes place in the form of a pyramid.” Albert may be credited with the initial clinical insight regarding the role of visual decussation, stating in De Sensu et Sensato that “…a soldier who was wounded in the left temple, lost vision of his right eye, which certainly happened because of the crossing of the nerves directed from the eyes to the front of the head…” (14). It was not physicians but Renaissance artists who took up the exercise of human dissection to refine the anatomical accuracy of their skills in painting, drawing, and sculpture. In the early 16th century, Leonardo da Vinci admonished his fellow artisans that “Those artists who are enamored of practice without science are like sailors… without rudder or compass…practice must always be founded on sound theory…” (15). Through human dissection, Leonardo produced the earliest existing anatomic representation of the chiasm (Fig. 3) in about 1504 CE. Given the wide breadth of his interests, Leonardo spent an inordinate amount of time dealing with the phenomenon of vision, the anatomy of the eye, and how images are formed. Leonardo did not alter the Galenic dictum that the optic nerves were hollow, but he did conclude that the lens was not the sensory organ and that the visual image was formed on the retina. By casual observations, he corroborated Al Haytham's earlier conclusion. In his notebook, Leonardo stated that: “If you look at the sun or other luminous object and then shut your eyes, you will see it again in the same form within your eye for a long time: this is a sign that the images enter within it.” Intrigued by the precision of conjugate eye movements, he asked, “whence the turning of the eyes when one draws the other after it?” and answered, as follows:FIG. 3: Leonardo da Vinci's anatomic drawing of the chiasm, 1504. (From Ref. 6, p. 56).“Saw a head in two between the eyebrows in order to find out by anatomy the cause of the equal movement of the eyes, and this virtually confirms that the cause is the intersection of the optic nerves” (15). Of course, here Leonardo is speaking of the chiasm itself! The extraordinary notebooks and pen and ink drawings of Leonardo could not be technically reproduced or disseminated, nor would he have permitted it. These treasures eventually came into the possession of Charles I of England and were not made available until they were finally published in the early 20th century. Although physicians occasionally took bold issue with visual humoralism, its legitimacy was not seriously challenged until 1543, when the 28 year-old Belgian Andreas Vesalius (Wesel) in Padua published De Humani Corporis Fabrica. Vesalius pointed out some 200 errors in Galen's description of anatomy, due primarily to its being based not on direct observation of the human body, but on fanciful and imaginative suppositions after animal dissections. According to Vesalius, “Each nerve of the first pair [optic] under the base of the brain where it rests on the sinus in which the gland which excretes the pituita from the brain is led forward somewhat obliquely, the right nerve extending toward the left and the left toward the right, and then both come together and are intermingled so that in no way can you separate the right from the left [my italics], so much so that it would be wholly fruitless to attempt to determine whether in this junction the right nerve remains on the right side or is led to the left side by an uninterrupted connection”(7, p. 163). Vesalius saw no tubes, but did not dispute the humor-tube theory propounded by the Greeks, so powerful was Galen's continuing influence. With Vesalius, however, true scientific methodology-the rejection of dogmatic technologic authority revered but not supported by reproducible critical observations and proofs-had been born. Of the personalities interested in the phenomenon of vision, the list reads like a “Who's Who of History.” In the 17th century, Rene Descartes (1596-1650 CE), who entered college at age 8 and was learned in astronomy, geometry, and optics and practiced occasional vivisection, postulated a novel concept of the intrinsic organization of the visual system. Descartes hypothesized that the left retina projected to the left ventricle of the brain and the right retina to the right ventricle. Why the ventricles? Galen had said so! According to Descartes, the afferent images are then transmitted to the pineal gland and merge there “…the mind and body interact in the pineal…” (7, p. 163). Why the pineal gland? Finding it the only unpaired midline organ of the brain, Descartes concluded that this was the seat of the soul and of conscious appreciation and that it received impulses from double sensory organs such as the retinas (Fig. 4). Thus, it was Descartes who first suggested cerebral retinotopic representation, calling the pineal the neural center for fusion of visual images.FIG. 4: Illustration of the visual pathway from Descartes' Tractatus de Homine, 1686, indicating his theory of the binocular stereoscopic visual system through topographic (retinotopic) projection to the brain. Note right nasal fibers 5-6 and left temporal fibers 5-6 meeting at point c on pineal H, forming a “syndynamical” idea. There is no chiasmal decussation. (From Ref. 12, p. 103).The London surgeon Thomas Willis, Sedleian Professor of Natural Philosophy at Oxford University, was the first to number the cranial nerves in the current order in his celebrated Cerebri Anatome published in 1664. Willis sought every opportunity for “…opening heads…to establish a more certain Pathologie of the Brain….” The illustrator of Willis's anatomical text was Sir Christopher Wren, the equally famous London architect who assisted in the rebuilding of London after the tragic fires of 1661. Wren had been a mathematician, astronomer, and anatomist before turning to architecture (16). Willis's experiments to elucidate cerebral arterial circulation owed their origin to Wren's technique of chirugia infusoria, the intravenous injection of various colored fluids such as “saffron, ink and milk” (17). Speculating that the fibers of the optic nerves are united and mixed at the chiasm, Willis stated that “The growing together of these optick nerves, and their again being separated, seems to be ordained for this end: that the visual images received from either eye might appear still the same and not doubled…” (18). Thus, a general surgeon, famous or otherwise, had declared that the chiasm served as a mechanism for the purpose of fusing the images of the two eyes and preventing double vision. Willis also stated that the optic nerves were not hollow, although humors of a sort still percolated and flowed along nerves. After 1400 years, the anatomical theories of Galen and the model of the hollow, humor-filled optic nerves of antiquity had finally been refuted. Wren was an acquaintance of the astronomer Edmond Haley, for whom the popular comet is named. These two founders of the Royal Academy of London questioned why the orbits of the planets were elliptical rather than strictly circular. They theorized that this phenomenon had to do with the effects of an “attraction” acting at a distance. It was to Newton, a brilliant mathematician at Cambridge, to whom they took this question. The young Lucasian Professor of Mathematics at Cambridge, a chair currently occupied by Stephen Hawkins, had declared that the attraction of bodies, or gravity, is inversely proportional to the square of the distance between them. With no firsthand working knowledge of anatomy, Newton published the earliest account of partial decussation of the optic nerve fibers (Fig. 5). Appended to the 1706 First Edition of Newton's book Opticks are a series of “Fifteen Queries.” Rhetorically, Query 15 asks:FIG. 5: Diagram of the hemidecussation concept of Newton, 1705-1706. Note right temporal point A joining left nasal point α in the chiasm at point p, continuing to theoretic cortex point a. (From Ref. 7, p. 648).“Are not the Species [images] of Objects