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Latent Heliotropism COMMENTARY 1327 Br J Ophthalmol: first published as 10.1136/bjo.86.12.1327 on 1 December 2002. Downloaded from Heliotropism reactions. By reducing instinctive behav- ................................................................................... iour to a series of tropisms and chained reflexes, Loeb postulated that the ani- mal, which appears to move purposively Latent heliotropism and of its own will, is actually forced to go where it is carried by its legs and M C Brodsky wings.910 Although many elements of the automaticity of animal behaviour ................................................................................... were subsequently disproved by Samuel 11 Our past is always with us O Mast and others, the importance of both plant and animal heliotropism is indisputable. (Philosophers such as he cover photographs and illuminat- horizontal plane and in postural abnor- Joseph Campbell have even contended ing essays that accompany each new malities. In this context, the tonus that plant heliotropism is a manifest Tissue of the BJO remind us that ocu- imbalance that accompanies human form of consciousness to the surround- lar anatomy and physiology can only be congenital hemianopia bears a striking ing environment!12) As summarised by fully understood as a function of evolu- resemblance to the heliotropic posturing Duke-Elder “ . .the control of the move- tion. I have become convinced that many of lower animals under conditions of ments of living organisms, both plants 7 evolutionary clues to human movement asymmetrical illumination. and animals, by light is a fundamental disorders lie buried within the ancient function of great phylogenetic age, pre- biology literature. For example, the torti- By redirecting our attention ceding the acquirement of vision and, collis that accompanies congenital ho- indeed, leading directly to its develop- monymous hemianopia may attest to the to the posture of our ment. The association of the functions of primitive role of vision in establishing equilibration and orientation with the baseline muscle tone. Humans born with patients with congenital visual system of higher animals is in homonymous heminanopia maintain a visual disorders, we can every sense basic.”13 curious head turn away from the side of begin to deduce the role Within certain limits, heliotropic re- the seeing visual field.12A patient with a sponses in plants and animals are a congenital right homonymous hemiano- of vision in regulating function of the Bunsen-Roscoe law of pia will tonically turn the head away human muscle tonus photochemical reactions.10 11 This law from the seeing left visual field and states that, in a light reaction, the effect towards the right shoulder, while the In 1832, Augustine P Candolle intro- is proportional to the simple product of eyes are maintained in a leftward devi- duced the term heliotropism to describe 12 the intensity of the stimulus and time. In ated position within the orbits. Since the turning movement (tropism) of visual fixation remains on the object of the robber fly, for example, covering one plants towards or away from the sun eye induces a flexion of the limbs on the regard even when the head is turned, 8 (helio). A bending towards the sun side of the animal with greater lumi- this torticollis does not seem to serve any came to be designated as positive helio- obvious compensatory function for nance and an extension of the limbs on tropism while a bending away from the 14 vision.3 Patients with congenital ho- the side with lesser luminance. The sun was designated as negative alterations of tonus are directed in such a monymous hemianopia are generally heliotropism.9–11 Since heliotropic behav- unaware of their abnormal head posture way as to assist in turning the body 14 iour was also recognised in unicellular http://bjo.bmj.com/ and are unable to explain why they towards the light. When a robber fly organisms as well as insects, fish, and maintain it. In patients with acquired climbs a vertical cylinder, covering one other vertebrates, ethologists at the end homonymous hemianopia, torticollis eye or covering the homonymous half of of the 19th century, influenced heavily does not manifest unless the causative the visual field of both eyes causes the by Darwinian theory, became vigorously hemispheric injury occurs in infancy.34 animal to move in a spiral trajectory engaged in the study of animal Proposed explanations assume that towards the light, with the number of heliotropism.9–11 Controversy ensued the torticollis must serve a compensatory circles per unit distance proportional to when Jacques Loeb, the objectivist Ger- the intensity of the light.14 function for visual orientation or naviga- on September 25, 2021 by guest. Protected copyright. tion. It has been suggested that the head man biologist, promulgated the icono- In 1944, Herbert S Wells argued that turn may be an adaptive response to clastic view that animal instinct could be humans possess a latent positive helio- frontalise the visual environment rela- reduced to the same elementary physico- tropism. In decerebrate animals, new- chemical laws which determine plant born infants, and functionally decer- tive to the body, that it may permit the 9 patient to use saccades to increase the tropisms. According to Loeb, animal ebrate or decorticate adult human effective visual field during ambulation, heliotropism occurred when the exciting subjects, rotation or tilting of the head to or that it may serve to minimise a stimulus (light from one side) induced the side results in extension of the fore subclinical nystagmus that is damped in or accelerated chemical reactions in a and hind limbs of that side and flexion of 910 one lateral field of gaze.1–4 portion of the retina. The mass of the limbs of the opposite side.15 During Torticollis is a manifest tonus imbal- photochemical reaction products formed levitation of the arms, turning or tilting ance of the neck muscles. Tonus de- in the retina passed directly through the the head to the right, turning the eyes scribes the state of excitation of a living central nervous system (which was con- strongly to the left, or shining a strong muscle during rest.5 Recognising that the ceptualised as a sort of protoplasmic light into the eyes from the left increases visual system exerts influence on tonus bridge) to influence the tension or abduction of the right arm and dimin- of the body musculature via the central energy production in the locomotor ishes or abolishes tonus in the left arm.15 nervous system, Meyer and Bullock pro- appendages on both sides of the body, A forerunner of this response can be seen posed that the eyes function not merely until a new plane of symmetry was in the classic fencing reflex of infants. as sensory organs but as components of a re-established between the two eyes and Turning an infant’s head to the left multimodally driven tonus pool that the source of light.910 causes the right arm to flex and come calibrates baseline muscle tone (that is, This radical theory challenged the over the head and the left arm to extend tonus inducing organs).6 In many inver- anthropomorphic and teleological con- in a classic fencing posture.16 The protec- tebrates, asymmetrical visual stimula- ception that animals sought or avoided tive function of this reflex is obvious— tion results in turning movements in the light based on voluntary or instinctive when starting to fall to one side, it is www.bjophthalmol.com 1328 COMMENTARY Br J Ophthalmol: first published as 10.1136/bjo.86.12.1327 on 1 December 2002. Downloaded from advantageous to turn the head towards in humans. Mechanistically, a heliotro- 5 Ewald JR. Physiologische Untersuchungen ber the side of the fall and to extend the arm pism would circumvent any element of das Endorgan des N Oktavus. Wiesbaden: will or choice on the part of the indi- Bergmann. on that side to block the fall. Since sun- 6 Meyer DL, Bullock TH. The hypothesis of light comes from the sky above, and ani- vidual; the head simply goes where the sense-organ dependent tonus mechanisms: mals in the earth’s gravitational field fall neck muscles pull it.9 Many patients with history of a concept. Ann NY Acad Sci to the ground below, a bright light from congenital hemianopia have an associated 1977;290:3–17. the left elicits the same reflex tonus hemiparesis, with increased flexor tonus 7 Brodsky MC. Vision-dependent tonus mechanisms of torticollis: an evolutionary imbalance as a fall to the right. on the side of the hemianopia (that is, perspective. Am Orthop J 1999;49:158–62. Another latent heliotropism may reac- opposite the side of the abnormal cerebral 8 De Candolle AP. Physiologie végétale. Paris: tivate when congenital strabismus pre- hemisphere).18 Thus, one could even ques- Béchet, 1832. cludes binocular vision in humans. Many tion whether the non-heliotropic tonus 9 Loeb J. Der Heliotropismus der Thiere und fish and insects exhibit a dorsal light inducing effects of a head turn might be seine Uebereinstimmung mit den Heliotropismus der Pflanzen. Wurzburg: reflex in which the eyes and body rotate in actively utilised in some patients to Georg Hertz. space to align with the direction of increase extensor tonus on the paretic 10 Loeb J. The organism as a whole. London: incident light.17 In humans with congeni- side. By redirecting our attention to the GP Putnam and Sons, 1916:253–85. tal strabismus, covering one eye causes it limb and body posture of our patients 11 Mast SO. Light and the behaviour of organisms. 1st ed. London: Chapman and to drift dorsally (towards the light). I have with congenital visual disorders, we can Hall, 1911. proposed that this dissociated vertical begin to deduce the role of vision in regu- 12 Campbell J.
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