Visual Perception of Accelerated Nitrogen Nuclei interacting with the Human Retina

During Apollo 11-15, astronauts observed randomly occurring light flashes and streaks.1 Among the explanations offered is that they are caused by direct interaction with fast, heavy primary cosmic rays with the retina2-6. Other suggestions include the assumption that Cerenkov light from relatavistic cosmic ray particles7,8 or fluorescence from the lens9 causes the effect. Previously we demonstrated that flashes are observed in single recoil events from fast nearons in the hundred MeV energy domain2,3 and by actions in the eye from fast neutrons of 25 MeV maximum kinetic energy6. Discrete flashes are generated by neutrons with energy ranges between 3 and 8 MeV (ref. 10), but not by neutrons of fission spectrum energies6. Recently, it was demonstrated that flashes and streaks are seen when single low velocity helium ions (about 240 MeV) stop in or cross the retina of the human eye11,12. These particles, with a linear energy transfer greater than 10 keV per micron, were seen by two subjects (T.F.B. and C.A.T.) with a 40% detection efficiency in a 4 mm diameter beam od 10 particles per second.

Visual phenomena have now been observed in high-energy nitrogen (7+) beams produced at the Berkeley Bevatron13. Using a nitrogen beam deflected at about 266 MeV/nucleon (or 3.72 BeV kinetic energy), three scientifically trained subjects made a series of observations (E. M. McMillan, physicist; P.K. Chapman, astrophysicist; and C.A.T.). These observations confirm earlier hypothesis and argue for electronic excitation in or near the outer segments as the important mechanism.

Figure 1 shows the general layout of experiments. Exposures were made near the focal point where horizontal and vertical dimensions were 3 mm and 20 mm respectively. A beam of 103 to 4 x 105 particles per pulse was detuned to decrease the particles to about 15/pulse by reducing the amplitude of the r.f. of the linear accelerator. Pulses were 1 ms duration every 4 seconds. The nitrogen particles of range 11.9 cm were slowed by an interposed variable water absorber14 so that they stopped in the vicinity of the eye or brain under study. The beam was collimated to the 6 mm diameter by a 7 cm long brass aperture. Particles emerging from the aperture passed through a plastic scintillator approximately 3 mm thick, which counted individual particles administered to the subjects; the ionization chamber gave measurable indication only if the beam pulse had more than 103, which did not occur. Elective and automatic safet controls on beam plug and Faraday cups were available. Our Human Use Committee gave a limit of 2,000 particles per person -- considerably fewer than the number of flashes experienced by astronauts in a lunar flight. The eye exposure was less than 0.1 rad.

From measurements of beam quality, we know that the beam had only insignificant contamination of heavily ionizing particles appearing to be fast, heavy secondaries or accelerated 16O8+ ions. Where the nitrogen particles stopped, the beam composition was greater than 60% (N7+) particles, the remainder being fragmentation products of nitrogen, fast protons, and helium ions15. The heavy fragments stopped in the vicinity of the N7+ range, whereas protons and helium ions had greater penetration. The scintillation monitor was biased so that 75% of the counts recorded by it were due to N7+.

Figure 2 is a simplified anatomy of the left eye in horizontal section showing three regions where various beam positions intercepted visual nervous structures. The accuracy of alignment was plus or minus 2 mm using a custom-moulded head holder.

When the beam was passed through the vitreous body or optic nerve, no streaks or flashes were seen. On passage through the posterior globe, discrete bright flashes, sometimes in clusters or interspersed with streaks, were seen as shown in Figure 3. The colourless streaks were usually horizontal, and most were observed in the upper visual field. C.A.T. thought the events observed in the nitrogen beam were brighter than events he observed in the helium ion beam11. 25% of the flashes may have been due to various heavy fragments, helium ions and stopping protons.

Two observers noted a balooning-out on the right (Fig. 3). The same observers described motion, mostly from left to right -- which was the trajectory of the beam. One careful observer (E. M. M.) did not note motion or shape to straks. Motion in streaks was also reported by C. A. T. and T. F. B. in the helium ion beam, and five subjects in the University of Washington neutron beams6. Particles transect the eye in about 10-10 seconds; thus the motion sense is a psychophysical phenomenon without clear explanation at present.

Throughout these experiments (and helium ion experiments) visual phenomena were seen only when the beam was positioned in the posterior portion of the eye (Fig. 2). In this position the efficiency vaties with observer, and probably with atomic number, velocity, and flux density11. In the nitrogen ion beam at 10 to 20 particles/pulse, observers reported stars, flashes, or streaks equivalent to 40% of particles in the beam path on entrance into the head.

From interviews with astronauts, we reconstruct the events seen by them in space flights as follows: Stars -- minute, dim light flashes; bright flashes -- like a camera photoflash 100 m distant; streaks -- pencils of light varying from dim, thin lines to bright, white lines; supernovae -- bright single flashes of slightly more than momentary duration surrounded by a halo less than half a degree in diameter; luminous clouds -- diffuse events resembling clouds illuminated from behind. These events are depicted in Figure 4. In the nitrogen beam, flashes and streaks were seen; E.M.M. and C.A.T. noted interrupted streaks, sometimes also termed "coincidences" or "doubles". P.K.C. described a thick streak ending in several individual streaks. This type of event might relate to nuclear fragmentation.

The best response was found when particles transversed the skin, zygomatic bone, and soft tissue, stopping just past the centre of the retina (approximately 4.6 grams per centimeter squared of residual range (in water)). When the beam was stopped in the right portion of the left retina, P.K.C. noted phenomena in the left visual field, and when stopped in the left retina, the particles appeared in the centre and right visual field, as expected.

Dependance of this phenomena on dark adaption was shown by E.M.M. who was light adapted before entering the positioning mask. He received 6 to 10 particles in three bursts separated by a few minutes. Approximately 12 minutes elapsed between commencement of dark adaption and experience of flashes. He reported these flashes as dimmer than those seen when fully dark adapted. This experience corroborates that with helium ions, and is consistant with astronaut observations.

In one series, 800 particles in bursts of 2 to 40 were passed through the vitreous humour 1 cm anterior to the posterior portion of the eyeball of P.K.C. -- no phenomena were seen. Fluorescence in the visual spectrum was less than 2 photons per micron when a beam of 104 nitrogen particles was passed through quartz chambers containing rabbit vitreous fluid, retina, and lens. Fluorescence in the visual spectrum is an unlikely mechanism; however, locally deposited ultraviolet photons are still candidates, as they would not be detected by the physical experiment.

Because Penfield16 and others have evoked visual phenomena in conscious patients by electrically stimulating the physical cortex during surgery, we explored the effect of nitrogen particles on the cortex of one dark-adapted subject (C.A.T). The 6 mm beam was aimed at the visuosensory area of the calcarine fissure, left occipital lobe. The subject wore a dark adaptation mask with a water-filled plastic compartment (Fig. 1) that allowed controlled penetration of the beam into the brain when used in conjunction with the variable water absorber. A total of 900 stopping nitrogen particles were used in 9 positions of various depths in the left occipital lobe. No visual phenomena were noted.

Local energy deposition ( linear energy transfer) necessary for this phenomenon, as determined with helium ions, appears to be greater than 10 keV per micron (stopping power of 108 eV are well localized and are different from the diffuse sensation grams per centimeter squared). Light flashes and streaks caused by single particles reported in fields of diagnostic X-ray machines or muons8.

Some degree of dark adaptation is necessary for perception of helium and nitrogen nuclei. This suggests the outer segment of rods and cones are involved. The mechanism is dissimilar from that of electric and magnetic phosphenes, as the latter do not require dark adaption17.

Professor E. M. McMillan and Dr P. K. Chapman gave critical advise and acted as subjects. The experiments were made possible by the cooperation of Dr H. A. Grunder and the Bevatron crew. We thank J. Howard, M. C. Wales, R. E. Walton, and Dr R. Thomas of the Lawrence Berkeley National Laboratory for assistance. Dr Webb Haymaker aided in exposure of the cerebral occipital lobe. This work was supported by the US Atomic Energy COmmission and National Aeronautics and Space Administration.

Thomas F. Budginer
John T. Lyman
Cornelius A. Tobias

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Lawrence Berkeley National Laboratory
and Donner Laboratory,
Universoty of California,
Berkeley, California 94720

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