The Tapetum Lucidum

William Bradley

The History of Science

April 2011

The Tapetum Lucidum

The variation among different creatures in the animal kingdom is astounding. There are so many different methods and mechanisms used in evolution, and we can see that in many of the creatures alive today. The tapetum lucidum (commonly shortened to “tapetum”) is no exception. It is a thin reflective layer in the eyes of many animals that reflect a specific color when light is shone upon it. This information is learned only by observing, just as Goethe did during the formulation of his theory of colors. His entire theory was based solely on his observations through experimentation and, possibly, dissection. This is similar to what we wish to achieve in this study. Through dissection, observation, and an assimilation of research articles, we can come closer to fully understanding the purpose and functions of the tapetum. We can then use this information to formulate hypotheses on related questions. For example, why is the tapetum not a part of every animal’s eyes, specifically humans? This question among others may easily be answered after we have collected and analyzed the results of this research and dissection.

Purpose:

To analyze and experience, through dissection, the tapetum lucidum in a sheep’s eye and pig’s eye.

Experiment and Observations:

Sheep eye: I started the procedure by trimming the excess fat off of the eye. I decided to use my camera and the flash on it to take a picture of the front of the eye to see if the tapetum was visible through the translucent cornea. Surprisingly, it is possible to see a blue tint through the cornea. Next, I cut the eye in two halves, front and back. Once the eye was separated and the fluid removed, I was able to see the cornea immediately in the back of the eye. This may be the result of a damaged specimen, since the layer of photoreceptors should be in front of the tapetum. I then was able to peel the tapetum out of the eye, and view it up close. There is a tiny dark spot in the middle of the tapetum. This spot is where the optic nerve met on the outside of the eye, and where the visual sensory information would be sent to the brain for processing. I initially observed that near this point on the tapetum, the tapetum was much brighter, reflective, and white. But, once you look at the tapetum at a different angle, the blue areas move around as they reflect the specific wavelength (bright blue ~ 475 nm) directly at your line of sight.

Pig eye: I basically began dissecting the pig’s eye in the same way that I dissected the cow’s eye. I trimmed off the excess fat, and cut it open into the same halves. I attempted to take a picture of the front, but I didn’t get any results of color, which I was initially looking for. It didn’t take long for me to realize that pigs actually do not have a tapetum. At first I was disappointed, but I realized I could use the pig’s eye as a comparison for the cow’s eye and the tapetum in general. The inside of the eye looked almost exactly identical, but where the tapetum would have been in the cow’s eye, the pig’s eye only had the blood vessels and grey matter, which would have been red had there been blood flowing through the organ.

Conclusions:

Viewing the tapetum in the cow’s eye allows us to get a better idea of the structure itself, and how it functions. The color reflection changes if you view it at a different angle, and it is interesting to see the focal point of the eye where the information is sent through to get to the optic nerve. In humans, this is the “blind spot” or the range in our line of sight that cannot be seen. In the pig’s eye, we can still find this spot, but it is much harder since a reflective coating doesn’t surround it. We can also use the pig’s eye as a comparison between animals that do have the tapetum and animals that do not. The pig’s eye had some grey matter where the tapetum would have been, but we can speculate that it would have been visible or red, had there been blood flowing through it.

Unfortunately, there exists a limit on what we can observe with the naked eye even in a dissection. The importance of dissection is underlined in many different sources. In Goethe’s Color Theory, it is almost impossible not to assume that he has performed a dissection on an eye, whether the eye was from a human or not. He knows much about the retina and the cornea, both of which are structures that no one at the time could have really discussed as he did unless they were well educated and had seen the structures fully. Barbara Stafford, in an article written on dissection, discusses the importance of it when she quotes P. N. Gerdy’s Anatomie des formes extérieures, “ . . . anatomy functioned like an enlarging glass. It magnified the smallest detail, rendering distinct hidden morphologies” (1). The dissection proves to us that the tapetum exists, and gives us a historical perspective into the research that has been done on the tapetum over time. But by using existing research, we can get an idea of the actual function of the tapetum, as well as how it has changed over time.

Through our observations, we find that the tapetum reflects one wavelength of light (which changes depending on the species). Basically, as laid out in a comparative study by F. J. Ollivier, D. A. Samuelson, D. E. Brooks, P. A. Lewis, M. E. Kallberg, and A. M. Komáromy, the tapetum, “normally functions at low light levels to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity” (2). The tapetum reflects a specific wavelength of light back into the photoreceptors of the eye after the light has already passed through one time. This allows for greater sensitivity to light in low light conditions. We saw this in effect in our dissection when the light from the camera reflected off of the tapetum.

Now that we have an idea of what the tapetum does, we need to know the reasoning behind the specific color of different animals. A different study by Ivan R Schwab, Carlton K Yuen, Nedim C Buyukmihci, Thomas N Blankenship, and Paul G Fitzgerald shows that, “tapeta have a tendency to reflect wavelengths most relevant to the animal” (3). This study showed that the tapetum reflects the wavelength of light important to the animal. For example, the study discusses the fact that the tapetum in deep-sea fish almost all reflect the light wavelength of 475 nm (cyan-green) because this is the only wavelength that can ever reach those depths. We can, perhaps, speculate that a cat or dog may reflect a more greenish color to reflect light off of plants or grass.

The next question that is important to us is the reason why there exists no tapetum in many animals. The study by Ollivier, Samuelson, Brooks, Lewis, Kallberg, and Komáromy states that the animals that do not have the tapetum are primates, squirrels, birds, red kangaroo and pig. These animals have very few things in common, so this helps us in identifying how the tapetum has evolved over time, which we will discuss later. These animals are all, however, diurnal creatures. This makes intuitive sense, since a creature that is active during the daytime will have less of a need for an increase in light reception. As we saw in our dissection of the pig’s eye, these creatures have a “red or orange to pale gray fundus reflection” (4). In our pig eye, the area where the tapetum would be was a pale grey. This could, however, be solely from an absence of blood in the separated and treated organ. In a live pig, the eye could be red as well. By observing human eyes, we see the red blood vessels in the back of the eye when a light is shone upon it. This causes the undesirable “red-eye” feature of many pictures taken of humans. Primates, along with the other creatures listed above, are diurnal creatures, and thus did not have an evolutionary need for a tapetum.

Given the “random assortment” of the animals with the tapetum, it is safe to assume that the tapetum developed independently among specific species. The “parent creature” could not have been very early in the mammalian evolutionary chain, thus, the evolution happened on multiple occasions. In the study by Schwab, Yuen, Buyukmihci, Blankenship, and Fitzgerald, they conclude that, “the tapetum may have arisen independently in both invertebrates and vertebrates as early as the Devonian period (390 to 345 million years ago)” (5). They base this conclusion on evidence found in other organisms. They use the assumptions that vertebrates evolve from pikaia, an invertebrate that is ancestor to other organisms that do not have the tapetum. Also, both hagfish and lampreys do not have a tapetum and they separated from an ancestor fish in the period before the Devonian.

The tapetum allows nocturnal creatures to see better in the darkness, and through dissecting, we can see what it looks like up close, as opposed to viewing it in a live animal. In doing this, we must dissect the organ and view it piece by piece. Understanding all of the functions of the eye, as well as the positions and purposes of each individual part of the eye is also important in understanding the tapetum. The cornea protects the eye, the iris expands and contracts to let specific amounts of light in, and the retina captures the image and sends it to the brain for processing. The tapetum is important in this process in that is reflects light back onto the photoreceptors of the retina for a second viewing. The tapetum is a fascinating aspect of the animal world. So many artistic endeavors have been based around the glow of an animal’s eyes. We see pictures and paintings of wolves and other large mammals where the glow of their eyes is the focal point of the picture over and over again. The tapetum is an important structure of the eye, and it is of great interest to humans in many aspects of biology, evolution, and even art. Not only does the tapetum have a scientific value, the impression that it leaves on humans, who are without it, is everlasting.

Endnotes:

1) Stafford, Barbara, Body Criticism: Imagining the Unseen in Enlightenment Art and Medicine (Cambridge, Massachusetts: Massachusetts Institute of Technology, 1991), 54.

2) F.J. Ollivier et al., “Comparative morphology of the tapetum lucidum (among selected species).” Veterinary Ophthalmology 7, no. 1 (2004): 12.

3) Ivan Schwab et al., “Evolution of the Tapetum.” Transactions of the American Ophthalmological Society 100 (2002): 197.

4) F.J. Ollivier et al., 12.

5) Ivan Schwab et al., 197.

Bibliography:

Goethe, Johann Wolfgang. Goethe’s Theory of Colors. New York: Van Nostrand Reinhold Company, 1971.

Ollivier, F.J. et al. “Comparative morphology of the tapetum lucidum (among selected species).” Veterinary Ophthalmology 7, no. 1 (2004): 11-22.

Schwab, Ivan, et al. “Evolution of the Tapetum.” Transactions of the American Ophthalmological Society 100 (2002): 187-200.

Sepper, Dennis. “Goethe, colour and the science of seeing,” in Romanticism and the Sciences, edited by Andrew Cunningham and Nicholas Jardine, 189-198. Cambridge: Cambridge University Press, 1990.

Stafford, Barbara. Body Criticism: Imagining the Unseen in Enlightenment Art and Medicine. Cambridge, Massachusetts: Massachusetts Institute of Technology, 1991.

Going Beyond Hermann von Helmholtz: The Octave Illusion with Respect to Handedness

Introduction
The octave illusion was initially produced by the stimulus configuration that is depicted in Figure 1a. Here there two tones that are spaced an octave apart, which are then repeated in alternation. This sequence was presented to both ears simultaneously, however, when the right ear received the high tone, the left ear received the low tone and vice versa with each ear receiving opposite tones. This presents the listener with a single continuous two-tone chord, but the ear in which each input is received switches repeatedly (Deutsch, 1981).
This sequence provokes various illusions. The most common illusion that arises from this sequence is illustrated in Figure 1b. This shows a single tone switched from ear to ear, whose pitch simultaneously switched back and forth from high to low. Thus, the listeners heard a single high tone in one ear, which alternated with a single low tone in the other ear (Deutsch, 1981).



The illusion is tried further when various subjects attempt to alter the ear in which the high and low tones are perceived by simply reversing the headphones. Most people still hear exactly the same thing; that is, the tone that was received in the right ear is still perceived in the right ear and the tone that was received in the left ear is still perceived in the left ear. The listener originally associated the difference in right versus left ear perception to the earphone, but the reversal of the headphones proves to the listener that it is not the headphones, but indeed the ears that perceive the different tones. This percept is illustrated in Figure 2. Figure 2 provides a written report by someone with absolute pitch (Deutsch, 1981).

The octave illusion was shown to be based on two factors: (1) the perception of the frequencies presented to a single ear (those presented to the other ear being suppressed), and (2) the localization of each tone to the ear receiving the high frequency signal. This is regardless to whether the higher or lower frequency was actually perceived (Deutsch, 1974). More recently, the octave illusion is explained on the foundation of selective attention.

Selective attention is expressed as the ability to selectively attend to certain stimulus; blocking out unimportant stimulus. Selective attention also explains how an individual in a room full of people can focus on only one conversation. The auditory system contains many centrifugal pathways, which extend from the auditory cortex, Brodmann’s areas 41 and 42 (Andorn, 1989), through the medial geniculate body, colliculi, olivary regions, and back to cochlea. Some of these structures are proposed to play a role in selective attention by modulating midbrain and auditory nerve responses or possibly the activity of the cochlear hair cells (Chambers, 2002).

In general, it was found that among the right-handers who obtained the percept that there was a single high tone in one ear, which alternated with a single low tone in the other ear, had a highly significant tendency to hear the higher tone on the right and the lower tone on the left. This observation did not hold true for left-handers (Deutsch, 1974). This is in accordance with the literature showing that although most right-handers have clear left-hemisphere dominance, the pattern of cerebral dominance among left-handers varies considerably (Deutsch, 2009). This helps explain why the proportion of listeners obtaining complex percepts was much higher in left-handers than in the right-handers (Deutsch, 1981). Furthermore, these results are consistent with neurological evidence.

Neurological evidence suggests that the majority of right-handers are left-hemisphere dominant; meaning their speech is represented in the left cerebral hemisphere. However, this is only true for approximately two-thirds of left-handers as opposed to the overwhelming majority of right-handers. The remaining one-third are right-hemisphere dominant, which means that although most left-handers have speech represented in the left cerebral hemisphere, a substantial amount of left-handers have speech in both hemispheres (Deutsch, 1981). The variation of the hemispheres may help account for the variation of tone localization that can be present in some left-handed populations. Since the brain exhibits contralateral tendencies, it would come as no surprise that right-handers would tend to strongly follow the information presented to their right side (the auditory cortex is on the left side of the brain; right-handed people have stronger pathways to the left hemisphere because of their right-handed dominance to the contralateral left hemisphere) (Deutsch, 1981).

The result of a substantial right-ear advantage for a sequence that is nonverbal is seemingly contradictory according to the widely held belief that the dominant hemisphere is used for verbal functions, while the non-dominant hemisphere is used for nonverbal or musical functions. Left-ear advantages have been obtained in dichotic listening tasks involving musical composition. Thus, it would then be the left ear that would perceive the higher tone in the sequence. Although, a left-ear dominance has been obtained for nonverbal sequences, right-ear advantages have also been obtained. It was found that when subjects were required to recognize two frequencies, dichotically, the ear advantage was purely right (Deutsch, 1981).

In short, this research in its current form focuses on repeating the octave illusion experiment. Based upon previous experiments, it is believed that the majority of right-handed subjects will perceive a high tone on the right alternating with a low tone on the left, while left-handed subjects will vary more in perception, but nonetheless, the majority of left-handed subjects will perceive a high tone on the left alternating with a low tone on the right. Additionally, musical training of the subjects will be assessed for a positive correlation in the ability to correctly establish the differences between the frequencies.

Methods

The current research was performed on 51 undergraduate students of varying degree study programs at the University of Michigan. Of the students that underwent the research, 28 students were female (54.9%) and 23 students were male (45.1%). The ages varied within a range of six years. The minimum age of the students was 17 with a maximum age of 22 and a mean of 19.08 years of age, with a standard deviation of 1.354 years. For a further breakdown of the ages of the participants please see Appendix A, Table 1. 32 of the 51 students were right handed (62.7%), 16 were left handed (31.4%) and 3 students were assessed as both handed (5.9%). Handed was assessed by the questionnaire in Table 1 (Varney, 1975). Answering eight or more in either the left or right category designated a participant to that respective category. Less than eight selections in either the left or right category alone designated a participant to both handed.

Of the right-handed participants, 14 were male (43.8%) and 18 were female (56.3%). From the left-handed participants, 7 participants were male (43.8%) and 9 were female (56.3%).

Furthermore, it was noted whether these 51 students had musical training, which was defined at three years or more learning or playing a musical instrument (Deutsch, 1981). 23 students did not have musical training (45.1%), while the remaining 28 students responded as having musical training (54.9%).

Each subject was tested individually. They were explained that they would hear a sequence of tones that would repeat. Upon the conclusion of the tones they were asked to indicate on a forced choice questionnaire which description best fit their perception of the tone(s), they had just heard (Deutsch, 1981).

The set-up for the testing was quite simple. JVC Gumy Ear Bud Headphones, HA-F150A stereo speaker headphones, with a frequency response of 16-20,000 Hz (JVC, 2010), was plugged into a MacBook Pro. The tone sequence was accessed and played back for the participant from an online source (Philomel Records). The tone consisted of an alternating pattern of 400 (G₄) and 800 (G₅) Hz (Deutsch, 1974). The clip was 30 seconds in length.

Figure 3

Screen Shot of the embedded music that the participants listened to.

At the conclusion of the clip, each participant was asked to pick one of the following options that best described their individual percept: (A) A high tone on the right alternating with a low tone on the left; (B) A high tone on the left alternating with a low tone on the right; (C) A tone switching from ear to ear with no change in pitch; (D) None of the above (explain) (Deutsch, 1981).

All data and statistics were processed by PASW, Predictive Analytics SoftWare, SPSS, Statistical Package for the Social Sciences, 18.0.

Results and Discussion

Of the 51 students who participated, 32 responded (A) a high tone on the right alternating with a low tone on the left (62.7%); 17 responded (B) a high tone on the left alternating with a low tone on the right (33.3%); 2 responded (C) a tone switching ear to ear with no change in pitch (3.9%). The data was then stratified for various variables. The first of the stratifications was handedness. The following is based from if the participant was right-handed. Of the students who were right-handed, it was found that 18 participants had musical training (56.3 %), while 14 did not (43.8%). Out of the right-handed participants (N=32), 30 participants responded with (A) a high tone on the right alternating with a low tone on the left (93.8%) and 2 participants responded with (B) a high tone on the left alternating with a low tone on the right (6.3%). The following is based from if the participant was left-handed. Of the students who were left-handed, it was found that 7 students had musical training (43.8%), while 9 did not (56.3%). Out of the left-handed participants (N=16), 0 students responded with (A) (0%), 15 students responded with (B) (93.8%) and 1 participant responded with (C) (6.3%). In general, this follows very well with the established trend; right-handed people hear the high tone on the right, while left-handed people are a bit more varied, but generally hear the high tone on the left.

The musical trend of this data set does not follow the pre-conceived notion that left-handed people are more likely to have musical training. Traditionally, the right hemisphere is viewed as the musical hemisphere, thus with the contralateral nature of the brain, left-handed people would stimulate the right hemisphere of the brain more often than right-handed people invoking musical tendencies. (Tramo, 2001). Further stratification of the data was performed to assess the trend in the data. It was found from the data set that if the participant was musical, he or she was more likely to be right-handed. But, due to the greater abundance of right-handed versus left-handed people in the study, it was also found that if a person was non-musical, he or she was more likely to be right-handed. For the complete statistical breakdown regarding handedness and musical training refer to Appendix A, Table 2 and Table 3.

The last set of data stratifications were performed with respect to each tone choice (A-C) perceived. 32 participants responded with having heard a high tone on the right, alternating with a low tone on the left, option A. Of these students, 20 students indicated having musical training (62.5%), while 12 did not (37.5%). Additionally, 30 of the 32 students who perceived the tone sequence associated with choice A were right-handed (93.8%) and 2 students were both-handed (6.3%). 17 students responded with having heard a high tone on the left, alternating with a low tone on the left, option B. Of these students, 6 indicated having musical training (35.3%), while the majority of this subset indicated not having musical training (N=11, 64.7%). Furthermore, 15 of the 17 students were left-handed (88.2%) and 2 were right-handed (11.8%). Lastly were those who perceived a tone switching from ear to ear with no change in pitch (C). Option (C) represented a very small portion of the data (N=2). Of these 2 students, both indicated having musical training, although one was left-handed and one was right-handed.

Conclusions and Recommendations

In general the results of the subjects from this experiment correlated well with those of previously published results. The majority of right-handed subjects responded by perceiving a high tone on the right alternating with a low tone on the left, while the results of the left-handed indicated that the majority of the left-handed subjects perceived a high tone on the left alternating with a low tone on the right. The most substantial difference between these results and previously published studies include the lack of variation within the left-handed participants. According to published findings, the perception of left-handed people should be more varied. The lack of variance in this current study can be attributed to a small sample size.

Additionally, the demographic statistics proved interesting. According to the data, musical training does not affect the tone that is perceived. Again, this could be attributed either to a lack of a relationship or a small sample size. To efficiently test this, a larger sample size is needed of both musically and non-musically trained participants that are equally divided with left- and right- handed subjects.

Overall, this research should be completed with more subjects that equally represent the left- and right-handed population. This would improve the sample and should, in theory, provide data that is more representative of the population.

References

Andorn, A.C., Vittorio, J.A., Bellflower, J.. “3H-spiroperidol binding in human temporal cortex (Brodmann areas 41-42) occurs at multiple high affinity states with serotonergic selectivity,” Psychopharmacology (Berl.), (1989:90), 4, 520-525.

Chambers, C. D., Mattingley, J. B., & Moss, S. A. “The octave illusion revisited: Suppression or fusion between ears?,” Journal of Experimental Psychology: Human Perception and Performance, (2002) 28(6), 1288-1302.

Deutsch, D. “An auditory illusion,” Nature (1974) 251, 307–309.

Deutsch, D. "An auditory illusion". Journal of the Acoustical Society of America (1974) 55, s18-s19.

Deutsch, D. “The Octave Illusion and Auditory Perceptual Integration,” Hearing Research and Theory, Volume 1 (1981), 1, 99-142, New York: Academic Press.

Deutsch, D. “The octave illusion in relation to handedness and familial handedness background,” Neuropsychologia (1983) 21 (3), 289–293

Deutsch, D. “Musical Illusions,” Encyclopedia of Neuroscience, (2009), 5, 1159-1167, Academic Press, Oxford.

JVC. 2010. HA-F150A Specifications. http://av.jvc.com/product.jsp?modelId=MODL028884&pathId=162&page=3

Philomel Records. Diana Deutsch’s Auditory Illusions. http://philomel.com/musical_illusions/example_octave_illusion.php

Tramo, M. J. “Music of the hemispheres,” Science. (2001), 5.

Varney, N. R. and Benton, A. L. “Tactile perception of direction in relation to handedness and familial handedness,” Neuropsychologia (1975), 13, 449-454.

Appendix A

Table 1

Age
		Frequency	Percent	Valid Percent	Cumulative Percent
Valid	17	6	11.8	11.8	11.8
	18	14	27.5	27.5	39.2
	19	11	21.6	21.6	60.8
	20	12	23.5	23.5	84.3
	21	6	11.8	11.8	96.1
	22	2	3.9	3.9	100.0
	Total	51	100.0	100.0

Ages of the participants

Table 2

Musical
		Frequency	Percent	Valid Percent	Cumulative Percent
Valid	Y	18	56.3	56.3	56.3
	N	14	43.8	43.8	100.0
	Total	32	100.0	100.0

Whether the participant had musical training (Y=Yes, N=No) with respect to the right-handed participants.

Table 3

Musical
		Frequency	Percent	Valid Percent	Cumulative Percent
Valid	Y	7	43.8	43.8	43.8
	N	9	56.3	56.3	100.0
	Total	16	100.0	100.0

Whether the participant had musical training (Y=Yes, N=No) with respect to the left-handed participants.