By Aysha Akhtar, M.D., M.P.H.
Neurological Experiments: Monkey See...But Not Like Humans
Rhesus (or macaque) monkeys are some of the most common animals used in neurological experiments concerning the visual system. Here are some recent examples of such experiments:
■At the University of Connecticut, electrodes were screwed into monkeys’ brains, the monkeys were strapped into restraining devices, and then they were trained to perform visual tasks, often involving electrical stimulation.1
■At the University of California, Davis, the brains of monkeys were surgically exposed and then directly injected with acid to destroy certain areas. The monkeys were then studied for visual learning skills.2
■At Columbia University, monkeys were implanted with scleral coils (electrical wires inserted into the eyes), head restraint devices, and recording chambers. Their eye movements were then recorded.3
Often, monkeys are deprived of food and water and then rewarded with these basic necessities. In addition to the fact that these and other similar experiments cause enormous stress and pain for the monkeys, they have little or no clinical usefulness.
Monkeys are used extensively in neurological experiments because of the assumption that they, out of all animal species, are most neurologically similar to humans. But how similar are they? The human brain is far more complex in architecture and physiology than the monkey brain. One indication of this is the length of time it takes for the brain to develop in its major phase: 136 days for monkeys and 470 days for humans.4 Here are a few of the many more specific examples of how the two species differ in neuroanatomy and neurophysiology:
■The human cortex has 10 times the surface area of that of a monkey.5
■The V 1 area (one of the predominant visual areas in the brain) makes up 10 percent of the total cortex in monkeys and only 3 percent of the total cortex in humans.6
■Similar visual areas perform very different functions in humans and monkeys.7, 8
■The number of synapses—or connections—a human neuron makes is between 7,000 and 10,000. In the rhesus monkey, that number is between 2,000 and 6,000.4
■The expression of at least 91 genes involved in a variety of neural mechanisms differ between monkeys and humans.9
■Humans have visual processing areas that do not exist in monkeys.10
As one primate researcher stated, “the human brain …is more than simply a large monkey or ape brain.”11 Undoubtedly, similarities exist in primate and human neurophysiology. However, given the advances in medicine today, the differences between species is far more important than the similarities. Technology has given researchers the ability to examine the nuances of physiological mechanisms in order to specifically target an intervention, such as a drug to boost or inhibit a specific cellular process. For this, we need the most accurate possible information about the neurological system of humans – not monkeys.
Researchers can study human neurology in an ethical manner. Many clinical centers use imaging and neurophysiologic tools to map and monitor the human visual and other neurological systems. Centers such as Princeton University, the University of Chicago, the University of Pennsylvania, and Minnesota State University use functional MRIs, PET scans, and evoked potentials (which record the brain’s electrical patterns) to collect relevant data on human neural processing and anatomy.12-15 With these and many more wonderful tools available for noninvasive study of the human brain, we can most effectively help patients who suffer from neurological diseases.
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3. Ipata AE, Gee AL, Goldberg ME, Bisley JW. Activity in the lateral intraparietal area predicts the goal and latency of saccades in a free-viewing visual search task. J Neurosci. 2006;26(14):3656-3661.
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11. Dehaene S, Duhamel J-R, Hauser MD, Rizzolatti G. From monkey brain to human brain: A Fyssen foundation symposium. Cambridge, MA: MIT Press, 2005:41.
12. McKeeff TJ, Tong F. The timing of perceptual decisions for ambiguous face stimuli in the human ventral visual cortex. Cereb Cortex. 2006. April 28 (Epub ahead of print).
13. Phan KL, Britton JC, Taylor SF, Fig LM, Liberzon I. Corticolimbic blood flow during nontraumatic emotional processing in posttraumatic stress disorder. Arch Gen Psychiatry. 2006;63(2):184-192.
14. Newberg AB, Wang J, Rao H, et al. Concurrent CBF and CMRGlc changes during human brain activation by combined fMRI-PET scanning. Neuroimage. 2005;28(2):500-506.
15. Page JW, Findley J, Crognale MA. Electrophysiological analysis of the effects of ginkgo biloba on visual processing in older healthy adults. J Gerontol A Biol Sci Med Sci. 2005;60(10):1246-1251.