“Auditory cortex of the marmoset monkey – complex responses to tones and vocalizations under opiate anaesthesia in core and belt areas.” Ramesh Rajan, Vladimir Dubaj, David Reser, Marcello Rosa. European Journal of Neuroscience, pp. 1-18, 2012.
“Many anaesthetics commonly used in auditory research severely depress cortical responses…there is an inherent limitation to this approach whenever the physiological data need to be combined with histological reconstruction or anatomical tracing”.
In this study, adult marmosets were used to determine the effects of an opiate-based anaesthetic regime to study the responses within areas of the animal’s brain. After eliciting responses in anaesthetised monkeys and recording from the same neuron over a period of several hours, the animals were killed. Individual cells and tissue involved with the auditory responses could be identified and mapped.
The study’s authors state that they have created the means to “combine stable recordings allowing precise correlations,” between relevant parts of the monkey brains and the underlying patterns of incoming and outgoing connections. With previously used anaesthetics there has been altered responsiveness in anaesthetised animals, meaning that they do not respond to all sounds. Many anaesthetics have been used in the past for this research and the authors note studies from 1994 through to 2010 using drugs such as pentobarbitone, ketamine and halothane on animals such as rats, cats and monkeys.
The application to humans for this research is unclear except for the statement by the authors that, “Studies on humans have shown that opiates may preserve important aspects of auditory processing”.
Experiments were approved by Monash University Animal Experimentation Ethics Committee and this committee also monitored the welfare of the animals.
The research was supported by a grant of $472,200 from the National Health and Medical Research Council (grant no.545982).
- After premedication, five adult marmoset monkeys were anaesthetised with intramuscular alfaxalone.
- A tracheotomy was performed. This is an opening into the neck, through the trachea and a tube inserted for respiration.
- Intravenous cannulation was performed to allow administration of drugs.
- Craniotomy was performed; this is a surgical opening into the skull where holes are drilled into the bone and a portion of the skull removed.
- An electrocardiogram was performed, blood pressure and oxygen levels monitored and the temperature was kept stable with the help of an anal probe.
- A head bar held in a stand was fixed to the forehead using a short screw and dental cement to hold the head rigid.
- The ear canals were cut open so that researchers could insert sound delivery tubes into the canal allowing recording and monitoring of the same cells for over two hours.
- The outer surface of the brain, the dura mater, was visible through the craniotomy and electrodes were advanced to the first depth of cells, at which, a response could be observed. Clusters of cells were tested at increasing depths until the white matter was reached.
- After the experiment the animals were given a lethal dose of anaesthetic, the monkey’s brains were preserved, sliced and the electrode tracks were reconstructed with relation to the surrounding tissues and cells.
How can a comparison of such a minute area of a monkey’s brain be representative of a human’s complex and interactive one?
In 2005 a review was conducted to explore the efficacy of using non human primates in research. The author, Jarrod Bailey, stated that for research to be useful it should be predictive of the human situation. However, the marmoset brain is not just a much smaller version of the human brain (our closest relative, the chimpanzee, has a brain one quarter the size of a human’s and the macaque is one quarter the size of a chimp’s, a marmoset is smaller still), there are vast differences in anatomical structure brought about by millions of years of environmental changes.
Specialised areas of the human brain are located differently to monkeys and humans possess areas that either do not exist in monkeys or they perform another function. Other variances include different cortex surface area, numbers of synapses or connections, and at least ninety one genes expressed differently between the species (Ménache, 2010).
With the advancement of medical imaging which shows real time human brain functioning, aren’t there alternatives that would be more relevant to the human condition than observing anaesthetised monkeys with their heads bolted in place?
Alternatives are: Positron emission tomography, magnetic resonance imaging, electroencephalography, magneto encephalography, transcranial magnetic stimulation (Ménache, 2010).
How can this research be deemed ‘humane’? Although there is no justification for this treatment of any animal, this is particularly barbaric as there is no clear benefit to the human condition.
Swiss law states that it will not accept the use of animals in research unless the benefit to society has been weighed against the suffering of the animal, and in Australia, the animal ethics committees are in place to determine exactly that, the benefit to society. However, ethics committees have a requirement that only a minimum of two of the possible five members are non-university employees so a vote in favour of the animal will always be ineffectual (HRA, 2009).
The chance of any human benefit arising from this research is remote and scientists are often accused of overstating the possible benefits of a study, in pursuit of funding. That is not even the case with this work as there is no clear description of what the application to humans will entail. This is not the first experiment of its kind as previous similar studies have been conducted over decades. And now that they can map these auditory areas will this be the last of its kind? No. In the conclusion, there is mention of the next opportunity to further explore the workings of the marmoset brain, but still no word on how it all applies to humans.
What you can do
NHRMC, GPO Box 142
Canberra ACT 261
Write to the Animal Ethics Committee that approved this and similar experiments :
Monash Animal Ethics Office
Faculty of Medicine, Nursing and Health Sciences
Building 13C, Wellington Road, Clayton Campus
MONASH UNIVERSITY Vic 3800
- Bailey, J. (2005). Non-human Primates in Medical Research and Drug Development: A critical review. Newcastle: University of Newcastle upon Tyne.
- Menache, A. (2010). The Replacement of Non Human Primates in Brain Research. Strasbourg: One Voice.
- Russell, D. (2009). Animal Ethics Committees. HRA Humane Research Australia , 11-12.