Elon Musk's implant company Neuralink has confirmed the monkeys died after they implanted computer chips in their brains.
However, the company - which aims to enable brains to connect and communicate with computers - has denied allegations of animal cruelty.
Neuralink addressed the rumors that their test subjects had died in a blog post on their website, saying: "two animals were euthanized at planned end dates to gather important histological data, and six animals were euthanized at the medical advice of the veterinary staff at UC Davis."
The statement went on to say that they are "absolutely committed to working with animals in the most humane and ethical way possible," and that "the use of every animal was extensively planned and considered to balance scientific discovery with the ethical use of animals."
However, they also stated that animal testing was unavoidable, saying "all novel medical devices and treatments must be tested in animals before they can be ethically trialed in humans."
The concern over their Macaque monkey test subjects came after the Physicians Committee for Responsible Medicine - a charity advocating against animal testing - wrote a letter to the US Department of Agriculture requesting an investigation into Neuralink's activities, CNN reports.
In the 700-page letter, the Physicians Committee refers to "apparent egregious violations of the Animal Welfare Act related to the treatment of monkeys used in invasive brain experiments," and claims that records for the 23 monkeys tested on expose a "pattern of extreme suffering and staff negligence."
Neuralink is reportedly developing Bluetooth-enabled implantable chips which are able to communicate with computers.
Last April, they demonstrated the potential of the technology by releasing a video in which a male macaque was shown playing a video game with just his mind.
The monkey - named Pager by the scientists - was able to move a cursor on a screen without the use of a joystick or any other physical controls. According to Neuralink, the chip implanted in Pager's brain fed information from the monkey's neurons into a decoder, which was then used to predict his intended movements.
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