Yale News

A recent study published in late March by psychologists and neuroscientists from Yale and other research institutions found that monkeys can process visual cues in conscious and nonconscious ways — a pair of mechanisms previously thought to only exist in humans. The study was conducted with funding from Fulbright, Rothschild and Lady Davis fellowships.

It is known that humans are able to register visual cues in both conscious and nonconscious ways — two distinct “stages” of visual processing, according to the study. But scientists and philosophers have long pondered whether non-human animals also have conscious and nonconscious processing of visual stimuli. To answer this question, Moshe Shay Ben-Haim, a postdoctoral fellow in psychology, brought together several psychology and neuroscience experts — including many from Yale — to conduct a study involving rhesus monkeys. Before this study, no one had explored if similar well-established “performance dissociations” exist in a nonhuman species.

“The question of animal consciousness has puzzled scientists and philosophers for a very long time, but we simply haven’t had good ways to test this question scientifically,” Laurie Santos, a psychology professor at Yale, wrote in an email to the News. “Our study was designed to separate the two modes of processing experimentally and provide clues to answering this difficult puzzle.”

Ben-Haim shared with the News that he has always been intrigued by the question of whether other animals are as consciously aware as humans, given that animals cannot speak their minds. According to him, “human consciousness has often been considered as the peak of our evolution as a species,” and the team’s research was crucial to exploring whether this feature is truly unique to humans. 

The experiment involved showing visual cues to both rhesus monkeys and humans and then observing their behavioral responses, according to Ben-Haim. He said that the team hypothesized that the monkeys would react to the tests just as humans do and show “separate behavioral outcomes in response to consciously accessible cues and to non-consciously presented cues.”

Ben-Haim noted that the same experiments were initially conducted on dogs in the Canine Cognition Center at Yale. The research was soon expanded to conducting the experiments on monkeys at the School of Medicine because according to Steve Chang, an associate professor of psychology and neuroscience, they have visual systems that are comparable to humans. The monkeys were quick to learn the task, so their results were published first, while the canine research is ongoing, according to Ben-Haim. 

Ben-Haim explained that in the experiments, the researchers tested visual consciousness by utilizing a “double dissociation task” on rhesus monkeys and humans, with the latter group serving as a control. 

The researchers presented two types of visual targets to the animals: ones that could be perceived consciously and ones that could not. The cues that could not be perceived consciously but were still processed by the brain were flashed for less than a few hundred milliseconds. Those that could be consciously perceived were shown for longer durations to the animals, according to Ben-Haim.

He explained that if the animals saw the cue, they would need to look on the opposite side of the screen in order to be rewarded. The monkeys only exhibited this behavior and were rewarded when the cue was consciously accessible to them. In other words, like humans, the rhesus monkeys displayed opposite reactions to visual stimuli that were processed consciously and non-consciously, according to Ben-Haim.  

“Both humans and monkeys exhibited evidence of having both conscious and non-conscious perceptual states in their visual processing,” Chang wrote in an email to the News. “The two states could be dissociated in an identical manner between the two species.”

Chang and Ben-Haim shared that it is a challenge to “prove” that non-verbal animals experience conscious stimuli.

However, by showing the two modes of processing exist in the monkeys, their research provides strong evidence to suggest that consciousness in visual awareness is not uniquely human. 

“Since the distinction between conscious and non-conscious processes is central to human cognition, we can now begin examining this distinction and its significance in other animals to better understand animal behavior as well,” Santos wrote. 

In particular, the researchers plan to conduct similar experiments on fish and continue with their related studies on dogs. Santos added that being able to study these phenomena in other animals will make it possible to explore the “phylogenetic origins” of consciousness among different species. 

“Consciousness still remains a big mystery,” Chang wrote. “Our research paradigm can be powerfully used to investigate how the human brain and the brains of non-human animals enable conscious perception of the world.”

The Yale School of Medicine is located at 333 Cedar St., New Haven, CT.

Eda Aker | eda.aker@yale.edu

Correction, Apr. 12: An earlier version of this story stated that Ben-Haim thought that humans did not appreciate that animals do have consciousness. In fact, it is more accurate to say he became interested in the work because animals cannot speak their minds. In addition, the original story said researchers studied the brain responses of the monkeys; however, they only studied the behavioral responses. The story has been updated.

Eda Aker is a WKND Editor and previously covered Yale Law School for the University Desk. She is a junior in Timothy Dwight College majoring in Global Affairs.