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Tara Thiagarajan Ph.D.

Can Neuroscientists Read Your Thoughts?

One challenge is that we don't know what a thought actually is.

Will we eventually be able to "read minds" and discern thoughts with neuroscience imaging methods? Perhaps—but the challenge is that we don't yet know what a thought actually is.

The Difficulty of Defining a Thought

Merriam Webster unsatisfactorily defines a thought as "the action or process of thinking" where thinking is to "have something in mind." Another (and maybe better) definition suggests that it is the "aim-oriented flow of ideas and associations that can lead to a reality-oriented conclusion" where ideas are mental representations of some sort. But what are the necessary and sufficient conditions of a thought?

Does a thought require retrieval of a memory? Must it be accompanied by conscious awareness? Does it require language? Is thought unique to humans? Or living organisms? Or complex open systems in general?

Even as we have an implicit sense of our own thoughts, the answers to these questions are far from resolved. The best answer we have as of now to all of the above questions is "Uh... maybe?"

Some linguists argue that a thought must necessarily contain language or some form of symbolic representation, which would suggest that thought may be uniquely human. On the other hand, there are some who raise the possibility that plants may, in fact, be able to think. It may not be anything like human thought, but plants have senses of smell (respond to chemicals), sight (the ability to detect light and dark), the ability to form memories and respond to stimuli in ways that could suggest they are thinking.

So which is it? Is there a general definition of thought that can manifest in different ways in different systems? Or is thought something very specific to the human mind? How would we arrive at these answers?

Is Thought a Measurable Physiological Object?

If we are not exactly sure what a thought is, how would we recognize a thought if we saw one? Is it even a measurable physiological object? Or does it exist in some realm of mind that defies measurement?

Neuroscientists assume a thought is represented in electrical activity of the brain. But if the linguists are right, then electrical activity in animal brains would not indicate thoughts, nor would it in babies. And yet the electrical activity in babies and animals has many structural similarities to language-enabled humans.

What if a thought is not constructed of electrical activity after all? This does not mean electrical activity has no influence on a thought. It may well. But it could be fully impossible to construct or reconstruct a thought out of a pattern of electrical activity alone.

Reading Thoughts in the Brain

This comes to the big question of whether we are on the path to reading human thought by measuring the activity of the brain.

In 2008, a widely publicized paper, "Identifying Natural Images From Human Brain Activity" in the journal Nature led to headlines like "Brain Decoding: Reading Minds" and "Scan a Brain, Read a Mind? In this study, people were asked to view natural scenes and these images were reconstructed (somewhat) from the early visual area of the brain using a method called fMRI, which measures differences in blood oxygen levels, to construct patterns of where there was greater activity. While this is in and of itself quite a feat, does this mean they were able to read a thought?

In their paper, the authors draw the conclusion: “Our results suggest that it may soon be possible to reconstruct a picture of a person’s visual experience from measurements of brain activity alone.” The early visual region is the part of the cortex where the visual signal first arrives from the retina via the thalamus. What this means is that they are essentially constructing a representation of the visual signal arriving in a spatially distributed way into the cortex—a representation of the input signal.

On the other end, there are studies like one titled "Real-Time Decoding of Question-and-Answer Speech Dialogue Using Human Cortical Activity," also in Nature, which garnered headlines like "Mind-Reading AI Turns Thoughts Into Words Using a Brain Implant." Here, they used an array of electrodes placed on the auditory and sensorimotor areas of the cortex to detect which response a person would make to a question from among a subset of possible answers. They were able to predict, with up to 76 percent accuracy, the sound that they were played (from auditory cortex) and, with up to 61 percent accuracy, the sound they produced (from sensorimotor cortex).

Here again, the auditory cortex is where sound input first arrives into the cortex, and the sensorimotor area is where motor output is essentially initiated—i.e. where the signal that gets transmitted to the muscles of the face and body to execute movement is initiated. And in the end movement, whether of vocal cords or limbs, is our only form of output. Altogether, what we have so far is quite a remarkable decoding of the input and output signals—what is going in and what is coming out.

But thought? Thought is what happens between the input and output. And we don’t know what that is yet.