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Brains May Predict The Future                                                                                     17

             To Make Sense of the

              Present, Brains May

               Predict The Future

                 Continued from Page 16

        The researchers observed a greater brain
        response when the study’s subjects came
        across the unexpected word “dog,”

        characterized by a specific pattern of
        electrical activity, known as the “N400
        effect,” that peaked approximately 400
        milliseconds after the word was revealed.
        But how to interpret it remained unclear.
        Was the brain reacting because the
        word’s meaning was nonsensical in the
                                                         mechanisms at play. While the idea that proposes               that    the    brain     makes
        context of the sentence? Or might it have
                                                         the brain is constantly making inferences probabilistic inferences about the world
        been reacting because the word was
                                                         (and comparing them to reality) is fairly based on an internal model, essentially
        simply unanticipated, violating whatever
                                                         well-established at this point, proponents calculating a “best guess” about how to
        predictions the brain had made about
                                                         of predictive coding have been seeking interpret what it’s perceiving (in line with
        what to expect?
                                                         ways to prove that their particular the rules of Bayesian statistics, which
                                                         version of the story is the right one — quantifies the probability of an event
        In 2005, Kutas and her team conducted
                                                         and that it extends to all of cognition.         based on relevant information gleaned
        another study that pointed to the latter
                                                                                                          from prior experiences). Rather than
        possibility. People were again asked to
                                                         Bayesian Brains and Efficient                    waiting for sensory information to drive
        read a sentence one word at a time on a                                                           cognition, the brain is always actively
        screen: “The day was breezy so the boy           Computing
                                                                                                          constructing hypotheses about how the
        went outside to fly ____.” Because “a                                                             world works and using them to explain
        kite” seemed the most likely way to              The foundational insight that the brain          experiences and fill in missing data.

        finish the sentence, the subjects expected       perpetually makes and evaluates its own          That’s why, according to some experts,
        to see “a” next, a word that had no              predictions about ongoing experiences            we might think of perception as
        intrinsic meaning but did predict the            wasn’t always taken for granted.  The            “controlled hallucination.”
        word to follow.  When the participants           view of neuroscience that dominated the
        saw “an” instead (as in “an airplane”),          20th century characterized the brain’s            In that vein, the Bayesian brain also
        they experienced an N400 effect,                 function as that of a feature detector: It       explains why visual illusions work: Two
        seemingly because the brain had to               registers the presence of a stimulus,            dots blinking in rapid alternation on a
        process a mismatch between its                   processes it, and then sends signals to          screen, for example, look like a single dot
        expectation and reality.  The effect was         produce a behavioral response. Activity          moving back and forth, so our brains

        apparently unrelated to the meaning of           in specific cells reflects the presence or       unconsciously start to treat them like a
        the word or any difficulty in processing         absence of stimuli in the physical world.        single object. Understanding how objects
        the presented stimulus itself.                   Some neurons in the visual cortex, for
                                                         instance, respond to the edges of objects        move is a higher-level type of
                                                                                                          knowledge, but it fundamentally
        The 2005 finding seemed like a great fit         in view; others fire to indicate the             influences how we perceive. The brain is
        for the predictive coding framework. But         objects’ orientation, coloring or shading.
                                                                                                          simply filling in gaps in information — in
        in April 2018, a paper published in eLife                                                         this case, about motion — to paint a
        reported that several labs have been             But the process turned out to be far less        picture that’s not entirely accurate.
        unable to replicate the result. Now, other       straightforward than it seemed. Further
        researchers have started to respond, some        tests found that as the brain perceives,         But in spite of the clear role that
        claiming that subtleties in the replication      say, a longer and longer line, the detector      generative models and expectations play

        methods still favor the prediction-based         neurons for lines stop firing even though        in brain function, scientists have yet to
        interpretation.                                  the line hasn’t disappeared. And the fact
                                                         that so much information seemed to be            pinpoint exactly how that’s implemented
                                                                                                          at the level of neural circuits. “The
        This back-and-forth reflects much of the         communicated through mysterious top-             Bayesian brain story is relatively agnostic
        debate that’s surrounded predictive              down feedback connections suggested              about what the underlying mechanisms
        coding. Experiments like Kutas’ are              that something else was going on.
                                                                                                          are,” said Mark Sprevak, a professor of
        subject to many interpretations. They can                                                         philosophy of mind at the University of
        be explained by models other than                That’s where the “Bayesian brain” comes          Edinburgh in Scotland.
        predictive coding, and they fall short of        into play, a general framework with roots

        definitive proof of the hypothesis               dating back to the 1860s that flips the                             (Continued on Page 18)
        because they don’t delve into the actual         traditional model on its head. The theory
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