The Frontal Cortex

Top-Down

Language is the stuff of thought. I’m reminded of this truism every time I sip a glass of wine and some pretentious snob (usually me) insists on saying something about the Chianti Classico smelling like cherries, or how the New Zealand sauvignon blanc exudes the perfume of pineapple. As soon as I hear those nouns, my olfactory cortex goes into dishonest overdrive and, before I know it, all I can smell is those damn cherries. My experience of the wine is completely altered by a few choice words; language subverts reality.

Of course, this isn’t always a bad thing. Parmesan cheese and vomit, for example, are both full of butyric acid, which has a pungent top note and a sweetish linger. As a result, blindfolded subjects in experiments will often confuse the two stimuli. In real life, however, such sensory mistakes are extremely rare. Common sense overrules our actual senses.

These gastronomic examples capture a basic truth of the mind, which is that our sensory impressions are always incomplete. As a result, we are constantly making judgments about what we think we are sensing: the mind needs a dash of top-down subjectivity to render the world whole. And this doesn’t just apply to the olfactory cortex, which is inundated with feedback from the “higher” parts of the brain, like the PFC. Atul Gawande, in his latest article, ably summarizes this somewhat disconcerting view of perception (at least it’s disconcerting if you’re a naive realist):

The account of perception that’s starting to emerge is what we might call the “brain’s best guess” theory of perception: perception is the brain’s best guess about what is happening in the outside world. The mind integrates scattered, weak, rudimentary signals from a variety of sensory channels, information from past experiences, and hard-wired processes, and produces a sensory experience full of brain-provided color, sound, texture and meaning.

A great new demonstration of this top-down process in action (courtesy of Science Faction) is this Bollywood music video, which features English mis-transliterations of the lyrics. What’s fascinating is that, once you read the fake subtitles, you can’t help but hear those exact words. The inputs of the auditory cortex are instantly rearranged:


Comments

  1. #1 jb
    June 27, 2008

    I happened to co-teach a course on Mahayana Buddhism last night and did some perception exercises with people to demonstrate the ‘emptiness’ of self and other. Just as a juice glass is full of air and empty of juice, tea, wine, beer, and other liquids we might put in it ( plus sand, ants, rabbits, whatever), so our actual experience of the world through the senses is empty of the many likes and dislikes, labels and uses we project onto appearances in our mind. It was nice to be able to refer people to Jonah’s book to show that artists have known this about the senses, memory and self for awhile and that neuroscience is just getting around to discovering it. A nice video for homework. Thank you , Jonah

  2. #2 Brian
    June 27, 2008

    Is that video an example of top down processing or is it an example of priming, where the audio and reading modules in are head plug into the same bank of words, and the bank is primed by the reading access, such that the audio access is influenced towards the primed words.

    This doesn’t quite seem to me to be the same thing as top down processing. It’s just a great example of priming. I don’t know for sure.

  3. #3 siddharth
    July 8, 2008

    It’s not a bollywood music video. Bollywood refers to only hindi movies which are usually directed by studios in bombay.

    “Benny Lava” is actually from a Tamil movie, Pennin Manathai Thottu. So, kollywood would be a more accurate term.

  4. #4 Thinkr
    May 21, 2011

    If our sensory perceptions are so “incomplete,” how do you explain (for example)a sports game where the fans go bonkers all together when their team scores?

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