The Colors, Man! The Colors!


Scientists have discovered direct neural correlates of synesthesia, a new study reports.

They sound like unicorns and rainbows.

Not only have they detected activation patterns corresponding to synesthesic activity (such as “seeing” certain colors when thinking of certain numbers or sounds) – they’ve isolated an actual functional difference in the brains of synesthesic people. And what’s more, they’ve discovered a way to crank up synesthesic activity.

Let’s break this down and talk about what they’ve done here.

To understand what’s going on, let’s take a quick glance at history. Synesthesia’s fascinated artists and scientists since way back – in fact, the first people to write about it were the ancient Greeks, who composed treatises on the “colors” of various musical sounds.

Centuries later, Newton and Goethe both wrote that musical tones probably shared frequencies with color tones – and though that idea turned out to be incorrect, it did inspire the construction of “color organs” whose keyboards mapped specific notes to specific shades.

The first doctor to study synesthesia from a rigorous medical perspective was Gustav Fechner, who performed extensive surveys of synesthetes throughout the 1870s. The topic went on to catch the interest of other influential scientists in the late 19th century – but with the rise of behaviorism in the 1930s, objective studies on subjective experiences became taboo in the psychology community, and synesthesia was left out in the cold for a few decades.

In the 1950s, the cognitive revolution made studying cognition and subjective experience cool again – but it wasn’t until the 1980s that synesthesia returned to the scientific spotlight, as neuroscientists and psychologists like  Richard Cytowic and Simon Baron-Cohen began to classify and break down synesthetic experiences. For the first time, synesthesic experiences were organized into distinct types, and studied under controlled lab conditions.

Today, most synesthesia research focuses on grapheme → color synesthesia – in which numbers and letters are associated with specific colors – because it’s pretty straightforward to study. And thanks to the “insider reporting” of synesthetes like Daniel Tammett, we’re getting ever-clearer glimpses into the synesthetic experience.

But as the journal Current Biology reports, today marks a major leap forward in our understanding of synesthesia: a team led by Oxford University’s Devin Terhune has discovered that the visual cortex of grapheme → color synesthetes is more sensitive – and therefore, more responsive – than it is in people who don’t experience synesthesia.

The team demonstrated this by applying transcranial magnetic stimulation (TMS) to the visual cortices of volunteers, which led to a thrilling discovery:

Synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. … These results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms.

In short, the visual cortex of a synesthete is three times more sensitive to incoming signals than that of a non-synesthete – which means tiny electrochemical signals that a non-synesthete’s brain might consider stray noise get interpreted into “mind’s-eye” experiences in a synesthete’s visual cortex.The question of what, exactly, causes this difference in the first place remains a Science Mystery, ripe for investigation.

But wait – this study gets much, much cooler.

There’s a technology called transcranial direct current stimulation (TDCS), which changes the firing thresholds of targeted neurons – making them more or less likely to fire when they get hit with a signal. The researchers applied TDCS to specific parts of the visual cortex, and found that they could “turn up” and “turn down” the intensity of the synesthesic experience:

Synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. A control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia.

In other words, they’ve discovered a technological mechanism for directly controlling the experience of synesthesia.

So Burning Question #1 is, Could TDCS be used to induce synesthesia – or create hallucinations – in non-synesthetes? With the right neurological and psychological preparation, it certainly seems possible. And Burning Question #2 is, could it be used to “turn down” the intensity of hallucinations in people with schizophrenia and other psychological disorders? It’ll take a lot more lab work to answer that question with any certainty – but I’d say it merits some lookin’ into.

In the meantime, I’m going to find some nice green music to listen to.

______________

1. This means synesthesia is somewhat similar to Charles Bonnet syndrome – in which blind patients see vivid, detailed hallucinations when their under-stimulated (and thus, hyper-sensitive) visual cortices catch a stray signal – and to musical ear syndrome, in which deaf people vividly hear singing. Here’s an amazing TED talk by Oliver Sacks on that very topic.

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