Let’s start off with this famous experiment done by neuroscientist V. S. Ramachandran and Edward Hubbard (Ramachandran and Hubbard, 2001). They asked American college undergraduates and Tamil Speakers in India “which of these shapes is bouba and which is kiki?” What do you think?
Did you pick the right one as “bouba” and the left one as “Kiki”? Yes, your instinct was correct. 95% to 98% of subjects responded the same way as you just did (Ramachandran and Hubbard, 2001). Another group of researchers tested this similar question to toddlers. The finding was that the associations of “kiki” to jagged shapes and “bouba” to rounded shapes were consistent even prior to language development (Maurer et al., 2006). These results suggested that no matter the test subjects were different native languages speakers or very young children, people were always able to make this association.
Ramachandran and Hubbard reasoned that because of the sharp form of the visual shape, subjects tended to map the name “kiki” onto the left figure, and because of the rounded auditory sound, subjects tended to map the name “bouba” onto the right figure (Ramachandran and Hubbard, 2001). Other researchers have proposed that perhaps this effect happened because when you say “bouba”, your mouth makes a more rounded shape, whereas when you say “kiki”, your mouth makes a more angular shape (D’Onofrio, 2014). It has also been suggested that this Bouba-Kiki effect (BK effect) could occur through cognitive mechanisms similar to those that underlie synesthesia (Ramachandran and Hubbard, 2001), the phenomenon in which someone experienced sensation in a particular modality (hearing, for example) when a different modality was stimulated (seeing a particular color, for example). To sum up, one thing that scientists agreed on was that in order for the BK effect to take place, some sort of integration of shapes and sound occurred in the brain (Spence and Deroy, 2013).
All these explanations made sense, right? But after learning about all BK effect in Dr. O’Toole’s class, I was still curious about how and where these integration processes happened in my brain when I selected “bouba” to the right figure and “kiki” to the left figure. To investigate this phenomenon one step further, two neuroscientists from Sorbonne University in Paris published their study using functional Magnetic Resonance Imaging (fMRI) (Peiffer-Smadja and Cohen, 2019).
These researchers had two questions in mind. Question #1: did this integration of shapes and sounds occur at an automatic or a controlled level? In other words, would participants show a BK effect even when no explicit judgment was required on audio-visual matching? Question #2: did this integration take place in our sensory cortices or in our supramodal regions (areas of the brain that have abstract functions to more one type of sensory input)?
In order to test the first question, the researchers designed a task called Implicit Association Test (IAT). The underlying trick is that responses are supposed to be faster and more accurate when concepts are strongly associated. In this case, we would predict that the response to be faster and more accurate whenever “kiki” sounds were paired with spiky shapes (congruent block) than whenever “kiki” sounds were paired with rounded shapes (incongruent block).
For each trial, participants were simultaneously presented with a pseudoword and a shape. The participants in this task were asked to decide if the pseudoword contained the sound “o” or the sound “i”. Then they had to decide if the shape was round or spiky. As anticipated, responses were faster and more accurate in congruent blocks than in incongruent blocks. This experiment was a clever twist to the traditional “BK” experiment. Here, the participants were never explicitly asked about matching the shapes and sounds. Still, the bouba-kiki sound-shape association had an impact on their behavior even when it was irrelevant to the task. The persistence of the BK effect even in this setting suggested that it may came at least in part from automatic perceptual stages of stimulus processing, which was separated from attention and task-related influences. The first mystery was solved.
Next, using fMRI, the authors were looking for which brain regions were activated when the subjects performed implicit BK matching tasks. Participants were simply asked to pay attention to both visual and auditory stimuli when sometimes the pairs were matching (bouba-round) and sometimes the pairs were mismatching (bouba-spiky). They found that cross-modal matching influenced activations in both auditory and visual sensory cortices. Moreover, they found higher activation in the prefrontal cortex to mismatching stimuli than to matching stimuli. Taken together, when the pairs were matching, the visual cortex (where visual information is processed by the brain) and the auditory cortex (where auditory information is processed by the brain) showed more activation. On the contrary, when the pairs were mismatching, prefrontal cortex showed more activation.
So, what could we conclude from these findings? Results indicated that BK matching had an effect on the early stages in sensory processing, while mismatching had an effect on the later stages of supramodal processing. As a follow-up, the authors hypothesized that the crossmodal BK effect perhaps was modulating the executive processes (processes that are necessary for the cognitive control of behavior) in the prefrontal cortex.
Bear in mind that these conclusions should be taken as preliminary findings. The common problem with fMRI study is that a structure active for a task does not mean it is critical for the task. So, the only certain inference we can make from the study is that prefrontal activation is related with part of the integration processes of BK effect. In the scientific literature, mechanisms involved in cross-modal integration is currently not well-understood (Peiffer-Smadja and Cohen, 2019). For hundreds of years, we have been investigating how our brain processes sensory information. And this BK effect perhaps now provides us a unique window to look into how our brain combines all these sensory information and create a coherent picture of how we perceive the world around us.
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