By Christian Jarrett

For most of us, it is difficult to imagine what it must be like to be a synesthete – that is, someone who experiences a crossing over of their senses, such as seeing sounds as colours, or perceiving shapes as having tastes. However, according to a new study in Consciousness and Cognition, it is actually relatively easy for people with normal perception to have a synesthetic experience (of the sound-to-vision variety). It merely takes a few minutes of visual deprivation, followed by a visual imagery task. The findings are not merely intriguing – and a fun idea for a psychology class experiment – they also have a bearing on the main theories for how synesthesia occurs.

Across three studies, involving dozens of non-synesthetic undergrad students, the University of Michigan psychologists Anupama Nair and David Brang asked their volunteers to sit with their eyes closed in a completely darkened room for five minutes. After this, the imagery task began, which was almost identical through the three studies: dozens of times the students heard one of four words followed by a letter, such as “symmetrical T” or “curve A”, and their job was to visualise the letter and judge whether it had the mentioned feature (so, in the previous examples, whether “T” is symmetrical and whether “A” contains any curves). The other two feature words used in the task were “diagonal” and “closed”, referring to whether the letter had a diagonal line in it or an entirely enclosed feature.

The purpose of the darkness was to remove competing visual perceptions, while the idea of the imagery task was simply to raise the activity levels within the visual cortex (visual imagery is known to recruit brain areas that overlap with those involved in visual perception).

Another key element of the task was that the researchers played occasional, unpredictably timed beeps throughout the task, to see if these induced a visual experience in their participants. Meanwhile, for the duration of the task, the participants were asked to press a button to indicate any instance in which a sound prompted them to have a visual experience in the darkness. At debriefing later, they were asked to provide more details about any visual experiences they’d had.

Overall, across the three studies, nearly 60 per cent of the participants reported at least one instance of experiencing a sound-induced visual sensation in the darkness, including: “colorful small circles, amorphous blobs, scintillation [flash or sparkles of light], extrusion [like visual pulsing or eruptions], and movement”. These experiences were more common on trials of the task in which a beep occurred, although they were sometimes induced merely by the task words. Louder beeps more often induced visual experiences than quieter beeps, and usually the visual sensations were perceived as occurring on the same side of space as where the beep came from.

It’s well-established that there is a lot of cross-wiring and cross-talk between the senses even in non-synesthetes and Nair and Brang said their findings, showing how quickly and easily synesthetic experiences can be induced in people with normal perception, are more consistent with the “disinhibited-feedback model” of synesthesia, which “proposes that synesthesia emerges when the balance of activity across the senses has been altered to allow one sensory modality the ability to exert stronger modulatory effects onto another sensory modality”.

In contrast, the new findings are less consistent with the “cross-activation model”, which states that synesthesia emerges through the development, over many years, of atypical connections between sensory areas in people with the condition. The researchers acknowledge, however, that both mechanisms could contribute to the emergence of the phenomenon.

An obvious criticism of the findings is that the participants may have been saying what they thought the researchers wanted to hear – indeed, the researchers did tell them at the start that they might experience visual sensations during the task. However, Nair and Brang think it unlikely that their participants were fabricating their experiences, not least because of the systematic patterns in the results (such as the visual perceptions occurring more often for louder beeps, and on the same side of space as the beeps).

“Collectively, these results demonstrate that mild visual deprivation facilitates auditory-visual percepts,” the researchers concluded, “indicating a higher prevalence of synesthesia-like experiences in the general population and a novel method to study both the relationship between synesthesia and normal multi-sensory processes and the experience of visual hallucinations in clinical populations.”

—Inducing synesthesia in non-synesthetes: Short-term visual deprivation facilitates auditory-evoked visual percepts

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest