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English Review - Senses: Do we have 5, 8 or 28?

The Relevance of our Senses

We all know the five traditional senses: taste, smell, hearing, sight, and touch. Since Ancient times, we have been convinced that they allow us to perceive the world. But how important are these senses? And do we only have five? In two lectures, the speakers challenged our conception of perception. During the first lecture, neurobiologist Tessa van Leeuwen from the University of Tilburg and the Donders Institute introduced a sensory deviation called synesthesia. The second lecture by Leon de Bruin, philosopher of mind and language at the VU Amsterdam and Radboud University, challenged the relevance of our senses and explored the way in which humans transform their perception. Both speakers were very excited to speak in front of a live audience, because “it is more stimulating to the senses” than an online event. After the readings, Marc Slors had a discussion with De Bruin and Van Leeuwen about the malleability of our perception and the importance of conscious perception.


During the first lecture, Van Leeuwen described the characteristics of synesthesia. Synesthesia is a perceptual deviation in which “senses get mixed up”. There are many kinds of synesthesia. For example, grapheme-color synesthesia, in which letters or numbers are associated with a particular color. Or time-space synesthesia, which means that points in time, e.g., months, are spatially laid out around the synesthete. An important characteristic of synesthesia is that the associations between senses do not change over time and happen automatically. Later in the evening, someone asked whether the production of saliva when seeing an orange is also a form of synesthesia. Van Leeuwen, who is herself a synesthete, said it is not. She explained that synesthesia is not only making associations between different sensory perceptions, nor does it include learned associations (e.g., associating blue with a cold temperature). Persons with synesthesia actually perceive a color when seeing a certain letter or number. Van Leeuwen showed this with an fMRI scan indicating that the part of the brain that is relevant for color perception is active in grapheme-color synesthetes even when there is no color to be perceived.

In the final part of her talk, Van Leeuwen discussed whether synesthesia really is a deviation. It is quite normal that our senses work together to make sense of the environment. By means of a quiz, Van Leeuwen showed that, without knowing the meaning of Japanese words, most people in the audience were able to pick the right image by listening to the sounds of the word. Synesthetes are better at this task, but synesthesia is not an “all or nothing” phenomenon. It seems that synesthesia is an extreme form of multimodal integration, which is something everyone does to a certain extent.

The Importance of our Senses

During the second lecture, De Bruin tried to convince the audience of the hypothesis that there may be an infinite number of senses and explored the possibility that our senses may become redundant in the future. De Bruin started his reading with the “intuitive idea that we have five senses” and argued that the importance of these senses relies on our phase of life and our bodies. “When you are a baby and you are low to the ground, smell is more important than sight. But when you start walking up straight, and you can look over things, vision becomes dominant.” De Bruin continued to argue that we might have more senses than these five. Think of proprioception, which allows us to “perceive the position of your own body and body parts”, or our vestibular system, which allows us to keep our balance. We do not only have additional senses, but we also train our senses and transform the way we use them by means of technical supplements. De Bruin used the example of a telescope. These kinds of techniques facilitate our perception by allowing us to see things that we were previously not able to see, like black holes. “We do not merely train what we can already see but engage in radical construction so that we can see something new”, argued De Bruin.

Finally, De Bruin introduced the work of David Eagleman, who investigates ‘sensory substitution’. This is the ability to receive information via unusual sensory channels, such as hearing via skin vibrations. De Bruin explained that Eagleman’s work reminded him of a paper by Churchland, who claimed that transferring information via language is very inefficient. Churchland proposed that we should instead infer this information from brain activity. Although his proposal was rejected, the idea has become more popular with the introduction of brain-to-brain interfaces. De Bruin ended his reading with an open question: “If we are able to plug in each other’s brains, will we still need senses in the future?”

The malleability of our senses

After the lectures, Slors asked the speakers about the extent to which our perception is malleable. Van Leeuwen answered that we are not “finished” when we are born. We are still “plastic” to the extent that our environment can determine how much we develop our senses. If our environment requires a good sense of smell, then we can develop a better sense of smell during our life. Finally, Slors asked about the relevance of conscious sensory perception: even though we have the same input and senses, there are individual differences in how people experience that input. Do these differences matter? Both speakers agreed that for social interaction, individual differences are not relevant. We believe that everyone experiences the world in the same way. Van Leeuwen added that conscious perception may allow humans to emphasize especially relevant information.

Someone in the audience asked De Bruin about the desirability of creating a super-sense by plugging in each other’s brains. De Bruin answered that many philosophers agree that it is not desirable. Still, it may be beneficial in some cases. For example, when someone cannot communicate because they suffer from locked-in syndrome. De Bruin concluded the evening by admitting that he was also not sure whether we should aim toward one super-sense.

Jikke Hesen