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For most of us, creating mental images from speech or memory is very simple.
When I say "cube," you probably already have one in mind (although people with aphantasia have little or no mental imagery).
You may not realize it, but you are probably also very good at translating physical sensations into mental images. Imagine you are in total darkness and holding a cube-shaped object. Chances are you can convert this tactile information into a mental image.
For centuries, scientists and philosophers have debated whether this is something we learn to do, or whether our nervous systems are prepared for it. My team's recent research with newborn chicks provided new insights into this question.
This topic has intrigued philosophers and scientists for centuries, dating back at least to 1688, when Irish philosopher William Molyneux wrote a letter to fellow philosopher John Locke. Molyneux's letter asked whether a person born blind, who learned by touch to distinguish between a cube and a sphere, would recognize these objects if he were given immediate sight. This question was also personal for Molyneux: his wife had become blind shortly after their marriage.
Because they focus on the role of experience in people's formation of ideas and beliefs, empiricists like Locke think that sensory experience is necessary to learn or understand the correspondence between tactile and visual information.
Yet both neuroscientists and philosophers are beginning to question this view. Some argue that sensory information can be processed in such a general and abstract way that the details become irrelevant at some level.
Others point to phenomena such as synesthesia, in which people experience stimulation in one sense (such as hearing) and have a corresponding experience in another sense (vision). This happens, for example, in 'colored hearing', where the experience of hearing sounds also produces the experience of colors.
Scientists once thought that only humans could associate sensory features across modalities, but research shows that other animals do this too.
For example, dogs match small pictures with high-pitched sounds and large pictures with low-pitched sounds. My team's 2023 research found that turtles do this too.
But scientists are still unsure whether animals learn through experience to match information between senses or whether this ability is a feature of the nervous system, which does not require experience.
Scientists are fascinated by the possibility that tactile sensations can spontaneously evoke visual representations, as a property of the nervous system. However, exploring these questions is not easy, especially when dealing with human subjects.
Although in some patients born blind, vision can be restored by removing cataracts. But it takes a few days for vision to fully recover. Patients tested within 48 hours of surgery were unable to solve the task of visually distinguishing shapes originally presented to them in tactile modality, but mastered the task in just five days, a 2011 study found.
Some researchers have studied babies to try to answer the question. In the 1979 study by researchers at the University of Washington, newborns were offered pacifiers of different shapes.
In another 2004 study, newborns were given objects of different shapes, such as cylinders and prisms, to touch. When the babies were shown new objects and shapes mixed with the objects they had touched, they tended to look longer at the new ones. This suggests that they were surprised by the new shapes.
But when several teams tried to replicate the studies, the results were mixed. It is also not possible to say with certainty what previous sensory experiences babies have had, as it would be highly unethical to raise them in sensory deprivation before the study.
In addition, vision in humans develops over time.
What we found
My team opted for an alternative approach with domestic chicks. These birds have a well-developed motor and sensory system upon hatching. In our experiment, we hatched chicks in complete darkness and maintained this environment during their exposure to tactile stimuli.
Each chick was housed in individual compartments with bumpy or smooth cubes. Over the course of 24 hours, the chicks became familiar with their bumpy and smooth environments, respectively. This arrangement mimics natural conditions, as chicks typically spend their first days nestled in darkness under the warmth of a hen.
Because chicks tend to develop attachment responses (imprinting) to the first stimuli they encounter, we thought that they might develop some kind of attachment to the tactile objects.
After the tactile exposure, we introduced chicks to their first visual experience. We placed them in a lit room with two objects: a smooth cube and a bumpy one.
Chicks exposed to the smooth stimuli approximated the smooth cube much more compared to chicks exposed to the bumpy stimuli.
This result would surprise Molyneux and Locke (and possibly modern empiricists), because it shows that the brain is wired to understand the complexity of the world before we have direct experience with it.
It is also consistent with research showing that the minds of newborn animals are born with expectations of stimuli. For example, my team's 2023 paper found that chicks have expectations of upward and downward movements, indicating an understanding of gravity, in the absence of visual experience.
We are also interested in investigating the neural mechanisms behind associations between senses during early development and other predispositions that help us cope with our environment in childhood.
This could help us understand the connection between sensory representations, imagination and the perception of reality.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
