Hearing body shapes
Photo credit: www.janeteresa.com
We can usually experience our own bodies in a variety of different ways. We can feel itches, tickles and pains. We can have proprioceptive and kinaesthetic experiences. We can see our bodies and we can touch them. Vision and touch also allow us to experience others’ bodies. However, as Ella Streim-Amit and Amir Amedi point out in their article, there are limitations to how much of either our own or others’ bodies we can experience haptically at any one time, as well as limits to how often and for how long we experience the human body through touch. By contrast, sighted individuals have extensive exposure to others’ bodies through vision, and in particular, can have experiences of the configuration of the whole body at once. We might therefore assume that vision is necessary for the emergence of body-shape representation in the brain.
Streim-Amit and Amedi sought to investigate this. They report that, after training using a vision-to-audition sensory-substitution device, presentation of special auditory stimuli that translate visual images of silhouettes and body shape outlines to congenitally blind subjects engages areas of the visual cortex associated with body shape representation. Subjects who have had no prior visual experience of others’ body shapes exhibit activation in the visual extrastriate body area (EBA) when presented with body shape soundscapes.
We are familiar with the idea that the brain exhibits crossmodal plasticity after sensory loss. The impairment of one sensory modality can result in the recruitment of brain regions associated with the impaired modality for the processing of inputs from other modalities. Streim-Amit and Amedi’s study doesn’t just report that a part of the visual cortex is recruited in the processing of auditory inputs. The study adds to a growing body of evidence that there is preservation of function in the visual cortex of the congenitally blind. During presentation of body shape stimuli, activity in the visual EBA was double that for face shape and non-body object stimuli. What is more, this selectivity for body shapes was confined to the visual EBA. There was no similar selectivity for body shape stimuli in the temporal or parietal cortices, brain areas associated respectively with auditory and somatosensory processing.
Streim-Amit and Amedi suggest that their study is unique in finding functional preservation for representation of the organization of the whole body, something that those who are blind do not usually experience. What seems to be absent for congenitally blind subjects, in virtue of the fact that they cannot see, is an experience of the configuration of the whole body. There is, then, retention of a function—representation of the structure of the whole body—without the sensory input that we might have assumed is required to establish that function.
The study contributes to our understanding of the way in which there can be preservation of function in the visual cortex in the absence of any visual experience. Yet, the authors make the further claim that there is preservation of function in the absence of any relevant sensory experience. This further claim is based on an unwarranted assumption: the authors assume that the same property will not be represented in the same way in different sensory modalities. For if the same property might be represented in the same way through different sensory modalities, then it is possible that the body shape selectivity Streim-Amit & Amedi observe in the visual EBA does not occur independently of any sensory input, but is conditional on experience of the body from the inside.
The claim that blind individuals fail to experience the shape of the whole body because they cannot see disregards the many kinds of experiences we have of our own bodies including, amongst others, proprioception, touch and kinaesthesia. This oversight seems to signal several assumptions about the differences between the sensory modalities. One assumption is that our experiences of our bodies through proprioception, kinaesthesia and so forth are never experiences of the configuration of the whole body. The other is that differences in phenomenology and differences in some of the properties presented through different modalities must be coincident with differences in the representation of properties presented across the modalities.
Some properties are perceptible in only one sensory modality. For example, we have visual but not haptic or auditory experiences of colours. However, other properties seem to be perceptible through more than one modality. Sometimes called amodal properties or, in Aristotelian terms, ‘common sensibles’, these include things like shape, size and location. One can, for example, see and feel the shape of an object that one can hold in one’s hand. Molyneux’s Question, first discussed in publication by John Locke, asks whether a congenitally blind adult able to distinguish between a sphere and cube by touch only and who is then given the ability to see will be able to say which object is which on the basis of vision alone. If we are prepared to accept that there are properties of objects such as shape that are accessible through a number of different sensory modalities, one question that presents itself is whether the same property is represented in the same way in each modality. Understood in this way, Molyneux’s Question asks whether different sensory modalities—in this case vision and touch—represent the same property—in this case shape—in the same way.
Admittedly, the phenomenology of our experiences of our own bodies from the inside seems to be different from both the phenomenology of our visual experiences of others’ bodies and the phenomenology of the auditory experience of body shape soundscapes used in the study. Yet, we should not assume that because of these differences, that there must be a difference in how body shape is represented across the senses. Neither should we assume that our experiences of our bodies from the inside do not involve experience of whole body configuration. We should therefore be cautious about Streim-Amit and Amedi’s claim that activity in the visual EBA of congenitally blind individuals in response to body shape soundscapes supports the view that there are innate domain-specific constraints, independent of any sensory input, on the function of particular areas of the brain. Nevertheless, their study contributes to our understanding of the ways in which there can at least be preservation of function in the visual cortex in the absence of any visual experience, and raises the question of whether our experience of the body in other sensory modalities contributes to the body shape selectivity that Streim-Amit and Amedi found in the visual cortex, and if so, how this comes about.