Smellosophy: What the Nose Tells the Mind, by A. S. Barwich (Harvard University Press), £28.96/$35
In Smellosophy: What the Nose tells the Mind, Ann-Sophie Barwich guides the reader into the fascinating science (both historical and contemporary) of odours and olfaction. The study of smell has tended to be neglected in both philosophy and in neuroscience, in comparison to vision. In reading Barwich’s book one quickly learns that this is not only the result of philosophical prejudice, but also due to the many complexities of both odours and the neurobiology of olfaction in comparison to vision. In detailing those difficulties, Barwich argues that olfaction challenges some assumptions about perception more broadly, and suggests a conception of olfaction (and perhaps other senses) as being not merely a means of detecting and classifying properties of external stimuli but also just as much or even more a matter of the internal milieu, functioning, and activity of the perceiver in navigating a constantly changing environment.
One of the major themes of the book is that there is no clear “objective representational content” associated with particular types of odour experiences. “Oflactory information is not computed into perspective-invariant perceptual objects as information in vision is.” The difficulties in assigning such a content arise at many different levels, and Barwich makes a strong case for this across several chapters.
First there is the challenge of describing and classifying odours in any systematic way. Here the contrast with vision seems especially apt. Colour appearances seem to be characterizable along the dimensions of saturation, hue, and brightness. And although we can discriminate many millions of hues, these can all be characterised in terms of amounts of red or green, blue or yellow, and white and black. For example, purples appear a little bit bluish and a little bit reddish. In the first, historical chapter of Barwich’s book we encounter a variety of attempts to classify odours, from physiology to perfumery. And here we quickly realise how difficult it is to develop a simple or intuitive classification of odours.
Later chapters provide an explanation. Whereas the eye and brain rely on the stimulation of just three types of cones in the eye to detect colour variation, the olfactory receptors are enormously multi-dimensional. As Barwich explains, “odour receptors deal with several thousand different molecular parameters in no particular order of continuity or scale. Therefore, there is no uniform way to carve the physical space of odorants ‘at its joints’ like visible wavelengths or audible frequencies.”
A further difficulty in understanding how the brain represents odours is that it turns out that our olfactory receptors do not “think” about chemistry in the way that a trained chemist does. Barwich explains that “the priority and hierarchy of features by which chemists and receptors determine chemical similarity” differ, and also these receptors often respond to unanticipated chemical features. Adding to the challenge is the fact that there are many inhibition and enhancement effects at the receptor level. This is part of the reason that some smells can suppress the perception of other odorants, a phenomenon known to perfumers and put to good use in the development of ways to neutralise unpleasant smells in waterless toilets.
In a later chapter, Barwich explains that “[t]here is no stereotypic, topographical mapping of odour, some generalisable order that links physical stimulus space to perceptual space in the brain of an organism That does not render smell without rules. The objective measure is the computational processes coding signalling bits and pieces into revisable perceptual judgments — an ongoing response proportional to the physiological conditions of the perceiver and the changing stimulus ratios in the environment.” We also learn that odour perception is highly context-sensitive and individually variable. A particular memorable example comes from the discussion of an experiment in which the same mixture of butyric acid and valeric acid is perceived differently depending on its label, in this case “vomit” or “parmesan cheese.”
Barwich often discusses the complexity of odour and odour perception via a contrast with vision. Here it sometimes seems that Barwich presents an overly simplified conception of colour perception. She sometimes implies that colour experiences simply represent wavelengths of light — but this is not a common view in philosophy for a variety of reasons, perhaps most importantly that most of the time we see colours as relatively stable properties at the surface of physical objects. There is also the phenomenon of metamerism: two objects with very different surface spectral reflectances can nonetheless look precisely the same hue under a given illuminant. This mirrors one of the difficulties Barwich notes about odour perception and the lack of a straightforward mapping from stimulus to odour. And colours, like odours, are context-dependent. The colour that an object will appear to have can vary with its surround (these are called “colour contrast effects”). And despite the fact that changes in illumination result in changes in the character of the light that reaches the eye, colour perceivers tend to see objects as having the same colour across varying lighting conditions (colour constancy).
That said, I suspect that Barwich would find these complexities in colour perception to further confirm her view, which seems to be that in understanding the senses we should not assume that their only or even primary function is to detect and model objective context-independent and perceiver-independent features of external objects. And even though this lesson might be a good one for those who study vision, Barwich makes a compelling case that it is unavoidable in the case of olfaction.
One could be overwhelmed by the sheer complexity of odour perception, and Smellosophy is rich in many of those details. Barwich does reasonably well at creating a book that is accessible to a wide audience. At times, things do get technical either from the point of view of chemistry, biology, or philosophy. But Barwich usually provides just enough background to guide the reader into a debate or theory and to help them understand why it is significant.
This book is also more than just a book about smells. It is also in many ways a book about the scientists who study smell, and the process of scientific discovery. Barwich frequently quotes from her interviews and discussions with dozens of the principal investigators in the field. Through these conversations and anecdotes, Barwich manages to convey some of the mystery of a scientific puzzle, the frustrations from theoretical dead-ends, and the excitement that comes from new breakthroughs.