Philosophy of Olfactory Perception

by Andreas Keller

I’ve had a background interest in the sense of smell since reading Luca Turin’s The Secret of Scent back in the day. I found it compelling because the complexity of olfaction seemed to exceed the capability of the receptor paradigm 

However, that was before my recent encounter with Keller’s Philosophy of Olfactory Perception, which describes the olfactory systems as consisting of over 400 receptors — the combinatorics of 400 receptors seems adequate to the complexity of olfaction. It certainly, at a minimum, puts receptors back in play and helps explain why Turin’s theory hasn’t gotten much empirical validation since publication 

The main thrust of the book is to describe Keller’s ambitious program of developing a perceptual quality space — an exhaustive construction of the similarities of all sensory modalities which would eventually allow for principled comparisons between modalities, both in the shapes of their perceptual quality spaces and also how particular stimulants might simultaneously stimulate multiple modalities. 

Keller emphasizes that this space should be developed using perception of similarities of stimuli rather than differences. I initially thought that this was simply isomorphic to Just Noticeable Difference with measurements with a flip of the distance bit. However, as I considered it more, it does seem likely that framing the task as the measurement of similarities rather than a measurement of differences allows you to come up with experimental frameworks that hadn’t been imagined previously. For example, the 3 stimulus similarity test he describes 

(given 3 stimuli: A, B and C, is B “perceived as being closer” to A or C) 

is something which I can’t recall having ever seen in a JND test. My intuition is that the forced binary of the 3 stimulus similarity test potentially allows better insight into perceptions that don’t rise to the level of consciousness.

That said, even characterizing the single sense of olfaction in this manner is a big lift. As Keller acknowledges, we don’t yet have a handle on any axes that capture our perception of distance between smells, let alone the appropriate axes to measure the principal components of these distances with any consistent methodology — as he says

For tastes and smells, pleasantness may even be the dominant perceptual dimension

Keller is fully aware of the of the complexity of the task, describing 3 barriers to this research

1. Using verbal labels for descriptions which he characterizes as establishing an odor descriptor space rather than a perceptual space. Which implies that there’s not only no attempt to assure that the descriptors are disjoint but also nothing to assure that they completely cover the olfactory capabilities of the test subjects

2. The difficulty of finding the right odorants to use for such a project. 

3. Most attempts to construct a smell space are based on individual odorous molecules, while most perceived smells are formed by a mixture of multiple odorants hitting the olfactory sensors.

He also delineates another set of difficulties in studying olfactory response

• Lack of spatial discrimination for olfaction
• Bias towards Novelty: Optimized to detect changes rather than constant values (although I think a lot of our senses have that kind of bias, vision, hearing)
• Broad similarity of response to various stimuli: Bitter tastants are therefore an excellent example of physically diverse stimuli that all require the same behavioral response

However, as he says, the goal of perception isn’t to achieve some Platonically ideal response, but just something that allows us to survive:

Indeed, all perception by a properly functioning perceptual system is by definition correct perception when correctness is defined in terms of the perceptual system

Keller ends with a section on consciousness and how it interacts with olfaction 

He references Edelman’s four level consciousness model: minimal consciousness, differentiated consciousness, subjective consciousness, and cognitive, subjective consciousness. ¹Edelman, G. M. (2003). Naturalizing consciousness: A theoretical framework. Proceedings of the National Academy of Sciences, 100(9), 5520–5524.

concentrating mostly on 

Keller then reviews how smell interacts with action (with or without conscious processing) delineating how our actions can be influenced by smells which aren’t consciously processed (unsurprising in the contexts of other work I’ve addressed in this blog) and how consciousness evaluates options presented by smells and serves to focus attention.

As is common in this type of book it’s ending is mostly a next steps/forward looking section. What is uncommon is it’s thoughtful, well-grounded program for exploring/delineating a perceptual mode that is so much on the tip of our tongues ²A quasi pun on how inaccessible smell is to verbal description and how intertwined it is with taste.

• 1
  Edelman, G. M. (2003). Naturalizing consciousness: A theoretical framework. Proceedings of the National Academy of Sciences, 100(9), 5520–5524.
• 2
  A quasi pun on how inaccessible smell is to verbal description and how intertwined it is with taste.