The Cinderella of the Senses: Smell as a Window into Mind and Brain?

What does this tell us about smell perception and the brain in general? Theories of perception, in effect, always have been theories of vision. Concepts originally derived from vision were made fit to apply to what’s usually side-lined as “the other senses.” This tendency permeates neuroscience as well as philosophy (Matthen 2005). However, it is a deeply problematic strategy for two reasons.

First, other sensory modalities (smell, taste, and touch but also the hidden senses of proprioception and interoception) do not resonate entirely with the structure of the visual system (Barwich 2014; Keller 2017; Smith 2017b). Second, we may have narrowed our investigative lens and overlooked important aspects also of the visual system that can be “rediscovered” if we took a closer look at smell and other modalities. 

Insight into the complexity of cross-modal interactions, especially in food studies, suggests that much already (Smith 2012; Spence and Piqueras-Fiszman 2014). So the real question we should ask is:

How would theories of perception differ if we extended our perspective on the senses; in particular, to include features of olfaction?

Two things stand out already. The first concerns theories of the brain, the other the permeable border between processes of perception and cognition.

First, when it comes to the principles of neural organization, not everything in vision that appears crystal clear really is. The cornerstone of visual topography has been called into question more recently by the prominent neuroscientist Margaret Livingstone (who, not coincidentally, trained with David Hubel: one half of the famous duo of Hubel and Wiesel (2004) whose findings led to the paradigm of neural topography in vision research in the first place). 

Livingstone et al. (2017) found that the spatially discrete activation patterns in the fusiform face area of macaques were contingent upon experience — both in their development and, interestingly, partly also their maintenance. In other words, learning is more fundamental to the arrangement of neural signals in visual information processing and integration than previously thought. The spatially discrete patterns of the visual system may constitute more of a developmental by-product than simply a genetically predetermined Bauplan.

From this perspective, figuring out the connectivity that underpins non-topographic and associative neural signalling, such as in olfaction, offers a complementary model to determine the general principles of brain organization.

Second, emphasis on experience and associative processing in perceptual object formation (e.g., top-down effects in learning) also mirrors current trends in cognitive neuroscience. Smell has long been neglected from mainstream theories of perception precisely because of the characteristic properties that make it subject to strong contextual and cognitive biases. 

Consider a wine taster, who experiences wine quality differently by focusing on distinct criteria of observational likeness in comparison with a layperson. She can point to subtle flavour notes that the layperson may have missed but, after paying attention, is also able to perceive (e.g., a light oak note). 

Such influence of attention and learning on perception, ranging from normal perception to the acquisition of perceptual expertise, is constitutive of odour and its phenomenology (Wilson and Stevenson 2006; Barwich 2017; Smith 2017a). Notably, the underlying biases (influenced by semantic knowledge and familiarity) are increasingly studied as constitutive determinants of brain processes in recent cognitive neuroscience; especially in forward models or models of predictive coding where the brain is said to cope with the plethora of sensory data by anticipating stimulus regularities on the basis of prior experience (eg, Friston 2010; Graziano 2015).

While advocates of these theories have centred their work on vision, olfaction now serves as an excellent model to further the premise of the brain as operating on the basis of forecasting mechanisms (Barwich 2018); blurring the boundary between perceptual and cognitive processes with the implicit hypothesis that perception is ultimately shaped by experience. 

These are ongoing developments. Unknown as yet is how the brain makes sense of scents. What is becoming increasingly clear is that theorizing about the senses necessitates a modernized perspective that admits other modalities and their dimensions. 

We cannot explain the multitude of perceptual phenomena with vision alone. To think otherwise is not only hubris but sheer ignorance. Smell is less evident in its conceptual borders and classification, its mechanisms of perceptual constancy and variation. It thus requires new philosophical thinking, one that re-examines traditional assumptions about stimulus representation and the conceptual separation of perception and judgment. 

However, a proper understanding of smell — especially in its contextual sensitivity to cognitive influences — cannot succeed without also taking an in-depth look at its neural underpinnings. Differences in coding, concerning both receptor and neural levels of the sensory systems, matter to how incoming information is realized as perceptual impressions in the mind, along with the question of what these perceptions are and communicate in the first place.

Olfaction is just one prominent example of how misleading historic intellectual predilections about human cognition can be. Neuroscience fundamentally opened up possibilities regarding its methods and outlook, in particular over the past two decades. It is about time that we adjust our somewhat older philosophical conjectures of mind and brain accordingly.