Tag Archives: smell

Making sense of the sense of smell

This is another post on Morsella’s ideas.

In developing the Passive Frame theory of consciousness, the group uses olfaction as the sensory source to focus on. This seems surprising at first, but they have good reasons for this.

First, it is an old system from an evolutionary viewpoint. As in this quote from Shepherd: “the basic architecture of the neural basis of consciousness in mammals, including primates, should be sought in the olfactory system, with adaptations for the other sensory pathways reflecting their relative importance in the different species”.

Second, its connections are simple compared to vision and hearing. Olfactory signals go straight to the cortex rather than arriving in the cortex via the thalamus and they enter an old part of the cortex, the paleocortex rather than the neocortex (which has primary processing areas for the other senses). The processing of smell is more or less confined to one area in the frontal region and does not extend to the extensive areas at the back of the brain where visual and auditory processing occurs. The sense of smell is much easier to track anatomically than the other ‘higher’ senses. To understand minimal consciousness, it is reasonable to use the least elaborate sense as a model.

Third, looking at what lesions interfere with olfactory consciousness, it seems that connections outside the cortex are not needed for awareness of odours. This implies that at a basic level consciousness does not require the thalamus or mid-brain areas (although consciousness of other senses does require those areas). Some links to the thalamus and other areas may be involved in further processing smell signals but not in being conscious of them.

Fourth, the addition of a smell into the contents of consciousness has a sort of purity. The sense is only there when it is there. We are aware of silence and of complete darkness but we are not aware of a lack of odour unless we question ourselves. If odours are at very low concentrations or if we have habituated to them because they are not changing in concentration, we are not conscious of those odours and also not conscious of their absence. “The experiential nothingness associated with olfaction yields no conscious contents of any kind to such an extent that, absent memory, one in such a circumstance would not know that one possessed an olfactory system.” So addition of a smell to the contents of consciousness is a distinct change in awareness and can of itself focus attention on it.

Fifth, olfaction is not connected with a number of functions. There are no olfactory symbols being manipulated and the like. It is difficult to hold olfactory ‘images’ in working memory. Also “olfactory experiences are less likely to occur in a self-generated, stochastic manner: Unlike with vision and audition, in which visually-rich daydreaming or ‘ear worms’ occur spontaneously during an experiment and can contaminate psychophysical measures, respectively, little if any self-generated olfactory experiences could contaminate measures.

As well as these reasons given by Morsella in justifying the choice of olfaction in developing the Passive Frame theory, it occurs to me that there is a significant difference in memory. There is a type of recall prompted by smell that seems instantaneous, effortless and very detailed. For example, when you enter a house that you have not been in since childhood and the house has changed in so many ways over the years, the first breath gives a forgotten smell and a vivid sense of the original house along with many images from memories you know you could not normally recall. There seems to be some direct line between the memory of a ‘place’ and the faint odour of that place.

This olfactory approach to consciousness does cut away much of the elaborations and fancy details of consciousness and allows the basic essentials to be clearer.

The smell of the land

The sense of smell is intriguing. It is not as readily conscious as sight, hearing, touch and taste; so it is often discounted. However, humans do have the ability is smell quite well and can learn to do so in sophisticated and conscious ways. Perfumers are an example of this. We also know that a particular smell can bring back the memory of a place in a flash that seems quite miraculous. It is the most important sense for many mammals – used to identify objects and places, track and navigate, and communicate emotional signals. There is no reason to think that we are that much different; we probably use smell as a background (largely unconscious) canvas on which to perceive the world.

Recent research (citation below) has indicated such a canvas. Jacobs and others experimented with human subjects to see if they could map their surroundings using odour gradients. They used a large room with two distinct sources of different odours. The subjects were disoriented, placed in a spot and asked to remember its smell. They were disoriented again and asked to find the spot using their memory of the odour. This was done first with sight and hearing blocked and only the sense of smell available, then repeated with sight as the only sense available, and finally with all three senses blocked. The subjects could come close to the target spot with scent alone, compared to the control of none of the three senses being available.

This is a distinct ability and not the same a tracking a smell or identifying an object or place. This is the formation of a map based on odour gradients. Spatial maps are created in the hippocampus and the olfactory bulb is strongly connected to the hippocampus. The authors address the relationship between the odour map, the sound map (echo location), and the visual map etc. “The ability to navigate accurately is critical to survival for most species. Perhaps for this reason, it is a general property of navigation that locations are encoded redundantly, using multiple orientation mechanisms, often from multiple sensory systems. Encoding the location with independent systems is also necessary to correct and calibrate the accuracy of any one system. As a general principle, then, navigational accuracy and robustness should increase with the number of unique properties exhibited by redundant orientation systems.

Abstract: “Although predicted by theory, there is no direct evidence that an animal can define an arbitrary location in space as a coordinate location on an odor grid. Here we show that humans can do so. Using a spatial match-to-sample procedure, humans were led to a random location within a room diffused with two odors. After brief sampling and spatial disorientation, they had to return to this location. Over three conditions, participants had access to different sensory stimuli: olfactory only, visual only, and a final control condition with no olfactory, visual, or auditory stimuli. Humans located the target with higher accuracy in the olfaction-only condition than in the control condition and showed higher accuracy than chance. Thus a mechanism long proposed for the homing pigeon, the ability to define a location on a map constructed from chemical stimuli, may also be a navigational mechanism used by humans.”

Citation: Jacobs LF, Arter J, Cook A, Sulloway FJ (2015) Olfactory Orientation and Navigation in Humans. PLoS ONE 10(6): e0129387. Doi:10.1371/ journal.pone.0129387