Olfactory Perception – Smell “…We have five senses, but only two that go beyond the boundaries of ourselves. When you look at someone, it’s just bouncing light, or when you hear them, it’s just sound waves, vibrating air, or touch is just nerve endings tingling. Know what smell is?... It’s made up of the molecules of what you’re smelling.” Angels in America, p. 17

Olfactory Perception – Smell What is smell? – The substance that you’re “smelling” has been emitted from that source as gas molecules, which in turn you take into your body – Different from light, sound & cutaneous stimulation, which involves transformations of receptors – Molecule detectors

Olfactory Perception – Smell We have not yet defined the physical parameters the way we have in other senses (i.e., frequency, spatial contrast) We have not yet defined the perceptual experiences like in other senses such as vision (brightness, color) or hearing (loudness, pitch) Very difficult to manipulate the stimulus- medium (i.e., how do you control the concentration of airborne molecules?) We know something about the receptors and neuroanatomy, but without understanding the stimuli we remain quite limited in what we know. Net result: smell is still largely not understood

Significance of Smell? General alarm system (smoke, disease) Component of flavor (quality of food) Part of communication (pheromones) Discrimination of social context (family, gender, attractiveness…) General rules: smells good  “safe” smells bad  “dangerous”

How can we classify odors? Already stated, we do not have the yardsticks of frequency or spatial measurement Largely done by description of examples – “Soap smells like lavender” – Some wines have a “woody” aroma

Olfactory system fun facts: human and dog receptors are the same Human receptors work as good as dogs even though the dog’s ability to smell is 300 to 10,000 times more sensitive then humans Difference? Number of receptors. Humans = 10 million Dogs = 1 billion

Olfactory system fun facts: human recognize their own odor and can distinguish between male v. female odors Russell (1976) had subjects where a shirt for 24-hours consecutively, then sealed the shirts in plastic bags Subjects could recognize there own shirt (from a set of 3) 75% of the time (chance: 33%) Subjects could recognize the strangers (male and female) also 75% of the time McKlintock (1998) women who live together for sufficiently long periods of time synchronize menstrual cycles… due to chemical signals in their sweat Savic et al. study: PET scans reveal differential responses in the Hypothalamus in response to opposite gender odors

Sekuler & Blake, 2002

Six primary (independent) odor qualities: fragrant, etheral, resinous, spicy, burned, putrid They form a basis to describe any smell sensation as combination of these 6 components (similar to Trichromatic color vision)

This corresponds to 6 types of receptors, which are reflected by specific anosmias ('smell blindness') – though the actual number of receptors are still disputed

Can we localize smell? Comparisons to other senses, we are very poor at this Experimentally, this problem is less easy to approach than in vision or hearing?! Some researchers invented some ingenious sniffo-meters… NOTE: other animals seem to perform much better (dogs)

Where are the receptors for olfaction?

Olfactory system from the top down Odors pass through the nose and mouth into the sinuses and across the Olfactory Mucosa (why don’t we smell much through our mouth?) Olfactory bulbs are above the Mucosa & bone Olfactory Receptor Neurons (ORN) run through the Mucosa Cilia of ORN are imbedded in base of the Mucosa (look like roots growing down into Mucosa)

From bottom up: Olfactory Receptor (on the end of the cilia)  Olfactory Receptor Neuron  Olfactory Bulbs (which are the beginning of the Central Nervous System (CNS))  Olfactory Tract (to higher (CNS) sites) Receptor pathway of smell:

The Olfactory Receptor The olfactory receptor at the end of the cilia are little proteins (like in the visual system) 1000 different protein patterns (each of which respond to the same group of odorants) All OR’s are the same on each ORN (1000 different types) The OR is the actual site of transduction from chemical reactions to electrical impulses

Organization of olfactory mucosa and olfactory bulb: zones 10 Million Olfactory Receptor Neurons (ORN) synapse on 1-to-2,000 Glomeruli Translation: 5,000-to-10,000 ORN’s per Glomerulus  lots of transduction at Olfactory Bulb

What do we know about neural coding or central (Brain) processing for odor? Almost nothing. Only thing clear? Different olfactory receptor neurons (ORN’s) fire in a coordinated pattern to an odorant. Piriform cortex is the primary receiving area, but we don’t know what it is doing. Orbital Frontal cortex is secondary receiving area where there is some evidence of differential responding to different combinations of odors.

Where is the Orbitofrontal Cortex