PSC 129 - Sensory Processes

Smell

General aspects of olfaction

• Most penetrating sense (e.g., most powerful effect on emotional state), good at triggering memories.
  
  
• Oldest sense - probably the first sense endowed to primordial creatures.
  
  
• Generally “underdeveloped” in humans, most keen in other animals (e.g., dogs, gypsy moths). Dogs are 300-10,000 times more sensitive than humans (depending on odorant).
  
  
• One of the so-called “chemical senses” (others are common chemical sense, internal receptors, and taste).
  
  
• Source of odors: volatile substances giving off vapors, fat soluble molecules. In aquatic animals these are water soluble.
  
  
• Odor segregation, the means by which we can separate out the various odors in a mixture (analogous to the “cocktail party problem” in audition) is a difficult and unsolved problem.
  
  
• Women are significantly better than men at discrimination of odors.
  
  
  

• Sex of a person can be identified from smell alone: from hand (perspiration), or breath odor (80% accuracy).
  
  
• People can identify their own odor (T-shirt study).
  
  
• Pheromones: used by other animals to find mates. - e.g., the male cabbage moth can sense minute concentrations of scent released from a female many miles away. In the gypsy moth, 1 molecule is sufficient to elicit a neural response. 200 molecules are sufficient to elicit change in behavior.
  
  
• Odor plays a role in synchronization of menstrual cycles.
  
  
• Animals mark their territories by smell.
  
  
  

• Shiffman’s MDS analysis - objective, two-dimensions suffice to span similarity space.
  
  
• No relation between shape of odorant molecule and odor similarity.
  
  
• Combinations are perceived as a superposition of several distinct sources (analogous to the way several tones can be simultaneously perceived in audition).

  ---

• Air is warmed and humidified by passing through baffles in nasal cavity.
  
  
• Nasal cycle: nostril dominance switches every 2-3 hours. Related to contralateral hemisphere activity.
  
  
• Olfactory epithelium consists of a mucus membrane, lined with cilia and olfactory receptor cells (neurons).
  
  
• The human olfactory epithelium contains 6-10 million olfactory receptor cells. Dogs have 200 million. Receptors themselves are equally sensitive - one molecule sufficient for measurable response.
  
  
• Axons of olfactory receptor cells make up the olfactory nerve.
  
  
• Olfactory receptor cells are constantly dying and being replaced - cycle is 5-8 weeks. Unique among sensory receptors, but no longer unique among neurons in the brain.
  
  
• Olfactory binding protein: ferries odorous molecules into mucus and onto receptor sites.
  
  
• Different olfactory receptor neurons have different receptor proteins which make them selective to different odorants. What these proteins code for though nobody knows. Broad selectivity. Approximately 1000 different types, each containing 1000 similar receptor proteins (contrast with three cone types each containing millions of pigments).
  
  
• Olfactory bulb: receives input from olfactory nerve, processes information and passes onto olfactory cortex, orbitofrontal cortex, and limbic system. No topographic “odor map.” Convergence: about 1000:1 amplification. Glomeruli within olfactory bulb receive their input from olfactory receptor neurons with similar receptor proteins. There are about 1000 glomeruli.
  
  
• Neural activity synchronized to inhalation cycle. Odor constancy is the ability for perceived concentration to remain constant despite fluctuations in flow rate.
  
  
  

• Mediated by free nerve endings.
  
  
• Association of pain, or “feeling” with smell.
  
  
  

• Anosmia - deficiency in manufacture of olfactory receptor proteins.
  
  
• Odor hallucinations.