Lecture 27 Flashcards
odor object
distal stimulus
will be omitting many (sometimes hundreds) of volatile molecules that we’re detecting
The ‘Proust effect’ describes how odor may help humans:
Have improved memory and more vivid recall of a
personal event.
helps the number of facts and the intensity of what you remember
model of odor detections (algorithmic)
links have been recently found between the
structure of molecules, olfactory quality, and patterns of activation in the olfactory system.
Researchers have found it difficult
to map perceptual experience onto
physical attributes of odorants
because:
- There is no specific language for odor quality (e.g. as color is associated with wavelength).
- Some molecules with similar structures smell different, and
some that have different
structures seem to have the same subjective smell - There is great variability
(~30%) between olfactory
receptors in people: basic machinery is different: different forms of the same receptor types
can’t use bottom up processes alone: no one molecule structure always associated with a certain perception
Mandairon et al. (2009)
looking for different patterns of activity in different parts of the olfactory system
found cross-species (humans and mice) similarities in odor preferences;
importantly, ratings of pleasantness could be predicted by the physiochemical properties of the odorant molecules.
suggests that there might be some properties out there where there’s a regularity: and that we prefer some of those regularities
but still not deterministic
Niessing & Friedrich (2010)
patterns of activity in the olfactory bulb
found that the olfactory bulb
has different patterns of activity that suggests a ‘categorical’
perception of odors.
patterns of activity are correlated with certain chemicals
What are the steps from odor
stimulus to identification?
- An odorant enters the nasal
cavity and makes contact with the olfactory mucosa.
2. Odorants are carried along the mucosa and dissolve into the surface. They then come into contact with the olfactory receptor neurons (ORNs).
- Signals are carried from
the ORNs to the glomeruli
in the olfactory bulb.
4. Signals from the olfactory bulb are sent to: – Primary olfactory (piriform) cortex in the temporal lobe and amygdala.
- Finally, signals arrive at the
secondary olfactory
(orbitofrontal) cortex in the
frontal lobe.
Step 1
there’s some odor object out in the environment producing a lot of volatile chemicals
chemicals turn into a gaseous form and they are the odorant
they enter the nasal cavity and makes contact with the olfactory mucosa
The olfactory mucosa is located at
the top of the nasal cavity.
it’s like the retina: organ that’s first detecting the signal as it comes in
Step 2
Odorants are carried along the mucosa and dissolve into the surface. They then come into contact with the olfactory receptor neurons (ORNs).
olfactory receptor neurons (ORNs).
ends poking out of the olfactory mucosa
• These neurons contain
molecules called little proteins called olfactory receptors (similar to role of retinal in the visual receptors). Each receptor is sensitive to a
narrow range of odorants.
• Humans have about 400
types of ORNs.
• We have roughly 10,000 of
each type of ORN.
molecules come in –> go across the mucosa —>
make contact with ORNs
olfactory receptors
little proteins that bond to the chemical coming into the nose
similar to role of retinal in the visual receptors.
Each receptor is sensitive to a
narrow range of odorants (like a tuning curve)
Only one type of receptor is
on each ORN.
Step 3
Signals are carried from
the ORNs to the glomeruli
in the olfactory bulb.
• ORNs of a particular type
converge their signals on
to one or two glomeruli within the olfactory bulb.
• Optical imaging has
shown that specific areas
of glomeruli may respond
to particular odorants (chemicals).
Step 4
Signals from the olfactory bulb are sent to: – Primary olfactory (piriform) cortex in the temporal lobe and amygdala. • Amygdala plays a role in emotional reactions to odors.
why you have a strong emotional response to smells
because you’re activating the amygdala really early on you have a strong emotional response to the smell
amygdala is directly activated by incoming odor!!
Step 5
Finally, signals arrive at the
secondary olfactory
(orbitofrontal) cortex in the
frontal lobe.
to classify and remember: what was that scary thing or that positive thing
what’s missing in this faster smell pathway?
the thalamus!!
emotional response: amygdala: flight or flight: PRIMAL
olfactory mucosa in the nasal cavity –> olfactory bulb –> immediately primary olfactory and…
amygdala, which is talking back to olfactory bulb, primary olfactory area and orbitofrontal cortex
Combinatorial code for odor (distributed coding) Theory in how odors are represented
not just one chemical that activates one set of neurons that is associated with one kind of perception
INSTEAD: lots of different areas responding to lots of different chemicals and then combining those signals to get that perception
– Proposed by Malnic et al. (1999) from results of calcium imaging experiments.
– Odorants are coded by patterns of activation of olfactory receptors called
recognition profiles.
– Molecules that have similar structures, but subjectively smell different, have
different recognition profiles.
– However, response profiles don’t always predict subjective qualities.
- Results: sometimes chemicals with similar structures have very different profiles
Cool thing: if it had a different profile in the olfactory receptor neurons that typically meant that we had different perceptual characteristics associated with it = it wasn’t the structure of the stimulus (how the molecule was shaped) but the pattern of activation of the neurons was associated with a different experience = some profile there that’s helping shape what the experience will be like
tough part: no one pattern for something like “rancid” have to look at the whole pattern of activity = still don’t have a basic language of neural activity always results in some perceptual quality but there did seem to be patterns that were associated with certain types of high level perceptions
Optical imaging method (Uchida et al. 2000)
– Cells consume oxygen when activated (like MRI).
– Red light is used to determine the amount of oxygen in the cells.
– Less oxygen (oxygen used up because the area is really active) reflects less red light.
– Measuring the amount of light reflected reveals which areas are most active (i.e. darker).
– Results: using this method, it was found that different chemicals activated different distinct clusters of glomeruli in the olfactory bulb.
a group of acids caused activity all the oxygen to be used up in the olfactory bulbs and a different group of alcohols caused a different region to be activated
suggests something in olfactory bulb is responding pretty strongly to particular kinds of odorants, which makes sense because the odorants of particular types converge onto one or two of the glomeruli
Radioactive tagging with 2-deoxyglucose (2DG)
also shows specific areas
of activation in response to different odorants.
– Supports the notion that a map of odorants exists in the olfactory bulb.
– Suggests that in the olfactory bulb you can create a map of different chemicals: This is called a chemotopic map. [Also called an odor map or odotopic map.] = really early on!
- all of the ORNs are converging on particular areas in olfactory bulb so that when those regions are activated its associated with particular molecules (oderants)
– Note that this provides something of a neural basis for a response to a
particular chemical, (but not necessarily an odor object, which contains many chemicals).
something of a neural-basis: responds to a chemical and not necessarily an odor object (doesn’t represent something like coffee)
What is the primary olfactory area?
Piriform cortex = primary receiving area
Experiment by Wilson (2003) on learning and the response properties of
the primary olfactory area.
– Measured response of neurons in the rat’s piriform cortex to two odorants.
* A single substance - isoamyl acetate (smells like banana) * A mixture - isoamyl acetate and peppermint
– Results: showed that with enough exposure (100 sniffs), piriform cortex cells
would fire more to single substance than mixture. That is, they could discriminate between the two odorants = cells distinguish that one element = didn’t always respond to a particular thing that was always present in the envirnoment = distinguished between the mixture and that substance within the mixture = detecting an odor object =
– this discrimination pattern was not seen in the olfactory bulb!!! which means that piriform cortex could do something in terms of classification that olfactory bulb could not !!!
chemotopic map tells us
that chemicals are present and have been detected
object recognition in piriform cortex
you start off with a scattered activation (chemicals present), but then the piriform cortex cells coordinate their activity and start to distinguish between the things that are active and things that are a distractor
it’s when these piriform cortex cells have enough experience that you get object recognition (later stage of processing)
object recognition =?
piriform cortex
pure detection =?
olfactory bulb
Experiment by de Araujo et al. (2005) on
top-down influences on odor perception
from secondary olfactory area.
– People were exposed to a mixture of a sweat-like smell and a cheddar cheese flavoring (two different stimuli present).
– On some trials, they saw the label “cheddar cheese,” and on others, the label “body odor.”
– Pleasantness ratings were higher for the “cheddar cheese” labeled trials,
regardless of which stimulus was actually present.
– fMRI scans showed activity in the orbitofrontal cortex to be associated with the pleasantness ratings.
orbitofrontal cortex
much more top down
involved with what’s your perception of it (pleasantness)
– like pain processing
Like all senses, odor perception is
constructive.
• Many molecules create a single perception (e.g. coffee aroma is made from more than
100 molecules, each with different ORN patterns).
• Odors occur concurrently, but the perceptual system separates them from one
another.
• Past experience and expectations have an impact on odor perception (like music). Other senses
also affect our identifications.
• Therefore, odor object perception (recognition) is both a bottom-up and topdown
process.
if you always expect to smell coffee in the morning, then…
you will be better at detecting coffee in the morning
expectations help you recognize and detect odors in the environment
presbyosmia.
Problems with our sense of smell become more common as we get older. After 55 years of age, more than 24% of us will have reduced sensitivity.
• ORNs in older adults become less selective for odors, responding
to multiple odorants.
• can’t detect when food has gone bad!
anosmia.
Loss of the sense of smell, from disease or injury,
Usually associated with a loss of taste
- Can be temporary (e.g. due to a cold or inflammation)
- Associated with some diseases (e.g. Parkinson’s, dementia)
- Can be environmental (e.g. smoking or medication – Zicam)
- However, it may be recovered due to neurogenesis of new olfactory receptor neurons.
- Long-term loss is associated with depression.
The chemical senses are construed as “gatekeepers” of
the body which:
- Identify things that should be consumed for survival.
- Detect things that would be harmful and should be avoided.
• Cause immediate affective responses to encourage and
remember the correct responses.
Why do we value taste?
• Along with the protective function afforded by taste in avoiding harmful substances, it also provides pleasure.
• Consuming to maintain
homeostasis is a necessary
biological function.
• Eating driven by palatability (i.e.hedonic value), is driven by
learning and reinforcement.
what is taste?
is the detection of molecules in liquid or solid form.
there is no perfect connection between tastes and function
of substances.
sweetness
the more calories something has the less of it you need to eat to maintain homeostasis
is usually associated with
substances that have nutritive value.
bitterness
is usually associated with
substances that are potentially harmful.
salty
indicates the presence of
sodium.
sour
indicates high acidity.
