senosry systems ii: perception of magnetic fields
TRANSCRIPT
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Senosry systems II: Perception of magnetic fields
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Amazing animal migrations
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Can animals sense magnetic fields?
The degree to which animals can sensemagnetic fields unclear
European Robins (Erithacus rubecula)Tend to migrate in a particular direction
Preferred flight direction is maintained in a closed room withno optical references – implies that vision is not necessary
If test cage is moved to all-steel chamber (a shieldagainst magnetic fields), bird no longer finds natural migratory direction
Artificially generated magnetic fields can influence the direction choice ofrobins
Other organisms shown to be affected by magnetic fields:Marine mollusks, Salmon, Salamanders, Turtles, Hornets, Honeybees
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Puzzle: The geomagnetic field is weak (ca. 0.5 Gauss). Three possible ways that animals can detect magnetic fields
Mechanical Reception – Same as a compass needle – requires the presence of ferromagnets (small magnetic particles in body)
Electric Induction – Any kind of movement in an induced electric fieldAllows animals to follow magnetic lines; However, electro-sensitive organs not found in all animals who can use magnetoreception
Chemical Reception – Chemical reactions inside body can be influenced by magnetic fields – poorly understood
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Honeybees = carry discrete bodies of ferromagnetic material that couldbe affected by earths magnetic field
Certain cells in their abdomen contain iron granules ~ 0.6 micrometersin diameter
These cells (trophocytes) are innervated by axons from the ventralnerve Cord
Mechanism poorly understood
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Newts: Can migrate up to five km to and from their natal ponds (and this is after being in a forest for five years)
How? Photoreceptor’s (light sensitive regions in the brain) facilitated migration
Sensory vision with a brightness pattern imposed (due to magnetic field)
Example (Quote from researchers):
"If the newt is pointing north, the newts' photoreceptors would align north-south and would show an excitation response, whereas if the newt points itself to the east it would still show an increased pattern but now the pattern will have rotated 90 degrees because the newt has now rotated,"
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Sea turtles migrate for thousands of kilometers to the same beach onwhich they were born
The migratory routes of turtles have unique combinations of magneticfield intensity and field line inclination
Hatchling loggerhead turtles can distinguish between different field intensities and magnetic inclination angles along routes
Turtles thus possess the ability to determine their global positionusing a bi-coordinate map
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Vision and sorting of information
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Vision and Lateral inhibition:
Lateral inhibition:The horseshoe crab (Limulus) hascompound eye in which individualreceptor units can be stimulated bya fine beam of light
However, the signals in the axon do not completely represent the stimulus
The pattern also depends on the amount of light falling on otherreceptor units
Why? Because each visual receptor is connected to its neighbors andInhibits their activity = lateral inhibition
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Potential advantage oflateral inhibition?
Enhanced edge effects
Consider 12 units that are stimulated by uniform lightof two intensities (bright and dim)
Because units 2-5 are borderedby other units getting brightIllumination, they are subject tolateral inhibition
Units 6 and unit 7 are not
Thus, their discharge frequencyis higher
=better picture of edge
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Information processing:
Ex. The eye of a frog
Rods and cones are distributed uniformly such that the structure of theretina is similar from place to place
Within the frog eye, there are neurons called ganglion cells. The axonsof these ganglions make up the optic nerve
There are about 500,000 ganglion cells, but there are about 1 millionreceptor cells (rods and cones) in the retina
Thus, there cannot be a 1:1 correspondence between the incoming andoutgoing messages
Fibers in the optic nerve are divided into five different classes on thebasis of the responses of the ganglion cells
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On fibers: Respond to the onset of illumination
Off fibers: Respond to only the termination of a light stimulus
ON-off fibers: Respond to either onset or offset of a light stimulus or itstermination
Edge receptors: Respond to the presence of a sharp edge in the visualfield
“Bug detectors”: Respond only to small, dark, moving objectsBut NOT to large dark moving objects or to stationary objects
In fact, switching a light on and off will not affect these final fibers
The retina of the frog contains a matrix of many kinds of specialized fibers,each of which is beneficial for a different purpose
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Recent article in Time magazine “The Science of Romance”
Could romance be driven by our senses?
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The idea of “love” as a biological phenomenon to promote mating and nurturing of children
May go all the way back to early human culture
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Hamsters and pheromones-dictates sexual behavior
Animal senses-A guide to humans?
OK, I thoughtthat we droppedthis!
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Within human females, college roommates tend to develop synchronizedMenstrual cycles (pheremones?)
If you apply sweat from a donor to a subject’s lip, the subjectsMenstrual cycle started an average of 9 days away from donor
About 4 months later: the difference was only an average of 3 days
No change in the control group
“DriverHormones”
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Pheromones may be linked to MHC genes
Strongly influence the immune system and tissue rejection
Idea: MHC matching
-Conceive a child with a person whose MHC is too similar to yours, and risk increases that the womb will expel the fetus
Find a partner with sufficiently different MHC, and you're likelier to carry a baby to term.
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-Lab mice can smell “too-similar” MHC in the urine of other mice
-Will avoid mating with those individuals
Human females smelled T shirts worn by anonymous males and then pick those that were most appealing
They chose the ones worn by men with a safely different MHC
MHC information may be contained within pheremones
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Confounding factor: birth control pill
Women who are on the Pill-which “simulates” pregnancy-tend to choose wrong in the T-shirt test
When they discontinue the daily hormone dose, the protective smell mechanism kicks back in
Pheromones may be a “natural” mechanism of partner selection-pregnancy may interfere with this
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Preference also relates to faces
Just before, during and after period - when women are least likely to become pregnant - more attracted to more feminine male faces
If woman is on the pill, no preference is shown through cycle
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Symmetric Female Face Original Female Face Asymmetric Female Face
Symmetric Male Face Original Male Face Asymmetric Male Face
How do researchers study facial symmetry? Digital manipulation!
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Composite of60 Faces
Composite of 15 MostAttractive Faces
Hyper-AttractiveComposite
Facial attractiveness researchers also create “composite” faces
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FemininisedFemale Face
MasculinisedFemale Face
FemininisedMale Face
MasculinisedMale Face
Researchers can also make more “male” and “female” faces
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Saliva also contains MHC genes
Kissing may be a “taste test” for future compatibility
Testosterone is also present in male saliva-can be passed ontoThe female
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Men may use pheromones too…
Evaluation of the subject as sexy
Bo
dy
od
or
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What IS attractive?
Deep voice in males (auditory signal)
Research on the Hadza tribesmen found that the richer and lower a man's voice, the more children he had
Sample group of 149 volunteers listened to recordings of men's and women's voices
Ranked them from "very unattractive" to "very attractive."
Voices that scored high on attractiveness also had physical features considered sexually appealing
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Ideas of beauty change among culturesand across time
Waist-hip ratio in women
Scientists have claimed that a certain waist-hip ratio is preferable Across cultures
Europe: Women with 0.7 WHR (waist circumference that is70% of hip circumference) are rated more attractive
Marilyn Monroe, Sophia Loren and the Venus de Milo: ratios ~ 0.7
Preferences range from ~ 0.6 in China to 0.8 or 0.9 in South AmericaOr Africa
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Things to consider
In many culture, men always have beards-but this has fallen outof favor in modern culture
The male beard – a sexually dimorphic trait
Common in some cultures, but increasingly more rare in ours
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Consider all the machinations that men take to get a hairless “attractive”body
Ouch!
There may not be one universal “best qualities” at any one time
If true - then what are pheromones signalling?