KHA258 Lecture Notes - Lecture 4: Neocortex, Left Fielder, Horseshoe Crab

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Behavioral Neroscience week 4: Vision
4.1: Transdction
-Cilliarly muscles: Control convergence of the eye: depth perception
determining
-Not just receptors that determine vision:
oMuscles: help keep visual image level
oAnd help with focusing
oDepth perception
-Convergence: eyes must turn slightly inwards when objects are close
-Binocular disparity: retinal images are slightly different due to different
positions
-Both of these processes are greater for closer objects
oThis provides the brain with a 3D image and distance information
-Retina and translation of light to neural signals:
oElectromagnetic system is light
oRetina is inside out:
Light passes through several cell layers before reaching
photoreceptors
Sensory receptors are at the very back
Physical interference before light reaches this
This is not the most efficient system to have:
Other animals have more sensible retina organisation
Have reflective surface so that light may hit and miss,
but is then reflected back and has another chance
Organisation:
Eyes think of light as particles
Rods: one category of rods
Cones: different categories
oInputs get integrated together for bipolar cells
Bipolar cells: influences activity of ganglion cells
Retinal ganglion cells: where light goes for further
processing
oWhat travels through optic nerve
This is known as a vertical pathway
Lateral communication: different cells act at different
stages
oHorizontal cells: multiple information from
different photoreceptors
Combine lots onto one bipolar cells
Influences this bipolar cells
Can influence a number at a time
oAmacrine cells:
Bipolar cell input gets integrated onto
amacrine cells
Making multiple photoreceptors have a
convergence effect onto smaller ganglion
cells
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Axons of ganglion
oFovea: high acuity area at centre of retina
Thinning of ganglion cell layer: allows more light through
More photons being caught by photoreceptors
Creates clearness
Reduced distortion and improved acuity
oBlind spot:
Retinal ganglion cells axons collect together at spot on the
retina where there is no photoreceptors at all
No receptors where optic nerve exits the eye
Completion: because of blind spot
Not treated as a blank space
Completes with information that hits retina of
surrounding areas
Filling-in
oRetina may be damaged:
Scotoma
Burning of the retina with too much light
Creates blind spots
-Duplexity theory:
oTwo receptor types create different visions
How they integrate
And their basic properties differ
oCones: photopic vision/daytime
Not particularly sensitive
Need lots of photons in order to be activated
Low convergence:
Small number of cones that converge onto a single
bipolar cell
One spot on the retina can be detected and identified
Few cones to the number of ganglion cells exiting
oLeads to better acuity
Sensitive to specific wavelengths of light
Mammalian eye
Responds to small portion of spectrum
Not equally sensitive to the entire thing
Some are more sensitive to one end than the other
Helps identify colours
High acuity, colour sensitive system
Location:
Fovea is central 2 degrees of vision
Contains only cones
Poor vision in low light
Colour perception low in low light
oRods: mediate scotopic vision (night time)
High convergence
One ganglion cells receives input from many rods
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oRods must take up bigger area of the retina
Not much detail provided because of this
Advantage of still being able to detect light even when not
much light around
High sensitivity
Enables low acuity vision in dim light
Lacks detail and colour information
Higher concentration in periphery
Good nighttime vision
Driving at night:
oToo bright for rods but low for cones
oRods not functioning as well
oLooking up at the sky:
Doesn’t brighten
Your rods begin to function better and so can see more
oIdea that they deal with different types of visual information and
different types of vision
-Spectral sensitivity: relationship between the measurable brightness of
particular wavelength of light and how sensitive the photoreceptor is to that
particular wavelength
oSpectral sensitivity curve
Different sensitivity for rods and cones
Photopic: cone
Peak sensitivity at wavelengths of around 550 nm
perceived as the brightest
Scotopic: rods
Peak sensitivity 500nm
Purkinje effect:
Means that if we are looking at a scene
Light of same intensity but different wavelengths may
be perceived as different brightness’s
Shift in perceptions based on which systems are being
used
Understanding physiology
-Visual transduction: the conversion of light to neural signals
oOperation of channels linked back to the process of sensation
What actually happens
oPhoton absorption initiates opening and closing of channels in
membrane
oRhodopsin: pigments in rods
Metatropic receptor activated
Leads to changes in neurotransmitter release
In the dark:
Na channels remain partially open (partial
depolarization)
Releases glutamate
oEffects the next cells in the line: bipolar cells
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Document Summary

Cilliarly muscles: control convergence of the eye: depth perception determining. Not just receptors that determine vision: muscles: help keep visual image level, and help with focusing, depth perception. Convergence: eyes must turn slightly inwards when objects are close. Binocular disparity: retinal images are slightly different due to different positions. Both of these processes are greater for closer objects: this provides the brain with a 3d image and distance information. Retina and translation of light to neural signals: electromagnetic system is light, retina is inside out: Light passes through several cell layers before reaching photoreceptors. Sensory receptors are at the very back. This is not the most efficient system to have: Other animals have more sensible retina organisation. Have reflective surface so that light may hit and miss, but is then reflected back and has another chance. Cones: different categories: inputs get integrated together for bipolar cells. Bipolar cells: influences activity of ganglion cells.

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