chapter 2/3 Flashcards
wavelength
distance between the peaks of electromagnetic waves in the electromagnetic spectrum
visible light
wavelength of light that humans can perceive - ranges from about 400 to 700 nanometers and is associated with different colours of the spectrum
colours and wavelength
blue - short
green - middle
yellow, orange, red - long
what makes up the electromagnetic spectrum
different wavelengths of light - continuum of electromagnetic energy that is produced by electric charges and is radiated as waves
examples of animals that perceive things we don’t
snakes that have holes in their faces (pit organs) which allow them to detect infrared radiation from their prey at night - example of how humans are only able to perceive a small portion of what is out there compared to other animals and things
rods vs cones
rods: scotopic vision - used in low light because they are very sensitive to light, low acuity and not good w colour
cones: photopic vision - less sensitive to light, used when a lot of light is available - high acuity, colour detail, mostly concentrated in the fovea
light pathway through the eye
light enters through pupil and is focused onto the retina by the cornea and lens. It travels all the way back to the retina to the photoreceptors -(rods and cones) which then send it back the way it came (but transduced to electrical energy) , through bipolar and finally ganglion cells
optical infinity
a quality of the lens that makes it so that anything further away from us than 20ft doesn’t need to be focused before entering the eye
accomodation
process by which the lens thickens or thins to keep an object in focus
macular degeneration
blurred central vision related to age
myopia
nearsightedness, meaning trouble seeing distant objects - images get projected in front of the retina instead of onto the retina
refractive myopia
cornea and lens overbend the light
axial myopia
eyeball is too long
hyperopia
farsightedness, meaning trouble seeing near objects - objects get focused beyond the retina because the eyeball is too short
presbyopia
trouble seeing near objects due to aging - lens becomes more rigid due to age
transduction in the eye
photoreceptors contain visual pigments in their rod/cone shaped outer segments - these pigments contain an opsin protein bound to retinal - when light hits the pigments, they change their binding configuration, triggering electrical signals
fovea
area of the retina that contains only cones - when we look directly at an object the object’s image falls on the fovea
peripheral retina
includes all of the retina outside of the fovea and contains both rods and cones, but many more rods (still a lot of cones)
retinitis pigmentosa
degeneration of the retina - in peripheral rod receptors = bad vision in peripheral field - is passed from one generation to the next, as opposed to macular degeneration which is usually caused by ageing
why don’t we see our blind spot?
because the brain fills in the places we don’t see - and its located off to the side of the visual field, where objects are not in sharp focus
how does the lens change shape
ciliary muscles increase its curvature when we need to bend/focus light
what happens when we look at an object 20ft away
the eye doesn’t have to do anything - rays of light enter the eye parallel to each other and focus on the back of the retina
what happens when we look at an object closer than 20ft
the focus point gets pushed back, past the retina so that the eye has to accomodate, by increasing the curvature of the lens (fatter) to increase its focusing power in order to bring the focus point of the object closer forward
accommodation
change in the len’s shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker
refractive errors
errors that affect the ability of the cornea and/or lens to focus the visual input onto the retina - presbyopia, myopia, hyperopia
visual pigment
light sensitive molecule contained in the outer segments of photoreceptors - contains opsins and retinal
isomerization
The process by which retinal molecules change shape when hit by light (bent to straight)- this results in a chemical chain reaction that activates thousands of charged molecules to create electrical signals in receptors, making it so that the isomerization of just one visual pigment molecule can lead to the activation of an entire photoreceptor
dark adaptation
process of increasing sensitivity in the dark, measured by a dark adaptation curve
how do we measure the dark adaptation curve?
using the method of adjustment - we have ps look at a fixation point while a light flashes in their periphery - they turn down the flight until it is just barely perceptible - this is their threshold. The same thing is done in the dark - as the p becomes more sensitive to light in the dark, we track how low they are able to dim the light
dark adapted sensitivity
sensitivity at the end of dark adaptation - about 100,000 times greater than the beginning
phases of the dark adaptation curve
first phase: sensitivity increases during the first 3 to 4 minutes then levels out
second phase: sensitivity starts to increase again at about 7 to 10 minutes and finally stops at around 20 or 30
cone adaptation
cones only go through the first phase - we can tell by measuring the threshold for light that falls only in the fovea
rod adaptation
rods adapt slower than cones and go through both stages - can tell by measuring the threshold for rod monochromats (people who have no cones)
rod cone break
the place where the rods begin to determine the dark adaptation curve instead of the cones
visual pigment bleaching
when retinal changes shape due to light (bent to straight) it detaches from the opsin causing it to become lighter in colour - in this state visual pigments are no longer useful for vision
visual pigment regeneration
process of reforming the visual pigment molecule - the increase in visual pigment concentration that occurs as the pigment regenerates in the dark is responsible for the increase in sensitivity we measure during dark adaption - happens faster in cones (6 min) than rods (30 min)
what determines our sensitivity to light
the concentration of visual pigment
what determines the speed at which our sensitivity increases in the dark
the regeneration of the visual pigment
detached retina
when the retina becomes detached from the pigment epithelium, a layer that contains enzymes necessary for pigment regeneration
spectral sensitivity
the eye’s sensitivity to light as a function of the light’s wavelength - measured by determining the spectral sensitivity curve
how do we measure a spectral sensitivity curve
we determine a person’s threshold for seeing monochromatic light across the spectrum at regular intervals - amount of light that is needed to see certain wavelengths - we often plot our curve in terms of wavelength, using sensitivity =1/threshold
how do we measure cone spectral sensitivity
by having participants look directly at the test light so that it stimulates only the cones in the fovea
how do we measure rod spectral sensitivity
we measure sensitivity after the eye is dark adapted so that only the rods control vision, and we present test flashes in the peripheral retina
what is the difference in rod and cone spectral sensitivity?
rods are more sensitive to short wavelength light (blue green) than cones -they are most sensitive to 500nm, while cones are most sensitive to 560 nm light - this means that as vision shifts from cones to rods in dark adapted eyes, our vision becomes more sensitive to short wavelength light
purkinje shift
enhanced perception of short wavelengths during dark adaptation
absoption spectrum
the plot of light absorbed by a pigment versus the wavelength of the light
absoption spectra for cones
short wavelength pigment: 419 nm
medium wavelength pigment: 531 nm
long wavelength pigement: 558nm
add together to result in a psychophysical spectral sensitivity curve that peaks at 560nm
rod is same to spectral sensitivity curve
horizontal cells and amacrine cells
signal travels between photoreceptors and bipolar cells in horizontal cells and between bipolar and ganglion cells through amacrine cells
