Chapter 2 Flashcards
electromagnetic spectrum
spectrum where energy is described as its wavelength, ranging from short wave gamma rays (10^-12m) to long length radio waves (10^4m). Visible light is a band in this spectrum and is identifiable by colour (short - blue, mid - green, long - yellow, orange, red)
photon
small packet of light energy
cornea
transparent, fixed covering of the front of the eye that accounts for 80% of the eye’s focusing power
lens
transparent, ciliary muscles adjust curvature of lens, accounts for 20% of focusing power of the eye,
retina
a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed; layered with 5 types of neurone - signals generated in receptors travel to bipolar cells and then to ganglion cells with long axons to transmit messages out retina; horizontal and amacrine cells connect neurons across retina
accommodation
change in lens shape that occurs when ciliary muscles in front of eye tighten, increasing curvature of the lens so that it gets thicker (increased curvature = increased light bending)
near point
distance at which you can no longer accommodate
myopia
inability to see distant objects clearly; refractive myopia - cornea/lens bend light too much, axial myopia - eyeball is too long
far point
distance at which light becomes focused on retina (a myopic can see an object clearly at far point)
laser assisted in situ keratomileusis
LASIK - cut a flap, sculpt cornea with laser, flap replaced
hyperopia
far sighted - focus is on a point behind the retina, the eyeball is too short
visual pigments
present in outer segments of rods and cones; opsin - long protein, retinal - smaller, light sensitive component; when it absorbs one photon of light, retinal component changes shape (bent to straight) in a process called isomerization. Triggers chemical reaction that activates thousands of charged molecules to create electrical signals in receptors, amplifying the effect of isomerization
isomerization
process by which one molecule is changed into another molecule with exactly the same components in a different organization
dark adaptation
eye adapts to darkness by increasing sensitivity to light
rods and cones
visual receptors with inner and outer segments interspersed throughout peripheral retina (+ rods [120 million] than cones [6 million-cones in fovea]); rods most sensitive to light at 500 nm, cones most sensitive to light at 560 nm
fovea
small area on retinal that has only cone receptors (50,000), looking directly at an image it falls here
macular degeneration
destroys cone rich fovea and area around it producing a blind region in central vision so that an object can not be seen if looked at directly
retinitis pigmentosa
hereditary degeneration of retina attacks peripheral rod receptors and results in poor peripheral vision
blind spot
has no receptors (brain “fills it in”, other eye compensates, located on edge of visual field) - location where optic nerve leaves eye
dark adaptation curve
function relating sensitivity, light and time in the dark and beginning when light is extinguished
light adapted sensitivity
sensitivity in light
method to measure dark adaptation
- observer looks at small fixation point centred on fovea while paying attention to flashing test light on peripheral retina
- in light, observer adjusts strength of flash with a knob so that it can barely be seen (threshold - converted to sensitivity high threshold = low sensitivity)
- lights are turned off and experiment is repeated, observer must continually reduce strength of flash as dark adaptation occurs
- adaptation occurs rapidly for 3-4 mins (cones) and levels out, at 7-10 minutes further adaptation occurs (rods) until observer has been in darkness for 20-30 mins
dark adapted sensitivity
sensitivity at the end of dark adaptation (100000 x greater than light adapted sensitivity)
measuring cone adaptation
must measure response of fovea
measuring rod adaptation
must use subjects with no cone receptors (people with rare defect called rod monochromats)
detached retina
retina detaches from pigment epithelium, retinal and opsin can’t recombine, blindness in area served by the separated retina
spectral sensitivity
responses to visual spectrum ( sensitivity to light as a function of light’s spectrum)
Purkinje shift
at night we are more sensitive to short wavelength light (blue/green)
spectal sensitivity curves
- use monochromatic light to determine threshold at different wavelengths
- threshold for light is lowest in the middle of the spectrum
- 1/threshold=sensitivity which produces curve
absorption spectrum
plot of amount of light absorbed vs wavelength of light (rods - 500 nm, cones - short @419nm, med @ 531nm, long @ 558 nm, add together for 560 nm); closesly matches spectral sensitivity curve
lateral geniculate nucleus (LGN)
signals through optic nerve end up in this group of neurons before arriving in visual receiving area of hte brain
resting potential
difference of charge inside and outside of neuron cell (resting potential -70 mV) measured by electrode tip inside and outside neuron
action potential
1 ms - charge in axon reaches +40 and returns generating a message
propagated response
message travels without decreasing in size
refractory period
after an action potential, a period when a neuron can not generate an action potential regardless of strength of stimuli
spontaneous activity
rate of action potentials generated in absence of stimuli
synaptic vesicles
packets of neurotransmitters in presynaptic cell
excitatory response
depolarization (+ positive)
inhibitory response
hyperpolarization (+ negative), important for inhibiting action potentials so brain has time to process
neural convergence
occurs when a number of neurons synapse into a single neuron (happens often in retina - 126 million receptors and only 1 million ganglion cells); rod signals converge more than cone signals, cones in fovea each have private lines to one ganglion cell; rods are more sensitive to light in darkness (less light required for response), cones are better for detail vision
convergence and summation
due to convergence, rods can summate excitation more quickly
acuity
ability to see detail - moving eyes to search is using fovea where acuity is highest
visual evoked potential
recorded by disc electrodes placed on head over visual cortex - researchers alternate grated and grey stimuli - electrical response is VEP
