Lesson 2: Begining of the Perceptual Process Flashcards
How does a tree become a perception of a tree?
Information about the tree (distal stimulus) is carried in light reflected from the tree and into the eye. When this light reaches the receptors in the retina, creating the proximal stimulus, it becomes transformed into electrical signals that contain information about the tree, which are transmitted to the brain, where eventually these electrical signals become transformed into a perception of the tree
The energy within the electromagnetic spectrum that humans can perceive
visible light
where does the light reflected from objects first enter the eye?
the pupil
where is an image focused in the eye?
the cornea and the lens
Where is a sharp image formed in the eye?
the retina, the network of neurons that covers the back of the eye and that contains the receptors for vision
what are the two types of visual receptors?
cones and rods
Where are the visual pigments located and what do they do?
located on the outer-segments of rods and cones. They are light-sensitive chemicals that react to light and trigger electrical signals.
cones and rods are different in (3)
- number
- shape
- distribution across the retina
what is the fovea? and contains which receptors?
The point of central focus. Contains only cones
when we look directly at an object, where does the image fall in the eye?
the fovea
what is the peripheral retina? Which receptors does it have?
all of the retina outside the fovea. Contains both rods and cones. contains more rods than cones.
how many cones and rods are there in the retina?
6 million cones
120 million rods
What happens when functioning receptors are missing from one area of the retina?
Macular degeneration is a condition in which the cone-rich fovea and a small area that surrounds it are destroyed. Most common in older people. Creates blind region in central vision.
why do we have a blind spot at 20 degrees in the eye? (2)
- Because there are no receptors since it is the area where the ganglion cells leave the eye to form the optic nerve.
- Brain “fills in” the place where the image disappears with a perception that matches its surrounding patterns.
retinitis pigmentosa
genetic disease in which the retina breaks down. Attacks the peripheral rod receptors and results in poor vision in the peripheral visual field. Eventually, in severe cases, the foveal cone receptors are also attacked, resulting in complete
blindness
forms and functions of the optical system?
lens: 20% of focusing, can change its shape to
adjust the eye’s focus for objects located at different distances.
This change in shape = ciliary muscles, which increase the focusing power of the lens (its ability to
bend light) by increasing its curvature
cornea: 80% of the eye’s focusing power, fixed in place so it
cannot adjust its focus.
Accommodation
change in the lens’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
presbyopia
loss of the ability to accommodate. lens hardening, weakening of ciliary muscles
myopia/nearsightedness
inability to see distant objects clearly
solution = corrective lens
point of focus for parallel rays of light = in front of the retina
Types of myopia and why?
- Refractive Myopia = cornea and/or the lens bends the light too much
- Axial Myopia = eyeball is too long
hyperopia/farsightedness
point of focus for the parallel rays of light = behind the retina,
because the eyeball is too short.
solution= for young ppl- accomodation, for old ppl= corrective lenses
Two parts of the visual pigments?
- opsin
- retinal
isomerization
change in the shape of the retinal from bent to straight. Causes a chain reaction that activates thousands of
charged molecules to create electrical signals in receptors.
Dark adaptation
Process of increasing sensitivity in the dark
Rod monochromats
no cones due to a rare genetic defect
visual pigment regeneration
process of reforming the visual pigment molecule by the retinal returning to its bent shape and becoming reattached to the opsin.
visual pigment bleaching
molecule becomes lighter in color due to the change in shape and separation of the retinal from the opsin
perception and physiology
- Our sensitivity to light depends on the concentration of a
chemical—the visual pigment - The speed at which our sensitivity increases in the dark
depends on a chemical reaction—the regeneration of the
visual pigment.
Detached retina
The retina becomes detached from the pigment epithelium, a layer that contains enzymes necessary for pigment regeneration
Monochromatic light
light of a single wavelenght
How do we measure cone spectral sensitivity?
by having a subject look directly at a test light so that it stimulates only the cones in the fovea
How do we measure rod spectral sensitivity?
measuring sensitivity after the eye is dark-adapted (so
the rods control vision because they are the most sensitive receptors) and presenting test flashes in the peripheral retina, off to the side of the fixation point
which receptors are faster at adapting to the dark? which one is more sensitive to the dark?
cones = faster. maxed out at 3 to 5 minutes
rods= catches up to cones at 7 min and goes further = more sensitive. maxed out at 20-30 min.
why? because visual regeneration occurs faster in cones
spectral sensitivity
eye’s sensitivity to light as a function of the light’s wavelength
which receptors are more sensitive to short-wavelength light?
rods
What is the Purkinje shift?
enhanced perception of short wavelengths during dark adaptation, that is the tendency for eyes to shift towards the blue and green colors of the visible spectrum
Absorption spectrum
a plot of the amount of light absorbed versus the
the wavelength of the light
How many receptors does each eye have ? and how many ganglion cells?
126 million R, 1 million G…. each ganglion cell
receives signals from 126 receptors
What are the group of neurons that receive signals from the optic nerve - 2
- lateral geniculate nucleus
- visual receiving area
both in the cortex
electrical signals are recorded from where? and how?
axons, using a small electrode
-70mV
resting potential, when the inside of the axon is 70mV more negative than the outside
What do we mean by an action potential being propagated response?
once the response is triggered, it travels all the way down the axon without decreasing in size
changing the stimulus intensity affects what
rate of firing. 500 to 800 impulses/second
refractory period
the interval between the time one nerve impulse
occurs and the next one can be generated in the axon. approx 1ms
spontaneous activity
Action potentials that occur in the absence of stimuli from the environment
What does the opening of sodium channels represent?
An increase in the membrane’s permeability
to sodium
What happens during the rising phase of the action potential?
Sodium enters the axon through channels. This increases the positive charge inside from -70mV to +40mV.
The falling phase of the action potential?
sodium channels close. potassium channels open, and potassium leaves the axon. This returns the charge from +40mV to -70mv.
The falling phase of the action potential?
sodium channels close. potassium channels open, and potassium leaves the axon. This returns the charge from +40mV to -70mv.
excitatory response (depolarization)
occurs when the inside of the neuron becomes more positive. Increases firing rate
inhibitory response (hyperpolarization)
occurs when the inside of the neuron becomes more negative. decreases firing rate
neural circuits
interconnected groups of neurons within the retina
Where do signals generated in the receptors travel to? (2)
- Bipolar cells (B)
- Ganglion cells (G)
neurons that connect neurons across
the retina (2)
- horizontal cells
- amacrine cells
neural convergence
occurs when a number of neurons synapse onto a single neuron
what does convergence entail for rods and cones?
(1) the rods = more sensitive because more convergence
(2) the cones = better visual acuity because of less convergence and receptors in the fovea are packed closely together
why is it easier to see stars if we look to its side and not straight on?
- Because the star’s image falls on the rod-rich peripheral
retina. - The rods have greater convergence than the cones
- takes less light to generate a response from an
individual rod receptor than from an individual cone receptor
Preferential looking (PL) technique - infant visual acuity
Two stimuli are presented. The experimenter looks at children’s eyes to see which one they look at more. This is to determine if infants see a difference between two stimuli.
visual evoked potential (VEP) - infant visual acuity
The electrical signal is recorded by disc electrodes placed on the infant’s head over the visual cortex. For this technique, researchers alternate a gray field with a grating or checkerboard pattern. If the stripes or checks are large enough to be detected by the visual system, the visual cortex generates an electrical response called the visual evoked potential
why do infants have poor acuity? (2)
- infant’s cones are spaced far apart
- visual area of the brain is poorly developed at birth,
with fewer neurons and synapses than in the adult cortex
