Section 4 Flashcards
Visual System
Human vision is limited as we can only see between
380 - 760 nm
Do we need light to see?
Yes. Not even nocturnal animals can see in the dark we all need some light to see
Visual field (definition)
the amount of space that is divided by the nose
Visual sensitivity
the neurons ability to detect
Visual acuity
refers to detail/resolution
Visual transduction
conversion of light energy to neural signals done by the receptors
What is the advantage of having one eye on each side of the head?
Larger visual field
What is the advantage of having both eyes in the front?
More visual acuity
Binocular disparity (definition)
the difference between two retinal images helps create 3D or depth perception - needed by animals who work with tools
Retina (definition)
has the receptors that regulate the transduction of light
The two unique spots of the retina are
fovea - 90% of receptors here
blind spot - no receptors
The layers of the retina (5)
photoreceptors (cones and rods) - transduction & synapse with bipolar cells
horizontal cells
bipolar cells - synapse with retinal ganglion cells
amacrine cells
retinal ganglion cells
Why is the blind spot of the retina “blind”
no photoreceptors are found in this part of the retina
Two types of photoreceptors and their properties
Cones:
- photopic vision
- predominant in bright lighting
- high acuity
- located in the center of retina
Rods:
- scotopic vision
- predominant in dim lighting
- high sensitivity
- located in the periphery/outer part of the retina
Convergence ratio of cone-fed circuits
low convergence 1 cone: 1 bipolar: 1 retinol ganglion
high acuity and low sensitivity
Convergence ratio of rod-fed circuits
high convergence 1 rod: 10 bipolar: 100 retinol ganglion
high sensitivity and low resolution
How can we have clear and detail vision if incoming light is distorted and there is a blind spot?
the fovea, perceptual completion, and eye movements
Fovea
allows high acuity vision due to the high density of receptors
Perceptual completion
the brain does “tricks” and fills in information in the blind spot based on context and past experience
Saccades
rapid eye movements; the perceived image is a sum of inputs received during the last few fixations (like quick camera snapshots)
Scotoma
an area of blindness caused by damage in the visual cortical area; damage in cortical areas affects signal processing due to retinotopic organization
Blindsight
the ability to see despite cortical-induced scotoma; happens due to alternative visual pathways (superior colliculus)
What happens if images are stabilized on the retina (the eye is not moving)?
The neurons stop firing and the image begins to fade (no input = image fades); the neurons respond to change
Differences between retinal images and photographs
photographs are exact copies while our visual system makes good “guesses”; what we see is our brain drawing on experiences and context
The lens of the eye projects on to the retina a _____, ______, and _____ image
focused, inverted, reversed
Photoreceptors transduce light into ____
neural firing patterns
How does the brain make sense of these sequences of impulses originating from different parts of the retina?
firing patterns are transmitted to the bipolar and horizontal cells, then transmitted to the endocrine and ganglion cells before bundling in optic nerve which is the sent to the brain (gets more ordered at each level of processing so you perceive the image focused and at the right orientation)
Retina-geniculate-striate pathway
major visual information processing pathway
Visual signal from each visual field goes to the ______ hemisphere
contralateral (e.g. objects in the left visual field => brain structures in the right hemisphere)
If there is a lesion in the right optic nerve, how is vision affected?
the right eye cannot see, the left eye is not impacted
If there is a lesion in the optic chiasm, how is vision affected?
damage with contralateral pathways (the LVF of left eye cannot see and the RVF of the right eye cannot see)
If there is a lesion in the right V1, how is vision affected?
the LVF of both eyes cannot see (left nasal and right temporal)
Visual Thalamus (LGN)
main subcortical nuclei that carries visual signals to the cortex
6 layers of cells
recieves 90% of retinal input
Input from each eye is ______ in the LGN
separate; monocular
Primary Visual Cortex
(also called V1 or striate)
6 layers of cells; LGN projects to layer 4
first stage of visual processing in cerebral cortex
Visual organization
Retinotopic organization
disproportionate cortical representation of the fovea
Higher concentration of receptors in a retinal area is associated with
larger cortical representation and higher acuity
What is the visual receptive field?
the area of the retina that when stimulate with light, changes the cell’s activity
has two areas, a center and a surround, producing opposite repsonses (light in center might be excitatory with the surround inhibitory or the opposite)
Two forms of visual receptive fields
circular (RGN, LGN)
rectangular (V1)
What are the properties of the on-center cell?
the on-center excites the neural firing while the off-surround inhibits firing
If light is shone in both the on-area and off-area at the same time the response is ____
weakend
What is the correlation between receptive field size and discrimination?
smaller receptive field = finer discrimination
How does the receptive field of neurons in the fovea compare to those in the periphery?
the RF of neurons in the fovea are smaller than in the periphery, allowing finer discrimination
Retinal Ganglion cells most respond to
small spots of light
Neurons in the V1 most respond to
bars of light
None of the receptors respond to
background or diffuse light; neurons respond to change/contrast
Two major classes of V1 neurons and properties
Simple cells:
- respond to lines/bars of specific orientation
- RF has a clear on/off area
Complex cells:
- larger RFs than those of simple cells
- binocular
- 3D vision begins here
Receptive fields of neurons in the foveal area of LGN or V1 are ____ than those in the periphery area
smaller
Contrast enhancement
mechanism in the visual system that enhances perception of brightness contrast between two areas
Perception is a _____ property
cortical
How does lateral inhibition work?
when a receptor fires, it also inhibits neighboring receptors
The Hermann Grid Illusion demonstrates
there is some lateral inhibition at the retinal level
If light alters the firing of a small group of receptors, what is the effect on the on-center bipolar cell that is directly connected to those receptors? What is the effect on the nearby cells? What causes this effect?
Excitatory; Inhibitory; Lateral Inhibition
Component theory
perception of color can be explained by different receptors (cones) in the retina
Opponent process theory
occurs at all levels of the visual system beyond receptors; opposing colors and complementary afterimage
Retinex theory
refers to the observation of color constancy and color contrast - the perception of object color is determined by its reflectance in a context
What behavioral phenomenon might have contributed to your percieved color?
color constancy and contrast
Which of the following most likely gives rise to the behavioral phenomenon of color constancy?
neural processing at the cortical level
Regions beyond the striate cortex are called
extrastriate areas
Cells in the extrastriate areas have ____ visual receptive fields
larger
Dorsal stream
where and action pathway; directing behavioral interactions with objects
Ventral stream
responsible for concious recognition (perception) of objects
Control of behavior vs. conscious perception
dorsal - directing behavioral interactions with objects
ventral - responsible for conscious recognition of objects
Visual Agnosia
inability to recognize visual image
Prospagnosia
inability to recognize faces
What about the existence of different visual agnosias tell us about visual processing in the brain
there might be separate object recognition processes in the visual system
Simple and complex cells are alike in that
they are unresponsive to diffuse light
V1 neurons are most responsive to
sine-wave gratings of light in particular orientation
A patient with a lesion to the ventral stream is able to interact with an object, and yet has no conscious perception of the object. This is explained BEST by which visual pathway theory?
The “control of behavior” vs. “conscious perception” pathway
What is a valid interpretation of the “Halle Berry neuron”?
The properties of this type of neurons may reflect broader face processing patterns found within the recorded region which may relate to emotional salience, semantic processes, or facial recognition
If you forced someone to guess what an object found within his scotoma is, he may be able to guess correctly due to:
alternative visual pathways
Why can the eye, in particularly the retinal layers, be described as “inside-out”?
Light must pass through multiple cell layers before reaching the cells capable of responding to light.
From the eye, visual information first travels mostly to which of the following in the mammalian brain:
the lateral geniculate nucleus
Even though we blink and move our eyes, we do not see objects around us as disappearing or blurry momentarily. This is because of:
temporal summation across recent inputs
Compared to the scotopic system, the photopic system
has less convergence.
What does the “component theory” propose?
color perception emerges from the relative activity in three types of photoreceptors
Even though we blink and move our eyes, we do not see objects around us as disappearing or blurry momentarily. This is because of:
temporal summation across recent inputs
The visual areas are organized like _________, which refers to the fact that _________
a map of the retina; adjacent neurons have adjacent receptive fields
Information from the left visual field of the right eye will travel to
the right LGN
What best explains the disproportionate representation of the fovea in the primary visual cortex?
the high density of visual receptors in the fovea
The fovea
Question options:
consists exclusively of rods.
creates the blind spot.
is the area of the retina that has the greatest sensitivity.
none of the above
none of the above
