VISUAL SYSTEM Flashcards
Sensory modality
Uses specially-designed receptors to convert particular stimuli from the outside world into action potentials
“Conscious sensations are different from physical properties of the outside
world, and are mental creations that do not exist outside of the brain”
-Sensory experiences like color, sound or taste are subjective and are generated entirely within the brain based on sensory inputs
-This idea highlights that our perceptions are not a one-to-one mirror of reality but are subjective interpretations created by our nervous system
Physiological coding of simple sensory experience
Stronger stimulus evokes more action potentials and is perceived as stronger
- example: if you touch something hot, the sensory neurons will fire rapidly, signaling to your brain that the heat is intense and may require a quick withdrawal from the source.
The constructive nature of visual processing
Refers to the way our brains actively interpret and build our visual perceptions
We perceive only a small fraction of the electromagnetic spectrum. TRUE/FALSE
TRUE, visible spectrum (380 to 750 nm)
How does our brain interpret and build our visual perceptions?
Our brains interpret, organize and construct visual experiences based on a combination of sensory information, past experiences and cognitive processes.
-eg. we can look @ orientation, color, contrast, disparity and movement of direction
Types of photoreceptors in the retina
Rods and Cones
Rods
- in the peripheral retina
- detect light in low-light conditions (black and white vision)
- they have low resolution levels
- humans have 100 million rods
Cones
- predominate at the fovea
- higher resolution
- detect color and detail
- sensitive to red, green and blue light
- humans have 6 million cones
Light detection process
Light enters through the cornea and cornea bends -> light focuses on the fovea in the retina -> activates photoreceptors -> sends signals to bipolar cells which send signals to ganglion cells which relay the information to the brain
How are photoreceptors packed in the fovea?
they are packed in a very dense and organized manner
Why is it important for photoreceptors to be tightly packed?
For high visual acuity (sharpness of vision)
What are receptive fields in the retina?
specific area where a stimulus can influence the activity of a particular sensory neuron
- eg. in the case of a visual sensory neuron: If light shines on a specific part of the retina that corresponds to that neuron’s receptive field, the neuron will become more active and fire more action potentials.
If the light is outside of this area, the neuron will not respond significantly.
How are retinal ganglion cells activated?
When light hits any photoreceptor contributing to ganglion cell receptive field
Receptive fields of somatosensory neurons on the skin
Particular area of skin where stimulation (such as pressure, touch, or temperature) can elicit a response (action potentials) from the neuron.
Types of somatosensory neurons
- Mechanoreceptors: Respond to mechanical stimuli (e.g., touch, pressure).
- Thermoreceptors: Respond to changes in temperature.
- Nociceptors: Respond to painful or harmful stimuli.
_____ have low convergence in retinal ganglion cells
Cones
_____ have high convergence in retinal ganglion cells
Rods
Contains a high density of cones for detailed and color vision
Fovea
Contains a high density of rods for low-light and motion detection, with fewer cones
Peripheral Retina
Night vision is exclusively …
Rod-based because color is not perceived
Types of cones
- S cones
- M cones
- L- cones
S-cones (short-wavelength cones)
- Sensitive to short wavelengths of light
- peaking around 437 nm - blue light
M-cones (medium-wavelength cones)
- sensitive to medium wavelengths
- peak around 533 nm (green light)
L- cones (Long-wavelength cones)
- Sensitive to long wavelengths
- peak around 564 nm (red light)
Why is there a difference in resolution between cones and rods?
- Packing density
- Degree of convergence
- Rods based circuits are more sensitive to light levels ( relatively low light is enough to activate because of spatial and temporal summation)
- Cones based circuits are less sensitive ( low levels of light will depolarize a bit but not enough to fire an action potential)
Degree of convergence in fovea
0.1 degrees
Degree of convergence in periphery
10 degrees
Portion of the retina where there are no photoreceptors (blind spot)
15-20 degrees away from the fovea
Why can’t rods see color?
Because they contain only one type of photopigment (rhodopsin) which does not differentiate between wavelengths
we need multiple cones to see color
TRUE/FALSE
TRUE
On-center ganglion cells
type of retinal ganglion cells that play a crucial role in how we perceive light and contrast
On-center receptive fields
(bipolar and ganglion cells)
On area (center) and off area (surround)
When does the on-center ganglion cell fires more?
when there is light in the center and no light in the surrounding area.
When does the on-center ganglion cell’s firing rate decreases?
when there is light in the surrounding area and darkness in the center (due inhibition)
What happens to the firing rate if full field has brightness?
light in the center and light in the surrounding area will cancel out. therefore no activity
What happens to the firing rate if center is bright and part of the surrounding (not all of it) is bright too?
it would still elicit spiking because of the complete activation of the on-center and not complete activation of the off-surround
Contrast enhancement via lateral inhibition happens in the_____
retina
Contrast enhancement via lateral inhibition
process by which an excited neuron reduces the activity of its neighboring neurons. In the context of the visual system, it enhances contrast by increasing the perceived difference between adjacent light and dark areas.
Retina - Thalamus - Visual Cortex Pathway
Contralateral vision
How Contralateral vision works:
- Each brain hemisphere processes visual information from the opposite visual field, crossing over at the optic chiasm.
- processing happens in the thalamus and cortex
Optic chiasm crossing
- Right visual field info is detected by the left side (nasal retina) of the right eye and the left (temporal retina) of the left eye. nasal fibers from the right eye cross at the optic chiasm to the left hemisphere.
- Left visual field info is detected by the right side (nasal retina) of the left eye and the right (temporal retina) of the right eye. Only the nasal fibers from the left eye cross at the optic chiasm to the right hemisphere.
Processing in the thalamus and cortex
some visual information stays on the ipsilateral field and others go to the contralateral side
Nasal fibers
the axons from the nasal retinas cross over at the optic chiasm to the contralateral side
Temporal fibers
the axon from the temporal retinas stay on the ipsilateral side and do not cross
How does the brain maintain separate visual inputs from each eye within the thalamus and primary visual cortex, and what structure is formed in the visual cortex as a result of this separation?
by keeping signals from each eye in different layers of the lateral geniculate nucleus. in the primary cortex these inputs remain separate forming ocular dominance columns
How does the retina’s organization of visual information accurately represent adjacent areas of the visual field in the brain, and what is this organization called?
By mapping adjacent visual field areas to adjacent areas of the retina, forming a retinotopic map that is preserved in the thalamus and visual cortex
The neocortex
- 6 layers
- Different stains reveal different features of the neocortex
-Layer 4 small, doesn’t receive many inputs
-Feedforward inhibition in every area of the brain
Layers of the neocortex
- Layers I-III mainly handle cortical communication,
- Layer IV is specialized for sensory input
- Layers V and VI are primarily output layers for signals leaving the cortex.
Receptive fields of simple cells
- are elongated rather than circular, meaning they respond to bars or edges of light rather than spots.
- consist of excitatory and inhibitory regions
- The precise arrangement of excitatory and inhibitory inputs allows these neurons to respond maximally when a stimulus (like a bar of light) is aligned with their preferred orientation.
- When a bar of light is presented at the preferred angle, it activates the excitatory inputs to the cell while minimizing the inhibition from surrounding regions. This causes a stronger output from the neuron.
- If the bar of light is presented at an angle that does not match the neuron’s preferred orientation, the inhibition may dominate, leading to a weaker response.
Columns in V1 have distinct receptive field properties
- Retinotopic map is preserved by:
- ocular dominance columns: presents stimuli only to the left or right eye (formed in the primary cortex) -> separates inputs from both eyes
- orientation columns: Other columns might respond to a bar of light that is horizontal, or vertical or any other orientation. - color blobs: found within orientation columns, these are sensitive to color info
Simple cells
- Allow us to understand the orientation of shapes as they only fire for specific orientations
- They will only fire if a sufficient number of on-center RFs that they receive input from are activated.
- The simple cells are in the V1 area of the occipital lobe, and they are activated when they receive input from the LGN.
Hypercolumn in V1
- Collection of columns for 1 specific point of view
-allow the brain to analyze various aspects of visual information (like orientation and eye input) at the same time. - contains blobs (color responsive neurons) for both eyes
