Lecture 11 Flashcards
Receptive fields
is the area of visual space where the presence of light influences the firing rate of the neuron
To find it, animal focuses on a point then shine light on areas near and far to the fixation point and see where in visual space a change alters the spiking activity of the neuron
Receptive fields and moving through the visual network
Outside the fovea, many photoreceptors go to one bipolar cell and many bipolar cells go to one ganglion cell
So the receptive fields get bigger
This happens to all visual information at deeper parts of the brain. At the deepest levels it is all of visual space and looks for patterns such as moms face
Fist cell in pathway
When correct wavelength of light is presented in a photoreceptors cell the photoreceptor hyperpolarizes and becomes less active - releases less glutamate
Second cell in pathway
ON/OFF bipolar cells
Respond differently because they have different glutamate receptors. ON only have inhibitory receptors, off have excitatory
ON bipolar cell
When light is presented in ON bipolar cells they depolarize and release glutamate
OFF bipolar cells
Hyperpolarize when light is presented to their visual fields. Less glutamate
Third cell in the pathway
Types of cells
Implication of all light or all dark
Mechanism of contrast detection
Retinal ganglion cells integrate info from many on/off bipolar cells
Their receptive fields have an CENTER SURROUND organizations.
ON CELL
Have activity when light is in the middle and inhibition when light is in the periphery.
OFF CELL
Have inhibition when light is in the middle and activity when light is in the periphery.
If all area has light or all area has darkness - not much change in firing.
BUT of there is a change in the visual input within this field like a line or an edge, then light from this will enter one part and not the other. This will hugely change neuronal activity.
And so these cells are particularly good at edges and contrast
Color vision in ganglion cells
Yellow on/blue off
Blue on/yellow off
Red on/Green off
Green on/Red off
So fire in distinct ways with specific color combinations greatly helping color vision.
What do Retinal Ganglion Cells (RGCs) do
Processing for color and edges
Path from ganglion to V1
Goes to the lateral geniculate nucleus LGN to V1
Receptive fields of V1
Are the sum of many RGCs.
Simple cells in V1
Are sensitive to the orientation of light
Can be understood as the sums of the afferent fibers
Also has center/surround orientation
If light is just off from its desired orientation, it fires less
There are cells sensitive for all orientations
Organized into columns with all orientations
Helps to ID edges etc. Downstream neurons will organise this
Visual association cortex
25% or cortex is for visual input
The part of the occp lobe that surrounds V1 is the visual association cortex
Each part recognises particular features of the visual environment such as shapes, movement etc
Striate/Extrastriate
V1=striate
V2+ = extrastriate
Where pathway
Deficit
Dorsal stream
Post parietal pathway
Spatial info
Deficit - Akinetopsia
What pathway
Ventral stream
Inferior temporal cortex
Identifies form, what the object is and its color
Deficit - cerebral achromatopsia No color vision Deny having it Everything is shades f grey Were not born with it, regular achromatopsia do not say this as it is not news for them, they were born with it
When color aspects of association cortex go, you also lose the memories of those colors
Depth perception
Monocular vision
Some V1 neurons respond to monocular depth cues and visual input from just one eye
Binocular vision
MOST respond to input from both
Depth perception - many monocular cues. Relative size, amount of detail, relative movement as we move eyes etc
Stereopsis
The perception of depth that emerges from the fusion of two slightly different projections of an image in the two retinas. Difference between the two is called retinal disparity
Improves depth perception
Agnosia
A deficit in ability to recognize or understand sensory information
Relates to a problem in an association cortex, or to problems with the sensory neurons themselves or to the primary areas
Predictive coding theory of perception
Most pathways in vision are bidirectional. Descending neural activity based on previous experience cancels out upward info so you only have the novel, differences left.
Every level except lowest makes these predictions via feedback connections. What propagates this is the prediction error signal which is used to improve future predictions.
Everyone agrees on what they see
Which implies that perception happens THEN cognition.
Not everyone agrees, some say it is all done together and thus how you see the world shapes what you see.
Visual information pathways (3)
(1) thalamus - LGN goes to V1
Determines what you are looking at. Creates a mental image of the entire visual space
(2) midbrain - Superior Colliculi
Fast, visually guided movements
Doesn’t know what you are looking at but knows where light is moving in visual space
(3) Hypothalamus: Circadian rhythm
Doesn’t know what you are looking at but knows how much light is present in your environment
3 parts of somatosensory system
Exteroceptive (cutaneous) responds to external stimuli
Interoceptive (organic senses) provides information about conditions within the body and is responsible for its maintenance (HR, Breathing, Hunger etc)
Proprioceptive system Position of body, posture and movement
Cutaneous system (4 senses)
Pressure - mechanical deformation of skin
Vibrations
Temp
Pain
Layers of skin
Epidermis - nob blood vessels, get O2 from air
Middle layer - dermis
Hypodermis or subcutaneous
Glabrous skin
Hairless. Usually sensitive
Meissner’s corpuscles - only found here, detect very light touch
Vibration senseors
Pacinian corpuscles
Free nerve endings
Temp and pain
Perception of temp
Free nerve endings in skin
2 cats of receptors: warmth and coolness
Poorly localized, unmyelinated, slow (touch before temp)
Some receptors that are sensitive to temp can be activated by ligands (capsaicin for heat, menthol for cold)
Thermal grill illusion
4 grills. some heated or cooled, others not
Put hand across all of them
Brain cannot differentiate the two so experiences this ambiguity as pain
Pain
Free nerve endings - skin
Many types
High-threshold mechanoreceptors (pressure receptor cells) are free nerve endings that spike in intense pressure like pinching or a strike
Others respond to extreme heat or presence of chemicals that should not be extracellular (like ATP)
2 pathways
(1) Spinothalamic tract
Poorly localized (crude touch, pain, temp)
Crosses over immediately in spinal chord and synapses there
Runs up to thalamus in spinothalamic tract
(2) Dorsal column Highly localized info (fine touch) Ascends ipsilaterally in dorsal column First synapse is in medulla Then crosses over to contralateral thalamus
Both get bundled together in the midbrain and synapse in the thalamus
Both go to somatosensory cortex
somatosensory cortex map
Relationship between cortical stimulations and body sensations is soma topic map
Somatosensory homunculus - the dude
Tactile agnosis
Cant id objects by touch alone
They an draw the object without looking and then recognize it
Phantom limb
Pain after amputation
Report feeling limb still exists and is hurt
Confusion on somatosensory cortex )primary and association). The brain gets nonsense sign als from cut axons and cannot interpret them.
Therapy
Often using mirrors
Re-organise brain
Taste
Transduces like synapses
Receptor protein changes membrane potential either directly or via g protein cascades
Tastes are due to activation of different receptor proteins
Taste buds are groups of 30-50 receptor cells which all express the same receptor (so are sensitive tot he same taste).
Replaced every 10 days as mouth is noxious
6 tastes
Sweentess (sugar) unami (Glutamate) bitterness (many molecules) saltiness (ions) Sourness (pH level and carbonation) Fat (Fatty acids)
Sugar and unami are instinctively rewarding and tasty. Bitter are repulsive instinctively although we can come to like them. Avoid poison
Primary Gustatory Cortex
Insula lobe of cerebral cortex
Smell
Specialized for identifying specific molecules called odorants
Change membrane potential - metabotropic g protein receptors
400 types
Odorant are volatile with molecular weight of 15-300. Not all molecules that meet this definition have odors
Olfaction
The olfactory epithelium is the tissue of the nasal sinus that sits under the skull and has the olfactory receptors. Each olfactory cell has only one type of receptor
All synapse in glomeruli in the olfactory bulb
They pass through the skull all together but are then sorted with each receptor and cell going to one glomerulus
400 receptors, can do 100000 smells based on combinations
Does not go to thalamus, goes to primary cortex the the temporal lobe directs and the amygdala
Nothing is inborne
Kids find all smells curious
Learn by association what is bad.
