10/30 Visual System - Dr. Fischer Flashcards
Objectives for Visual Lecture
- Describe the structure of the retina including layers, location of photorecetors, bipolar cells, and ganglion cells.
- Describe the basic circuitry of the retina: photoreceptor to bipolar cell to ganglion cell to lateral geniculate nucleus to visual cortex.
- Describe differences in activation of on-center and off-center bipolar cells.
- Describe the significance of the fovea.
- Define basic differences between the properties of rod and cone photoreceptors.
- Differentiate P and M ganglion cells.
- Describe the pathway from the retina to the lateral geniculate to the visual cortex including laterality of visual field.
- Differentiate simple and complex cells in the primary visual cortex.
- Differentiate and define ocular dominance column, orientation columns, blobs
- Differentiate between the “what” and the “where” pathways from the primary visual cortex.
The Retina is Layered
PHOTORECEPTORS:
RODS & CONES
> > > BIPOLAR CELLS
> > > GANGLION CELLS
> > > LATERAL GENICULATE
NUCLEUS
> > > CEREBRAL CORTEX
Light passes through layers to get to the photoreceptors.
The types of neurons in the retina are very diverse
Circuitry responsible for generating receptive field center responses of ______ .
Circuitry responsible for generating receptive field center responses of retinal ganglion cells.
In the dark, _____ have resting membrane potentials ~__mV;
In the dark, cones have resting membrane potentials ~-40mV;
When cones are depolarized, they release _____.
When cones are depolarized, they release glutamate.
Glutamate excites _____ cells and inhibits ____ cells.
Glutamate excites off-center bipolar cells and inhibits
on-center bipolar cells.
Light ______ cones resulting in ______ release of glutamate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, ____ cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
Depolarization of the on-center bipolar cell causes ____ of glutamate at their terminals which _____ ganglion cells (sign conserving).
Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving).
Light hyperpolarizes cones resulting in _____ release of glutamate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With ___ glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.
With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they hyperpolarize.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving)
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Receptive Field
the particular region from which a light stimulus will trigger the firing of a photoreceptor.
Simple cells
Neurons in Primary visual cortex that are orientation selective.
Input is from neurons in the lateral geniculate nucleus.
Represents input from multiple retinal ganglion cells and LGN cells that have particular alignment when put together.
Complex cells
Neurons in Primary visual cortex that respond primarily to direction of movement across the receptive field and increase or decrease in contrast.
Input is from multiple simple cells.
Orientation Columns
Organized regions of neurons that are excited by visual line stimuli of varying angles.
These columns are located in the primary visual cortex and span multiple cortical layers.
Blobs
Clusters of neurons interspersed between orientation columns.
Mostly concerned with color.
Ocular Dominance Columns
Stripes of neurons in the visual cortex that respond
preferentially to input from one eye or the other.
Alternate in visual cortex.
Primarily found in layer IV where LGN terminates.
Vision facts
(1) Nearly one-half of the Brain is involved in vision
(2) The human visual system recognizes a multitude of objects with ease. This task is still a “challenge” for computerized imaging systems
(3) The impact of vision disorders in the US was estimated at $51.4 billion in 2007 (The Impact of Vision Problems, National Symposium)
(4) Worldwide, about 39 million people are blind, about 230 million have some kind of vision impairment, and as many as 3.2 billion people have refractive error (need glasses)
(5) 80% of vision problems worldwide are avoidable or curable.
The retina receives an inverted image
Incident light is “focused” by the cornea, restricted by an aperture (the pupil), and focused again by the lens to project upon the retina
Incident light is “focused” by the ______
Incident light is “focused” by the cornea
aperture
an opening, hole, or gap.
Incident light is restricted by an aperture, the _____
Incident light is restricted by an aperture: the pupil
Incident light is focused again by the ____ to project upon the ____ .
Incident light is focused a second time by the lens to project upon the retina
Types of neurons can be distinguished based on…..
(1) morphology
(2) electrophysiology
(3) expression profile (calcium-bufferring proteins, neurotransmitters, neuropeptides, etc.)
The purpose of the fovea:
(1) Responsible for central vision and highest visual acuity
(2) Contains densely packed cone photoreceptors
– but contains NO ROD photoreceptors
= high acuity day-time vision
(3) Blood vessels excluded and inner retinal neurons are physically skewed to the side, hence “a pit” is formed, and light is not distorted by vessels or inner layers of the retina
rods
- high light sensitivity, long integration time
- more photopigment
- high amplification
- low temporal resolution (12 Hz)
- more sensitive to scattered light
- severely affected by quantum noise
cones
- low light sensitivity, short integration time
- less photopigment
- lower amplification
- high temporal resolution (55Hz)
- most sensitive to axial rays (+-15 deg)
- not affected by quantum noise
rod system
- low acuity: not present in the fovea
- highly convergent retinal pathways
- achromatic = one pigment only
cone system
- high acuity: concentrated in the fovea
- almost no convergence in the fovea
- chromatic, 3 types of cones, each with a distinct pigment that is most sensitive to a different part of the visible spectrum
Example of convergence:
The convergence of signals in the rod pathway is very large
Color vision deficiencies:
Congenital
affect photopigments
Color vision deficiencies:
Acquired:
lesions in visual pathways, usually after stroke
Red cone pigment missing
Protanope
Green cone pigment missing,
Deuteranope
only 2 active photopigments
dichromate
no color vision
achromatopsia
Blue cone pigment missingents
Tritanope
Types of neuronal electrical activity in the retina can be segregated to different cell types
Non-spiking and spiking (action potentials) of retinal neurons in different layers
“analog”
more finely graded response,
but becomes “noisy” over distances.
“digital”
more “steps”, but little or no degradation over long distances.
X-chromosomal recessive
Protanopia
Deuteranopia
Blue Cone Monochromacy
chromosome 7
autosomal dominant
Tritanopia
autosomal recessive
Achromatopsia
complete & incomplete
Color vision testing
“Ishihara pseudoisochromatic plates”
no red opsin
protanope
no blue opsin
tritanope
no green opsin
deuteranope
Separation in ON and OFF channels occurs at the _____ .
Separation in ON and OFF channels occurs at the cone terminals
all photoreceptors ____ in response to light, whereas they _____ in response to dark
all photoreceptors hyperpolarize in response to light, whereas they depolarize in response to dark
ON
depolarize (get “excited”) in response to light
OFF
depolarize (get “excited”) in response to dark
OFF CHANNELS
glutamate opens cation (Na+) channels through ionotropic receptors
spatial:
homogenous illumination doesn’t excite the ganglion cells because of the ON/OFF input to their receptive fields
–> redundant information is ignored
- saves energy
temporal
The dichotomy into ON cells and OFF cells, doubles the dynamic range.
- without this the maximal spike frequency must be increased
Why have separate ON and OFF channels?
spatial :
homogenous illumination doesn’t excite the ganglion cells because of the ON/OFF input to their receptive fields
–> redundant information is ignored
- saves energy
temporal :
The dichotomy into ON cells and OFF cells, doubles the dynamic range.
- without this the maximal spike frequency must be increased
Action potentials sent by retinal ganglion cells to higher visual centers code for:
(1) Changes in color
(2) Changes in brightness/darkness
(3) Movement across the visual field
(4) Position within the visual field
(5) Edges = a changes in brightness/darkness
Summary
Information sent from the retina is divided into several parallel channels to segregate different important image features
Neuronal signals (Action Potentials) from the retina represented data reduction to convey only relevant information = the retina is a change detector
To convey only the important information, there is an enourmous amount of image processing by relatively few retinal cells
Information sent from the retina is divided into several parallel channels to ______
Information sent from the retina is divided into several parallel channels to segregate different important image features
Neuronal signals from the retina represented data reduction to convey only relevant information = the retina is a ___ detector
Neuronal signals (Action Potentials) from the retina represented data reduction to convey only relevant information = the retina is a change detector
To convey only the important information, there is an enourmous amount of image processing by ___ ___ ____ cells.
To convey only the important information, there is an enourmous amount of image processing by relatively few retinal cells.
Organization of the
central visual pathways
Most of the projections (axons) from the retina to the brain end in the LGN (lateral geniculate nucleus), which in turn project to the primary visual cortex
Visual information from the retina to the cortex is compressed, yet a significant portion of the cortex participates in visual processing
retina to LGN to cortex 0.3% of the cortex (retina: ~100 mg)
> > > > 50% of the cortex participates in visual perception
There is a concentration of ___ cells in the fovea.
P cells
M and P cells
ganglion cells that project into the M or P pathway
M cells
NO color contrast
higher luminance contrast
higher temporal frequency
lower spatial frequency
P cells
YES color contrast
lower luminance contrast
lower temporal frequency
higher spatial frequency
higher spatial frequency?
P cells
lower temporal frequency?
P cells
higher temporal frequency?
M cells
color contrast?
P cells
higher temporal frequency
M cells
Lateral geniculate nucleus
LGN
90% of ganglion cell axons terminate in the LGN
- lesions in the optic tracts cause “blindsight”
- some visual sensation is maintained by retinal projections to the Superior Colliculi neurons.
Retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina.
Strong representation of fovea (50% of all).
Segregation of Magno- and Parvocellular pathways
M & P pathways carry information of contralateral visual field but inputs from both eyes remain segregated
LGN neurons maintain center/surround receptive fields that are similar to the retina’s
_____ of ganglion cell axons terminate in the LGN
90% of ganglion cell axons terminate in the LGN
lesions ____ cause “blindsight”
lesions in the optic tracts cause “blindsight”
some visual sensation is maintained by retinal projections to the _____.
some visual sensation is maintained by retinal projections to the Superior Colliculi neurons.
The LGN has retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina.
Strong representation of fovea (____ of all).
The LGN has retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina.
Strong representation of fovea (50% of all).
M & P pathways carry information of ____ visual field, and inputs from both eyes ___ ____.
M & P pathways carry information of contralateral visual field but inputs from both eyes remain segregated
LGN neurons maintain center/surround receptive fields are similar to retina
LGN has segregation of Magno- and Parvocellular
pathways
Primary visual cortex
V1 or Area 17
superior colliculus (SC)
Bilateral structure on the roof of the midbrain; direct projection from retina
(map of contralateral visual field
= ipsilateral retina)
Projection through pulvinar (thalamus) to cortex (“indirect pathway”)
- mediates “blindsight”
Extensive input from cortex (not only visual)
Multiple sensory maps superimposed in a topographic fashion (auditory, somatosensory)
Visual field representation in the visual cortex
V1 = striated cortex = Area 17
(Brodmann area 17)
LGN and SC receive information exclusively from the contralateral visual field
striated cortex
Primary visual cortex = V1 = Area 17
axons from the LGN all terminate in layer ____ of the cortex
axons from the LGN all terminate in layer 4 of V1 (v1 = primary vision, area 17) of the cortex
“simple” cells
orientation selective
excitatory & inhibitory zones in Receptive Field
“complex” cells
larger Receptive Fields
not clearly defined on/off
zones
movement of bar particularly effective
Receptive Fields of
“simple cells” in V1
retina and LGN: circular anuli On/Off center
V1: elongated zones with excitatory or inhibitory flanks. Retinotopic and orientation selective
V1
elongated zones with excitatory or inhibitory flanks.
Retinotopic and orientation selective.
Receptive fields of complex cells:
orientation is important, but not really position-dependent within the receptive field
no clear inhibitory or excitatory zone
input from several simple cells
Orientation of an edge, the direction of movement across the receptive field, plus the increase or decrease in contrast is what matters to a complex cell.
what matters to a complex cell?
Orientation of an edge, the direction of movement across the receptive field, plus the increase or decrease in contrast is what matters to a complex cell.
Types of stimuli that V1 cells have preferential responses to:
orientation columns
cells in columns have identical retinal position and axis of orientation (orientation columns)
using tangential penetration with microelectrode, an orderly shift in axis from one column to the next was shown
- about 3/4 mm for one full rotation
Types of stimuli that V1 cells have preferential responses to:
Blobs
First identified via cytochrome oxidase staining (measure of metabolic activity: “heightened neural activity”)
Mostly concerned with color, not orientation
orientation columns divided in
ocular dominance columns
unilateral injection of a label into one eye is transported to the cortex to identified regions of layer 4 in V1 that recieve input from 1 eye
Normally, most cortical neurons ____ receive input from both eyes
Normally, most cortical neurons above layer IV receive input from both eyes
Visual experience is required for the establishment of ocular dominance columns in the cortex
neurons outside of layer IV receive primary input from the left eye
hypercolumns
(about 1 mm2)
Each hypercolumn recieves and processes information from a defined part of the visual field
Each hypercolumn contains
- a complete set of orientation columns
- one set ocular dominance columns
- several blobs (color processing)
Regular repetition of these modules across the surface of V1
retinal projections (ganglion cells axons) connect to the \_\_\_\_ and to the \_\_\_\_\_
retinal projections (ganglion cells axons) connect to the superior colliculus and to the lateral geniculate nucleus
V4
color
Dorsal parietal cortex:
motion and depth
Where is it located?
Inferior temporal cortex:
face and object recognition
What is it?
Responses of cells in the inferior temporal cortex
shape selective cells
color selective cells
Responses of a neuron in the inferior temporal cortex to complex stimuli:
example of a face-sensitive neuron
RFs no retinotopic organization, sometimes whole visual field,
“position invariance” –> recognition of form anywhere (hands, faces, facial expressions…)
Where?
Dorsal
Motion, motion direction
Where is it located?
Dorsal parietal cortex: motion and depth
What?
Ventral
What is it?
Inferior temporal cortex:
face & object recognition
RFs no retinotopic organization, sometimes whole visual field, “position invariance” –> recognition of form anywhere (hands, faces, facial expressions…)
ON channels
glutamate closes cGMP-controlled Cation channels through metabotropic receptors
Sensory maps superimposed
in a topographic fashion (auditory, somatosensory)?
superior colliculus (SC)
In V1, there is interruption of orientation columns by ____ in the visual cortex.
In V1, there is interruption of orientation columns by “blobs” in the visual cortex.
