4. The Visual Brain Flashcards
crossover point
- optic nerves from from retinal ganglion cells (RGCs) meet at the optic chiasm
e.g., object in the left visual field( not the left eye but the left half of your FOV):
- image passes through lens and projects on the right part of the retina in both eyes
- at optic chiasm, optic nerves from right side of both eyes meet
- optic tracts (optic nerve after the optic chiasm) project to the LGN in the thalamus
- then to the right half of the visual cortex in the occipital lobe
Lateral geniculate nucleus LGN
layers of lateral geniculate nucleus (LGN) of the thalamus (sensory processing centre)
parvocellular
RCG input: p-cells
small cell body
many of these
slow conduction speed
sustained response type
small receptive field
high spatial detail
colour
magnocellular
RCG input: m-cells
large cell body
few in number
rapid conduction speed
transient (responding to change) response type
large receptive field size
motion sensitive
black and white
there are 2 visual pathways
- geniculostriate
- tectopulvinar
geniculostriate visual pathway
- P-cells, some M-cells (P and M pathways) → LGN
- LGN has 6 layers; each has a reinopic map: adjacent neurons correspond to spatially related points on the retina
(LGN also receives substantial feedback connections from the cortex)
- then, optic radiations project to cortex in occipital lobe:
- primary visual cortex (V1, “striate cortex”)
- and secondary visual cortex (V2)
tectopulvinar visual pathway
- remaining M-cells → superior colliculi of the tectum: part of brain stem; guide visual attention
- then projects to thalamus: pulvinar and lateral posterior nuclei, then to V2 and beyond
- controls eye movements/fixations; detection/orientation to visual stimuli, motion and location
this process bypasses the primary visual cortex
The Striate Cortex
- has 6 layers (magno → 4Cα, parvo → 4Cβ)
- layers 2 and 3 contain: blobs and interblobs
blobs
sensitive to wavelength, but not orientation (mostly receive input from parvo)
interblobs
areas between blobs; sensitive to orientation, but not wavelength (receive input from parvo only)
Cells in the Primary visual cortex/V1
simple cell,
complex cell,
hypercomplex/end-stopped cell
David Hubel & Torsten Wiesel (1962, 1968):
- shared 1981 Nobel Prize for research on information processing by cortical cells in the visual system
simple cell
(layer 4) responds best to:
▸ a bar, line, or edge of light
▸ in a particular location on the retina
▸ having a specific orientation
complex cell
(layers 2/3 and 6) responds best to:
▸ a bar, line or edge of light
▸ in a particular location on the retina
▸ having a specific orientation
▸ and moving in a certain direction
cells respond to stimuli that have the same ______
when inserting electrode perpendicular to surface of cortex, cells responded to stimuli having the same PROPERTY
this includes: location columns, ocular dominance columns, orientation columns, and hyper columns
hypercomplex/end-stopped cell
(beyond V1) responds best to:
▸ a bar, corner, or angle having a certain length and/or width
▸ in a particular location on the retina
▸ having a specific orientation
▸ moving in a certain direction
location column
cells respond to stimuli from same retinal location
ocular dominance column
cells respond to stimuli presented to one eye only
orientation column
cells respond to line stimuli having the same orientation; adjacent orientation columns differ in orientation selectivity by 10°
hyper column
region containing a single location column, which contains left and right ocular dominance columns, which contain the set of orientation columns from 0° to 180°
the occipital lobe is made of areas…
V3 and V4
V3
cells sensitive to moving edges of a certain orientation
- believed to handle perception of forms and local motion (local motion is one thing moving, global is your whole FOV moving)
- projects to temporal lobe
V4
cells respond to perceived colour of a surface (not wavelength)-the difference in color can be perceived but not identified
- projects to temporal lobe
Achromatopsia
Verrey (1888):
▸ 61-year-old female stroke patient with cerebral Achromatopsia : unable to perceive colour in the right half of her visual field
▸ could perceive but not identify different colours
e.g., purple and orange look different, but colours could not be identified
the temporal lobe is comprised of:
the inferior and the medial temporal cortex
inferior temporal (IT) cortex (fusiform gyrus)
involved in identifying stimuli (waving monkey paw example)
has primary cells (respond to simple stimuli like dots and squares) and elaborate cells (respond to more complex shapes, or shapes combined with colour/texture)
- face detector cells associated with prosopagnosia, the inability to recognize faces due to IT damage
Medial temporal cortex (MT) aka, V5
sensitive to overall motion (and direction) of object–but not colour
- projects to parietal lobe
Akinetopsia
Zihl, von Cramon, & Mai (1983):
▸ 43-year-old female: small lesion due to vascular disorder, had both of her V5s knocked out
▸ cerebral akinetopsia (a.k.a. motion agnosia): unable to see objects in motion, she was seeing at 2 FPS
Extrastriate Pathways
Two somewhat distinct neural pathways (Ungerleider and Mishkin, 1982):
ventral/temporal pathway:
parvo → V1 → V2 → V4 → IT
dorsal/parietal pathway
magno → V1→ V2 → V3 → V4, & MT (V5) → parietal lobe
ventral/temporal pathway
- parvo → V1 → V2 → V4 → IT
- concerned with object recognition and identification
- a.k.a. “what” system
dorsal/parietal pathway
- magno → V1→ V2 → V3 → V4, & MT (V5) → parietal lobe
- involved in locating objects, motion, spatial relationships, depth
- a.k.a. “where” system
object discrimination
shown object (e.g., brick), then presented choice task (brick and cylinder): if target (brick) was moved, it would uncover a hidden well that held food reward
landmark disrimination
one object presented; food hidden in well closest to object
visual agnosia
failure or deficit in perceiving or recognizing visual objects (Freud, 1891); from Greek, meaning “without knowledge”
e.g., prosopagnosia : disruption of face perception; inability to recognize/identify friends and family; inability to read facial expressions/emotions
2 types: (implies 2 steps in the brain)
* may not be able to perceive a face
* or may have intact perception of facial features, but inability to identify face
apperceptive agnosia/visual form agnosia, or visual space agnosia
failure to form a holistic percept; deficit in perception of whole objects, they can see things but not fully make sense of them
- inability to extract global structure, despite intact low-level sensory processing (acuity, colour, and brightness discrimination intact)
- cannot recognize, discriminate, or copy complex visual forms, like shapes
- but they can grasp objects they cannot identify, they can find a coffee mug if you gave them coffee
- neuropathy: caused by diffuse lesions to posterior occipital cortex (e.g., due to CO or mercury poisoning)
- likely a failure of “binding” of features together, due to damage at early stage of visual processing
associative agnosia/visual object agnosia
deficit in associating percept with meaning (“recognition without meaning”)
- cannot draw from memory
- able to copy pictures (after a long time), but cannot identify them
- can use other senses (e.g., touch, smell)
e.g. “convoluted red form with a linear green attachment” = rose (Sacks, 1970)
- neuropathy is not consistent; different subtypes may involve different perceptual impairments
- whole object not identified, likely due to damage to later stage of visual processing that connects visual object to semantic knowledge
category specific agnosia
inability to identify living (or nonliving) objects, metals, fruit, vegetables, musical instruments, fabrics, or gemstones
orientation agnosia
able to recognize drawings of objects rotated in picture-plane, but impaired at recognizing picture’s orientation; drawings often copied perpendicular to original
simultagnosia
inability to perceive more than one aspect of a visual stimulus and integrate details into coherent whole
- dorsal: can only perceive one of a number of overlapping objects; unattended objects not perceived; often cannot localize objects; bilateral damage to occipitoparietal regions
- ventral: can perceive more than one object at a time, but cannot identify more than one; may be able to describe one aspect of a scene without understanding the whole; can localize objects; damage to left inferior temporo-occipital cortex
pure alexia
cannot read words; must do letter-by-letter reading; can write to dictation but cannot read back what has been written; can copy words, which leads to recognition
- may be the same as ventral simultanagnosia
topographic agnosia
impaired recognition of scenes and landmarks; get lost easily; damage to inferior medial occipito-temporal cortex (similar to prosopagnosia)
Double-Dissociation of Extrastriate Pathways
in one case study, one ability is functioning, but another ability is not; and vice-versa in another case study
ex: a study with someone that has a damaged what pathway but an intact where pathway AND someone with an intact what pathway and damaged where pathway
Balint’s syndrome
- patient had bilateral damage to superior posterior parietal lobes (very rare)
- optic ataxia: inability to reach for and grasp objects in field of view
- optic apraxia : inability to guide eye movements or change visual fixation
- simultanagnosia
- could recognize individual objects, but could not tell where they were located or reach for them
- intact “what” system, but damaged “where” system?
What is the optic chiasm? Describe how retinal ganglion cells cross over in the optic chiasm. How does this crossing over affect the representation of the visual world in the LGN?
The optic chiasm is a structure located at the base of the brain where the optic nerves from each eye meet and partially cross. In the optic chiasm, retinal ganglion cell axons from the nasal (inner) retina of each eye cross over to the opposite hemisphere, while axons from the temporal (outer) retina remain on the same side. This crossover allows the visual information from the right visual field (from both eyes) to be processed in the left hemisphere and vice versa. As a result, the lateral geniculate nucleus (LGN) represents visual information from both eyes, providing a coordinated view of the visual world.
What are the three types of layers in the LGN? What kind of retinal ganglion cell innervates each? What is the functional significance of each layer?
The LGN consists of six layers, but they can be categorized into three main types:
Magnocellular layers (Layers 1 and 2):
- Innervation: Receive input primarily from M (magnocellular) ganglion cells (large, fast-conducting).
- Function: Process motion and depth information; sensitive to low contrast and high temporal frequency.
Parvocellular layers (Layers 3 to 6):
- Innervation: Receive input from P (parvocellular) ganglion cells (small, slow-conducting).
- Function: Process fine detail and color; sensitive to high contrast and low temporal frequency.
Koniocellular layers (interspersed between the magnocellular and parvocellular layers):
- Innervation: Receive input from koniocellular ganglion cells.
- Function: Involved in processing color and aspects of visual attention.
What is the function of the superior colliculus? Where in the brain is it found?
The superior colliculus is located in the midbrain and plays a crucial role in visual processing, particularly in coordinating eye movements and orienting the head and eyes toward visual stimuli. It integrates sensory information and helps with the rapid processing of visual cues to facilitate reflexive movements, such as saccades (quick eye movements).
What is meant by the term cortical magnification? How does it explain the retinotopic organization of V1 of the visual cortex?
Cortical magnification refers to the phenomenon where areas of the visual field that require higher visual acuity (like the fovea) are represented by a disproportionately larger area of the visual cortex (V1) than areas with lower acuity (like the periphery). This magnification results in a retinotopic organization, where adjacent neurons in V1 correspond to adjacent areas of the visual field, with a greater density of neurons devoted to the central visual field, enhancing our ability to process fine details in that region.
Describe the receptive fields of simple and complex cells in V1. What is the function of each type of cell?
Simple cells:
Receptive fields: Have distinct excitatory and inhibitory regions arranged in a linear fashion, sensitive to specific orientations of light bars or edges.
Function: Detect edges and orientations, helping to form the basis of shape and contour recognition.
Complex cells:
Receptive fields: Respond to light of a specific orientation over a larger area without distinct excitatory/inhibitory regions; they also respond to motion.
Function: Process motion and direction of moving stimuli, integrating information from multiple simple cells to provide a more comprehensive understanding of visual input.
What are ocular dominance columns? How were they discovered by Hubel and Wiesel? What role do they play in the hypercolumns of V1?
Ocular dominance columns are vertical columns in the primary visual cortex (V1) that are preferentially responsive to input from one eye over the other. Hubel and Wiesel discovered these columns through experiments involving electrodes implanted in V1 of cats and monkeys. They found that neurons within these columns exhibited a stronger response to visual stimuli presented to one eye compared to the other. Ocular dominance columns are essential for binocular vision and depth perception, and they contribute to the organization of hypercolumns, which are units in V1 that contain neurons processing information from both eyes, orientation, and spatial frequency.
What are V2 and V3? Where are they found in the brain, and what is their functional role in vision?
V2 and V3 are secondary visual areas located adjacent to the primary visual cortex (V1) in the occipital lobe.
V2:
Location: Surrounds V1 and is part of the visual processing pathway.
Function: Processes more complex visual information, such as texture, contours, and depth, and plays a role in integrating information from both eyes.
V3:
Location: Found adjacent to V2, also in the occipital lobe.
Function: Involved in processing dynamic visual information and motion perception, contributing to the understanding of spatial relationships in the visual field.
What is the dorsal pathway? What is the ventral pathway? What is the functional significance of each pathway?
Dorsal pathway:
Pathway: Extends from the primary visual cortex (V1) to the parietal lobe.
Function: Known as the “where” pathway, it processes spatial awareness, motion, and the location of objects, enabling the perception of how to interact with the environment.
Ventral pathway:
Pathway: Extends from V1 to the temporal lobe.
Function: Known as the “what” pathway, it processes object identification, form, and color, helping to recognize and categorize objects in the visual field.
What is blindsight? Describe its expression in neuropsychological patients. What is the likely anatomical cause of blindsight?
Blindsight is a phenomenon where individuals with damage to the primary visual cortex (V1) report an inability to consciously see objects but can still respond to visual stimuli, indicating some level of visual processing. This condition is often observed in neuropsychological patients with lesions in V1 who can detect motion or orientation of objects without conscious awareness.
The likely anatomical cause of blindsight is the functioning of alternative visual pathways (e.g., the superior colliculus and other subcortical structures) that bypass V1, allowing for some visual processing despite the lack of conscious visual experience.
What are congenital cataracts? What happens to adult patients when congenital cataracts are removed?
Congenital cataracts are cloudy areas in the lens of the eye that are present at birth, which can obstruct light from reaching the retina and impair visual development.
When congenital cataracts are removed in adult patients, they often experience significant visual improvement. However, the extent of recovery can depend on the duration of the cataract and whether any additional visual processing issues developed during early childhood. Adults may require rehabilitation, including corrective lenses, visual training, or therapy, to adjust to changes in their vision following cataract removal.
Bistratified retinal ganglion cells (K cells)
retinal ganglion cells that project to the koniocellular layer of the lateral geniculate nucleus; they represent 10% of ganglion cells, possess low sensitivity to light, and are sensitive to wavelength
Contralateral organization:
opposite-side organization; in the visual system, the nasal retina projects to the opposite side of the brain
Contralateral representation of visual space:
the arrangement whereby the left visual world goes to the right side of the brain, and the right visual world goes to the left side of the brain
Cortical magnification
the allocation of more space in the cortex to some sensory receptors than to others; the fovea has a larger cortical area than the periphery
End-stopped neurons
neurons that respond to stimuli that end within the cell’s receptive field
Extrastriate cortex
the collective term for visual areas in the occipital lobe other than V1
Hypercolumn
a 1-mm block of V1 containing both the ocular dominance and orientation columns for a particular region in visual space
Inferotemporal cortex
the region in the temporal lobe that receives input from the ventral visual pathway; one of its functions is object identification
Ipsilateral organization:
same-side organization; in the visual system, the temporal retina projects to the same side of the brain
Koniocellular layers
layers of the lateral geniculate nucleus with very small cells that receive input from K ganglion cells (bistratified retinal ganglion cells)
Koniocellular pathway (K pathway)
a pathway that starts with bistratified retinal ganglion cells and projects to the koniocellular layers of the lateral geniculate nucleus
Lateral geniculate nucleus
a bilateral structure (one is present in each hemisphere) in the thalamus that relays information from the optic nerve to the visual cortex
Magnocellular layers
layers of the lateral geniculate nucleus with large cells that receive input from M ganglion cells (parasol retinal ganglion cells)
Magnocellular pathway (M pathway)
a pathway that starts with the parasol retinal ganglion cells and projects to the magnocellular layers of the lateral geniculate nucleus
Midget retinal ganglion cells (P cells)
retinal ganglion cells that project to the parvocellular layer of the lateral geniculate nucleus; they represent 80% of ganglion cells, possess low sensitivity to light, and are sensitive to wavelength
MT (V5)
an area of the occipital lobe in the dorsal pathway, specific to motion detection and perception
Ocular dominance column
a column within V1 that is made up of neurons that receive input from only the left eye or only the right eye
Optic tract
the optic nerve starting at the optic chiasm and continuing into the brain
Orientation column
a column within V1 that is made up of neurons with similar responses to the orientation of a shape presented to those neurons
Orienting tuning curve
a graph that demonstrates the typical response of a simple cell to stimuli or different orientations
Parasol retinal ganglion cells (M cells)
retinal ganglion cells that project to the magnocellular layer of the lateral geniculate nucleus; they represent 10% of ganglion cells and possess high sensitivity to light
Parvocellular layers
layers of the lateral geniculate nucleus with small cells that receive input from P ganglion cells (midget retinal ganglion cells)
Parvocellular pathway (P pathway)
a pathway characterized by the retinal ganglion cells known as midget retinal ganglion cells
Primary visual cortex (V1)
the area of the cerebral cortex that receives input from the lateral geniculate nucleus, located in the occipital lobe and responsible for early visual processing
Retinotopic map
a point-by-point relation between the retina and V1
Saccades
the most common and rapid of eye movements; sudden eye movements that are used to look from one object to another
the most common and rapid of eye movements; sudden eye movements that are used to look from one object to another
the second area in the visual cortex that receives input; often considered the area that starts with visual associations rather than processing the input (sometimes called the prestriate cortex)
Smooth-pursuit eye movements
voluntary tracking eye movements
Superior colliculus
a structure located at the top of the brain stem, just beneath the thalamus, whose main function in mammals (including humans) is the control of eye movements
Ventral pathway
starts with midget and bistratified retinal ganglion cells and continues through the visual cortex into the inferotemporal cortex in the temporal lobe; often called the “what” pathway, as it codes for object identification as well as color vision
