The Visual System (3) Flashcards
Electromagnetic energy that we can see
Light
The white outer layer, the “eyeball
Sclera
The eye’s clear outer covering
Cornea
Reflects our eye colour
Iris
The black hole in the iris
Pupil
Focuses light rays
Lens
Nearsightedness
Myopia
Farsightedness
Hyperopia
“Old-sightedness”
Presbyopia
The eye does not focus light evenly on the retina
Astigmatism
What are the three main roles of photoreceptors?
- Convert light energy into action potentials
- Discriminate between wavelengths
- Distinguish between light intensities
What is the orientation of objects projected onto the retina relative to what we see?
Upside down and backwards
Region at the center of the retina that is specialized for high visual acuity
Fovea
Why is vision “sharpest” at the centre of the visual field rather than the outer areas?
Higher density of photoreceptors at the centre of the retina and slight depression at the fovea
Region of the retina where axons forming the optic nerve leave the eye and where blood vessels enter and leave
Blind spot
Intracranial pressure increase leading to the swelling of the optic disc
Papilledema
What is the energy conversion pathway enacted by the retina’s photoreceptors?
Light energy to chemical energy to neural activity
What are the characteristics of rods?
- Longer than cones, cylindrical
- More numerous than cones
- Sensitive to low light levels
- Used for night vision
- One type of pigment
What are the characteristics of cones?
- Tapered
- Sensitive to bright light
- Colour and high visual acuity
- Three types of pigment (red, blue, green)
- Randomly distributed across the retina
What are the four cell types in layer one of the retinal neurons?
Horizontal, bipolar, Amacrine, Muller
Links photoreceptors and bipolar cells
Horizontal cell
Receives input from photoreceptors
Bipolar cell
Links bipolar cells and retinal ganglion cells (layer two)
Amancrine cell
Glial cells that help channel light to the retina
Muller cell
Collection of axons at the optic disc that leave the eye and give rise to the optic nerve.
Retinal ganglion cells (RGCs)
What are the two major categories of RGCs?
Magnocellular cells, parvocellular cells
What are some characteristics of M-Cells?
- Large (magno)
- Layers 1 and 2 of LGN
- Receives input primarily from rods (no color)
- Sensitive to light and moving stimuli
- Throughout the retina, including periphery
What are some characteristics of P-Cells?
- Small (parvo)
- Layers 3 to 6 of LGN
- Receives input primarily from cones
- Sensitive to colour and fine detail
- Mostly in fovea
Describe the geniculostriate pathway
Retina, to the lateral geniculate nucleus (thalamus), to the primary visual cortex (occipital lobe), to extrastriate regions, then on to other cortical areas (temporal/parietal lobes)
Which layers of the LGN receive input from which area of the retina?
Layers 1, 4, and 6: Input from contralateral retina
Layers 2, 3, and 5: Input from ipsilateral retina
Describe the tectopulvinar pathway
Retina, to the superior colliculus (tectum), to the pulvinar (thalamus), then to the parietal and temporal visual areas
Sends connections to the parietal lobe
Medial pulvinar
Sends connections to the temporal lobe
Lateral pulvinar
Pathway that originates in the occipital cortex and projects to the parietal cortex
Dorsal visual stream
Pathway that originates in the occipital cortex and projects to the temporal cortex
Ventral visual stream
What is the dorsal visual stream responsible for?
The how pathway (how action is to be guided toward objects)
What is the ventral visual stream responsible for?
The what pathway (object recognition and identification)
Primary visual cortex
Occipital lobe striate cortex (V1)
Secondary visual cortex
Extrastriate cortex (V2 to V5)
Upon arriving at V1, what categories are projections from the LGN segregated into?
Colour, form, motion
What are the areas of the temporal lobe responsible for visual functions?
Fusiform face area, parahippocampal place area
What is the FFA responsible for?
Recognizing faces
What is the PPA responsible for?
Recognizing scenes
What are the areas of the parietal lobe responsible for visual functions?
Lateral and anterior intraparietal areas, parietal reaching region
What is the LIP responsible for?
Eye movements
What is the AIP responsible for?
Visual control of grasping
Region of the visual world that is seen by the eyes
Visual field
What are the ways through which the brain determines location of visual information?
- Divided into left and right fields (side)
- RGC receptive fields (limited circular location)
- Light hits retina at specific locations depending on direction (more precise location)
- Segregation continues into LGN
- Topographic organization of V1 and beyond
What happens to neurons in response to visual stimulation?
Increase or decrease firing rate (deviate from baseline)
How is our perception of shape constructed?
Light information and edges (contrast)
How do on and off-centre cells respond to light?
On-centre cells are excited when light hits centre portion of receptive field and inhibited when light falls onto periphery
Off-centre cells are excited when light falls on the periphery and inhibited when light falls on the centre
The amount of light reflected by an object in contrast to its surroundings
Luminance contrast
How do cells in V1 contribute to the perception of shape?
V1 neurons have much larger receptive fields than RGCs allowing them to respond to complex stimuli and perceive orientation by responding to bars of light in specific directions
The colour we see is determined by the mixture of the responses of the three different cone types.
Trichromatic theory
Theory that we perceive 4 basic colors in opposition
Opponent processes
Destruction of the retina or optic nerve of one eye, producing loss of sight in that eye
Monocular blindness
Blindness of an entire left or right visual field
Homonymous hemianopia
Blindness of one quadrant of the visual field
Quandrantanopia
Scotoma
Small blind spot in the visual field caused by small lesions in V1
Hemiachromatopsia
No colour present in one visual field
Blindsight
The ability of people who are cortically blind to respond to visual stimuli that they do not consciously see
The inability to process sensory information, a loss of the ability to recognize objects, person, sounds, smells, etc
Agnosia
Inability to recognize objects or drawings of objects
Visual-form (visual object) agnosia
Generally, what results from injury to the ventral stream?
Issues with object recognition
Generally, what results from injury to the dorsal stream?
Issues with visually guided movements