Lecture 6 - Vision Flashcards
Retina and Associated Neurons
The retina is the light-sensitive reflective surface at the back of the eye composed of photoreceptors with multiple layers of transparent neurons on top.
There are 4 different types of neurons in this structure: horizontal, bipolar, amacrine, and ganglion.
The horizontal cells link the photoreceptors with bipolar cells.
Bipolar cells are interneurons responsible for conducting information from photoreceptors to the retinal ganglion cells.
The amacrine cells link bipolar cells to the retinal ganglion cells.
The retinal ganglion axons amass around the optic disc and form the optic nerve which carries information to the brain.
Fovea
The region of highest visual acuity in the retina; a dense collection of photoreceptors allows for such sharp vision.
Lens
A structure in the eye that helps focus an image on the retina; the ciliary muscle adjusts the shape of the lens to supplement the cornea - this changes our percept on the basis of distance.
Terms for Vision Problems
Some terms for vision problems” astigmatism, hyperopia, myopia, presbyopia.
Astigmatism refers to atypical curvature of the cornea or lens causing distortions in vision - most commonly blurred vision.
Hyperopia, also known as farsightedness, is caused by an abnormally short eye or flat cornea. This focuses light behind the retina, distorting the perception of nearby objects.
Myopia, also known as nearsightedness, is caused by an abnormally long eye or steep cornea. This focuses light in front of the retina, distorting the perception of far-away objects.
Presbyopia, an age-related change in visual acuity, is caused by an inflexible lens that leads to distortions in vision.
Photoreceptors
Specialized neural cells (rods and cones) that convert light energy to chemical energy, and then into neural activity. A series of chemical reactions lead to a change in membrane potential that produces a change in neurotransmitter release.
The rods are plentiful and scattered; they function in a broader range of conditions than do the cones. They are sensitive to low light and contain only one color receptor.
The cones are smaller, have a higher acuity, and more densely packed than the rods; they are sensitive to bright light and contain the majority of our color receptors.
Bipolar Cells
Bipolar cells are interneurons responsible for conducting information from photoreceptors to the retinal ganglion cells.
Retinal Ganglion Categories
There are two categories of RGC: M-Cells (Magnocellular) and P-Cells (Parvocellular).
This distinction is maintained throughout the visual pathway.
M-Cells are sensitive to light and receive their information from the rods. They are found throughout the retina, including the periphery and so are sensitive to movement but not color and fine detail. They project their axons to the magnocellular layer of the lateral geniculate nucleus in the thalamus.
P-Cells are sensitive to color and receive their information from the cones. They are localized in the fovea and so are sensitive to color and fine detail. They project their axons to the parvocellular layer of the lateral geniculate nucleus in the thalamus.
Optic Chiasm
The junction of the optic nerves where axons from the nasal retina cross to the brain’s opposite sides.
In each eye, information received at the nasal retina crosses to the opposite side of the brain via the medial pathway. Information received at the temporal retina travels straight back along the lateral pathway.
Thus, information from each visual field is represented on both sides of the brain.
Geniculostriate System
Processes the object’s image (color form and motion).
Predominated by P-Cell axons of retinal ganglia. Continues from the retina to the LGN of the thalamus, and then to layer IV of the primary visual cortex in the occipital lobe (striate cortex). The pathway then bifurcates, some information is sent to the parietal lobe and others sent to the temporal lobe.
Primary Visual Cortex
V1, also known as the striate cortex, is located in the occipital lobe around the calcarine fissure. It is the location of the first stage in the cortical processing of visual information.
Calcarine Fissure
The deep sulcus located on the medial surface of the occipital lobe encompassed by the primary visual cortex.
Retinotopic
A neurological map of the visual field within V1 that remaps a retinal image onto the cortical surface.
Each point in the visual field maps onto a group of neurons in V1. These neurons are selective for position, orientation, as well as spatial and temporal frequency.
Ventral Stream
The visual processing pathway from the visual cortices that leads to the temporal cortex.
Projections from V1 and extrastriate areas V2 and V4 lead to the inferior temporal cortex.
In the temporal lobe is the fusiform face area and parahippocampal place area.
Dorsal Stream
The visual processing pathway from the visual cortices that leads to the parietal lobe.
Projections from V1 and each extrastriate area lead to the posterior parietal cortex.
In the parietal lobe is the intraparietal sulcus area (control of eye movements, visual control of grasping) and parietal region (visual control in reaching).
Center-Surround Receptive Field
A region of space, characteristic of retinal ganglion cells, in which a stimulus will alter a sensory neuron’s firing rate; a central area that excites neural responses and a surrounding area that suppresses neural responses.
On-Center and Off-Center
On-Center: Retinal ganglion cells that depolarize (become excited) when an increase in illumination strikes at the center area of the concentric receptive field; illumination in the surround inhibits them.
Off-Center: Retinal ganglion cells that depolarize in response to a decrease in illumination at the center area of the concentric receptive field; illumination in the center inhibits them.
V1, V2, V3, V4, V5/MT
The extrastriate areas (secondary visual cortex); all visual cortical areas outside of the primary visual cortex (V1).
V2 contains similarly tuned cells as in V1; contributes to visuospatial map.
V3 receives inputs from V1 and V2; contains ventral projections toward the inferior temporal cortex; also has dorsal projections to the parietal cortex.
V4 receives inputs from V1 and V2; contains strong projection to the inferior temporal cortex and plays a role in color vision and simple shape recognition.
V5, the mid-temporal visual area (MT), receives inputs from V1, V2, and V3; it plays a major role in the perception of motion.
Cortical Magnification
Refers to the fact that the number of neurons in the visual cortex responsible for processing the visual stimulus of a given size varies as a function of the location of the stimulus in the visual field.
