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Dorsal and Ventral Streams

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Ethan D

on 20 March 2013

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Transcript of Dorsal and Ventral Streams

Visual Cortex located in the sensory area of the cerebral cortex. Light Enters the Eye It is then detected by Rods and Cones on the Retina and turned into a nerve impulse This then leaves the eye by the Optic Nerve It Then Travels Along the Optic Nerve To the Lateral Geniculate Nucleus Located in the Thalamus This then Relays the Signal on to the Visual Cortex A closer Look at what the Visual Cortex does with the Signal it receives The impulse is received by the primary visual cortex (V1) The Primary Visual Cortex It was the first part of the visual cortex to develop.
It is highly specialized for processing information about static and moving objects
It is also excellent at recognizing patterns It starts both the Ventral and Dorsal streams Ventral Dorsal Dorsal Stream Travels from V1 to V2 and then travels through the Dorsomedial area into the Visual Area (MT or V5) Known as the "Where?" & "How?" Pathway Spatial Awareness Guidance of Actions Main Functions & Ventral Dorsal Ventral Stream Begins in V1 and travels through V2 and V4 before finishing in the inferior temporal cortex Known as the "What?" Pathway Involved in Form Representation and Object Representation It is also associated with Long -Term Visual memory The Dorsal and Ventral
Visual Streams

By: Ethan Diamond Retina - layer of cells at the back of the eye that transforms information about the visual world before it gets to the brain.
Made up of 2 layers of cells: Ganglion layer and Receptor Layer
The photoreceptors are cells that help process information about light through the cone and rod system.
Cones: Fovea (central vision), high light, preserves more info about spatial location of light because there is no convergence
Rods: peripheral vision, low light , convergence
Retinal Ganglion cells: Last part of the visual pathway that happens in the retina. Parvocellular (perceive color) and Magnocelluar (perceive motion) are the two different types. Organizational stage between the retina and the brain.

LGN has RETINOTOPIC MAPPING which means that the objects you perceive in the world are mapped directly on to the brain in a corresponding way (ex. keys on a keyboard corresponding to characters on a screen)

Left LGN is only getting info about right half of the world, but it gets that info from both sides and vice versa AKA BROADMANN'S AREA 17, V1, STRIATE CORTEX 2 paths out of the retina:
Tectopulvinar path
(specialty: moving from peripheral to foveal)
Geniculostriate path (specialty: color) Why might blindsight happen?
Because the tectopulvinar path is still intact and you can still sense motion but not fine detail.

Because the LGN is still functioning and the LGN sends info to places other than V1 (like the Koniocellular layers), so those other areas might be intact and able to sense a little motion.


Because some parts of V1 are not damaged, and may be compensating and limping along a little even though the majority of it is gone. 1. Every object can be oriented in a lot of different ways (allocentric-position relative to other objects)
2. Many different objects can be classified in the same category (ex. different chair types)
3. You have to be able to distinguish the object from the background in order to identify something (aka figure-ground segregation) V4 is another place after the primary visual cortex where vision is processed. The cells in V4 are more complicated than in V1. They have color constancy (meaning that they can see that something is red even if there is a green light shining on it). We know that V4 is important for color perception but there are a lot of theories as to what it actually does. V4 has retinotopic mapping. achromatopsia: after brain damage to v4 area, people have the perception of the world being devoid of color

visual agnosia: inability to recognize objects when you see them. Important for distinguishing that there is a disconnect between the ventral and dorsal streams because different areas of damage cause different defects. 2 types:
aperceptive: means that can see individual aspects (like light and dark), but you can't form a meaningful whole

associative: means that you cannot link the information to previously stored knowledge prosopagnosia - you can't recognize faces, which means that faces are special because you can still recognize other things Problems in Ventral Stream Posterior Parietal Cortex (PPC)
Helpful with spatial cognition
3D frames of reference Dorsal stream contains binocular cells because binocular cells are helpful for the perception of depth and the identification of the "where." Problems of the Dorsal Stream Optic ataxia: means that you can identify an object, but you have a hard time reaching for it and finding it in the right place. You can't identify the "where."
Ocular motor apraxia: inability to direct gaze to a visual target
Akinetopsia: Very rare, inability to perceive motion in visual field. Occurs with damage to V5 Egocentric-information is relative to observer Possible Conclusions:
Animals
Infancy
Visual Streams + Consciousness Called binocular cells because we have two eyes, like binoculars

Slightly different images are created as a result of different position of eyes
Differences are known as binocular disparity
The dorsal stream passes this information along to the brain where it calculates depth using the angle between these differences in a process known as stereopsis
Also known as Parallax (used by astronomers to calculate distance to planets)
Modern Day Criticisms
Dissociation of two streams has been over emphasized
Some say the dorsal stream is a semi autonomous function that operates under guidance of executive functions
These executive functions themselves are informed by ventral stream processing
Emerging neurophysiological perspective is that there is considerable interaction between the two streams
Therefore, the streams have begun to be generalized as vision-for-action (dorsal) and vision-for-perception (ventral)
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