Vision II PDF

Vision II LG1) What is LGN and what happens there? located in thalamus Receives informaion form the optic tract Sends information via the optic radiations to the primary visual cortex (V1) Function: § Regulation of information flow : signal is smaller afte...

Vision II LG1) What is LGN and what happens there? located in thalamus Receives informaion form the optic tract Sends information via the optic radiations to the primary visual cortex (V1) Function: § Regulation of information flow : signal is smaller after passing the LGN § Feedback processing: LGN receives more signal from the cortex than it does from the retina. This suggests that feedback from the brain plays a role in shaping the information that the LGN sends to the cortex. § Maintaining receptive fields: neurons in the LGN have concentric receptive fields. (ON-Cell) -> suggests that the LGN is important for detecting contrast and edges in the visual scene. § Color processing: The LGN is organized into six layers. The four outer layers are called parvocellular layers, and contain P Ganglion Cells. The two inner layers are called magnocellular layers and contain M Ganglion Cells. Magnocellular layers receive input from large ganglion cells which do not discriminate between wavelengths of light and are not involved in color vision. Parvocellular layers receive input from cones, which are sensitive to diOerent wavelengths of light. P Ganglion Cells can discriminate between wavelengths, and some exhibit spectral opponency, meaning they are excited by some wavelengths and inhibited by others. -> Basis for color perception feature M Ganglion Cell P Ganglion Cell Cell size Large Small Input Source Rods (and broader Cones (specific cone types) photoreceptor input) Temporal Resolution High (motion detection) Low Spatial Resolution Low (less detail) High (fine detail) Color Sensitivity No (achromatic) Yes (color vision) Pathway Magnocellular layer of LGN Parvocellular layer of LGN Role Motion, flicker, large pattern Detail, color, fine spatial analysis  § Relaying information between eyes and brain: essential for vision, as it ensures that information from the eyes is processed correctly and eOiciently LG2) What happens in the first picture Monocular zone = zone that is only visible by one eye, more limited input Binocular Zone = zone that is visible for both eyes (were the monocular zones overlap -> 3D vision) 1) The light form the zones hit the eye and retina and from there on go through the optic nerve/tract, which consits of the axons oft he ganglion cells located in the retina 2) At the optic chiasm the optic nerves meet : - Retina is divided into nasal hemiretina (closer to nose) and the temporal hemiretina (closer to temple) - At optic chiasm, axons from nasal hemiretinas cross over to the opposite side oft he brain (contralateral = opposite side) – the temporal hemiretinas remain on the same side (Ipsilateral = same side) 3) Lateral Geniculate Nucleus: The optic tracts carry visual information to the LGN in the thalamus. The LGN has six distinct layers: - Layers 1 and 2 (Magnocellular pathway) process motion and broad outlines, primarily detecting movement and contrast. - Layers 3 to 6 (Parvocellular pathway) process finer details, including color and texture 4) Visual Cortex: The signals are transferred from the LGN tot he primary visual cortex (V1) in the occipital lobe, where the information gets further porcessed - Magnocellular pathway (blue lines) contributes to processing motion and spatial relationships. - Parvocellular pathway (red lines) contributes to fine detail and color perception. LG3)How is the primary visual cortex organized? Retinotopic map = point-to-point correspondence between locations on the retina and locations in V1 I.e. if a light shines on a particular point on the retina, neurons in a specific location in V1 will fire Cortical Magnification: Retinotopic map is distorted, with more cortical space dedicated to the fovea This phenomenon, known as cortical magnification, reflects the importance of the fovea for detail visual processing Larger cortical representation for fovea allows for finer discrimination of detail in the central visual field Columnar Organization: V1 is organized into columns, which specialize in processing specific aspects of visual information Location Columns: Contain neurons that all have their receptive fields at approximately the same location on the retina This means that all neurons within a single location column respond to stimuli presented at roughly the same position in the visual field Orientation Columns: ð Sub-columns withing each location column Neurons in an orientation column share a preference for stimuli oriented at a specific angle è Each column responds to a diOerent angle of a visual stimuli As you move across the cortex, the preferred orientation of neurons in each column changes systematically Adjacent (benachbart) orientation columns often shift by 90° This organization allows V1 to detect and analyze edges, and shapes Ocular Dominance Columns: Consist of neurons that have a preference for input from one eye over the other (ocular dominance)  columns are arranged in a striped pattern, alternating (abwechseln) between left- eye and right-eye Color Blobs: Clusters of neurons specialized in processing color information Present in parvocellular pathway Parvocellular & Magnocellular Pathways: V1 processes two main streams of input from the LGN: - Parvocellular: Processes motion, depth, and broad outlines of objects - Magnocellular: Processes fine details and color LG4) What are the dorsal and ventral systems? 1. Retina – the cells carry visual input from retina to LGN 2. LGN – information from the cells, gets further processed and organized before moving on to the primary visual cortex 3. Area 17/primary visual cortex 4. Area 18 (V2 & V3) information gets further processed  & Dorsalup) Pathwa , - Ventralbelow Pathway Dorsal Pathway Ventral Pathway Alternative “Where” Pathway, “How” Pathway, “What” Pathway Names Action Pathway Function Processes information object Processes information about object location and guides actions toward identity. objects. Contributes to conscious visual May operate outside of conscious perception. awareness. Brain Areas Parietal Lobe, Middle Temporal Area Occipitemporal and inferior Involved Temporal Areas, Lateral Occipital complex (LOC) Input Receives input from both M and P Receives input from both M and P cells in the retina cells in the retina Evidence from Damage to the dorsal stream can Damage to the ventral stream can Neuropsychology lead to difficulty interacting with lead to visual from agnosia, objects, despite intact object impairing object recognition, while recognition. preserving the ability to interact with For example; patients can describe objects. an object but cannot accurately For example; patients cannot reach for it. identify an object but can still reach for and grasp it. Example Guiding hand movements to grasp a Recognizing a pen as a pen pen LG5) How do lesions aDect the two pathways? - connect to patients R.F. and D.F. Evidence from Damage to the dorsal stream can Damage to the ventral stream can Neuropsychology lead to difficulty interacting with lead to visual from agnosia, objects, despite intact object impairing object recognition, while recognition. preserving the ability to interact with For example; patients can describe objects. an object but cannot accurately For example; patients cannot reach for it. identify an object but can still reach for and grasp it. Examples Case of patient R.F.: damage to Case of patient D.F.: damage to dorsal stream ventral system Had trouble to accurately reach for Led to visual form of agnosia objects (trouble recognizing) But could easily recognize them But could still easily reach for objects LG6) Go over receptive fields and on/oD centers again on-center off-center from above surround t Center - + + t Receptive Fields light hits activity when + increases ↑ when light hits activity inhibits retina back surround center surround T Photo receptors I - · 1 1. Horizontal Cell A E BipolarCell t 2 - -Amarine & i - 3 Ganglion Cell = E retina front LG7) Why does the illusion in Vision I happen? Herman Grid Illusion: At intersections, more light from the surrounding areas falls onto the surround of the receptive field compared to areas along the straight lines. This stronger lateral inhibition at intersections causes the brain to interpret these areas as being dimmer, creating the illusion of gray dots.