From Eye to Brain PDF

From Eye to Brain Retina to Cortex Visual Pathway From retina to the primary visual cortex (V1) through lateral geniculate nucleus (LGN) Optic nerves from each eye meet at the optic chiasm ◦ Left visual field of each eye go to the right LGN ◦ Right " " " " " " to the...

From Eye to Brain Retina to Cortex Visual Pathway From retina to the primary visual cortex (V1) through lateral geniculate nucleus (LGN) Optic nerves from each eye meet at the optic chiasm ◦ Left visual field of each eye go to the right LGN ◦ Right " " " " " " to the left LGN LGN Part of the thalamus Arranged in layers ◦ 1 & 2: magnocellular layer from M (parasol) ganglion cells ◦ 3 - 6: parvocellular layers from P (midget) ganglion cells ◦ 1,4 & 6: receive input from the contralateral eye ◦ 2, 3 & 5: receive input from the ipsilateral eye ◦ Koniocellular layers may be involved in colour vision ◦ Get input from K type ganglion cells Pathways 1. Dorsal: where or how pathway 2. Ventral: what pathway The visual system maintains the pathways separate for processing different information Primary Visual Cortex (V1) Aka striate cortex Circular receptive fields are replaced by elongated (stripe) receptive fields Topographic mapping Retinotopic mapping: relative position of objects within the 2 halves of our visual field ◦ Maintained in V1 Cortical magnification: relative amount of V1 dedicated to processing visual information from the fovea ◦ relative size of the retinal image ◦ More neurons of V1 needed to process central vision than peripheral vision Visual crowding Visual crowding: peripheral objects are hard to detect because of clutter Objects in peripheral vision have lower resolution than central vision Object can be discernable but those around can be missed Spots to Stripes Neurons in V1 respond best to bars of light vs spots Types of V1 neurons 1. Simple cells: respond best to bars of light (or dark but not both) with a particular orientation and location on the retina Strong response to bars of a specific orientation Less response to bars of similar orientation Contrast has no effect the orientation Contrast affects the magnitude of response 2. Complex cells: respond equally well to bars of light and dark with a particular orientation anywhere within their receptive field Orientation-tuned response but location of stimulus is not important Highly responsive to moving bars within their receptive field Receptive field is usually larger than simple cells End-stopped cells: are selective for the length of the bar of light they are stimulated with In V1 V1 Organization V1 Layers V1 columns: information that have preferred ocular dominance (orientation) Each ocular dominance column contains smaller columns with shared orientation selectivity Combining the 2 ocular dominance columns results in hypercolumns Visual streams Different visual processing streams are maintained in the layered structure of V1 Selective Adaptation Adaptation: reduction in response to prior or sustained stimulation Selective adaptation: technique where we can selectively deactivate groups of neurons through adaptation by presenting a stimulus for an extended period of time Neurons in V1 have a preferred orientation But still fire in response to other close orientations Perception = center of overall response Extrastriate Cortex We can compare visual areas based on: 1. Types and distributions of neurons 2. Other areas of the brain they connect to 3. Properties to which the neurons are tuned Motion, colour, orientation, form) 4. Retinotopoc map of visual field Segregation occurs early and is maintained throughout the visual pathways Dorsal stream (Where or how pathway) V1 > V2 > MT > parietal lobe Represents properties that relate to objects' motion or location Guided action Ventral stream (What pathway) V1 > V2 > V4 > inferotemporal cortex Represents properties that represent objects' identities Shape, colour Lesions In dorsal stream = impaired performance in visually guided tasks (optic ataxia) In ventral stream = impaired performance in object recognition (visual form agnosia) Manual estimation: use the distance between index finger and thumb to estimate the size of an object Grasp calibration: adjust the distance between thumb and forefinger to match the size of the object as you reach it Functional modules Functional areas Beyond V1, visual areas become more specific in the type of information they process V4 = colour, edges, curvatures Multiple simple cells Slightly different orientation selectivity Synapse onto a neuron in V4 Form a contour-selective receptive field  LOC (lateral occipital cortex) & IT (inferotemportal cortex) = objects, faces, places Fusiform face area (FFA): responds best to faces, or context that implies faces Parahippocampal place area (PPA): responds best to places and layouts (rooms) Extrastriate body area (EBA): responds best to full bodies and body parts MT = motion Grandmother cell hypothesis: a single neuron is responsible for recognizing your grandmother - not likely Distributed coding Distributed coding: where multiple neurons firing at specific rates will code for a person or object - with different firing rates for different faces