Chapter 3: Spatial Vision: From Spots to Stripes
• contrast- the difference in luminance between an object and the background or between lighter and darker parts of the same object
• contrast acuity – the smallest spatial detail that can be resolved (with smallest amount of contrast) • visual information processing- info goes from retina (optic disc) to -> down optic nerve -> to optic chiasm where fibers cross to opposite hemispheres of brain -> down optic tract -> to LGN (of thalamus) -> thalamus sends info to visual cortex in occipital lobes of each hemisphere • visual pathways in the brain – optic nerve- leaves each eye at optic disk; information reaches optic chiasm, where medial fibers cross to opposite hemispheres of brain. Optic tract- carries info to LGN of the thalamus and also to superior colliculi (motion) Thalamus- sends info to visual cortex in occipital lobes of each hemisphere – where detailed analysis of scene takes place • lateral geniculate nucleus (LGN) – 6 distinct layers of cells; where axons of ganglion cells first synapse 2 types of layers: magnocellular (bottom 2 layers – large fast moving objects) parvocellular (top 4 layers – details of stationary objects) Each LGN has a topographical map of half of the visual field
• magnocellular vs. parvocellular layers- magno-bottom 2 layers of the LGN; cells are physically larger. Para- any of top 4 layers of LGN; cells are physically smaller • contralateral vs. ipsilateral- contra- referring to the opposite side of the brain Ipsil- referring to the same side of the brain
• topographical mapping of LGN- the orderly mapping of the world in the LGN and the visual cortex; each LGN layer contains a highly organized map of a complete half of the visual field. Provides us with a neural basis for knowing where things are in space • striate cortex/primary visual cortex/area V1- “layered” organization; consists of 6 major layers; fibers from LGN project mainly to layer 4; more than 100 times as many cells as LGN- major visual transformation takes place here; 2 important features are topography and magnification; cortical magnification of foveal images • cortical magnification- the amount of cortical area devoted to a specific region in the visual field • receptive fields in the cortex- neurons in cortex are selectively responsive to specific elements of a visual scene; individual neuron will respond best when the line or edge is at just the right orientation or angle • orientation tuning- tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others • ocular dominance- tendency of neurons in striate cortex to respond more rapidly when a stimulus is presented in one eye; could contribute to behavioral habits such as baseball player choosing to bat left handed • simple and complex cells- simple- a cortical neuron with clearly defined excitatory and inhibitory regions. Complex- a neuron whose receptive-field characteristics cannot be easily predicted by mapping with spots of light; will respond regardless of where the stripe is presented, as long as it is somewhere within the cell’s receptive field • feature detection- cells’ response to certain orientations of stimulus (stripe) • columns and hypercolumns- columns- vertical arrangement of neurons that runs perpendicular to the layers; all neurons in column share specific sensitivity for certain orientation. Hypercol- 1-mm block of striate cortex consisting of a collection of columns; each hypercol. Corresponds to one area of the visual field; evaluates all possible orientations and in which eye a stimuls has been received • Amblyopia- reduced visual acuity in one eye because of abnormal early visual experience; “lazy eye” • strabismus- where one eye is turned so that it is receiving a view of the world from an abnormal angle; “crossed eyes” • anisometropia- where the two eyes have very different refractive errors; (one eye is...