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3 3 Visual Acuity • Acuity: The smallest spatial detail that can be resolved Spatial Vision 3 Visual Acuity (cont’d) 3 Discrete Sampling of Gratings • Measuring visual acuity: – Eye doctors use distance (e.g., 20/20) – Vision scientists use the smallest visual angle of a cycle of grating 1 3 Snellen Letters 3 • Herman Snellen invented method for designating visual acuity in 1862 Contrast Sensitivity and Spatial Frequency • Spatial Frequency: The number of cycles of a grating per unit of visual angle (usually specified in degrees) • Contrast: The difference in illumination between a figure and its background (difference between darker and lighter regions) 3 3 Visual Acuity: Oh Say, Can You See? (cont’d) • Contrast Sensitivity Function 2 3 Retinal Ganglion Cells and Stripes 3 Response of ON-center Retinal Ganglion Cell Pathways from the Eye 3 The Lateral Geniculate Nucleus • Retinal cells: like spots of light (of the right size) • Also like gratings (of the right size) 3 • Two lateral geniculate nuclei (LGNs): Axons of retinal ganglion cells synapse there 3 3 The Lateral Geniculate Nucleus (cont’d) 3 • Two types of layers in LGN: Magnocellular vs. Parvocellular The Lateral Geniculate Nucleus (cont’d) • Each layer – only receives input from one eye (hemiretina ) – Magnocellular layers (1&2) receive inputs from Parasol RGC • Layers 2,3,5 – Ipsilateral: Referring to the same side of the body (or brain) – Parvocellular layers (3-6) receive inputs from Midget RGC • Layers 1,4,6 – Contralateral: Referring to the opposite side of the body (or brain) • Left LGN input from right visual field (left half of each retina) • Right LGN input from right visual field (right half of each retina) 3 Lateral Geniculate Nucleus (LGN) • Part of Thalamus • 80% RGCs project to LGN – other 20% to Superior Colliculus 3 Layers of LGN -- Retinotopic organization • Adjacent areas in LGN receive input from adjacent areas of retina • Each layer -- complete map of hemiretina • Function: Plays a role in Attention – ~80% input areas other than retina (higher visual areas) – May help to control the flow of visual information • Unattended regions may be filtered 4 3 Receptive Field Properties 3 • Definition of receptive field? Parvo Layers – smaller r.f.’s – broadband & color opponent – slow - sustained response • R. F. properties of LGN neurons – circular – surround antagonism – size increases as we move to periphery – Parvo: .1-.2 º in fovea / 3-5º in periphery 3 LGN Role in Perception Parvo vs. Magno Layers of LGN: RF Properties Magno Layers 3 x larger r.f. broadband only rapid - transient 3 • Magno and Parvo Layers – Specialized for different types of visual information – Parvo: Form analysis and object recognition – Magno: Motion, location, and temporal analysis • Schiller & Logothetis – Chemically Lesioned Magno or Parvo layers of LGN in Macaque monkey – Tested perceptual skills • Singleton detection task – Make an eye movement to the stimulus that is “different” 5 3 3 Perceptual Tasks: Locate target based on: • • • • • • • • Task Parvo Lesion Color Severe Texture Severe Fine Shape Severe Coarse Shape Mild 3-D Severe Motion None Flicker None Magno Lesion None None None None None Moderate Severe 6