Visual System Sensory systems Sensory transduction Receptor potential

Visual System

Sensory systems • Sensory transduction • Receptor potential

Visual system anatomy - Eyeball • Light passes through: 1. Conjunctiva 2. Cornea 3. Pupil 4. Lens • Accommodation

Visual system anatomy - Retina • Retina • Rods (scotopic) • Cones (photopic) • Fovea • Optic disk

Visual system anatomy - Retina • Retinal layers 1. Photoreceptor layer 2. Bipolar layer – – Bipolar cells Horizontal and amacrine cells 3. Ganglion cell layer

Sensory transduction • Photoreceptors: • Contain molecules of photopigments • Steadily release glutamate in the dark • Light exposure causes the photopigments break apart, causing a hyperpolarization of the receptor membrane • What happens to glutamate release?

Sensory transduction • Effect of decreased photoreceptor Glu release on bipolar cells (light is on!) • On-center bipolar cells: • Less glutamate is depolarizing • Cells release more glutamate • Off-center bipolar cells: • Less glutamate is hyperpolarizing • Cells release less glutamate • Ganglion cells are always depolarized by glutamate • On-center ganglion cells • Off-center ganglion cells

Primary visual pathway 1. 2. 3. 4. 5. 6. Visual field Retina Optic nerve Optic chiasm Lateral geniculate nucleus Primary visual cortex • The PVC is also called “striate cortex” or V 1 Visual field

Visual processing – Striate cortex • Striate cortex contains a map of the contralateral visual field • Disproportionate representation

Striate cortex integrates information • Each retinal ganglion cell responds to its own receptive field Striate cortex neuron • Portion of the visual field • LGN neurons receive information from multiple retinal ganglion cells • Striate neurons receive information from multiple LGN neurons • Integration allows for visual perception of orientation, movement, depth and color Visual field LGN cell Retinal cells

Visual processing in the striate cortex – Orientation and movement • Orientation-sensitive neurons only respond to objects in a particular spatial placement • Simple cells – detect orientation • Complex cells – detect orientation and movement

Visual processing in the striate cortex – Depth • Binocular vision provides the most acute depthperception • Stereopsis • Retinal disparity • Binocular cells in the visual cortex respond strongly to retinal disparity

Visual association cortex • Striate cortex (V 1) organizes visual information. • Extrastriate cortex (visual association cortex) combines information from V 1 • Perception

Visual association cortex • Two streams of visual information project from V 1 to extrastriate cortex: • Dorsal stream • Movement • Ventral stream • Form • Color http: //www. colorado. edu/intphys/Class/IPHY 3730/image/figure 7 -3. jpg

Visual agnosias • Damage to the visual association cortex can result in a visual agnosia – the inability to perceive or identify a stimulus, despite normal visual sensation • Achromatopsia – damage to the extrastriate cortex in the medial occipital lobe may result in loss of color vision • Akinetopsia - damage to V 5 can result in the inability to perceive movement • Apperceptive visual agnosia – inability to perceive and identify common objects by sight • Prosopagnosia – inability to identify a familiar face