Transcript The Eye
The Eye A Model for Sensing Three components: Stimulus detection – a specialized sensory neuron Reception – where neurons receive information from the sensory neurons Integration – where information from receivers is processed All the human sensory systems have these components. The Visual System Retina Optic nerve (axons of ganglia in eye) Layers in the thalamus (LGN) Secondary paths to SCN (circadian rhythms) and superior colliculi to control eye movement Primary visual cortex Adaptation Adaptation -- diminishing receptiveness of a sensory receptor neuron. Habituation – also diminishing receptiveness but at a different level (within a neuronal circuit not a cell). Adaptation is essential for the perception of change. Dark and light adaptation permit vision under different environmental conditions. Parts of the Eye Cornea – protects eye and initiates focusing Lens – focuses light on the appropriate spot on the retina Iris – adjusts focus by opening and closing pupil to admit light Pupil – hole that admits light More Parts of the Eye Aqueous and vitreous humor – fluid inside eye Sclera – whites of eye Retina – layer of photoreceptors at the back of the eye, responsive to light Blind spot – place where optic nerve exits the eye Fovea – spot of best focus and densest cones Types of Photoreceptors Rods – used for brightness perception and motion Cones – used for color and form (shape) perception Cones are wavelength-specific: Blue = 430 nm Green = 530 nm Red = 560 nm Mixing all three equally produces white Transduction Photoreceptors release the neurotransmitter glutamate (glu) when depolarized. Only ganglion cells have action potentials. Depolarized in the dark. Hyperpolarized by light. Photoreceptors produce graded response that provides input aggregated by bipolar cells. Magno ganglion cells receive input from rods, parvo ganglion cells from cones Bipolar Cell Receptive Fields The receptive field is the area of the retina capable of changing the bipolar cell’s membrane potential Two kinds of receptive fields: OFF cell – excitatory ON cell – inhibitory OFF and ON refers to light, not the cell Center and surround are opposites Edge Detection The center-surround organization of the receptive fields of ganglion cells exaggerates the contrast at borders. Visual processes “fill in” what occurs between borders (edges). Contrast effects occur because we notice variations, not absolute magnitudes of light. Color Contrast Cones respond to specific wavelengths of light that determine hue. Color cells have complementary surrounds that heighten contrast and strengthen their signal. Opponents are: red/green, blue/yellow.