Construction of the Visual Image Anne L. van de Ven 8 Sept 2003 BioE 492/592 Sensory Neuroengineering Lecture 3
Visual Perception Light Photoreceptors Interneurons Visual Processing Ganglion Neurons Optic Nerve Visual Cortex Visual Processing
Outline Anatomy of the eye Structure & function of photoreceptors Mechanism of photo-transduction Visual processing in the retina Photoreceptors Interneurons Ganglion neurons Global view
Image Acquisition Cornea Lens Optic nerve Retina Cones Rods
Anatomy of the Retina Light Ganglion Neuron Amacrine Cell Bipolar Cell Horizontal Cell Rod Cone Pigment Epithelium
Photoreceptor Structure & Function Rods detect dim light Cones mediate color vision Cones provide better spatial and temporal resolution
Light is absorbed by visual pigments in the photoreceptors Opsin Retinal Rhodopsin
Folding of the plasma membrane creates a high surface area 10 8 rhodopsin molecules / cell
Phototransduction Mechanism 11-cis rhodopsin Light hyperpolarization of photoreceptor all-trans rhodopsin closure of cgmp-gated Na + channels all-trans retinal opsin active G-proteins phosphodiesterase GMP Why an inhibitory reaction? - SPEED!! cgmp
Graded changes in receptor potential are conveyed to ganglion cells Light Ganglion neuron Interneurons Rod Cone Photoreceptors continuously release glutamate Hyperpolarization reduces glutamate release Glutamate levels influence the potential of interneurons Interneurons modulate ganglion neuron activity
Processing of the visual image starts in the retina Photoreceptors are specialized filters Interneurons combine signals from multiple photoreceptors Different classes of ganglion cells carry different information
Photoreceptors are specialized filters RODS CONES Achromatic Low threshold, high sensitivity High signal amplification Slow signal integration (~100ms) Chromatic Low sensitivity Low signal amplification Fast signal integration (~10ms)
Color Filtering Rods and cones have unique opsins absorption of different light wavelengths
Spectral Sensitivity Rods are sensitive to changes in light intensity Cone sensitivity is highly dependent on: - Light wavelength - Light intensity
Processing of the visual image starts in the retina Photoreceptors are specialized filters Interneurons combine signals from multiple photoreceptors Different classes of ganglion cells carry different information
Visual Acuity The convergence of multiple photoreceptors reduces visual acuity FOVEA > 1:1 ratio of cones to ganglion neurons > receptive field ~2um PERIPHERY > 100:1 ratio of rods to ganglion neurons > receptive field ~1mm = 500x lower resolution
Visual Acuity The fovea centralis has the sharpest vision CONES Blind Spot Highest density of cones RODS Less neural layers High (~1:1) ratio of cones to ganglion cells
Processing of the visual image starts in the retina Photoreceptors are specialized filters Interneurons combine signals from multiple photoreceptors Different classes of ganglion cells carry different information
Ganglion Cell Specialization Cell behavior is dependent on the location of a stimulus within the receptive field Different ganglion cells monitor different parts of the receptive field 2 classes: On-center Off-center Center - - - + + - + - - Surround RECEPTIVE FIELD
ON-CENTER NEURON OFF-CENTER NEURON - - On area - + - + + - - + + + On area - - + + - + Central light LIGHT LIGHT Peripheral light Diffuse light
Ganglion Cell Specialization Cells respond best when light intensities in the center and surround are different Specialized to detect CONTRAST
Contrast information is carried in parallel pathways Cone Hyperpolarize glutamate Off-center bipolar cell Hyperpolarize Depolarize On-center bipolar cell Off-center ganglion cell On-center ganglion cell
The Overall View Visual processing starts in the retina Selective filtering Signal amplification Parallel channels of information Processing is facilitated by the differences between rods and cones More complex data can be encoded with the addition of more neural circuitry
Next Time Light Photoreceptors Interneurons Ganglion Neurons Optic Nerve Visual Processing Color, Motion, and Form Visual Cortex Visual Processing
References Essentials of Neuroscience and Behavior. Kandall ER, Schwartz JH, Jessel TM. Prentice Hall 1995: 367-488. Sensation and Perception, 6 th ED. Goldstein BE. Wadsworth 2003: 109-225. http://www.webvision.med.utah.edu/ http://www.cis.rit.edu/people/faculty/montag/vandplite/pages http://www.psychology.psych.ndsu.nodak.edu/mccourt/websit e/htdocs/homepage/psy460/color%20vision/color%20vision.html