The visual system

1. The visual system Part I

2. In general, our visual system represents the world: Imperfectly Accurately Better than reality Distinction between: -- transduction -- coding

3. Light • Photons – discrete particles of energy • travel through space at 300,000 kilometers/sec (186,000 miles/sec) • Waves of electromagnetic energy • 380 to 760 nanometers in length

4. Electromagnetic spectrum nm = nanometer

5. What other animals see… Honeybees can see Ultraviolet light Rattlesnakes can see infrared light

6. Rats can see ultraviolet light

7. Properties of light and perception In general: • Wavelength – color (hue) perception • Intensity – brightness perception • Saturation – purity perception

8. Light enters the eye • through the pupil • size of the pupil is regulated by the iris • The lens focuses light on the retina Note: that the retinal image is upside down.

9. Pupil size • Adjusted in response to changes in illumination, which is a tradeoff between: • Sensitivity – ability to detect the presence of dimly lit objects • Acuity – ability to see the details of objects • When illumination is high, pupils are constricted allowing a greater depth of focus of the image falling on the retina • When illumination is low, pupils dilate in response to low activation of receptors allowing more light to enter the eye but sacrificing acuity and depth of focus

10. Ones to know Anatomy of the eye Ligament

11. Accomodation • Process of adjusting the configuration of the lens to bring images into focus on the retina • Focus on a near object • ciliary muscles contract • putting less tension on the ligaments • allowing the lens to take its natural cylindrical shape • thus increasing its ability to refract (bend) light • Focus on a distant object • Ciliary muscles relax • Increasing tension on the ligaments • flattens the lens • thus decreasing its ability to refract (bend) light

12. Binocular disparity • The difference in the positions of the same image on the two retinas • Is greater for close objects (eyes must converge or turn slightly inward) • The degree of binocular disparity enables the visual system to construct 3-D perception from two 2-D retinal images

13. The retina • Composed of 5 layers of neurons • Receptors (photoreceptors) • 1 rod • 3 cones • Horizontal cells (2 subtypes) • Bipolar cells (10 subtypes) • Amacrine cells (25-30 subtypes) • Ganglion cells (10-15 subtypes)

15. The cellular structure of the retina • Appears to be inside-out • Light passes through the 4 cell layers before reaching the receptors • After receptor activation, signals are transmitted back out to the ganglion cells whose axons project across inside surface of the retina, gathering at the optic disk where the optic nerve begins as the ganglion cell axons leave the eye.

16. Two visual problems result from the inside out arrangement: • Incoming light is distorted as it passes through the cell layers • There is a blind spot (no receptors or cells) at the optic disk where the axons gather to exit the eye

17. Solutions • The fovea is an area (0.33 cm diameter) in the center of the retina where there is a thinning of the retinal ganglion cell layer. • Less distortion of light • Specialized for high-acuity vision (seeing details) • Completion – the visual system uses information from receptors around the blind spot to fill in the gap in the retinal image.

18. photopic and scotopic vision The two systems are “wired” differently • Cones – low degree of convergence (a single ganglion cell receives signals from a few cones). • Rods – high degree of convergence (a single ganglion cell receives signals from hundreds of rods).

20. photopic and scotopic vision • Cones are concentrated in the fovea, which contains no rods. • Rods are concentrated 20 degrees from the fovea and in the nasal hemiretina (retina half of both eyes near the nose).

21. Spectral sensitivity curve • In general, more intense light appears brighter. However, wavelength also has an effect on the perception of brightness. A graph of the relative brightness of lights of the same intensity but at different wavelengths is called a spectral sensitivity curve (see Pinel p. 138).

22. Spectral sensitivity curves • There are two spectral sensitivity curves. • The photopic spectral sensitivity curve has a peak brightness at 555 nm (yellow-green) • The scotopic spectral sensitivity curve has a peak brightness at 507 nm (green-blue) • The Purkinje effect – walking through his garden, Purkinje noticed that his yellow and red flowers were brighter than the blues ones just before dusk; just a few minutes later the trend was reversed (blue flowers appeared as brighter greys).

23. Transduction - Conversion of one form of energy to another. • Visual transduction – conversion of light to neural signals. • Rhodopsin – the red pigment in rods becomes bleached when exposed to light. • It is a G-protein-linked receptor that responds to light.

25. Light activation of rods: Light bleaches rhodopsin molecules. cGMP is broken down, closing sodium channels Sodium ions cannot enter the rod resulting in hyperpolarization. Glutamate release is reduced Transduction of light by rods demonstrates that signals can be transmitted through neural systems by inhibition.

26. signal transduction • Light bleaches rhodopsin • Opsin separates from retinal molecule • Transducin (G-protein) activated • α-subunit breaks away and activates PDE6 • PDE breaks down cGMP • With cGMP broken, the Na+ channel closes

27. From retina to primary visual cortex • Pathway: retina  lateral geniculate nucleus (LGN)  primary visual cortex • ~90% of axons of retinal ganglion cells make up this pathway • LGN channels • Parvocellular (P layers) run through top 4 layers of LGN – responsive to color and fine detail (input from cones) • Magnocellular (M layers) run through bottom 2 layers of LGN – responsive to movement (input from rods) • Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV

28. Is retinotopic – each level is organized like a map of the retina Temporal hemiretina does not cross Nasal hemiretina crosses Right visual fields of Both eyes Left visual fields of Both eyes Bottom of visual field to dorsal cortex Top of visual field to ventral cortex Note that 25% of V1 is dedicated to fovea

29. Seeing Edges • A visual edge is “nothing” • Where two different areas of an image meet. • A perception of contrast between two adjacent areas of the visual field

30. Mach bands

31. Lateral inhibition of ommatidia receptors in lateral eye of horseshoe crab • A, B & C fire at the same rate (same high level of light stimulation and same degree of lateral inhibition) • D fires more (same high level of stimulation from light but less lateral inhibition from E) • F, G & H fire at the same rate (dim light) • E, fires less because of greater inhibition from D.

32. Saccades • The eye continually scans the visual field and makes a series of brief fixations (3/sec) connected by quick eye movements called saccades. • The fixations are integrated to produce greater color and detail than the restricted foveal region can produce if it remained stationary • stabilized retinal images, projected from a contact lens that moves with the eye; image disappears in a few seconds.

33. Brief fixations associated with saccades while a person views different pictures Making visual saccades to items of interest is a function of the superior colliculus

34. Visual fields Hemi-retinas Optic chiasm LGN P layers M layers Optic radiations Striate (primary visual) cortex Retina-geniculate-striate pathways

35. LGN