Vision. The Eye External View. The Eye in Cross-Section

Transcription

1 Vision The Eye External View cornea pupil iris The Eye in Cross-Section Light enters via cornea, Is focused by cornea and lens, Forming image on retina, Which contains photoreceptors. 1

2 The Retina Photoreceptors (innermost layer) Bipolar cells Ganglion cells (outermost layer) Axons form optic nerve. Two other layers of nerve cells. Central Versus Peripheral Vision Central area of vision located in pit-like area called the fovea. Vision here relatively sharp. Remaining area is the periphery. Vision here is relatively fuzzy. Foveal vision is sharper because A thinner layer of cells cover receptors. One axon to brain per 10 photoreceptors in fovea; one per 100 photoreceptors in periphery In the next slide, focus on one word near the center and try to read other words a few lines away without moving your eyes. Bet you can t! 2

3 Test of Foveal Versus Peripheral Vision The Visual Pathways of the Brain Optic Nerve Optic Chiasm Optic Tract Thalamus (Lateral Geniculate Nucleus) Pathways to Occipital Lobes of Cortex Visual Areas of the Cortex Theories of Color Vision Trichromatic Theory (Helmholtz) Three types of color receptor responding to three different wavelengths of light. Perceived color results from the ratios of stimulation of the three receptors. Opponent Process Theory (Hering) Activities of Red-Green, Blue-Yellow, and Black-White opponent pairs determine perceived color. 3

4 Frequency Responses of Rods and Cones Both Theories of Color Vision Are Partly Correct! As the trichromatic theory asserts, there are three types of cone receptors in the retina that respond to different wavelengths of light. But cells functioning as opponents can be found among the retinal neurons and in the thalamus and cortex, as expected by the opponent process theory. Evidence for Opponent Colors Comes from studies of color-blindness and from the observation of color afterimages. Next slide demonstrates color afterimages. Stare at the + in the center of the image until the image goes away. Be sure to keep your gaze fixed on the + at all times. When the slide changes to white, pay attention to what you see. 4

5 Explanation for Negative Afterimage While you stared at the oddly-colored American flag, the opponent cells being stimulated by those colors were becoming fatigued. When the flag was replaced by the white screen, the fatigued receptors were not able to respond as well to the white light as their opponents, so instead of perceiving white, you perceived the opponent colors. 5

6 Seeing Colors that Aren t There Color TV tubes have clusters of three phosphors that glow only in red, green, and blue (respectively) when struck by the electron beam. Yet we can see many more colors (e.g., yellow) on the screen. The yellow isn t there, but the brain sees yellow when the red and green glow about equally and the blue, little or none. Seeing Yellow Real Yellow stimulates R and G about equally, B little or none. Shining both red and green light simultaneously does the same. In both cases, you perceive yellow. Perceive Yellow Perceive Yellow Feature Detection Frog Research Jerome Letvin presented several types of visual stimulus to a frog while recording impulses from individual ganglion cells, with results like this: 6

7 What the Frog s Eye Tells the Frog s Brain Tests with frogs revealed five types of feature detectors in the retina of the frog s eye: On detectors Off detectors On-off detectors Line or edge detectors Bug detectors The Human Visual System Retina does relatively low-level processing: contrast enhancement (lateral inhibition) early color processing Extraction of features takes place mainly in cerebral cortex. Occipital lobes: primary visual area shapes, spatial layout, depth. Projection areas: color, motion etc. The Homunculus The homunculus : your conscious self seems like a little person in the head, watching the Cartesian Theater of the mind. But visual system doesn t project an image of the outside world into the head. it analyzes the visual input for specific features and constructs a visual world based on those features and prior experience. no little person passively watching the show Instead, a collection of brain processes actively analyzing the sense data and constructing your visual reality moment by moment. 7

8 My Homunculus... Me 8