The Visual System

Vision: The Stimulus Light = electromagnetic radiation; travels as a wave Amplitude: height of waves; perception of brightness Wavelength: distance between peaks; perception of color * The visible spectrum is only a portion of the total range of wavelengths

Figure 4.5 Light, the physical stimulus for vision

Vision: The Stimulus *Visual input must be converted into neural impulses that are sent to the brain Purity: how varied the mix of wavelengths is Saturation: relative amount of whiteness in a color, or richness of colors

The Eye: Converting Light into Neural Impulses Two purposes of the eyes: Channeling light to neural tissue that receives it Housing the tissue

The Eye: Converting Light into Neural Impulses Pathway for Vision: Cornea: where light enters the eye Lens: focuses the light rays on the retina

The Lens: Converting Light into Neural Impulses Accommodation: the curvature of the lens adjusts to alter visual focus Problems with this lead to: Nearsightedness: close objects seen clearly; distant objects blurry Farsightedness: distant objects seen clearly; close objects appear blurry

The Eye: Converting Light into Neural Impulses Iris: colored ring of muscle, constricts or dilates via amount of light Pupil: regulates amount of light coming into the eye

The Retina: An Extension of the CNS Retina: absorbs light, processes images, sends visual info to the brain

Optic nerve – Bundle of neurons that carries visual information from the retina to the brain Optic Disk/Blind Spot – Point where the optic nerve exits the eye and where there are no photoreceptors

The retina is composed of specialized photoreceptor cells that convert light energy into nerve energy. Only about 10% of the light arriving at the cornea reaches these receptors Rods: black and white/low light vision; more sensitive in dim light – peripheral vision; greatest density just outside the fovea Cones: color and daylight vision; do not respond well in dim light

**rods outnumber cones by a huge margin **cones provide better visual acuity (sharpness/precise detail) Figure 4.9 The retina

Retina Review: Retina – Light-sensitive layer at the back of the eyeball Photoreceptors – Light-sensitive cells in the retina that convert light energy to neural impulses Rods – Sensitive to dim light but not colors Cones – Sensitive to colors but not dim light Fovea – Area of sharpest vision in the retina; densely packed cones

ADAPTATION Dark Adaptation: eyes become more sensitive to light in low illumination (enter dark theater on bright day) Complete in 30 minutes; major progress in first 10 minutes Light Adaptation: eyes become less sensitive to light in high illumination (leaving school to go to your car/bus)

Information Processing in the Retina Light strikes the photoreceptors and triggers neural impulses - - sent to optic nerve Bipolar Cells: combine info from photoreceptors; send results to ganglion cells Ganglion Cells: integrate info into single firing rate to optic nerve Horizontal Cells: connect receptors Amacrine Cells: connect bipolar to bipolar; ganglion to ganglion

Receptive field: area on the retina that, when stimulated, affects the firing of that cell Come in a variety of shapes & sizes Lateral antagonism: neural activity in a cell opposes activity in surrounding cells

Directions: Look at this grid – you will see dark spots at the intersections of the white bars, except in the intersection you’re staring at directly.

The Retina and the Brain: Visual Information Processing Light  rods and cones  neural signals  bipolar cells  ganglion cells  optic nerve  optic chiasm  opposite half brain Main pathway: lateral geniculate nucleus (thalamus)  primary visual cortex (occipital lobe) Magnocellular Channels: where Parvocellular Channels: what Second pathway: superior colliculus  thalamus  primary visual cortex

Figure 4.13 Visual pathways through the brain

Neural Pathways in the Human Visual System

Pathways to the Brain

Figure 4.15 The what and where pathways from the primary visual cortex

Hubel and Wiesel: Feature Detectors Early 1960’s: Microelectrode recording of axons in primary visual cortex of animals Discovered feature detectors: neurons that respond selectively to lines, edges, etc.

Simple Cells: respond most strongly to bars of light in their “favorite” orientation Complex Cells: respond most strongly to moving bars of light in their “favorite” orientation Hypercomplex Cells: respond most strongly to moving bars of light of a particular length or angle Later research: cells specific to faces in the temporal lobes of monkeys and humans

Intensity (amplitude) Basics of Color Vision Wavelength Intensity (amplitude) Color Brightness

Basics of Color Vision (cont.) Wavelength determines color Longer = red / shorter = violet Amplitude determines brightness Purity determines saturation

Hue: the qualitative experience of color of the light stimulus Saturation: purity/vividness of color sensations Brightness: intensity of light Figure 4.18 The color solid

Color: Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world Subtractive Color Mixing: remove wavelengths of light leaving less there Additive Color Mixing: superimposing lights, putting more light in the mixture than exists with one light by itself

Theories of Color Vision Trichromatic Theory - Young and Helmholtz Receptors for red, green, blue – color mixing Opponent Process Theory – Hering All color experiences arise from 3 systems, each of which includes 2 opponent elements red/green, blue/yellow, black/white Current perspective: both theories necessary

Complementary Colors Afterimages: Visual sensations that linger after the stimulus is removed; color of the image is the complement of the color you originally stared at

The Queen                                                       

Color Blindness the inability to distinguish colors affects more males than females most common: inability to distinguish red from green

Perceiving Forms, Patterns, and Objects Reversible figures: drawings that have two interpretations that can shift back and forth

Figure 4.31 A famous reversible figure

Perceiving Forms, Patterns, and Objects Perceptual sets motivational forces can foster perceptual sets Inattentional blindness - failure to see visible objects because attention is focused elsewhere

Perceiving Forms, Patterns, and Objects Feature detection theory - bottom-up processing Form perception - top-down processing

Figure 4.25 Feature analysis in form perception

Perceiving Forms, Patterns, and Objects Subjective contours – perceive contours where none actually exist Gestalt psychologists: the whole is more than the sum of its parts Reversible figures and perceptual sets demonstrate that the same visual stimulus can result in very different perceptions

Figure 4.27 Subjective contours

Principles of Perception Gestalt principles of form perception: Figure-ground Proximity Similarity Continuity Closure Simplicity

Principles of Perception Recent research: Distal (stimuli outside the body) vs. proximal (stimulus energies impinging on sensory receptors) stimuli Perceptual hypotheses Context

Depth and Distance Perception Binocular cues – clues from both eyes together retinal disparity convergence Monocular cues – clues from a single eye motion parallax accommodation pictorial depth cues

Stability in the Perceptual World: Perceptual Constancies Perceptual constancies – stable perceptions amid changing stimuli Size Shape Brightness Hue Location in space

Optical Illusions: The Power of Misleading Cues Optical Illusions - discrepancy between visual appearance and physical reality Cultural differences: perceptual hypotheses at work

The Ames Room

Muller-Lyer Illusion

Ponzo Illusion

Poggendorff Illusion

Upside-Down T Illusion

Zollner Illusion

Impossible Figures