Sensation and Perception. Sensation: What is it? The process by which a stimulus in the environment produces a neural impulse that the brain interprets

Sensation and Perception

Sensation: What is it?

• The process by which a stimulus in the environment produces a neural impulse that the brain interprets as a sound, image, odor, etc.

Transduction

• Transduction – Transformation of one form of energy into another – especially the transformation of stimulus information into nerve impulses

• Receptors –Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse

Transduction

• Sensory pathway – Bundles of neurons that carry information from the sense organs to the brain

Sensory Adaptation

• Sensory adaptation – Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while

Thresholds

• Absolute threshold – Amount of stimulation necessary for a stimulus to be detected

• Difference threshold – Smallest amount by which a stimulus can be changed and the difference be detected (also called just noticeable difference – JND)

Thresholds

• Weber’s law – The JND is always large when the stimulus intensity is high, and small when the stimulus intensity is low

The senses all operate in much the same way, but each extracts different information

and sends it to its own specialized processing region

in the brain

How Are the Senses Alike? How Are They Different?

Sense Stimulus Sense Organ Receptor Sensation

Vision Light waves Eye Rods and cones

Color, brightness, motion

Hearing Sound waves Ear Hair cells Pitch, loudness

Skin Senses

External Contact

Skin Nerve endings Touch, warmth, cold

Smell Volatile Substances

Nose Hair cells Odors

Taste Soluble Substances

Tongue Taste buds Flavors

Pain Intense or Extreme Stimuli

Pain Fibers all over body

Pain receptors Pain

Kinesthetic & Vestibular

Body position or balance

Semi-circular canals; joints, tendons

Hair cells; specialized neurons

Body position





Skin Senses

External Contact








Pain receptors Pain





Body position





Skin Senses

External Contact








Pain receptors Pain





Body position





Skin Senses

External Contact








Pain receptors Pain





Body position





Skin Senses

External Contact








Pain receptors Pain





Body position





Skin Senses

External Contact








Pain receptors Pain





Body position





Skin Senses

External Contact








Pain receptors Pain





Body position

The Anatomy of Visual Sensation

Fovea – Area of sharpest vision in the retina

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 dimlight but not colors

Cones – Sensitive tocolors but not dim light


• Optic nerve – Bundle of neurons that carries visual information from the retina to the brain

Blind spot – Point where the optic nerve exits the eye and where there are no photoreceptors

Transduction of Light in the Retina


• Visual cortex –Part of the brain – the occipital cortex – where visual sensations are processed

Neural Pathways in the Human Visual System

How the Visual System Creates Color

Color –

Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world

How the Visual System Creates Color

• Electromagnetic spectrum – Entire range of electromagnetic energy, including radio waves, X-rays, microwaves, and visible light

• Visible spectrum – Tiny part of the electromagnetic spectrum to which our eyes are sensitive

Two Ways of Sensing Color

• Trichromatic theory– Three different types of cones that sense different

parts of the visible spectrum (i.e., red, green, & blue)– Explains initial stages of color vision

• Opponent Process Theory– From bipolar cells onward, visual system processes

color in either-or, complementary fashion (i.e., red vs green or blue vs yellow)

– Sensations of one color (e.g., red) inhibits sensation of its complementary color (i.e., green)

– Explains negative afterimages & color blindness

Afterimages

• Afterimages – Sensations that linger after the stimulus is removed

• In the following slide, fix your eyes on the dot in the center of the flag

Color Blindness

Perception

Perception

• Same sensory input can give rise to very different perceptions

• Perceptual Set– Readiness to perceive stimuli in specific ways

• Reversible figures – drawing that is compatible with two different interpretations

Reversible Figures

Reversible Figure

Perception

• What is Perception?

– Active process in which we organize and interpret sensory information

• i.e., How we make sense of (or understand) what we see, hear, feel, taste, and smell

Object Perception

• Distal Stimulus – Stimuli that lie in the distance (i.e., in the

outside world)– Three-dimensional

• Proximal Stimulus– Stimulus that impinges directly onto your

sensory receptors (i.e., the retina)– Distorted and two-dimensional

Distal Stimulus

Proximal Stimulus

Images Projected on the Retina

Object Perception

• Feature Analysis – Analyze individual features and put them

together to form a whole

Bottom-Up Processing

• Start with the elements and progress to the whole

• Evidence– Hubel and Wiesel – cells in the cortex

operate as highly specialized feature detectors

Top-Down Processing

• Start with the whole and work towards the elements

– What we perceive is influenced by what we expect to see

• Context and prior experience are important

Object Perception• A number of Gestalt psychology principles can

help explain how we organize information in order to perceive a coherent whole

• Figure/Ground• Proximity• Similarity• Continuity• Common Fate• Closure• Goodness of Form or Pragnanz

Figure / Ground

Proximity

Similarity

Continuity

Common Fate

Closure

Goodness of Form or Pragnanz

=

Images Projected on the Retina

Proximal Stimulus

Depth Perception

• Binocular Depth Cues– Binocular disparity – Each eye has a slightly different

view of the world (i.e., the distal stimulus)• The brain thus has two different BUT overlapping images of

the world• The difference between these two retinal (i.e., proximal)

images is used to compute distances to nearby objects

• For example:– Object at 25 feet – image projected to slightly different

locations on the right and left retina– Closer objects project images on locations that are

further apart on the right and left retina

Depth Perception

• Monocular Depth Cues– Occlusion– Relative Size– Familiar Size– Linear Perspective– Texture Gradient– Position relative to horizon

• Motion cues for Depth– Motion Paralax– Optic Flow

Proximal Stimulus

Distal Stimulus

Size Perception

• The size of the retinal image depends on the distance of the object from the observer

– Further away = smaller retinal image

– Thus, to determine size, visual system must know how far away the object is

Linear Perspective

B

A

Which box is bigger, A or B?

Moon Illusion

Final Thoughts

• Visual system is exceptionally good at analyzing the outside world– i.e., perception is a faithful representation of

the distal stimulus

• Understanding how the visual system is tricked (i.e., by studying common illusions) helps us to understand how it works

Documents

Sensation and Perception. Sensation: What is it? The process by which a stimulus in the environment produces a neural impulse that the brain interprets