Sensation and Perception. Psychophysics zthe study of how physical stimuli are translated into...

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Sensation and Perception

Psychophysics

Vision Audition Touch Chemical Senses KinestheticSystem

VestibularSystem

Our Senses Attention

Psychophysics

the study of how physical stimuli are translated into psychological experience

Sensation vs PerceptionOliver Sak’s “Dr. P”

Psychophysics

Sensation the stimulation of sense organs

Perception the selection, organization and interpretation

of sensory input“Dr. P”

visual sensation was intact visual perception was severely impaired “visual agnosia”

Basic Concepts of Psychophysics

stimulus any detectable input from the environment

threshold dividing point between energy levels that do

and do not have a detectable effect absolute threshold

minimum amount of stimulation that an organism can detect

defined as the level where the stimulus intensity is detected 50% of the time

JND (Just Noticeable Difference)smallest difference in amount of stimulation

that a specific sense can detectWeber’s Law

• the size of a JND is in proportion to the size of the initial stimulus

• as stimuli increase in magnitude, the JND becomes larger

Fechner’s Law• constant increments in stimulus intensity produce

smaller and smaller increases in the perceived magnitude of sensation

Subliminal Perception

registration of sensory input without conscious awareness “Eat popcorn” rock music “backward masking” self-help tapes priming

altering participants’ attitude towards target person depending on type of subliminal photos seen

visual priming/semantic priming

Signal Detection Theorydetection of a stimulus involves sensory processes

but also emotional and cognitive processes“noise” from irrelevant stimuli in the environment

Sensory Adaptationgradual decline in sensitivity to prolonged

stimulationautomatic, built-in process designed to keep people

tuned in to changes rather than constants in their sensory input

• Shouldn’t need constant confirmation that your clothes are still on!

W. W. Norton

Psychophysics

VestibularSystem

Vision: The Stimulus

Light electromagnetic waves amplitude brightness wavelength colour purity “richness” or “saturation”

incoming visual input must be converted into neural impulses

Nature of the stimulus:

-one small band of electromagnetic radiation.

W. W. Norton

Optic Disk: point where the ganglion cell axons converge and exit the retina.

-creates a blind spot in the visual field.

Retina

-contains: Photoreceptor cellsInterneurons.Retinal ganglion cells.

1. Outer Nuclear Layer:

-contains the photoreceptor cells.

}Outer Nuclear Layer

2. Inner Nuclear Layer:

-contains the interneurons:bipolar, amacrine & horizontal cells.

-transmit signals from the photoreceptor cells toganglion cells.

}Inner Nuclear Layer

3. Ganglion Cell Layer:-contains ganglion cells.

-output cells of the retina.

-axons form the optic nerve.

}Inner Nuclear Layer

}Ganglion Cell Layer

Photoreceptor CellsTwo types:

RodsCones

Rods:-very sensitive to light.

-used in dim light.

-dense in periphery of retina

Fovea:

-center of the retina.

-densely packed with cones.

Cone vs Rod Vision

Perception of an item and the perception of color are separate.

A volunteer please!

Trichromatic Theory of Color Vision-three types.

-each sensitive to a particular wavelength of light

-red, blue, green

Trichromatic Theory of Color Vision

Opponent Process Theory of Color Vision

Ganglion cells (and neurons) depend on receptors that respond best to pairs of colours.

Yellow onBlue off

Blue onYellow off

Red onGreen off

Green onRed off

Theory of Color Vision

Cones appear to follow trichromatic theory

Lateral Geniculate Nucleus and Visual Cortex appear to follow opponent process theory

so … takes both theories to explain color vision

Colour Blindness

2 - 8% of males..03% of females.

due to a loss or mutation of pigment gene.Red/Green is most common.

First Stage of Visual Processing

Information Processing in the Retina

Info from about 130 million rods and cones converge upon 1 million axons in the optic nerve

bipolar and ganglion cells in the intermediate layers integrate and compress signals from many receptors.

Axons from the ganglion cells form the optic nerve

Information Processing in the Cortex

Feature Detectors (Hubel & Wiesel) neurons respond selectively to very specific

features of more complex stimulisimple cells

respond best to line of correct width, oriented at the correct angle and located in correct position in receptive field

complex cells respond best to specific width and orientation but

any position in receptive field

Parallel processing simultaneously extracting different

kinds of info from same input Parvocellular channel Magnocellular channel

Parallel processing Parvocellular channel

process details about the objects“what” channelhandles perception of color, form and texture__________ lobeprosopagnosia

Magnocellular channelprocesses “where” the ojbects areperception of motion and depth__________ lobe

Review

1. Physical Shapeelongated stubby

2. number 125 million 6.4 million

3. area of retinawhere dominant

Periphery Center/fovea

4. critical to colorvision?

No Yes

5. critical toperipheralvision?

Yes No

6. sensitivity todim light

Strong Weak

Dimension Rods Cones

CENTRAL VISUAL PATHWAYS

Optic nerve: fibres from retina to the optic chiasm.

Optic chiasm: optic nerves from both eyesconverge and partially cross over.

Optic tract: optic fibres beyond the chiasm.

W. W. Norton

Visual Perception

The same visual input can result in radically different perceptions.

W. W. Norton

Perceptual Set

How we perceive the stimulus depends on our expectations.

Perceptual Theories

Bottom-up processingTop-down processing

Bottom-up Processing

Feature Analysis process of detecting specific elements

in visual input and assembling them into a more complex form

progression from individual elements to the whole

supported by work of Hubel and Wiesel

Top-Down Processing

Gestalt psychologyStep 1: formulate perceptual

hypothesis about the nature of the stimulus as a whole

Step 2: select and examine features to check hypothesis

Step 3: recognize stimulus

Top-Down Processing

Whole can be greater than the sum of its parts visual illusions take advantage of this most common visual illusion is

televisionsee smooth motion but actually objects just

moving from slightly different positions in successive frames

Gestalt Principles of Top-Down Processing

figure-ground divide visual displays into figure and

ground

figure-groundproximity

things that are near one another seem to belong together

similaritycontinuity

tend to follow in whatever direction they’ve been lead

figure-groundproximitysimilaritycontinuitysimplicityclosure

you may “complete” figures that actually have gaps in them

Visual Perception

Need both top-down bottom-up

Perception of Depth

Monocular depth perceptionBinocular depth perception

Monocular Depth Perception

Van Gogh

The Garden of St. Paul’s Hospital

Occlusion (near objects block distant ones)

Relative SizeFamiliar SizeLinear Perspective

W. W. Norton

Occlusion (near objects block distant ones)

Relative SizeFamiliar SizeLinear PerspectiveTexture GradientPosition Relative to Horizon

Binocular Depth Perception

Clues about the distance based on the differing views of the two eyes “retinal disparity”

convergence sensing the eyes converging towards each

other as they focus on closer objects

Perceptual Constancy

Tendency to experience a stable perception in the face of continually changing sensory input

table looks like a table no matter what angle

door looks like a door even if partly openfriend does not shrink before your eyes if

walking away

Psychophysics

VestibularSystem

The Stimulus: Sound

-waves of changing air pressure-generated by vibrating objects

Frequency: Pitch.

Frequency = cycles per second (hertz)

20 Hz up to 20,000 Hz

Amplitude: loudness (decibels)

Sensory Processing in the Ear

External ear - vibration of air moleculesMiddle ear - vibration of moveable bonesInner ear - waves in a fluid

converted to stream of neural signals sent to brain

system designed to convert relatively large movements with little force into smaller motions with greater force

. No r

Inner Ear:

Cochlea:

-circular structure filled with fluid.

-regions of the basilar membrane preferentiallyvibrate in response to sound waves of specific frequencies

100 m 500 m

Low FreqHigh Freq

18,000 Hz

-connected to bipolar ganglion cells of the spiralganglion (auditory nerve).

Theories of Auditory Perception

Place theory perception of pitch/frequency

corresponds to vibration of different portions, or places along basilar membrane

Place theoryFrequency Theory

frequency/pitch corresponds to the rate, or frequency, at which the entire basilar membrane vibrates

Place theoryFrequency Theory both are valid in part

sounds under 1000 Hz - frequency theory (entire membrane vibrates)

sounds between 1000 and 5000 Hz - combination

sounds above 5000 Hz - place coding only

Auditory Pathway

1. Cochlear Nuclei:

3. Inferior Colliculus (IC)

4. Medial Geniculate Nucleus (MGN)

5. Auditory Cortex

Auditory Localization

Intensity loss of sound intensity with distance “shadow” effect of head

Timing sound takes longer to reach ear that is

farther away

Primary Auditory Cortex (A1):

-lies along the transverse temporal gyrus.

Tonotopically organized:

-high frequency sounds caudal.-low frequency sounds rostral.

100 m 500 m

Organization is not fixed:

Trained to recognize 4K sound

Association Areas:Planum temporale.

-contains Wernicke’s area.-speech comprehension.

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