Lecture 9: Sensation and Perception

Transduction

conversion of external energies or substances into a nervous system signal (inhibition or excitation)

Sense Receptors

transduce specific stimuli

Sensory adaptation

activation is highest when the stimulus is first detected, then sensory adaptation occurs

Perception

the process of organizing and interpreting sensory information

Psychophysics

the study of how we perceive sensory stimuli based on their physical characteristics

Absolute threshold

smallest stimulus energy needed for the nervous system to detect [50% of the time]

Gustav Ferchner

Suggests that human error increases as stimuli get weaker

Just noticeable difference

smallest change in intensity of a stimulus that we can detect 50% of the time

Weber’s law

the stronger the stimulus, the greater the change needed to detect

Signal detection theory

provides a way to detect and account for subjects’ biases

parallel processing

can attend to many senses at once

• Bottom-up versus top-down processing (co-exist)

Bottom-up processing

sensory receptors register information about the external environment and send it up to the brain for interpretation

• takes information and tries to make sense of it

Top-down processing

starts with cognitive processing in the brain

• begins with some sense of what is happening and applies that framework to incoming information from the world

Perceptual sets

The relationship between a stimulus and its context

Perceptual constancy

size, colour, shape are consistent across conditions

Selective attention

process of focusing on on sensory channel and ignoring others

Filter theory of attention

theory that posits we can pay attention to important information while filtering out the rest

Dichotic listening task

Broadbent used this task to demonstrate his filter theory

Cocktail party effect

can pick out an important message in a conversation that does not involve us

Binding problem

how our brains take multiple pieces of information and combine them to represent something concrete

How light enters the eye

• sclera

• iris

Sclera

The white portion of the eye

Iris

opening that modifies the amount of light permitted through the pupil

Focusing light

• Cornea

• lens

Cornea

refracts light to focus it on back of eye

Lens

changes curvature (accommodation) to refract light onto back of eye

Changing light into neural activity

• Retina

• Fovea

• Receptor cells (rods/cones)

• Ganglion Cells

Retina

membrane at back of eye

Receptor cells

Contain photopigments that change on exposure to light

• Rods: low levels of light

• Cones: high acuity, colour vision

Ganglion cells

Axons leave the retina (at the blind spot), forming the optic nerve

Optic Nerve

propagates visual signals to the visual areas in the brain

Perception and the Visual Cortex

Visual Perception: Shape and Contour

Different cortical cells respond maximally to different types of stimuli

Detecting lines and edges

Simple cells and complex cells

Simple cells

Orientation-specific slits of light in a particular location

Complex cells

orientation-specific but less dependent on loaction

Feature detection

using minimal patterns to identify objects

Hierarchical model of processing

feature detector cells respond to increasing shape complexity with higher levels of cortical processing, from lines and edges to complex shapes and moving objects

Subjective contours

the brain fills in

Gestalt principles

the perception of objects as wholes within a context, not isolated lines and curves

We perceive objects as wholes when they display:

• Proximity

• Similarity

• Closure

• Symmetry

• Figure-ground

• Continuation

• Pragnanz

Proximity

close together

Similarity

are similar

Closure

have missing contours

Symmetry

having symmetrical arrangement

Figure-ground

a central figure

Continuation

lines are seen as following the smoothest path

Pragnanz

reality reduced to the simplest form possible

Face recognition

• extract key features, fill in from context and memory

• cells in the lower temporal lobe fire in response to particular faces

Motion detection

• brain compares visual frames

• from there, it “estimates” what is moving

Colour

Theories of colour perception:

• Trichromatic theory

• Opponent process theory

Trichromatic theory

colour vision is based on our sensitivity to the three primary colours (red, blue, green)

Opponent process theory

colour vision is a function of complementary, opposing colours: red versus green or blue versus yellow

3D Relations

Monocular depth cues versus binocular depth cues

Monocular Depth Cues

• Relative size

• Texture gradient

• Interposition

• Linear perspective

• Height in plane

• Light and shadow

• Motion parallax

Binocular Depth Cues

• binocular disparity

• binocular convergence

Present early in development

Infants between 6 and 14 months hesitate to crawl over the visual cliff used by Eleanor Gibson

Moon Illusion

the moon appears larger when it is near the horizon than when it’s high in the sky

The Ames Room

Trapezoidal room with slanted floor and ceiling, makes occupants appear to vary in height (exploits relative size principle)

Change Blindness

difficulty detecting obvious scene changes when eyes are moving, lights are flickering, or when watching a video

Synesthesia

a condition in which people experience cross-modal sensations