Chapter 4: Sensation and Perception Psychophysics
• Ian Howard - was known as a pioneer in sensation/perception research, particularly in
the area of binocular vision and human spatial orientation.
• Three types of cues to determine which way is up: visual, gravity, and body direction.
• Sensation - the stimulation of sense organs.
o Involves the absorption of energy (light, sound wave) by sensory organs (ears,
• Perception - the selection, organization, and interpretation of sensory input.
o Involves organizing and translating sensory input into something meaningful.
Psychophysics: Basic Concepts and Issues
• Psychophysics - The study of how physical stimuli are translated into psychological
• Gustav Fechner - Important contributor to psychophysics.
Thresholds: Looking for Limits
• Sensation begins with a stimulus - any detectable input from the environment.
• Detectability depends on who or what is doing the detecting.
o i.e., One may not be able to detect a weak odour in a dog that already has a
• Threshold - a dividing point between energy levels that do and do not have a detectable
• Absolute Threshold - lowest amount of stimulation one can detect.
o Define the boundaries of an organism's sensory capabilities.
Sense Absolute Threshold
• Vision • A candle flame seen at 50km on a
dark clear night.
• Hearing • The tick of a clock 20 feet away.
• 5mL of sugar in 7.5L of water.
• Smell • A drop of perfume in a six-room
• A wing of a fly on your cheek, dropped
• As stimulus intensity increases, subjects' probability of responding to stimuli gradually
Weighing the Difference: The JND
• A Just Noticeable Difference (JND) - the smallest difference in the amount of
stimulation that a specific sense can detect.
o Vary by sense and detection depends on original stimulus. o Demonstrated by Ernest Weber, thus called Weber's Law
• Weber's Law - states the size of a just noticeable difference is a contrast proportion of
the size of the initial stimulus.
o i.e., One can detect the difference between a 300-gram weight and a 310-gram
weight. However, they won't be able to detect the difference between a 900-gram
weight and a 910-gram weight.
• As stimuli increases in magnitude, the JND becomes larger.
• Fechner's Law - states the magnitude of a sensory experience is proportional to the
number of JNDs that the stimulus causing the experience is above the absolute
• Constant increments in stimulus intensity produce smaller and smaller increases in the
perceived magnitude of sensation.
o i.e., one is in a dark room with a lamp consisting of three bulbs of same watts.
Turns the switch and one bulb lights. There would now be a difference in the
lighting after being in complete darkness. Turns the switch again, and second
bulb lights. The light is doubled, but it's not twice as bright. Turns third bulb on,
difference is barely noticeable. Thus, three equal increases in stimulus intensity
(the amount of light) produce smaller difference in the magnitude of sensation
• Perception can't be measured on absolute scales.
• Signal-Detection Theory - proposes that the detection of stimuli involves decision
processes as well as sensory processes which are both influenced by a variety of factors
besides stimulus intensity.
o How a subject responds to a stimulus condition
Refer to figure 4.3 on page 140.
Perception Without Awareness
• Subliminal Perception - the registration of sensory input without conscious awareness.
o Below threshold
• Research has shown that perception without awareness can take place.
• Sensory Adaptation - gradual decline in sensitivity due to prolonged stimulation.
o i.e., If garbage in the kitchen has a really bad odour, and one stays in the kitchen
without changing the garbage, the smell would soon start to fade. The smell is
still present, but with continued exposure, your sensitivity to it decreases.
• Built-in process that keeps people tuned in to the changes rather than the constants in
their sensory input.
Review of Key Points: • Psychophysicists use a variety of methods to relate sensory inputs to subjective
perception. They have found that absolute threshold are not really absolute.
• Weber's Law states that the size of a Just Noticeable Difference is a constant
proportion of the size of the initial stimulus. Fechner's Law asserts that larger and
larger increases in stimulus intensity are required to produce Just Noticeable
Differences in the magnitude of sensation.
• According to signal-detection theory, the detection of sensory inputs is influenced by
noise in the system and by decision-making strategies. Signal-detection theories
replaces Fechner's sharp threshold with the concept of detectability and emphasizes
that factors besides stimulus intensity influence detectability.
• In recent years, a host of researchers, using very different conceptual approaches,
have demonstrated that perception can occur without awareness. However, research
indicates that the affects of subliminal perception are relatively weak and of little or no
• Prolonged stimulation may lead to sensory adaptation, which involves a reduction in
sensitivity to constant stimulation.
Our Sense of Sight: The Visual System
• Sight is an essential sense for the humans.
The Stimulus: Light
• Light is needed in order to see.
• Light waves vary in:
o amplitude (height) - affects perception of brightness
o wave length (the distance between peaks) - affects perception of colour.
• The lights seen by humans are mixtures of several wave lengths, thus light can also vary
in its purity (how varied the mix is).
• Purity - influences perception of saturation or richness of colours.
The Eye: A Living Optical Instrument
• The eye serves two purposes: they channel light to the neural tissue that receives it,
called the retina, and they house that tissue.
• Cornea - transparent "window" -> light enters through here. The cornea and the
crystalline lens then forms an upside-down image of objects on the retina.
• Lens - the transparent eye structure that focusses the light rays falling on the retina.
o Made up of soft tissues.
o Capable of adjustments that facilitate a process called accommodation.
• Accommodations occur when the curvature of the lens adjusts to alter visual focus.
o When you focus on a close object, lens of the eyes gets rounder to give clear
image. o When you focus on distant objects, the lens flatten out to give you a better image
of the object.
• Visual deficiencies -> caused by focusing problems for defects in lens.
• Nearsightedness - close objects are seen clearly but distant objects appear blurry.
o Focus of images in front of retina instead of on the retina, resulting in blurred
o Occurs when the eyeball becomes too long and prevents incoming light from
focusing directly on the retina.
• Farsightedness - distant objects are seen clearly but close objects appear blurry.
o Light entering the eye is focused behind the retina instead of directly on it.
o Occurs when eyeball is too short.
• Iris -> surrounds the pupil.
• Pupil - the opening in the centre of the iris that helps regulate the amount of light
passing into the rear chamber of the eye.
o Black centre of the eye.
o When pupil constricts, it lets less light into the eye, sharpening the image falling
on the retina. When it dilates (open), it lets more light in, but image is less sharp.
• The pupils usually constrict (get smaller) when you shine light into it and they dilate (get
bigger) when it is dark or when light is dim.
• Saccades - eye movements.
o Essential to good vision.
o Small reduction will degrade vision.
The Retina: The Brain's Envoy in the Eye
• Retina - the neural tissue lining the inside back surface of the eye; it absorbs light,
processes images, and send visual information to the brain.
o Piece of central nervous system that is located in the eyeball.
o Contains specialized cells arranged in layers.
• Optic Disk - a whole in the retina where the optic nerve fibres exit the eye.
o Hole in the retina.
o Known as a blind spot because you cannot see part of the image that falls on it.
Visual Receptors: Rods and Cones
• Retina contains many cells that are sensitive to light. So light must pass through several
layers of cells before it gets to the receptor that can detect it.
o Located in the innermost layer of the retina.
• Retina contains two types of receptors: rods (100-125 million) and cones (5-6.4 million).
• Cones - specialized visual receptors that play a key role in daylight vision and colour
o Handles daytime vision.
o Do not respond well to dim light. o Provides better visual acuity -> sharpness and precise detail.
• Fovea - a tiny spot in the centre of the retina that contains only cones; visual acuity is
greatest at the spot.
• Rods - specialized visual receptors that play a key role in night vision and peripheral
o Peripheral vision, or side vision, is that part of vision that detects objects outside
the direct line of vision. For instance, when you read a word on a page, you are
using your central vision, but it’s your side vision that tells you if the word is at the
beginning or end of a sentence, or at the top or bottom of a page.
Dark and Light Adaptation
• Dark Adaptation - the process in which the eyes become more sensitive to light in low
o i.e. Going from a very bright area to a dim one. Being outside on a bright sunny
day just after a fresh snow fall. Then you walk indoors. It will be difficult to see for
several minutes just after you went indoors.
o Takes about 30 minutes to complete.
• Light Adaptation - the process whereby the eyes become less sensitive to light in high
o i.e. You spent the afternoon in the movies and when you left and the sun was still
shining, your eyes would've almost hurt when you got outdoors. Or perhaps, on
a cold winter day you go from indoors where the lighting is moderate and you
could see very well to the outdoors. Especially, at high noon when there is lots of
fresh snow on the ground you find that it is difficult to see for some seconds,
perhaps as long as a minute.
o Happens very quickly, usually less than a minute.
Information Processing in the Retina
• Optic nerve - a collection of axons that connect the eyes with the brain.
o Departs from the eye through optic disk, carrying visual information to the brain.
• Receptive Field of a Visual Cell - the retinal area that, when stimulated, affects the
firing of that cell.
o Receptive fields that are circular with a centre-surround arrangement
Light falling in the centre has the opposite effect of light falling in the
• When receptive fields are stimulated, retinal cells send signals both toward the brain and
laterally (sideways) toward nearby visual cells.
• Laterally -> later signals allow visual cells in retina to have interactive effects on each
• Lateral Antagonism (lateral inhibitation) - occurs when neural activity in a cell
opposes activity in surrounding cells.
o Basic of the interactive effects.
o Responsible for the opposite effects on centre-surround receptive fields. o helps to explain a number of "optical illusions" and more importantly, provides an
excellent example of how the brain is organized to actively "make sense" of the
information it gets, rather than to simply absorb and respond to it. In doing so, it
provides some valuable insights into the sources of our sense of "reality".
Vision and the Brain
Visual Pathways to the Brain
• Optic Chiasm - the points at which the optic nerve from the inside half of each eye
cross over and then project the opposite half of the brain.
o Ensures that signals from both eyes go to both hemispheres of the brain.
• Process of how information is sent to the brain: check on page 149.
• Two pathways in which the optic nerve fibres diverge along:
1. lateral geniculate nucleus (LGN) - the primary relay center for visual information
received from the retina of the eye. The LGN is found inside the thalamus of the
2. superior colliculus - an essential visual center between the retina and the striate
• Main visual pathway is subdivided into two specialized pathways, which are based on
the layers of the LGN they synapse in:
• Parallel Processing - involves simultaneously extracting different kinds of information
from the same input.
o Magnocellular and parvocellular engage in this.
Information Processing in the Visual Cortex
• Visual cortex -> located in occipital lobe.
• David Hubel and Torsten Wiesel - found a solution to how the cortical cells in the primary
visual cortex respond to light.
• Individual cells in primal visual cortex -> sensitive to lines, edges.
• Hubel and Wiesel identified various types of specialized cells in primary visual cortex
that respond to different stimuli.
o i.e., simple cells -> respond best to what?
o Complex cells -> respond best to what?
• Cells in the visual cortex are highly specialized and are characterized as featured
• Feature Detectors - neurons that respond selectively to very specific features of more
• After visual input is processed in the primary visual cortex, it is often sent to other
cortical areas for additional processing. These signals travel through two streams:
1. The ventral stream -> processes the details of what objects are out there
(example, perception of form and colour). 2. Dorsal stream -> processes where the objects are (example, perception of
motion and depth).
• Visual Agnosia - an inability to recognize objects.
o Occurs although their eyes functions perfectly.
o Probably due to damage somewhere along the pathway that handles object
• Prosopagnosia - an inability to recognize familiar faces.
• Neurons that usually serve as face detectors were "retuned" to be responsive to other
Multiple Methods in Vision Research
• McCollough effect - a form-contingent color after-effect.
o Effect is meditated by the area of the brain known as VI or the primary visual
cortex and this affect does not depend on conscious form perception.
Review of Key Points:
• Light varies in terms of wave length, amplitude, and purity. Light enter the eye through
the cornea and pupil and is focused upside down on the retina by the lens. Distance
objects appear blurry to nearsighted people and close objects appear blurry to
• The retinal is the neural tissue in the eye that absorbs light, process images, and send
visual signals to the brain. Cones, which are concentrated in the fovea, play a key role
in day light vision and colour perception. Rods, which have their greatest density just
outside the fovea, are critical to night vision and peripheral vision. Dark adaptation and
light adaptation both involve changes in the retina's sensitivity to light, allowing the eye
to adapt to changes in illumination.
• The retina transforms light into neural impulses that are sent to the brain via the optic
nerve. Receptive fields are areas in the retina that affect the firing of visual cells. They
vary in shape and size, but centre-surround arrangements are common. The optic
nerves from the inside half of each eye cross at the optic chiasm and then project to
the opposite half of the brain.
• Two visual pathways engage in parallel processing and send signals to different areas
of the primary visual cortex. The main pathway is rooted through the LGN in the
thalamus. After processing in the primary visual cortex, visual information is shuttled
along the what and where pathways to other cortical areas.
• Noble prize-winning research by Hubel and Wiesel suggest that the visual cortex
contains cells that function as feature detectors. The discover of the what pathway and
the neurons inside it that respond specifically to faces have shed new light on visual
disorders that have perplexed scientists for decades.
• Vision researchers employ multiple, converging methods when trying to explain the
role of the brain in visual experience. This was illustrated with research on the
Viewing the World in Colour The Stimulus for Colour
• Wavelength -> related to hue, amplitude to brightness and purity to saturation.
• Two kinds of colour mixing
o Subtractive colour mixing - works by removing some wavelengths of light,
leaving less light than was originally there.
o Additive colour mixing - works by superimposing lights, putting more light in the
mixture than exists in any one light by itself.
Trichromatic Theory of Colour Vision (SEARCH ONLINE)
• First stated by Thomas Young and later modified by Hermann Von Hemholtz.
• Trichromatic theory of colour vision - holds that the human eye has three types of
receptors with differing sensitivities to different light wavelengths.
• Colour blindness - encompasses a variety deficiencies in the ability to distinguish
o Occurs much frequently in males than females.
• People who are colour-blind are dichromats -> make do with only two colour channels
• Three types of dichromats with each insensitive to one of the primary colours - red,
green, or blue.
Opponent Process Theory of Colour Vision
• Complementory Colours - pairs of colours that produce grey tones when mixed
• Afterimage - a visual image that persists after a stimulus is removed. Opponent
process theory of colour vision - holds that colour perception depends on receptors
that make antagonistic responses to three pairs of colours. (CHECK ONLINE)
o Proposed by Ewald Hering.
• Grapheme-colour synesthesia - (CHECK ONLINE)
Reconciling Theories of Colour Vision
• George Wald - found that the eyes have three types of cones with each type being most
sensitive to a different band of wavelengths.
Effects of Colour on Behaviour
• Andrew Elliot -> believes that colours can have automatic, unconscious affects on
o Assert that effects are rooted on two bask source:
People learn associations based on certain colors being paired repeated
with certain experiences (example, red ink is usually used to mark errors).
Certain colours may have had adaptive significance for survival or
reproduction (example, blood and fire, both red, can signal danger).
• The color red has a negative impact on performance. Perceiving Forms, Patterns, and Objects
• Reversible Figure - a drawing that is compatible with two interpretations that can shift
back and forth.
• The same visual input can result in radically off perceptions.
• Perceptual set - a readiness to perceive a stimulus in a particular way.
• Inattentional blindness - failure to see fully visible objects or events in a visual display.
o Leads to focus most of their attention on a specific feature in a scene.
Feature Analysis: Assembling Forms
• Feature Analysis -the process of detecting specific elements in visual input and
assembling them into a more complex form. (FIND EXAMPLES ONLINE)
• Bottom-up processing - a progression from individual elements to the whole.
• Top-down processing - a progression from the whole to the elements.
• Subjective contours - the perception of contours where none actually exist.
o Depends on viewing stimulus configurations as wholes, then filling in the blanks.
Looking at the Whole Picture: Gestalt Principles
• Gestalt psychologists -> demonstrates that the whole can be greater than the sum of its
• Phi phenomenon - the illusion of movement created by presenting visual stimuli in rapid
• Principles that describe how the visual system organizes the scene into discrete forms:
o Figure and Ground
Figure -> thing being looked at -> appear closer to viewer and stands out.
Ground -> background against which the thing stands.
Things that are close to one another seem to belong together.
Completing figures that have gaps in them.
Elements that are similar tend to be grouped together.
Viewers tend to organize elements in the simplest way possible.
Concept of simplicity plagued by ambiguity.
Viewers tend to see elements in ways that produce mood continuation.
• In addition there’s several other factors that influence our perception: o Familiarity: we perceive things differently depending whether its new to us or
we’re used to it
o Context: of which it occurs
Stimulus can be interpreted differently depending on the context
o Expectations: we often see what we expect to see
Formulating Perceptual Hypotheses
• Understanding problems requires distinguishing between two kinds of stimuli:
o Distal stimuli - stimuli that lie in the distance (that is, in the world outside the
Your eyes don't touch the objects.
o Proximal stimuli - the stimulus energies that impinge directly on sensory
• Perceptual Hypotheses - an interference about which distal stimuli could be
responsible for the proximal stimuli sensed.
Review of Key Points:
• Perceptions of colour (hue) are primarily a function of light wavelength, while
amplitude affects brightness and purity affects saturation. There are two types of
colour mixing: additive and subtractive. Human colour perception depends on
processes that resemble additive colour mixing.
• The trychromatic theory hold that people have three types of receptors that are
sensitive to wavelengths associated with red, green, and blue. The opponent process
theory hold that colour perception depends on receptors that make antagonistic
responses to red versus green, blue versus yellow, and black versus white. The
evidence now suggests that both theories are necessary to account for colour vision.
• Reversible figures and perceptual sets demonstrate that the same visual input can
result in very different perceptions. Form perception depends on both the selection
and interpretation of sensory inputs. According to feature analysis theories, people
detect specific elements in stimuli and build them into recognizable forms through
bottom-up processing. However, form perception also involves top-down processing,
which progresses from the whole to the elements.
• Gestalt psychology emphasized that the whole may be greater than the sum of its