Chapter 4: Sensation and Perception Sensory adaptation: sensitivity to prolonged stimulation tends to
decline over time as an organism adapts to current conditions.
Our senses encode the information our brains perceive:
• Sensation: simple stimulation of a sense organ; basic
registration of light, sound, pressure, odor or taste as parts of 1. The length of a light wave determines its hue, or what humans
the body interact with the physical world. perceive as color.
2. The intensity or amplitude of a light wave-how high the peaks
• Perception: takes place at the level of your brain: it is the
organization, identification and interpretation of the sensation are-determines what we perceive as the brightness of light.
in order to form a mental representation.
3. Purity is the number of distinct wavelengths that make up the
• Our senses are the means by which we experience the world; light. Purity corresponds to what humans perceive as saturation,
or the richness of colors.
everything we learn is detected by sense organs and
transmitted to our brains by sensory nerves
The human eye
• Senses depend on the process of transduction: what takes
place when many sensors in the body convert physical signals • Light reflected from a surface enters the eyes via the
transparent cornea, which bends the light wave and sends it
from the environment into encoded neural signals sent to the through the pupil, a hole in the colored part of the eye- the iris
CNS. (controls the size of the pupil and hence the amount of light that
Psychophysics: can enter the eye.)
Fechner: developed this approach to measure sensation and perception: • Behind the iris, the thickness and shape of the lens adjust to
methods that measure the strength of a stimulus and the observer’s focus light on the retina (a light sensitive tissue lining the back
of the eyeball) where the image appears upside down and
sensitivity to that stimulus.
Absolute threshold: the simplest quantitative measurement minimal
intensity needed to just barely detect a stimulus; is a boundary. • Accommodation: process by which the eye maintains a clear
image in the retina.
JND (just noticeable difference)- the minimal change in a stimulus that
can just barely be detected. weber’s law: the JND of a stimulus is a • If eyeball is too long, images are focused in front of the retina,
constant proportion despite variations in intensity. leading to nearsightedness (myopia)
Signal detection theory: response to a stimulus depends both on a • If eyeball is too short, images are focused behind the retina,
person’s sensitivity to the stimulus in the presence of noise and on a leading to farsightedness (hyperopia).
person’s decision criterion. Phototransduction in the retina • The bundled retinal ganglion cells axons form the optic nerve,
which leaves the eye through a hole in the retina. The hole in
• Two types of photoreceptor cells in the retina contain light- the retina doesn’t contain rods or cones and hence has no
sensitive pigments that transduce light into neural impulses. mechanism to sense light and this creates a blind spot, which is
a location in the visual field that produces no sensation on the
• Human retina contains two general types of photoreceptors: retina.
125 million rods (function mainly in dim light; are very sensitive
to light but cannot detect change in hue. Visual info they Receptive fields
provide lacks sharpness) and 6 million cones (function when the
level of illumination is bright enough to see things clearly. • Most retinal ganglion cells respond to input not from a single
Responsible for acute daytime vision and for color perception) retinal cone or rod but from an entire patch of adjacent
photoreceptors lying side by side, or laterally, in the retina.
• Cones detect color, operate under normal daylight conditions,
• A particular RGC will respond to light falling anywhere within
and allow us to focus on fine detail.
that small patch, which is called its receptive field, the region of
• Rods become active under low-light conditions for night vision. the sensory surface that, when stimulated, causes a change in
the firing rate of that neuron.
• Rods contain the same photo pigment and provide no
information about color and sense only shades of gray. Perceiving colour
• Fovea- an area of the retina where vision is the clearest and • S.I.Newton color is nothing but our perception of wavelengths
there are no rods at all. from the spectrum of visible light.
• Shortest visible wavelength purple
• the absence of rods in the fovea decreases the sharpness of • Longest visible wavelength red
vision in reduced light.
Trichromatic color representation in the cones
• The cones are densely packed in the foveathe distribution of • Pattern of responding across the three types of cones provides a
cones directly affects visual acuity and explains why objects off unique code for each colour.
to the side, in your peripheral vision, aren’t so clear. The light
reflecting from those peripheral objects has a difficult time • A genetic disorder in which one of the cone types is missing and
in some very rare cases, two or all three, causes a color
landing in the fovea making the resulting image less clear. deficiency a.k.a color blindness.
Color opponent system
• The retina is thick with cells. The photoreceptor cells form the • Pairs of visual neurons work in opposition.
innermost layer. The middle layer contains bipolar cells, which
collect neural signals from the rods and cones and transmit
The visual brain
them to the outermost layer of the retina, where neurons called • Streams of action potentials containing information encoded by
retinal ganglion cells organize the signals and send them to the the retina travel to the brain along the optic nerve,
• Half of the axons in the optic nerve that leave each eye come
from the retinal ganglion cells that code information in the right visual field, whereas the other half code information in the left regions. This dorsal stream allows us to locate objects, to track
visual field. their movements, and to move in relation to them.
• The two nerve bundles link to the left and right hemispheres • Visual-form agnosia: brain damage to the ventral stream; the
respectively. inability to recognize objects by sight.
• The optic nerve travels from each eye to the lateral geniculate • Optic ataxia: brain damage to the parietal section of the dorsal
nucleus located in the thalamus. (Thalamus receives inputs from stream; difficulty using vision to guide their reaching and
all of the senses except smell). grasping movements.
• From there, the visual signal travels back to the brain, to a
location called area V1, the part of the occipital lobe that Recognizing what we perceive:
contains the primary visual cortex.
• There, the information is systematically mapped into a • binding problem: how features are linked together so that we
see unified objects in our visual world rather than free-floating
representation of the visual scene. or miscombined features.
Neural systems for perceiving shape
• Perceiving shapes depends on the location and orientation of an • Illusory conjunction: a perceptual mistake where features from
object’s edges. multiple objects are incorrectly combined.
• Area V1 is specialized for encoding edge orientation. • Feature integration theory: the idea that focused attention is
• Neurons in the visual cortex selectively respond to bars and
edges in specific orientations in space. not required to detect the individual features that comprise a
• Some neurons fire when an object in a vertical orientation is stimulus BUT is required to bind those individual features
• Other neurons fire when an object in a horizontal orientation is • Perceptual constancy: even as aspects of sensory signals
• Other neurons fire when objects in a diagonal orientation of 45 change, perception remains constant.
degrees are perceived, and so on. • Perception is sensitive to changes in stimuli, but perceptual
constancies allow us to notice the differences in the first place.
• One brain system identifies people and things and another Principles of perceptual organization: before object recognition, the
tracks their movements, or guides our movements in relation to
visual system must group the image regions that belong together into a
them. representation of an object. We tend to perceive a unified, whole object
• Two visual streams project from the occipital cortex to visual rather than a collection of separate parts is the foundation of gestalt
areas in other parts of the brain. psychology.
• One interconnected visual system forms a pathway that courses
from the occipital visual regions into the lower temporal lobe. 1. Simplicity
This ventral stream enables us to identify what we see. Includes
brain areas that represent an object’s shape and identity. 2. Closure: we tend to fill in missing elements of a visual scene,
• Another interconnected pathway travels from the occipital lobe allowing us to perceive edges that are separated by gaps as
through the upper regions of the temporal lobe into the parietal belonging to complete objects. 3. Continuity: edges or contours that have the same orientation • Inattentional blindess: a failure to perceive objects that are not
have what the Gestaltists called “good continuation” and we the focus of attention.
tend to group them perceptually.
4. Similarity: regions that are similar in color, lightness, shape or
texture are perceived as belonging to the same object. • Vision is based on the spatial pattern of light waves on the
5. Proximity: objects that are close together tend to be grouped • The sense of hearing is about sound waves- changes in air
pressure unfolding over time.
6. Common fate: elements of a visual image that move together Sensing sounds:
are perceived as parts of a single moving object.