Chapter 4 - Sensing and Perceiving Our World
The long, strange, trip from sensation to perception
- Sensation is the stimulation of our sense organs by the outer world
- Eyes are sensitive to to light waves, ears to sound, skin to touch and pressure, tongues to
tastes, and noses to odors
- Perception is the act of organizing and interpreting sensory experience. It is how our
psychological world represents our physical world
- If you had not been taught to read, the symbols on this page would not be words, but rather meaningless marks
Basic sensory processes
- Our sensitivity diminishes when we have constant stimulation, a process
known as sensory adaptation
- Sensory adaptation ensures that we notice changes in stimulation more than
stimulation itself
Principles of perception
- Absolute threshold: when do we go from not sensing an object or event to
sensing it? What is the softest sound you can hear? These questions concern
absolute threshold, the lowest intensity level of a stimulus we can detect half
of the time
- Signal detection theory: attempts to separate “signal” from “noise” and takes into account both stimulus intensity and the
decision-making processes people use in detecting a stimulus
- Difference threshold: Absolute thresholds involve perceiving or not perceiving a stimulus, but what about perceiving when a stimulus
changes? Once we already perceive a stimulus, how much does it have to change before we notice that change? This threshold is known as
a difference threshold.
- Difference thresholds are relative thresholds and are also referred to as just noticeable differences (JND) because they involve the
smallest difference that is noticeable.
- The laws of just noticeable differences in sensory perception go back to Ernst Weber, who in 1834 discovered that the size of the JND is a
constant fraction of the intensity of the stimulus, a finding that came to be known as Weber’s law
- our frame of mind, which is ultimately coded in the brain, can impact how we perceive things
- The effect of frame of mind on perception is known as perceptual set
Sensing visual stimuli
- What does the eye do? It bends light, converts light energy to neural energy, and sends that
information to the brain for further processing
- Vision and the eye: Light enters the eye at the cornea, a clear, hard covering that protects the
lens. It then passes through liquid until it reaches a hole called the pupil. Light enters the
interior of the eye through the pupil. The colored part of the eye, the iris, adjusts the pupil to
control the amount of light entering. The light then passes through the lens, which bends the
light rays. Through a process known as accommodation, muscles around the lens alter its shape
, to adjust to viewing objects at different distances and to allow the lens to focus light on the retina.
- Finally, the retina is a thin layer of nerve tissue that lines the back of the eye, whose main function to to convert light energy into
neural energy.
- The deepest layer of cells, where processing of light energy begins, is made up of photoreceptors. The two types of photoreceptors in the
retina—rods and cones—convert light energy into neural impulses.
Rods
- play a key role in night vision, as they are most responsive to dark-and-light contrast
- They work well at low illumination
- At first, everything is completely dark. Then, with a bit of time, we begin to see shapes and forms, although we cannot really see colors
- The process of adjustment to seeing in the dark, known as dark adaptation, can take up to 30 minutes
and reflects the rods at work
- Rods are very sensitive, however, and sudden exposure to light can quickly cancel out their effectiveness
Cones
- are responsible for color vision and are most functional in conditions of bright light
- The fovea, a spot on the back of the retina, contains the highest concentration of cones in the retina
- We see images with the greatest clarity when they are focused on the fovea
- So visual acuity, or our ability to see clearly, depends on our cones
- Animals that have the most cones have the best acuity
Vision and the brain
- After transduction at the photoreceptor layer, visual information is processed by different layers of cells in the retina
- One of these layers is made up of ganglion cells, the axons of which make up the optic nerve.
- The optic nerve transmits signals from the eye to the brain
- each half of the retina (the area at the back) sends out its own axons
- each optic nerve has two strands
- One strand contains axons that travel from the retina to the thalamus and on to the visual cortex
of the s ame side of the brain as the eye from which the axons come
- The other strand crosses to the o pposite side of the brain in an area called the optic chiasm
Vision and specific neurons
- after leaving the retina, optic fibers go to the visual portion of the thalamus (the LGN) and then travel to the visual cortex in the
occipital lobes
- Hubel and Wiesel were able to record specialized activity of individual cells in the brain’s vision area by implanting electrodes into the
visual cortex of cats
- in the visual cortex they discovered neurons called feature detectors, which analyze the retinal image and respond to specific aspects of
shapes, such as angles and movements
- Hubel and Wiesel discovered three types of neurons in the visual cortex that act as feature detectors:
- Simple cells
- Complex cells
- Hypercomplex cells
Simple Cells
The long, strange, trip from sensation to perception
- Sensation is the stimulation of our sense organs by the outer world
- Eyes are sensitive to to light waves, ears to sound, skin to touch and pressure, tongues to
tastes, and noses to odors
- Perception is the act of organizing and interpreting sensory experience. It is how our
psychological world represents our physical world
- If you had not been taught to read, the symbols on this page would not be words, but rather meaningless marks
Basic sensory processes
- Our sensitivity diminishes when we have constant stimulation, a process
known as sensory adaptation
- Sensory adaptation ensures that we notice changes in stimulation more than
stimulation itself
Principles of perception
- Absolute threshold: when do we go from not sensing an object or event to
sensing it? What is the softest sound you can hear? These questions concern
absolute threshold, the lowest intensity level of a stimulus we can detect half
of the time
- Signal detection theory: attempts to separate “signal” from “noise” and takes into account both stimulus intensity and the
decision-making processes people use in detecting a stimulus
- Difference threshold: Absolute thresholds involve perceiving or not perceiving a stimulus, but what about perceiving when a stimulus
changes? Once we already perceive a stimulus, how much does it have to change before we notice that change? This threshold is known as
a difference threshold.
- Difference thresholds are relative thresholds and are also referred to as just noticeable differences (JND) because they involve the
smallest difference that is noticeable.
- The laws of just noticeable differences in sensory perception go back to Ernst Weber, who in 1834 discovered that the size of the JND is a
constant fraction of the intensity of the stimulus, a finding that came to be known as Weber’s law
- our frame of mind, which is ultimately coded in the brain, can impact how we perceive things
- The effect of frame of mind on perception is known as perceptual set
Sensing visual stimuli
- What does the eye do? It bends light, converts light energy to neural energy, and sends that
information to the brain for further processing
- Vision and the eye: Light enters the eye at the cornea, a clear, hard covering that protects the
lens. It then passes through liquid until it reaches a hole called the pupil. Light enters the
interior of the eye through the pupil. The colored part of the eye, the iris, adjusts the pupil to
control the amount of light entering. The light then passes through the lens, which bends the
light rays. Through a process known as accommodation, muscles around the lens alter its shape
, to adjust to viewing objects at different distances and to allow the lens to focus light on the retina.
- Finally, the retina is a thin layer of nerve tissue that lines the back of the eye, whose main function to to convert light energy into
neural energy.
- The deepest layer of cells, where processing of light energy begins, is made up of photoreceptors. The two types of photoreceptors in the
retina—rods and cones—convert light energy into neural impulses.
Rods
- play a key role in night vision, as they are most responsive to dark-and-light contrast
- They work well at low illumination
- At first, everything is completely dark. Then, with a bit of time, we begin to see shapes and forms, although we cannot really see colors
- The process of adjustment to seeing in the dark, known as dark adaptation, can take up to 30 minutes
and reflects the rods at work
- Rods are very sensitive, however, and sudden exposure to light can quickly cancel out their effectiveness
Cones
- are responsible for color vision and are most functional in conditions of bright light
- The fovea, a spot on the back of the retina, contains the highest concentration of cones in the retina
- We see images with the greatest clarity when they are focused on the fovea
- So visual acuity, or our ability to see clearly, depends on our cones
- Animals that have the most cones have the best acuity
Vision and the brain
- After transduction at the photoreceptor layer, visual information is processed by different layers of cells in the retina
- One of these layers is made up of ganglion cells, the axons of which make up the optic nerve.
- The optic nerve transmits signals from the eye to the brain
- each half of the retina (the area at the back) sends out its own axons
- each optic nerve has two strands
- One strand contains axons that travel from the retina to the thalamus and on to the visual cortex
of the s ame side of the brain as the eye from which the axons come
- The other strand crosses to the o pposite side of the brain in an area called the optic chiasm
Vision and specific neurons
- after leaving the retina, optic fibers go to the visual portion of the thalamus (the LGN) and then travel to the visual cortex in the
occipital lobes
- Hubel and Wiesel were able to record specialized activity of individual cells in the brain’s vision area by implanting electrodes into the
visual cortex of cats
- in the visual cortex they discovered neurons called feature detectors, which analyze the retinal image and respond to specific aspects of
shapes, such as angles and movements
- Hubel and Wiesel discovered three types of neurons in the visual cortex that act as feature detectors:
- Simple cells
- Complex cells
- Hypercomplex cells
Simple Cells
