Sensation and Perception
Sensation: Input from the physical world received by sensory receptors. Involves detection of
external stimuli (e.g., light, sound).
Perception: The process of selecting, organizing, and interpreting sensory input. Involves how
the brain interprets stimuli.
1. Vision (Sight)
Visual Perception and Processing
● Retina: Contains photoreceptor cells (rods and cones) that detect light; the fovea is the
area for sharp vision and color detection.
● Wavelength: The distance from one peak of a wave to the next; light perceived as color
depends on wavelength (hue), while brightness depends on amplitude.
● Blind Spot: The point where no visual information is processed due to the optic nerve.
● Monocular and Binocular Cues: Depth perception relies on one eye (monocular) or
two eyes (binocular) for cues.
● Linear Perspective: A monocular cue for depth perception where parallel lines
converge.
● Top-Down Processing: Interpretation of sensations influenced by available knowledge,
experiences, and thoughts.
● Bottom-Up Processing: Perceptions are built from sensory input.
● Inattentional Blindness: Failure to notice a visible stimulus because attention is
focused elsewhere.
○ Simons & Chabris Study (1999):
■ Participants asked to count basketball passes between players in white
shirts.
■ A person in a gorilla costume walks through the game.
■ ➡Nearly half of participants did not notice the gorilla, despite its visibility
for 9 seconds.
■ Conclusion: Attention can block perception of unexpected stimuli.
● Follow-Up Study (Most, Simons, Scholl, & Chabris, 2000):
○ Participants observed moving images on a screen.
○ Instructed to focus on white or black objects and ignore the other color.
○ When a red cross moved across the screen, about one third of participants failed
to notice it.
○ Conclusion: Focused attention can lead to inattentional blindness to other
stimuli.
,Color Perception Theories
● Opponent-Process Theory: Color perception is based on opposing pairs: black-white,
yellow-blue, and red-green.
● Trichromatic Theory: Color vision results from the activity across three groups of cones
responsive to red, green, and blue light.
Anatomy of the Visual System
● Eye: The primary sensory organ for vision.
○ Cornea: Transparent covering over the eye.
■ Serves as a barrier and focuses light entering the eye.
○ Pupil: Small opening through which light passes.
■ Dilates in low light to allow more light in.
■ Constricts in bright light to reduce light entry.
○ Iris: Colored portion of the eye, controls pupil size.
○ Lens: Curved, transparent structure providing additional focus.
■ Adjusts shape to focus light from near or far objects.
○ Fovea: Small indentation in the back of the eye.
■ Part of the retina, where images are focused.
Photoreceptors in the Retina
● Retina: Light-sensitive lining of the eye.
○ Cones: Photoreceptor cells concentrated in the fovea.
■ Function best in bright light.
■ Provide spatial resolution and color perception.
○ Rods: Photoreceptor cells located throughout the retina.
■ Work best in low light.
■ Involved in peripheral vision and movement detection.
■ Lack the spatial resolution and color detection of cones.
Visual Adaptation
● Transition from Bright to Dim Light:
○ Bright environment: Vision dominated by cone activity.
○ Dim environment: Vision dominated by rod activity.
■ Takes time to adjust due to delayed activation of rods.
■ Night blindness: Difficulty seeing in low light due to improper rod
function.
,Retinal Ganglion Cells and the Optic Nerve
● Retinal Ganglion Cells:
○ Connected to rods and cones through interneurons.
○ Axons form the optic nerve, which carries visual information to the brain.
● Blind Spot:
○ Point where the optic nerve exits the eye.
○ Not consciously noticed due to:
■ Different views from each eye preventing blind spot overlap.
■ The brain "filling in" the missing visual information.
Optic Nerve and Optic Chiasm
● Optic Nerve: Carries visual information from each eye.
○ Merges below the brain at the optic chiasm.
○ Optic Chiasm: X-shaped structure below the cerebral cortex at the front of the
brain.
■ Information from the right visual field (from both eyes) is sent to the left
side of the brain.
■ Information from the left visual field is sent to the right side of the brain.
Visual Processing in the Brain
● Visual information is sent to the occipital lobe for processing via multiple pathways.
○ What Pathway (Ventral Pathway): Involved in object recognition and
identification.
○ Where/How Pathway (Dorsal Pathway): Involved in spatial location and
interaction with visual stimuli.
○ Example: Seeing a ball roll:
■ The what pathway identifies the ball.
■ The where/how pathway tracks its movement.
Amplitude and Wavelength
● Amplitude: Height of a wave, measured from the crest (highest point) to the trough
(lowest point).
● Wavelength: Distance from one peak to the next.
○ Wavelength is related to frequency, which measures how many waves pass a
point in a given time.
○ Longer wavelengths = lower frequencies.
○ Shorter wavelengths = higher frequencies.
, Light Waves and the Electromagnetic Spectrum
● Visible Spectrum: Portion of the electromagnetic spectrum visible to humans.
○ Ranges from 380 to 740 nanometers (nm).
● Other species, such as honeybees, can detect ultraviolet light, and some snakes can
detect infrared radiation.
Perception of Color and Brightness
● Light Wavelength: Determines color perception.
○ Red: Longer wavelengths.
○ Green: Intermediate wavelengths.
○ Blue and Violet: Shorter wavelengths.
○ Mnemonic for visible spectrum: ROYGBIV (Red, Orange, Yellow, Green, Blue,
Indigo, Violet).
● Amplitude: Determines brightness or intensity.
○ Larger amplitudes appear brighter.
Color Vision
● Cones: Normal-sighted individuals have three types of cones that mediate color vision.
Each cone type is sensitive to different wavelengths of light.
Trichromatic Theory of Color Vision (Young-Helmholtz Theory)
● All colors are produced by combining red, green, and blue.
● Each type of cone is receptive to one of these three colors.
Opponent-Process Theory
● Color is coded in opponent pairs: black-white, yellow-blue, and green-red.
● Visual system cells are excited by one color and inhibited by the other in the pair.
○ Example: Cells excited by green are inhibited by red, and vice versa.
● Implications:
○ We do not experience greenish-red or yellowish-blue as colors.
○ This theory explains negative afterimages, which are visual sensations that
continue after the stimulus is removed.
■ Example: Staring at the sun briefly, then looking away and seeing a spot
of light.
■ When color is involved, the afterimage appears as the opposing color.
Sensation: Input from the physical world received by sensory receptors. Involves detection of
external stimuli (e.g., light, sound).
Perception: The process of selecting, organizing, and interpreting sensory input. Involves how
the brain interprets stimuli.
1. Vision (Sight)
Visual Perception and Processing
● Retina: Contains photoreceptor cells (rods and cones) that detect light; the fovea is the
area for sharp vision and color detection.
● Wavelength: The distance from one peak of a wave to the next; light perceived as color
depends on wavelength (hue), while brightness depends on amplitude.
● Blind Spot: The point where no visual information is processed due to the optic nerve.
● Monocular and Binocular Cues: Depth perception relies on one eye (monocular) or
two eyes (binocular) for cues.
● Linear Perspective: A monocular cue for depth perception where parallel lines
converge.
● Top-Down Processing: Interpretation of sensations influenced by available knowledge,
experiences, and thoughts.
● Bottom-Up Processing: Perceptions are built from sensory input.
● Inattentional Blindness: Failure to notice a visible stimulus because attention is
focused elsewhere.
○ Simons & Chabris Study (1999):
■ Participants asked to count basketball passes between players in white
shirts.
■ A person in a gorilla costume walks through the game.
■ ➡Nearly half of participants did not notice the gorilla, despite its visibility
for 9 seconds.
■ Conclusion: Attention can block perception of unexpected stimuli.
● Follow-Up Study (Most, Simons, Scholl, & Chabris, 2000):
○ Participants observed moving images on a screen.
○ Instructed to focus on white or black objects and ignore the other color.
○ When a red cross moved across the screen, about one third of participants failed
to notice it.
○ Conclusion: Focused attention can lead to inattentional blindness to other
stimuli.
,Color Perception Theories
● Opponent-Process Theory: Color perception is based on opposing pairs: black-white,
yellow-blue, and red-green.
● Trichromatic Theory: Color vision results from the activity across three groups of cones
responsive to red, green, and blue light.
Anatomy of the Visual System
● Eye: The primary sensory organ for vision.
○ Cornea: Transparent covering over the eye.
■ Serves as a barrier and focuses light entering the eye.
○ Pupil: Small opening through which light passes.
■ Dilates in low light to allow more light in.
■ Constricts in bright light to reduce light entry.
○ Iris: Colored portion of the eye, controls pupil size.
○ Lens: Curved, transparent structure providing additional focus.
■ Adjusts shape to focus light from near or far objects.
○ Fovea: Small indentation in the back of the eye.
■ Part of the retina, where images are focused.
Photoreceptors in the Retina
● Retina: Light-sensitive lining of the eye.
○ Cones: Photoreceptor cells concentrated in the fovea.
■ Function best in bright light.
■ Provide spatial resolution and color perception.
○ Rods: Photoreceptor cells located throughout the retina.
■ Work best in low light.
■ Involved in peripheral vision and movement detection.
■ Lack the spatial resolution and color detection of cones.
Visual Adaptation
● Transition from Bright to Dim Light:
○ Bright environment: Vision dominated by cone activity.
○ Dim environment: Vision dominated by rod activity.
■ Takes time to adjust due to delayed activation of rods.
■ Night blindness: Difficulty seeing in low light due to improper rod
function.
,Retinal Ganglion Cells and the Optic Nerve
● Retinal Ganglion Cells:
○ Connected to rods and cones through interneurons.
○ Axons form the optic nerve, which carries visual information to the brain.
● Blind Spot:
○ Point where the optic nerve exits the eye.
○ Not consciously noticed due to:
■ Different views from each eye preventing blind spot overlap.
■ The brain "filling in" the missing visual information.
Optic Nerve and Optic Chiasm
● Optic Nerve: Carries visual information from each eye.
○ Merges below the brain at the optic chiasm.
○ Optic Chiasm: X-shaped structure below the cerebral cortex at the front of the
brain.
■ Information from the right visual field (from both eyes) is sent to the left
side of the brain.
■ Information from the left visual field is sent to the right side of the brain.
Visual Processing in the Brain
● Visual information is sent to the occipital lobe for processing via multiple pathways.
○ What Pathway (Ventral Pathway): Involved in object recognition and
identification.
○ Where/How Pathway (Dorsal Pathway): Involved in spatial location and
interaction with visual stimuli.
○ Example: Seeing a ball roll:
■ The what pathway identifies the ball.
■ The where/how pathway tracks its movement.
Amplitude and Wavelength
● Amplitude: Height of a wave, measured from the crest (highest point) to the trough
(lowest point).
● Wavelength: Distance from one peak to the next.
○ Wavelength is related to frequency, which measures how many waves pass a
point in a given time.
○ Longer wavelengths = lower frequencies.
○ Shorter wavelengths = higher frequencies.
, Light Waves and the Electromagnetic Spectrum
● Visible Spectrum: Portion of the electromagnetic spectrum visible to humans.
○ Ranges from 380 to 740 nanometers (nm).
● Other species, such as honeybees, can detect ultraviolet light, and some snakes can
detect infrared radiation.
Perception of Color and Brightness
● Light Wavelength: Determines color perception.
○ Red: Longer wavelengths.
○ Green: Intermediate wavelengths.
○ Blue and Violet: Shorter wavelengths.
○ Mnemonic for visible spectrum: ROYGBIV (Red, Orange, Yellow, Green, Blue,
Indigo, Violet).
● Amplitude: Determines brightness or intensity.
○ Larger amplitudes appear brighter.
Color Vision
● Cones: Normal-sighted individuals have three types of cones that mediate color vision.
Each cone type is sensitive to different wavelengths of light.
Trichromatic Theory of Color Vision (Young-Helmholtz Theory)
● All colors are produced by combining red, green, and blue.
● Each type of cone is receptive to one of these three colors.
Opponent-Process Theory
● Color is coded in opponent pairs: black-white, yellow-blue, and green-red.
● Visual system cells are excited by one color and inhibited by the other in the pair.
○ Example: Cells excited by green are inhibited by red, and vice versa.
● Implications:
○ We do not experience greenish-red or yellowish-blue as colors.
○ This theory explains negative afterimages, which are visual sensations that
continue after the stimulus is removed.
■ Example: Staring at the sun briefly, then looking away and seeing a spot
of light.
■ When color is involved, the afterimage appears as the opposing color.
