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Lecture 4

PSYC 1010 Lecture Notes - Lecture 4: Color Vision, Torsten Wiesel, Visual Cortex

Course Code
PSYC 1010
Agnieszka Kopinska

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4 | HH/PSYCH 1010 Sensation and Perception
Chapter 4: Sensation and Perception
Sensation and Perception
Sensation is the stimulation of sense organs, the detection of physical energy by our sense organs,
including our eyes, ears, skin, nose and tongue.
oSensory Receptors detect environmental stimuli and convert stimulus energy into electro-
chemical signals (Vision, Hearing, Smell, Taste, Touch, etc.).
Perception is the the brain’s interpretation of raw sensory inputs that allows us to assemble these
signals into something meaningful through selection, organization, and interpretation, of sensory input.
Psychophysics is the study of how physical stimuli are translated into psychological experience.
oGustav Fencher
oA German scientist who published work on the subject in 1860, and founded the concept of
the threshold.
oA Threshold is a dividing point between energy levels that do and do not have a detectable
oAn Absolute Threshold for a specific type of sensory input is the lowest level/smallest
strength of stimulation that an organism can detect about 50% of the time.
Absolute Thresholds for Various Senses
Sense Modality Absolute Threshold
Vision Candle flame seen at approximately 50km on a
clear dark night.
Hearing Tick of a watch under quiet conditions at
approximately six meters,
Taste 1 teaspoon of sugar in approximately 7.5 liters of
Smell One drop of perfume diffused into the entire
volume of a large apartment.
Touch Wing of a fly or bee falling on your cheek from a
distance of one centimeter.
oJust Noticeable Difference (JND) is the smallest difference in the amount of stimulation that
specific sense can detect.
Example: Playing a song on an MP3 player but the volume is turned to low so that
we cant hear it, if we nudge the volume dial up to the point at which we can just
begin to make out the song, that’s a JND.
oWeber’s Law states that the size of a JND is a constant proportion of the size of the initial
stimulus, the stronger the stimulus, the greater the change needed to detect.
Example: Imagine how much light we would need to add to a brightly lit kitchen to
notice an increase in illumination compared with the amount of light we’d need to
add to a dark bedroom to notice a change in illumination.
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.
oDetectability is measured in terms of probability and depends on decision-making processes
as well as sensory processes.
o4 Possible Outcomes:
Hits (Detecting signals when they are present)
Misses (Failing to detect signals when they are present)
False Alarms (Detecting signals when they are not present)
Correct Rejections (Not detecting signals when they are absent)
Subliminal Perception is the registration of sensory input without conscious awareness.
oExistence vs. Practical Effects.
Sensory Adaptation is a gradual decline in sensitivity due to prolonged stimulation.
Our Sense of Sight: The Visual System
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4 | HH/PSYCH 1010 Sensation and Perception
The visual system transforms light energy into an electrochemical neural response that represents
characteristics of objects in our environment such as size, colour, shape, and location.
Light is form of electromagnetic radiation that travels both as a particle and a wave.
oA Wavelength of a light is the distance between peaks and of one complete cycle of the wave.
Visible light has wavelengths from about 400 nm (blue-violet) to 700nm (red).
A wavelength determines hue of colour.
The Eye: A Living Optical Instrument
oThe Cornea, a transparent “window” at the front, is where light enters the eye.
oThe Lens is the transparent eye structure that focuses the light rays falling on the retina.
Nearsightedness, close objects are seen clearly but distant objects appear blurry –
The focus point falls in front of the retina.
Farsightedness, distant objects are seen clearly but close objects appear blurry --
The focus point falls behind of the retina..
oThe Pupil is the opening in the centre of the iris that helps regulate the amount of light
passing into the rear chamber of the eye.
oThe Iris is the coloured ring of muscle surrounding the pupil that constricts or dilates via
amount of light.
oThe Retina is the neural tissue lining inside the innermost layer of the eye; it absorbs light,
processes images, and sends visual information to the brain.
The axons that run from the retina to the brain converge at the Optic Disk, a hole in
the retina where the optic nerve fibers exit the eye/blind spot.
oReceptor Cells:
Rods are specialized visual receptors that play a key role in night vision and
peripheral vision.
Cones are specialized visual receptors that play a key role in daylight vision and
colour vision – The Fovea is a tiny spot in the centre of the retina that contains only
cones; Visual Acuity, sharpness and detail.
Processing Visual Information
Bipolar Cells are neurons that connect rods and cones to the ganglion cells.
Ganglion Cells are neurons that connect to the bipolar cells; their axons form the
optic nerve.
The Optic Nerve is a collection of axons that connect the eye with the brain.
Optic Chiasm is a point in the brain where the optic nerves from each eye meet and
partly crossover to opposite sides of the brain.
Dark and Light Adaptation
oDark Adaptation is the process in which the eyes become more sensitive to light in low
oLight Adaptation is the process whereby the eyes become less sensitive to light high
Cones adapt after 5 minutes.
Rods adapt after 30 minutes.
Information Processing in the Visual Cortex
oFeature Detector Cells are neurons that respond selectively to very specific features of more
complex stimuli, cells that can detect lines and edges.
Early 1960’s: David Hubel and Torsten Wiesel: Microelectrode recording of axons in
primary visual cortex.
The Ventral Stream which processes the details of what objects are out there (e.g.
the perception of form and colours).
The Dorsal Stream which processes where the objects are (e.g. the perception of
motion and depth).
oVisual Agnosia is an inability to recognize objects.
oProsopagnosia is an inability to recognize familiar faces.
Viewing the World in Colour
oOur visual system interprets differences in the wavelength of light as colour.
oRods are colour blind, but with the cones we an see different colours.
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