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Chapter 5

Psychology 1000 Chapter Notes - Chapter 5: Detection Theory, Absolute Threshold, Color Vision

Course Code
PSYCH 1000

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-synesthesia: a condition where stimuli are experienced by more than one sense.
Literally means “mixing of the senses” can experience sounds as colours or
tastes as touch sensations that have different shapes
- Sensory receptors must translate information into nerve impulses. Then,
specialized neurons break down and analyze the specific features of the stimuli.
At the next stage, these numerous stimulus pieces are reconstructed into a neural
representation that is then compared with our knowledge of what particular
objects look, smell, or feel like. This allows us to recognize the stimulus.
-Sensation: the stimulus-detection process by which our sense organs respond to
and translate environmental stimuli into nerve impulses that are sent to the brain
-Perception: making “sense” of what our senses tell us – the active process of
organizing this stimulus input and giving it meaning
- more than just the 5 classical senses vision, audition, touch, gustation (taste),
and olfaction (smell)
- also senses that provide info about balance and body position, pressure, pain,
temperature, foreign invaders in the immune system, etc
-psychophysics: studies relations between the physical characteristics of stimuli
and sensory capabilities concerned with two types of sensitivity: absolute
limits of sensitivity, and the differences between stimuli
The Absolute Threshold:
- asks “how intense must a stimulus be before we can detect it’s presence?”
- researchers present stimuli of varying intensities and ask ppl if they can detect it
-the absolute threshold: the lowest intensity at which a stimulus can be detected
correctly 50% of the time (lower the absolute threshold, greater the sensitivity)
Signal Detection Theory:
- level of sensitivity for each sense can vary from person to person
- can also vary based on factors such as fatigue, expectation, or potential
significance of the stimulus
-signal detection theory: concerned with the factors that influence sensory
- ask participants if they hear a tone 4 outcomes: hit, miss, false alarm, or correct
rejection (figure 5.3 pg 162)
- at low stimulus intensities, the decision criterion is influenced by both the
participant’s and the situation’s characteristics bold participants say “yes”
more than conservative participants, and manipulating the rewards or costs for
correct or incorrect responses can put more pressure to answer correctly
The Difference Threshold:
- the smallest difference between two stimuli that ppl can perceive 50% of the time
- also called the just noticeable difference (jnd)

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-Weber’s law: the jnd is directly proportional to the magnitude of the stimulus
with which the comparison is being made, and can be expressed as a Weber
Eg, the jnd value for weights is a Weber fraction of approx. 1/50 this
means that if you lift a weight of 50 grams, a comparison weight must
weigh at least 51 grams for you to be able to say it was heavier
Does not work at extremely high and low intensities
Table 5.2 pg 165
Sensory Adaptation
-Sensory adaptation: sensory neurons are engineered to respond to a constant
stimulus by decreasing their activity, and diminishing sensitivity to that stimulus
(also known as habituation)
- R.M. Pritchard demonstrated that if tiny involuntary eye movements did not
keep images moving around the retina, stationary objects would fade from sight
- Sensory adaptation allows us to pick up informative changes in the environment
by freeing our senses from the constant and mundane stimuli around us
- stimulus for vision is electromagnetic energy, or light waves, measured in
nanometers (nm) humans can detect from about 700nm (red) to about 400nm
The Human Eye
- light waves enter the eye through the cornea, a transparent protective structure at
the front of the eye
- Behind the cornea is the pupil, an adjustable opening that can dilate or constrict
to control the amount of light that enters the eye. The pupil’s size is controlled by
muscles in the coloured iris
- Behind the pupil is the lens, an elastic structure that becomes thinner to focus on
distant objects and thicker to focus on nearby objects
- The lens focuses the visual image on the light-sensitive retina, a multi-layered
tissue at the rear of the fluid-filled eyeball
-Myopia (nearsightedness): the lens focuses the visual image in front of the retina,
resulting in a blurred image for faraway objects. Usually occurs because the
eyeball is longer than normal
-Hyperopia (farsightedness): occurs when the lens does not thicken enough and
the image is focused on a point behind the retina. Aging typically causes the
eyeball to become shorter, which is why many older ppl need reading glasses
Photoreceptors: Rods and Cones
- the retina contains two types of light-sensitive receptor cells: rods and cones
there are about 120 million rods and 6 million cones in the human eye

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-rods function best in dim light, and are primarily black-and-white brightness
receptors. They are about 500 times more sensitive to light than cones, but cannot
sense colour.
-Cones are colour receptors, and function best in bright light
- in humans, rods and cones are found throughout the retina except in the fovea, a
small area in the center of the retina that contains only cones. Cones decrease in
concentration as one moves away from the center of the retina
- rods and cones send their msgs to the brain via bipolar cells and ganglion cells
-bipolar cells have synaptic connections with the rods and cones, then they
synapse with a layer of about one million ganglion cells, whose axons are
collected into a bundle to form the optic nerve
- Typically, many rods are connected to the same bipolar cell. They therefore can
combine their individual electrical msgs to the bipolar cell, where the effect of the
many signals may be enough to fire it. That is why we can detect a faint stimulus
more easily if we look at it off to the side so that it does not fall on the fovea, but
on the peripheral of the retina, where the rods are most dense
- Cones in the fovea each have their own private line to a single bipolar cell, so our
visual acuity, or ability to see fine detail, is best when an image is focused right
on the fovea eagles and hawks have 2 fovea to allow them to see small prey
- The optic nerve formed by the axons of the ganglion cells exits through the back
of the eye producing a blind spot, where there are no photoreceptors we are
unaware of the blind spot because our perpetual system fills in the missing parts
Visual Transduction
-transduction: the process whereby the characteristics of a stimulus are converted
into nerve impulses
- rods and cones translate light waves into nerve impulses through the action of
protein molecules called photopigments absorb light to produce a chemical
reaction that changes the rate of neurotransmitter release at the receptor’s synapse
with the bipolar cells greater the change, the stronger the signal passed on to
the bipolar cell and then the ganglion cells triggering of all three levels
(rod/cone, bipolar cell, ganglion cells) the msg is sent to the brain
Brightness Vision and Dark Adaptation
- sensitivity of rods and cones depends on the wavelength of the light
- rods are least sensitive to red, and cones are most sensitive to yellow-green
- switch to yellow-green fire trucks for cones, and blue runway lights for rods
-dark adaptation: the progressive improvement in brightness sensitivity that
occurs over time under conditions of low illumination
- after absorbing light, a photoreceptor is depleted of its pigment molecules for a
period of time – during dark adaptation, the photopigment molecules are
regenerated and sensitivity increases
- cones reach maximum sensitivity after 5-10 minutes, and rods after about ½ hour
- red light keeps rods in state of dark adaptation because they are insensitive to red
wavelengths, thus retain high levels of photopigment and remain sensitive
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