threshold). The absolute threshold is the intensity at which a stimuli is detected 50% of the time.
Signal detection theory is concerned with factors that influence decisions about whether or not a
stimuli is present
Research indicates that subliminal stimuli, which aare not consciously perceived, can influence
perception and behavior in subtle ways, but not strongly enough to justify concerns about the
subconscious control of behavior through subliminal messages
The difference threshold or just noticeable difference (jnd) is the amount by which two stimuli
must differ for them to be perceived as different 50% of the time. Studies of the jnd led to weber’s
law which states that the jnd proportional to the intensity of the original stimulus and is constant
within a given sense modality
Sensory systems are particularly responsive to changes in stimulation and adaptation occurs in
response to unchanging stimuli.
The Sensory Systems
Electromagnetic energy, or light waves which are measured in nanometers.
Our visual system is only sensitive to wavelengths extending from about 700 nanometers (red)
down to about 400 nanometers (blue-violet).
Remember: ROY G BIV (red, orange, yellow) (Green) (Blue, Indigo and violet)
The Human Eye
Light waves enter the eye through the cornea. Behind the cornea is the pupil. The pupils size is
controlled by muscles in the colored Iris that surrounds the pupil.
Behind pupil is lens
Retina- a multi layered tissue at the rear of the fluid filled eyeball
If you have good vision for nearby objects but have difficulty seeing far away objects then you
probably suffer from MYOPIA
Hyperopia (farsightedness) occurs when the lens does not thicken enough and the image is
therefore focused on a point behind the retina (too far from the lens). The aging process typically
causes the eyeball to become shorter over time, contributing to the development of hyperopia and
the need for many middle-aged people to acquire reading glasses.
Photoreceptors: The Rods and Cones
The retina is an extension of the brain
Retina contains 2 types of light sensitive receptor cells called rods and cones, because of their
shapes. These are about 120 million rods and 6 million cones in the human eye.
Rods function best in dim light = are primarily black and white brightness receptors. They are
about 500 times more sensitive to light than are the cones, but they do not give rise to color
Cones are color receptors, function best in bright illumination.
In humans rods are found throughout the retina except in the FOVA- a small area in the centre of
the retina that contains only cones.
Cones decrease in concentration as one moves away from the centre of the retina, and the
periphery of the retina contains mainly rods.
Bipolar cells have synaptic connections with the rods and cones. The bipolar cells in turn synapse
with a layer of about one million GAGLION CELLS- whose axons are collected into a bundle to
form the OPTIC NERVES. Thus, input from more than 126 million rods and cones is eventually
funneled into only one million traffic lanes leading out of the retina towards the higher visual
Rods and cones not only not only form the rear layer of the retina, but their light- sensitive ends
actually point away from the direction of the entering light so that they receive only a fraction of
the light energy that enters the eye. Furthermore, the manner in which the rods and cones are
connected to the bipolar cells accounts for both the greater importance of rods in dim light and our
greater ability to see fine detail in bright illumination, when the cones are most active.
Many rods are connected to the same bipolar cell.
They can combine or funnel their individual electrical messages to the bipolar cell, where the
additive effect of the many signals may be enough to fire it.