Chapter 5: Sensation and Perception
Sensation = the sense organs’ responses to external stimuli and the transmission of
these responses to the brain.
Perception = the processing, organization and interpretation of sensory signals; it
results in an internal representation of the stimulus.
Sensory coding = our sensory organs’ translation of stimuli’s physical properties
into neural impulses.
Transduction = a process by which sensory receptors produce neural impulses
when they receive physical or chemical stimulation.
Connecting neurons then transmit information to the brain in the form of neural
impulses. Most sensory information first goes to the thalamus, a structure in the
middle of the brain. Neurons in the thalamus then send information to the cortex,
where incoming neural impulses are interpreted as sight, smell, sound, touch or
Sense Stimuli Receptors Pathways to the
Gustation (Taste) Molecules Taste cells in taste Portions of facial,
dissolved in fluid buds on the tongue glossopharyngeal,
on the tongue and vagus nerves
Olfaction (Smell) Molecules Sensitive ends of Olfactory nerve to
dissolved in fluid olfactory neurons the olfactory bulb
on mucous in the olfactory below the frontal
membranes in the epithelium lobes
Haptic sense Pressure on the Sensitive ends of Trigeminal nerve
(Touch) skin touch neurons in for touch above
skin neck, spinal nerves
Audition (Hearing) Sound waves Pressure-sensitive Auditory nerve
hair cells in
cochlea of inner
Vision Light waves Light-sensitive Optic nerve to the
rods, cones, primary visual
bipolar, amacrine cortex in the
and horizontal occipital lobe at
cells in retina of the back of the
eye. Then one head million ganglion
Our brains need qualitative and quantitative information about a stimulus. Different
sensory receptors respond to qualitatively different stimuli. In contrast, quantitative
differences in stimuli are coded by the speed of a particular neuron firing – a more
rapidly firing neuron is responding at a higher frequency.
Coarse coding = sensory qualities are coded only be a few different types of
receptors, each of which responds to a broad range of stimuli.
Psychophysics = a subfield developed during the nineteenth century by the German
researchers Ernst Weber and Gustav Fechner, examines our psychological
experiences of physical stimuli. It assesses how much physical energy is required for
our sense organs to detect a stimulus. To test this, researchers present very subtle
changes in stimuli and observe how participants respond. They study the limits of
humans’ sensory systems.
The absolute threshold = the minimum intensity of stimulation that must occur
before you experience a sensation, or the stimulus intensity detected above chance.
The difference threshold = the just noticeable difference between two stimuli – the
minimum amount of change required for a person to detect a difference.
Weber’s Law states that the just noticeable difference between two stimuli is based
on a proportion of the original stimulus rather than on a fixed amount of difference.
Signal Detection Theory (SDT) = a theory of perception based on the idea that the
detection of a faint stimulus requires a judgment – it is not an all-or-none process.
Sensory adaptation = a decrease in sensitivity to a constant level of stimulation.
Gate control theory = for us to experience pain, pain receptors must be activated
and a neural “gate” in the spinal cord must allow the signals through the brain. Pain
signals are transmitted by small-diameter nerve fibres, which can be blocked at the
level of the spinal cord by firing of larger sensory nerve fibres.
Lateral inhibition = a visual process in which adjacent photoreceptors tend to
inhibit one another.
Subtractive color mixing = a way to produce a given spectral pattern in which the
mixture occurs within the stimulus itself and is actually a physical, not
psychological, process. Additive color mixing = a way to produce a given spectral pattern in which different
wavelengths of light are mixed. The percept is determined by the interaction of
these wavelengths with receptors in the eye and is a psychological process.
A receptive field = the region of visual space to which neurons in the primary visual
cortex are sensitive.
Simultaneous contrast = an optical illusion in which identical stimuli appear
different when presented against different backgrounds. One theory to explain this
effect is that lateral inhibition in the retina emphasizes the difference between an
object and its background.
Kinesthetic sense = refers to sensations we gather from receptors in muscles,
tendons and joints, that pinpoint the position in space and the movements of both
our body and our limbs.
Vestibular sense = uses data from receptors in the semicircular canals of the inner
ear. These canals contain a liquid that moves when the head moves, bending hair
cells at the ends of the canal. The bending generates nerve impulses that inform us
of the head’s rotation and is thus responsible for a sense of balance.
Nonhuman animals’ sensory systems:
- sonar senses
Senses bases on electrical fields.
Perception occurs in the Brain
With the exception of olfaction, all sensory information is relayed to cortical and
other areas of the brain from the thalamus. From the thalamus, the information
from each sense is projected to a specific region of the cerebral cortex.
In these primary sense areas, the perceptual process begins.
Hearing – auditory neurons in the thalamus extend their axons to the primary
auditory cortex (A1), which is in the temporal lobe. Neurons in this region code the
frequency of auditory stimuli. Neurons towards the rear of A1 respond best to lower
frequencies, such as that of a foghorn, whereas those toward the front of A1 respond
best to higher frequencies, such as that of a train whistle.
Touch – touch information from the thalamus is projected to the primary
somatosensory cortex (S1), which is in the parietal lobe. Neighboring body parts
tend to be represented next to each other in S1, so that the body is effectively
mapped out there according to physical proximity. More sensitive body parts have
relatively larger amounts of cortical tissue dedicated to them. Vision – the primary visual cortex (V1) is in the occipital lobe. The neural pathway
from the retina to the occipital lobe preserves spatial relationships, so that adjacent
areas of the retina correspond to adjacent areas in V1. Some neurons in the V1
respond more to lines of particular orientations than to any other type of stimulus,
including lines tilted at a different angle. For example, some neurons increase their
firing rate when a vertical line segment is presented in their receptive field. The
firing rate of these cells, termed simple cells by Hubel and Wiesel, decreases as the
orientation of the line segment is rotated away from the preferred orientation.
Neurons in different parts of the brain tend to have different types of recep