Lecture #6: Part I: Sensation & Perception
Sensation involves the detection of external stimuli (e.g. light, pressure or
odours), the responses to the stimuli and the transmission of these responses to the
brain. This is the first step, which senses the stimulus. Perception involves the
processing, organization and interpretation of sensory signals in the brain, which
results in an internal representation of the stimuli – and your conscious experience
of it. This is the second step, which makes sense of the stimuli to experience it.
Everything is experienced in your brain; even though it is felt in the
periphery, it is experienced in the brain. The world we live in is constructed by us.
Context and change is important to respond and adapt. A stimulus needs to be
translated into chemical or electrical signals for the brain to understand them
(neuronal impulses). Transduction is the process by which sensory receptors pass
impulses to connecting neurons when they receive stimulation. These impulses
travel to the thalamus, which then directs it to the appropriate brain location in the
cortex (unless the case is smell, which directly bypasses the thalamus). It is in these
regions that the information is interpreted as one of the senses.
Sensory adaptation: sense initial change, but prolonged stimulation causes
adaptation, which eventually cause the system to ignore the stimulus.
Sensing Chemicals: Taste & Smell:
When chemical substances dissolve in the saliva, the taste receptors found in
the taste buds are activated. A taste bud can have more than one receptor. Every
taste experience is composed of a mix of five basic qualities: sweet, salty, sour, bitter
and umami (savoury). Contrary to popular belief, the tongue is not divided among
flavour; all of the receptors are found dispersed all over the tongue. Taste is also
dependent on smell and texture. Since the taste experience occurs in the brain, it is
able to integrate all of these signals. The amount of taste receptors can vary between
humans. Those with a greater amount are called supertasters.
The stimuli for smell are chemical substances from the external environment
that dissolve in fluid on the mucous membranes in the nose. The olfactory
epithelium is a thin layer of tissue embedded with smell receptors, which transmit
the neural signal to the olfactory bulb, which is the brain center for smell (since it
bypasses the thalamus). A smell is determined by is pleasantness versus its
intensity. The intensity of a smell is determined by the amygdala. The brain is good
at distinguishing good smells from bad smells, however it is poor at naming a smell
if the source is not seen.
Touch: Temperature, Pressure and Pain:
The sense of touch is otherwise referred to as the haptic sense. The skin
contains three kinds of receptors: temperature, pressure and pain. The pressure
receptors are located at the hair follicle and the pain receptors and thinner and
more dendritic. People cannot tickle themselves because the coordinated movement required to do so comes from the cerebellum. Therefore, the cerebellum also
predicts that something is about to happen, so it is not unexpected and not ticklish.
When nerve signals enter the thalamus, they are projected to the primary
somatosensory cortex in the parietal lobe (front part of the lobe). Areas that are
most sensitive take up more space in the lobe. This is how phantom limb works.
When a person gets a body part amputated, even though the limb is gone the part is
the brain that corresponds to it is still present. It is possible for the person to
experience feeling in the phantom limb due to this. However, over time due to brain
plasticity, the brain can remap to remove the unnecessary region.
Fast, myelinated fibres in the body are responsible for sharp, immediate pain.
This allows for protection to remove that part of the body away from danger (i.e.
touching a hot stove). Slow, non-myelinated fibres in the body are responsible for
dull, steady pain. This acts as a recuperation mechanism because it tells the body
that the area needs conscious attention over time to heal properly (i.e. the burn
received afterwards). Pain is a perceptual experience, as it occurs in the brain,
which means there is a partial emotional component.
Gate Control Theory of Pain: For pain to be experienced, pain receptors must be
activated, and the neural gate in the spinal cord must allow signals to get through to
the brain. Only when the gate is open is pain experienced. This is often why soldiers
don’t experience pain until after the war is over. This is also why rubbing a bite
prevents pain. The pressure from rubbing activates the larger sensory pressure
nerve fibres, which causes the neural gate to close, shutting off the pain signal.
Children can fall down, but the context of the situation depends on whether the pain
is felt or not. It is only felt if there is focus upon the pain. If their attention is
distracted from the pain, their spinal cord gate closes, preventing them from feeling
the pain since the sensory information isn’t being processed.
Parts of the eye:
Cornea: clear window in the front of the pupil, and with the help lens focuses
the image on the retina. It is the first place where the light is refracted
(gathered and bent)
Pupil: the hole in the iris that controls the amount of light that enters the eye
Lens: focuses light onto the retina by bending at an appropriate angle
Retina: receives lights and records visual messages. This is where
Sclera: eye’s strong outer coat
Optic Nerve: carries visual messages from the retina to the brain
Accommodation is the