Download Sensory Modalities and Adaptation

Sensory Modalities and Adaptation
The receptors that provide information for somesthetic senses come in a
variety of anatomical and functional types. Each specific receptor will
respond to only one type of functional signal (such as mechanical or
chemical, but not both). If the information is transmitted all the way to
the brain’s cortex, we perceive a sensation. The type of perception that
this information leads to is called a modality. There are four main
modalities typically recognized as part of the somesthetic senses. These
include temperature, touch, pain (nociception), and position and
movement (proprioception). Yet each of these can be further
subdivided
into
sub-modalities
(sometimes
called stimulus
modalities). For example, the modality of pain can be subdivided into
sharp, dull and aching.
There is not general agreement over how sensory modalities should be
categorized and subcategorized. In some classification schemes the
general sense of touch is replaced with pressure. In others, there may be
differences in the subcategories of touch. One classification scheme is
presented in the table below.
One Scheme for Categorizing Sensory Modalities
In subdivision 2, touch is a sensation produced under conditions of very little skin
displacement, while pressure requires displacement of the skin and underlying
tissues. Flutter is a sensation produced by a stimuli acting at a lower frequency
than vibration.
Main Modality
Subdivision 1
Touch
Crude touch
Discriminative touch
Subdivision 2
Touch
Pressure
Vibration
One Scheme for Categorizing Sensory Modalities
Flutter
Pain (Nociception) Sharp
Dull
Aching
Temperature
Hot
Cold
Proprioception
Position
Muscle length
Muscle tension
Joint pressure
Movement
Muscle length
Muscle tension
Joint pressure
Joint angle
The stimulus modalities shown in the subdivision 1 and subdivision 2
columns in the table might contribute to more complex sensations when
combined under certain conditions. For example, there is evidence that
a tickle occurs with simultaneous activation of certain touch and pain
receptors – as long as other conditions are appropriate. As you are
probably aware, others can tickle you, but you generally cannot tickle
yourself, even if you can activate the same sensory receptors. Thus input
from other systems seems to be able to affect our sensations. There is
still a lot for scientists to learn about sensory perception.
Sensory Adaptation
When first jumping into cool water, you may endure a wave of sensory
information “reminding you” that the water is cool. Yet minutes later,
you may be “used to” the water temperature, or have adapted to it. This
change in perception did not occur because the water warmed up, but
because the sensory receptors that originally responded to the change in
temperature are no longer sending signals to the CNS, or are sending
them at a decreased rate. This example indicates how a rapidly adapting
receptor might function; it provides significant signals to the CNS about
the original change in temperature that the body experiences, but then
adapts such that it sends fewer signals thereafter.
Not all sensory receptors are rapidly adapting. Certain pain receptors
seem to have little adaptation, or are very slowly adapting, so that as
long as the pain stimulus is applied, the person continues to “get the
message”! The general rates of adaptation for specific sensory receptors
are indicated in the above table.