Chapter 5: Sensation and Perception

Olfactory Perception

The sense of smell is more primitive than any other senses we have. First, the odor molecules enter the nasal cavity. The odorants attach to the olfactory epithelium, which are odor receptors. There are thousands of odor receptors; each respond to certain number of odorants. When an odorant binds to a bipolar neuron, also known as an olfactory receptor, a signal is sent to the neurons in the olfactory bulb, which are called glomeruli. They then leave the olfactory bulb and go to the primary olfactory cortex. The primary olfactory cortex is located at the intersection of the frontal and temporal cortices. Then the neurons in this area connect to the orbitofrontal cortex, which is considered the secondary olfactory processing center.

There are a few reasons why olfactory perception has not been studied as much as the other senses. It is hard to control how much of an odor you are giving to a participant. It is also hard to know when an odor is no longer present. Most odors are quite subtle and take investigating to identify and detect the smell.

Vision

Vision is the most studied out of all the senses. Visual information is in the light reflected from objects. As light passes through the eye, the image in inverted and projects on the retina. Photoreceptors can be found in the deepest layer of the retina. There are two types of photoreceptors: rods and cones. Rods are most effective with lower levels of light; however, cones are sensitive to higher levels of light. The fovea contains many cones. The optic nerve transmits visual information to the central nervous system. Since the retina is curved the temporal half of either the left or right retina is stimulated by the opposite visual field. Each optic nerve splits into different pathways depending where they terminate in the subcortex. Most of the axons are projected from the retina to the lateral geniculate nucleus. The remaining axons stimulate other subcortical structures.  The final projection to the visual cortex is through the geniculocortical pathway and then heads to the primary visual cortex.

The area that is activated in the color foci is called area V4 and the area activated in the motion task is area V5. There is a big difference between species when it comes to the position of the color and motion areas in the brain.

Achromatopsia is a deficit in seeing color. People with achromatopsia have lesions around the V4 area. One study found that individuals are better at differing reflectance differences than they are at distinguishing hue differences. Each subject was shown three color patches. Two of them were identical and the third was different in either hue or reflectance.

Akinetopsia is a deficit in motion perception. M.P. could not see objects moving at a continual speed; instead, she saw an object in one position and then another. She was not able to determine the direction or speed of a moving object. The area in the brain that was damaged was the area around the V5.