- Sensory system
A sensory system is a part of the
nervous systemresponsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the braininvolved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic sensation(touch), tasteand olfaction(smell).
receptive fieldis the specific part of the world to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field; the light that each rodor cone can see, is its receptive field. [Kolb & Whishaw: "Fundamentals of Human Neuropsychology" (2003)] Receptive fields have been identified for the visual system, auditory systemand somatosensory system, so far.
Sensory systems code for four aspects of a stimulus; type (modality), intensity, location, and duration. Certain receptors are sensitive to certain types of stimuli (for example, different
mechanoreceptors respond best to different kinds of touch stimuli, like sharp or blunt objects). Receptors send impulses in certain patterns to send information about the intensity of a stimulus (for example, how loud a sound is). The location of the receptor that is stimulated gives the brain information about the location of the stimulus (for example, stimulating a mechanoreceptor in a finger will send information to the brain about that finger). The duration of the stimulus (how long it lasts) is conveyed by firing patterns of receptors.
stimulus modality(sensory modality) is a type of physical phenomenon that can be sensed. Examples are temperature, taste, sound, and pressure. The type of sensory receptoractivated by a stimulus plays the primary role in coding the stimulus modality.
memory-prediction framework, Jeff Hawkinsmentions a correspondence between the six layers of the cerebral cortexand the six layers of the optic tractof the visual system. The primary visual cortexhas areas labelled V1, V2, V3, V4, V5, MT, IT, etc. Thus Area V1 mentioned below, is meant to signify only one class of cells in the brain, for which there can be many other cells which are also engaged in vision.
Hawkins lays out a scheme for the analogous modalities of the sensory system. Note that there can be many types of senses, some not mentioned here. In particular, for
humans, there will be cells which can be labelled as belonging to V1, V2 A1, A2, etc.:
Visual Area 1, or V1, is used for vision, via the
visual systemto the primary visual cortex. See the illustration above.
A1 (auditory - hearing)
Auditory Area 1, or A1, is for hearing, via the
auditory system, the primary auditory cortex.
1 (somatosensory - touch)
Somatosensory Area 1, or S1, is for touch and
proprioceptionin the somatosensory system. The somatosensory system feeds the Brodmann Areas 3, 1 and 2 of the primary somatosensory cortex. But there are also pathways for proprioception(via the cerebellum), and motor control(via Brodmann area 4).
G1 (gustatory - taste)
Gustatory Area 1, or G1, is used for taste.
O1 (olfactory - smell)
Olfactory Area 1, or O1, is used for smell. In contrast to vision and hearing, the
olfactory bulbs are not cross-hemispheric; the right bulb connects to the right hemisphere and the left bulb connects to the left hemisphere.
Human sensory system
The Human sensory system consists of the following sub-systems:
Visual systemconsists of the photoreceptors, optic nerve, and V1.
Somatosensory systemconsists of the receptors, transmitters (pathways) leading to S1, and S1 that experiences the sensations labelled as touch or pressure, temperature(warm or cold), pain(including itchand tickle), and the sensations of muscle movement and joint position including posture, movement, and facial expression (collectively also called proprioception).
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