Stimulus Modality
Labelled line theory
Premises:
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individual receptors preferentially transduce information about an adequate stimulus
- individual primary afferent fibres carry information from a single type of receptor
Conclusions:
- pathways carrying sensory information centrally are therefore also specific, forming a "labelled line" regarding a particular stimulus
Evidence:
- the brain associates a specific modality (the adequate stimulus) with a signal coming from a specific receptor
- e.g., "light" is detected by the photoreceptors, even if the stimulus is pressure on the eye
- in the cortex , perception is localized by modality, resulting in specific cortical areas for each sensation
- within each sensory area, sensations are "mapped" in ways that maintain an orderly representation of the stimulus (e.g. the humunculus that is maintained within somatosensory cortex; tonotopic maps that are formed in the auditory system; topographic maps in the visual cortex)
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Cortical sensory areas |
Humunculus |
Pattern theory
Premises:
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some sensory systems (esp. taste and olfaction) integrate information across multiple primary afferents
- a few types of afferent endings are multimodal (more than one sensation can be evoked by their activation)
Conclusions:
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an ascending pathway can convey sensory information by altering the temporal pattern of action potentials among multiple responding primary afferents
- under this theory, it is the pattern of activation of neural networks that forms the basis of perception
Evidence:
- examples are found in the olfactory system, and in colour perception
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Olfactory receptor codes |