Senses and perception
How the brain turns raw signals from eyes, ears, skin, and body into a seamless experience - and what rehabilitation can do when sensing goes wrong.
Every sense begins with a receptor in the body - but perception is entirely a brain process. The difference between detecting a sound and understanding it as speech is the distance from ear to cortex.
From signal to meaning
| Sense | Receptor | Primary cortex | Higher processing |
|---|---|---|---|
| Vision | Retina (rods & cones) | Occipital lobe | Temporal (what) + parietal (where) |
| Hearing | Cochlea (hair cells) | Superior temporal gyrus | Wernicke’s area (speech meaning) |
| Touch / pain | Skin & joint receptors | Somatosensory cortex (parietal) | Insula, thalamus |
| Smell | Olfactory epithelium | Olfactory cortex (direct - no thalamic relay) | Amygdala, hippocampus |
| Proprioception | Muscle spindles, joints | Somatosensory cortex | Cerebellum (movement calibration) |
Two visual pathways
The visual cortex splits incoming signals into two processing streams:
- The “what” pathway (ventral stream - temporal lobe) - identifies objects, faces, colours, and written words.
- The “where / how” pathway (dorsal stream - parietal lobe) - tracks locations, guides reaching, and detects motion.
Damage to one stream without the other produces striking dissociations: a person may fail to recognise a face but reach for it accurately, or describe an object correctly but fail to grasp it at the right angle.
Perception is prediction
The brain doesn’t passively receive data - it predicts what signals will arrive and only updates when reality differs from prediction. This is why habits feel automatic and why a subtle sensory loss (like mild neglect) can go unnoticed by the person themselves.
When senses go wrong after injury
- Hemianopia - loss of half the visual field after stroke affecting the visual cortex; scanning strategies and prism glasses help.
- Unilateral neglect - the parietal lobe stops updating attention to one side; scanning and cueing training can retrain spatial awareness.
- Auditory agnosia - sounds are heard but not recognised (temporal lobe); distinct from hearing loss.
- Sensory loss / hypersensitivity - touch can feel absent or painfully amplified after spinal cord or brain injury; graded sensory re-education helps.
- Tinnitus - phantom ringing linked to altered auditory cortex activity after hearing damage; sound therapy and CBT reduce distress.
Rehab applications
Sensory re-education systematically reintroduces graded textures, temperatures, and proprioceptive cues to rewire the somatosensory cortex after peripheral nerve or brain injury.
Scanning training teaches compensatory strategies for visual-field loss or neglect - systematic head and eye movements to cover the neglected side.
Mirror therapy uses visual feedback from the intact hand to “trick” the motor and sensory cortex into activating pathways for the affected hand.