How Astereognosis Impacts the Brain

by Eva D 

Astereognosis is a condition that affects the brain’s ability identify objects by touch and falls under two categories: The primary recognition deficit (morphognosia), for which the patient is unable to recognise the physical features of objects through touch; and the secondary recognition deficit, for which the patient can recognise physical features of an object through touch but not the object itself. The area in your brain responsible for receiving sensory input is your parietal lobe - one of 5 lobes in the brain. Its main function is to process information about temperature, pressure, vibration and pain, which is sent from mechanoreceptors in the skin, through nerves and sensory pathways. In addition, the parietal lobe ‘remembers’ precise movements such as those involved in writing or playing a musical instrument - this becomes easier as you practise more because the information is stored. It is no surprise then that damage to this particular area, most commonly from head trauma, would result in an impaired sense of touch and or memory relating to touch. Similarly, strokes can have the same effect as they may restrict blood flow to cortical regions of the brain including the parietal lobe. In order for someone to have manual stereognosis (the ability to identify objects by touch), mechanoreceptors pass information about touch to the dorsal column medial lemniscus tract where the information is then passed to the ventroposterior lateral nucleus and finally to the somatosensory cortex in the parietal lobe.

Mechanoreceptors are a type of somatosensory receptor, the movement of which, relative to one another, helps to perceive 3D structures. These receptors can be further divided into 4 types: Ruffini corpuscles which respond to skin stretching, movement, finger position and warmth (they are deeper in the skin than receptors detecting the cold which is why humans detect cold stimuli before hot stimuli); Meissner corpuscles which respond to low frequency vibrations, fine touch and pressure; Pacinian corpuscles which respond to high-frequency vibrations, transient pressure and fine textures; and Merkel cell receptors which respond to slow moving stimuli, shapes and edges. A theory proposed by scientists is that Merkel cells are responsible for fingerprint patterns because fingerprint formation starts at the 10th week of pregnancy right after the arrival of merkel cells. This could interestingly show that if someone were to burn off their fingerprints, their perception of edges would be more confused yet other mechanoreceptors would be unaffected. This suggests someone with burnt fingertips may have more trouble typing on a keyboard.

One of the key areas of the parietal lobe is the somatosensory cortex which processes information about touch and tells our brain what physical features we are feeling. Another is the somatosensory association cortex which plays a part in memory retrieval and associating identities to objects through touch. Damage to this particular area would result in the secondary recognition deficit of astereognosis. By this point you should be able to deduce that the primary recognition deficit is a lot more common than the secondary recognition deficit. This is because damage such as a tumour or head trauma is likely to affect more than one area of the brain including various areas of the parietal lobe, causing not only the primary recognition deficit, but also other medical conditions. For someone to have the secondary recognition deficit however, only a specific area just posterior to the primary somatosensory cortex has to be damaged without damage to the primary somatosensory cortex itself. As previously explained, this is because people with the secondary recognition deficit are still able to recognise physical features - this would not be possible with damage to the somatosensory cortex as a whole.  It is possible to have a tumour located in the somatosensory association cortex and not the primary somatosensory cortex but it would most likely spread to other areas in the parietal lobe causing not just secondary recognition deficit but also other conditions. Isolated secondary recognition deficit of astereognosis would be possible but extremely unlikely as it would require a head injury in that specific area of the brain. Even more uncommonly, it is possible for damage to be limited to one side of the somatosensory cortex. Damage to the left side would cause astereognosis on the right side of the body and vice versa. While astereognosis is incurable, the treatment for a patient with astereognosis depends on the cause of the condition. A patient that experienced a stroke may receive statins or have blood clots removed, while a patient with a tumour may receive surgery, chemotherapy or radiotherapy. All Astereognosis patients would be encouraged to do cognitive rehabilitation therapy which would involve relearning the textures of specific objects and identities and manual exploration of different surfaces, shapes and textures.