Sensory Homunculus, Btarski, Creative Commons 3.0 License
Looking more closely at the Primary Somatosensory Cortex (PSC) in an effort to characterise the properties of the Brodmann Areas, i’m starting with an overview of the PSC with reference to a variety of research papers highlighting the results of different research approaches. The PSC comprises Brodmann Areas 1, 2 and 3. The diagram above illustrates the results of the classic Penfield studies in which the Canadian Neurosurgeon identified through surface electrode stimulation the sensory correlate of each point on the cortical surface. For the PSC there was a distorted body map with an exaggerated representation of the lips and hands. Whilst Penfield’s original findings have been supported by subsequent research more recent studies have characterised the PCS as a dynamic structure in which maps are continuously reorganised and in which subtly variations of these maps are represented in other parts of the cortex.
The authors of this review article suggest that the face region of the PSC is involved in the integration of sensory and motor data in semi-automatic and voluntary orofacial movements and also in pathological disorders involving these movements. The authors of this review suggest three types of body schema – the somatosensory representations characteristic of the PSC, a secondary representation with body size and shape and third representation with kinaesthetic and tactile stimuli leading to postural maps. The authors of this review paper suggest that the somatosensory cortex is organised into micro and macrocolumns which compete with each other to more accurately represent sensory stimuli, features characteristic of neural network architectures. Differences between Brodmann Area 3 in humans and other apes were found in this post-mortem study which also characterised some of the important difference in other areas between species. An fMRI study in 10 subjects finds that there is a consistent representation for the little finger but not the index finger suggesting that maps for the two fingers develop differently. This fMRI study correlated greater contralateral PSC activation with active rather than passive touch. In other words where the hand moves over a stimulus rather than the stimulus moving under the hand. In this fMRI study, painful muscle stimulation activated brain regions associated with emotions (e.g Amygdala) in contrast with painful skin stimulation which activated contralateral PSC.
In a study of 20 people with traumatic peripheral nerve injury, nerve blockade reversed the pain. Magnetoencephalography was used to identify the corresponding cortical activation changes and the PSC was identified as the most likely correlate of this change in cortical activity. This supports the hypothesis that chronic pain is associated with reorganisation of the PCS. The authors of this review paper however suggest there is no abnormal processing of somatosensory stimuli in the facial area of the PCS. In one study, Transcranial Magnetic Stimulation over the contralateral PCS results in altered sensory perception in people with Parkinson’s Disease treated with dopaminergic therapy compared to controls. The researchers suggest that dopamine may be associated with sensory symptoms in Parkinson’s Disease. In a small PET study (n=15) recovery in hemiplegic stroke was associated with increased activity in the contralateral PSC compared to controls. In orofacial dystonia there was found to be an asymmetric somatosensory body map in this fMRI study. The authors of this review paper find that evoked responses in the PSC are modified by Migraines with or without aura. Increased somatosensory evoked potentials are identified in Rett Syndrome but are probably secondary to widespread cortical hyperexcitability.
Appendix – Articles Reviewed in relation to Brodmann Areas or other Structures
Brodmann Area 1 – Somatosensory Cortex
Area 6 (Agranular Frontal Area 6)
Areas 13 and 14 (Insular Cortex)
Area 15 (Anterior Temporal Lobe – Controversial Area in Humans)
Area 27 (Piriform Cortex)
Area 28 (Entorhinal Cortex)
Areas 45, 46, 47 (Inferior Frontal Gyrus)
Medial Temporal Lobe
Miscellaneous Subcortical Structures
Generic Articles Relating to Localisation
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