Is there a quantitative report for cortical homunculus?

Question

Almost all websites that I see for cortical homunculus only show the final picture with some qualitative descriptions such as "hands have more dexterity and occupy more motor cortex."

But is there a numerical table for their percentages?

Lips and tongue are big, but how big? 13%? 17%?

Is there a quantitative report for this?

Answer 1

I think you'd appreciate reading an 80-year update (Catani, 2017)) of the original paper (Penfield & Boldrey, 1937).

Four things come to mind (with commentary from Catani, (2017) sprinkled throughout:

1. I would guess that certain inconsistency between individuals makes such a quantitative approach difficult.

2. Known overlap between body parts in any given region of the cortices makes such quantification tricky

   • From Catani, (2017):

     Surprisingly, the picture that emerged from their work was a clear degree of functional overlap between stimulation fields rather than an orderly sequence of segregated areas as their homunculus suggests....

     These maps indicate a high degree of overlay between different body parts, especially in the proximity of the central sulcus. Interestingly, the range of functional overlaps varies from expected—such as tongue and mouth, or arm and hand—to intriguing—such as mouth, arm and hand

3. How one determines the "regions" that each body part maps to can be done in various ways that would each create a different homunculus wth parts therefore associated with slightly different region.

   • See Figures 2 and 4 in Catani, (2017) to see how redrawn homunculi vary in their relative proportions depending if they are proportioned based on stimulation areas, number of stimulations, or vertical length of surface maps for each body part.

4. The homunculus was never meant to represent quantitatively-discernable regions

   • From one of the original authors (Penfield) in a later publication:

     such drawings may easily become confusing if too much significance is attributed to the shape and comparative size (Penfield and Rasmussen, 1950)

   • From Catani, (2017):

     this little man, like many other figures that may naively populate our collective imagination, is just a metaphor for the complex neurological mechanisms that we strive to comprehend in their entirety

Note: I'm sure someone has tried to quantify as you've asked, so I encourage you keep looking. I would just guess that such efforts would best be found using search terms like "high field functional MRI" rather than "homunculi."

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Citations:

• ^{Marco Catani, A little man of some importance, Brain, Volume 140, Issue 11, November 2017, Pages 3055–3061, https://doi.org/10.1093/brain/awx270}

• ^{Penfield W , Boldrey E. 1937Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60: 389–440.}

• ^{Penfield W , Rasmussen R. The cerebral cortex of man. New York, NY: The Macmillan Company; 1950.}