There are two competing theories regarding the neural basis of word deafness. Empirically we know that it can occur following unilateral left or bilateral lesions in and around auditory cortex (STG).

**The unilateral theory**says that the relevant speech perception network is left dominant. Unilateral lesions cause word deafness by simultaneously disconnecting acoustic input from ascending auditory pathway AND by disconnecting callosal fibers from the intact right hemisphere. Bilateral lesions do the same but clip the right hemisphere inputs at their source.

**The bilateral theory**says that the relevant speech perception network is bilateral. Bilateral lesions disrupt the speech perception network, therefore causing word deafness. Unilateral cases are anomalies, perhaps reflecting atypically strong left dominant language organization.

Here's an argument in favor of the bilateral account based on probability distributions. Note that there are several simplifying assumptions here, but I believe these bias things in favor of the unilateral theory.

According to the
CDC (as of September 6, 2017), 795,000 people in the United States have a
stroke every year, 610,000 of which are first time strokes. From these numbers, and assuming a 50/50
chance of having a left or right hemisphere stroke, we can estimate the
following in a given year:

·
Number
of people with first-time left hemisphere strokes: 610,000 * .5 =

**305,000**
·
Number
of people who have a second (or 3

^{rd}or 4^{th}…) stroke: 795,000 – 610,000 = 185,000
·
Number
of people whose second stroke is in the opposite hemisphere from their first
stroke: 185,000 * .5 =

**92,500**
·
Number
of people with either a left hemisphere stroke or a bilateral stroke: 305,000 +
92,500 =

**397,500**
Now, with the
three bolded numbers, we can calculate expected proportion of a single left
hemisphere stroke compared with a bilateral stroke

*in this sample of 397,500 with one or the other*:
·
Proportion
of single left hemisphere strokes: 305,000/397,500 = .767

·
Proportion
of bilateral strokes: 92,500/397,500 = .233

With these
proportions we can estimate the

*expected*frequency of word deaf cases with unilateral versus bilateral strokes, assuming an equal frequency of occurrence for either etiology for a given sample size. Here we use a sample of 59 cases with confirmed lesions, as discussed by Poeppel (2001). The expected frequencies are 45.3 (59 * .767) cases with unilateral left hemisphere lesions and 13.73 (59 * .233) cases with bilateral lesions. The*actual*frequencies are 17 cases with left hemisphere lesions and 42 cases with bilateral lesions. We can then plug these numbers into a binomial probability calculation--assuming a biased coin flip with the bias equal to .767 in favor of unilateral--to determine the probability of getting such an*actual*outcome. When we do this, we find that the observed outcome is extremely unlikely and would be expected to occur by chance only 1 time in 1 million samples (p = 0.000001).
This seems like pretty strong evidence in favor of the bilateral theory.

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