Revisiting the relation between speech production and speech perception: Further comments on Skipper et al.

Continuing the "discussion" of Skipper, Devlin, and Lametti's (SDL) recent and in my opinion badly misguided review of the relation between speech perception and production, let's consider this quote on page 84:

Miceli, Gainotti, Caltagirone, and Masullo (1980) found a strong relationship between the ability to produce speech and discriminate syllables in 69 fluent and nonfluent aphasics. Specifically, contrasts between groups with and without a phonemic output disorder showed that patients with a disorder were worse at discriminating phonemes, particularly but not limited to those distinguished by place of articulation

This is misleading. "Ability to produce speech" in this paper is defined basically as a presence of phonemic paraphasias in the absence of articulatory difficulty, which will tend to identify fluent aphasics with posterior lesions like Wernicke's and conduction aphasia.  This is a rather odd measure of "ability to produce speech" but nonetheless the article reports that patients with "phonemic output disorder" (POV+) so defined were compared with those without (POV-) on a syllable discrimination task and the POV+ group performed worse, which is what SDL note and call a "strong relationship." However, when Miceli et al. dug deeper to ask whether there was a correlation between severity of POV+ and severity of syllable discrimination deficit, no relation was observed.  More importantly, Miceli et al. go on to report dissociations between POV+ and comprehension measures, which is the point I've been making for quite a while.  

Thus, rather than providing evidence for a relation between speech output ability and the ability to perceive speech, the report shows (i) that the severity of the production deficit is not correlated with the severity of performance on a syllable discrimination task and (ii) the presence of production deficits (POV+) dissociates from measures of auditory comprehension.  

SDL also claim in the same section that "Both children and adults with cerebral palsy have been shown to perform worse on phoneme discrimination and this is often related to articulatory
abilities" citing in support of their claims Bishop et al. 1990.  This is one of my favorite studies because it clearly shows how incredible important task selection is to understanding speech perception.  It is true that people with cerebral palsy performed worse on syllable discrimination tasks but that the same participants had NO IMPAIRMENT relative to controls when the same speech sounds were comprehended (using a very cool task) rather than discriminated.  See my blog post about the Bishop et al. study here.

SDL also use Parkinson's disease--"a degenerative movement disorder that results in reductions in premotor, SMA, and parietal cortex metabolism, linked to the basal ganglia"--as evidence that motor impairment affects speech perception.  I've addressed these findings previously in the Myth of Mirror Neurons, but noticed a new paper in the citation list by Vitale et al., so I looked it up and noted a fascinating conclusion from this large scale (N>100) study.  Here's an extended quote from the abstract:

Our patients with Parkinson's disease showed age-dependent peripheral, unilateral, or bilateral hearing impairment. Whether these auditory deficits are intrinsic to Parkinson's disease or secondary to a more complex impaired processing of sensorial inputs occurring over the course of illness remains to be determined. Because α-synuclein is located predominately in the efferent neuronal system within the inner ear, it could affect susceptibility to noise-induced hearing loss or presbycusis. It is feasible that the natural aging process combined with neurodegenerative changes intrinsic to Parkinson's disease might interfere with cochlear transduction mechanisms, thus anticipating presbycusis

So people with Parkinson's disease have peripheral hearing loss.  It seems to me that might be a better explanation of the speech perception deficit than damage to the motor system as SDL try to argue.