Selective Impairments of Nouns and Verbs and their Neuroanatomical Correlates
Theoretical linguistics suggests that the representation of verbs (V) and nouns (N) is independent, reflecting the functional difference of these word classes in syntax. Observation of brain damaged patients demonstrates the existence of double dissociations with disproportionate impairments of either Vs or Ns. However, the mechanisms underlying N-V dissociations are not yet well defined.
The aim of the present study was the identification of the anatomical loci of the lesions causing selective impairments of Vs and Ns.
Fifteen aphasic patients suffering from disproportionate impairment of verbs and five patients suffering from disproportionate impairment of nouns participated in the study. N-V dissociations were identified by evaluating naming abilities of Ns and Vs through a picture-naming task. The degree of fine motor planning (actionality) underlying the action denoted by each of the verbs was tested on a group of 25 healthy undergraduate students.
Lesions were reconstructed and mapped onto a lateral template of the brain.
In the majority of the noun impaired patients, the lesion involved the medial part of the middle and inferior left temporal gyri. On the contrary, the lesions found in V impaired patients clustered in two major subsets: in a first group, the focal damage involved the posterior part of the left temporal lobe and the inferior parietal gyrus. In a second group, the lesion caused complete destruction of the left hemisphere language areas. Isolated frontal lesions were not present in any of the V impaired cases.
A further analysis tested whether the temporo-parietal region stores lexical representations of action Vs only or whether it is a functional unit involved in processing all types of Vs. High- and low-actionality Vs were equally impaired in the four patients with temporo-parietal lesions.