Neurology of anomia in the semantic variant of primary progressive aphasia

Abstract

The semantic variant of primary progressive aphasia (PPA) is characterized by the combination of word comprehension deficits, fluent aphasia and a particularly severe anomia. In this study, two novel tasks were used to explore the factors contributing to the anomia. The single most common factor was a blurring of distinctions among members of a semantic category, leading to errors of overgeneralization in word-object matching tasks as well as in word definitions and object descriptions. This factor was more pronounced for natural kinds than artifacts. In patients with the more severe anomias, conceptual maps were more extensively disrupted so that inter-category distinctions were as impaired as intra-category distinctions. Many objects that could not be named aloud could be matched to the correct word in patients with mild but not severe anomia, reflecting a gradual intensification of the semantic factor as the naming disorder becomes more severe. Accurate object descriptions were more frequent than accurate word definitions and all patients experienced prominent word comprehension deficits that interfered with everyday activities but no consequential impairment of object usage or face recognition. Magnetic resonance imaging revealed three characteristics: greater atrophy of the left hemisphere; atrophy of anterior components of the perisylvian language network in the superior and middle temporal gyri; and atrophy of anterior components of the face and object recognition network in the inferior and medial temporal lobes. The left sided asymmetry and perisylvian extension of the atrophy explains the more profound impairment of word than object usage and provides the anatomical basis for distinguishing the semantic variant of primary progressive aphasia from the partially overlapping group of patients that fulfil the widely accepted diagnostic criteria for semantic dementia.

Figure 1

Percentage of pointing errors in the word–picture and picture–word matching tasks that consisted of semantically related items. The asterisks indicate percentages that were significantly greater than chance at the individual patient level (Experiment 1).

Figure 2

Reaction times in the computerized word-to-picture matching task in trials where the two choices on the screen were semantically related or unrelated. Asterisks indicate significant differences at the individual level (Experiment 2).

Figure 3

Reaction times in trials on the computerized task where both choices were living or non-living objects. Asterisks indicate significant differences at the individual level (Experiment 2).

Figure 4

Distribution of cortical thinning in group of five PPA-S patients (P1, P3–P5 and P6) compared to a control group. f = fusiform (occipitotemporal) gyrus; i = inferior temporal gyrus; m = middle temporal gyrus; p = temporal polar cortex; ph = parahippocampal gyrus; s = superior temporal gyrus. Significance is displayed as a log₍₁₀₎ P-value.

Figure 5

T₁-weighted MRI scans of P2 and P6. The sections are in the coronal plane and are displayed according to the radiological convention so that the left hemisphere is on the right side of each section. The arrows point to asymmetric widening of the left perisylvian region. For each patient, the section on the left is most anterior, at the level of the temporal pole and amygdala, the middle section is at the level of the anterior hippocampus and the section on the right is at the level of the mid-to-posterior fusiform gyrus. a = amygdala; f = fusiform (occipitotemporal) gyrus; h = hippocampus; i = inferior temporal gyrus; m = middle temporal gyrus; p = temporal polar cortex; ph = parahippocampal gyrus; s = superior temporal gyrus.