University of Pennsylvania School of Medicine, Department of Neurology, Philadelphia, Pennsylvania, PA, USAAccepted 6 July 2006IntroductionSemantic dementia (SD) results in a progressive loss of conceptualknowledge in the context of preserved syntactic, auditory, and visual spa-tial abilities (Grossman & Ash, 2004; Hodges, 2003; Paterson, Knott, H moderate: 66). The mean MMSEwas 18.75 (mild: 24; moderate: 13.5). Semantic memory impairment asmeasured by the Pyramids and Palm Trees test (Howard & Patterson,1992) averaged, for pictures and words, 43.75/52 (mild: 47.25; moderate:40.25). Average confrontation naming on the Boston Naming Test (Kap-lan, Goodglass, & Weintraub, 1976) averaged 5.75 of 15 (mild: 9; moder-ate: 2.5). All patients passed a pure tone hearing screening at 1, 2, and4 kHz. Patients were instructed through training sessions before each task.All auditory stimuli were digitized and administered by headphones with afrequency response range of 8–25 kHz.Structural MRI scans of milder SD revealed atrophy in the left ventral,anterolateral cortex (see Fig. 1a for VBM analysis of significant corticalatrophy in a mild SD patient). Patients with moderate SD showed atrophyspread to the posterior temporal cortex (see Fig. 1b for a VBM analysis ofsignificant cortical atrophy in a moderate SD patient).Experiment 1: Pure tone acoustic discriminationPure tones varying in frequency from 300 to 5000 Hz were presented inpairs, with a 1000 ms interstimulus interval (ISI). Half the tone pairs wereidentical; the remainder differed by increments of 25–4000 Hz. ParticipantsindicatedwhethereachtonepairwastheSAMEorDIFFERENTwithakeypress.ResultsOverall accuracy of pure tone discrimination was 82.5% (p < .001,binomial test differing from chance). The within-group contrast of mildSD versus moderate SD demonstrated greater discrimination difficultyin moderate SD (mild accuracy: 90%; moderate accuracy: 75%;v