Background We studied a young woman (LM) with auditory agnosia, caused by brain stem trauma damaging the left inferior colliculus. DTI tractography demonstrated that connectivity of the auditory pathways from the acoustic nerve to primary auditory cortex was not compromised. She was impaired in recognising words, environmental sounds, melodies and emotional expression. We measured the temporal resolution of her auditory system by measured 2-click discrimination as well as her ability to distinguish words differing in voice onset time (VOT) or place of articulation (POA). VOT refers to the time between aspiration and the vibration of the vocal cords. When the duration of the unvoiced interval is manipulated, words differing only in VOT (duck-tuck) are perceived categorically. That is, there is an abrupt boundary such that the listener will either hear one sound or another but never a hybrid of the two.
Method Two, two-alternate forced-choice experiments were performed, with accuracy as the dependent measure.
Experiment 1 Two brief clicks were presented with random intervals between them, and the patient reported whether she heard one or two clicks.
Experiment 2 L listened to one of eight spoken words through headphones and then immediately saw two written words, one above the other on a computer monitor. One word in each word-pair was the correct response and the other differed in VOT (duck-tuck), POA (duck-buck) or both (duck-puck). She indicated by key press which of the two words she had heard. Before the experiment she had listened to each of the eight words and reported that she could not understand any of them.
Results Whereas two clicks can normally be identified with intervals of less than 10 ms, LM did not reliably hear two clicks until the interval was 170 ms. With free report she could not distinguish any of the words. In the forced choice task, she was impaired at distinguishing word pairs differing both in VOT (66% correct) and POA (71%) although performance was above chance for both. Discrimination was best when word pairs differed in both VOT and POA (78%). fMRI contrasting silence versus sounds (vocal and environmental) revealed activation of parts of primary auditory cortex (A1) but not auditory belt or parabelt.
Conclusion The inferior colliculus is likely to have a role auditory temporal resolution, dependent on discrimination based on VOT and POA.