Imaging study provides new information on how the brain processes sounds of diffe...( rations were presented with an off pe...)

rations were presented with an "off" period of 40 s following each train. Noise burst repetition rate within each train was always 35/s. Each train duration was presented once per run (89 runs; 310 s per run) with the order of durations randomized across runs. Supplementary information concerning the effects of train duration was obtained in two additional experiments that used a single, long-train duration (60 s) and 35/s noise bursts.

Additional elements: Separately for each ear, the subject's threshold of hearing to 10/s noise bursts was determined in the scanner room. For all experiments, the stimuli were presented binaurally at 55 dB above this threshold. Subjects were instructed to listen to the noise burst stimuli. At the end of each scanning run, subjects reported their alertness on a qualitative scale ranging from 1 (fell asleep during run) to 5 (highly alert). Subjects were imaged using a whole-body scanner and a head coil. The scanners were retrofitted for high-speed imaging. Experiments I and II were conducted at 1.5 T. experiments III and IV were conducted at 3 T, except for one of the supplementary sessions of experiment IV.

Results
There was a systematic change in the form of fMRI response rate-dependencies from midbrain to thalamus to cortex.

• In the inferior colliculus, response amplitude increased with increasing rate while response waveshape remained unchanged and sustained.

• In the medial geniculate body, increasing rate produced an increase in amplitude and a moderate change in waveshape at higher rates (from sustained to one showing a moderate peak just after train onset).

• In auditory cortex, amplitude changed somewhat with rate, but a far more striking change occurred in response waveshape--low rates elicited a sustained response, whereas high rates elicited an unusual phasic response that included prominent peaks just after train onset and offset.

• The shi
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Contact: Donna Krupa
djkrupa1@aol.com
703-527-7357
American Physiological Society
12-Nov-2002


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