The medial temporal lobe supports mnemonic discrimination for event duration

The Medial Temporal Lobe Supports Mnemonic Discrimination for Event Duration

ABSTRACT
Time has an integral role in episodic memory and previous work has implicated the medial temporal lobe in both representing time and discriminating between similar information. Here we developed a novel paradigm to test mnemonic discrimination for temporal duration, as previous work has largely focused on order or maintaining information over time. Thirty-five healthy, young adults completed a continuous-recognition, temporal discrimination task in which participants were tasked with detecting a change in stimulus duration on the order of 0.5 seconds. Whole-brain high-resolution fMRI data were acquired. Behavioral results indicate that participants were successful at detecting whether or not the stimulus duration changed. Further, fMRI analyses revealed that the left entorhinal and perirhinal cortices were differentially involved in encoding and retrieval, respectively. These findings support the notion of the entorhinal cortex being involved in the encoding of temporal context information as well as the perirhinal cortex representing the conjunction of item and context during retrieval.

MRI METHODS
Functional and structural data were collected on a Siemens Tim Trio 3T MRI scanner at the Brigham Young University MRI Research Facility, using a 32-channel head coil. T1-weighted, T2-weighted, and T2*-weighted scans were collected from each participant. The T1-weighted structural scan was acquired using a magnetization-prepared rapid acquisition with gradient echo (MP-RAGE) sequence, according to the following parameters: TR = 1900 ms, TE = 2.26 ms, flip angle = 9°, FoV = 218 × 250, voxel size = 0.97 × 0.97 × 1 mm, slices = 176 interleaved. A high-resolution T2-weighted scan was used to image hippocampal subfields according to the following parameters: TR = 8020 ms, TE = 80 ms, flip angle = 150°, FoV = 150 × 150 mm, voxel size = 0.4 × 0.4 × 2 mm, slices = 30 interleaved; the T2 data were acquired perpendicular to the long axis of the hippocampus. High-resolution multi-band echo-planar scans were collected from each participant according to the following parameters: Multi-band factor = 4, TR = 2200 ms, TE = 45.2 ms, flip angle = 90°, acquisition matrix = 100 × 100 mm, voxel size = 1.8 mm3, slices = 72 interleaved; the first 11 TRs were discarded to allow for T1 equilibration. Finally, a reverse blip echo-planar scan was collected using the same parameters as the previous protocol save that direction of acquisition was opposite (P>>A) and only 10 volumes were collected.