Compensation of Trial-to-Trial Latency Jitter Reveals the Parietal Retrieval Success Effect to be Both Variable and Thresholded in Older Adults

Abstract

Although the neural mechanism supporting episodic recollection has been well characterized in younger adults, exactly how recollection is supported in older adults remains unclear. The electrophysiological correlate of recollection-the parietal retrieval success effect-for example, has been shown to be sensitive to both the amount of information recollected and the accuracy of remembered information in younger adults. To date, there is mixed evidence that parietal effect also scales with the amount of information remembered in older adults whilst there is little evidence that the same mechanism is sensitive to the accuracy of recollected information. Here, we address one potential concern when investigating Event Related Potentials (ERPs) among older adults-namely, the greater potential for single-trial latency variability to smear and reduces the amplitudes of averaged ERPs. We apply a well-established algorithm for correcting single-trial latency variability, Residual Iteration Decomposition Analysis (RIDE), to investigate whether the parietal retrieval success effect among older adults is sensitive to retrieval accuracy. Our results reveal that similar to younger adults, older adult parietal retrieval success effects scale with the accuracy of recollected information-i.e., is greater in magnitude when recollected information is of high accuracy, reduced in magnitude when accuracy is low, and entirely absent when guessing. The results help clarify the functional significance of the neural mechanism supporting recollection in older adults whilst also highlighting the potential issues with interpreting average ERPs in older adult populations.

Keywords: cognitive aging; episodic recollection; parietal ERP effect; residual iteration decomposition analysis; retrieval accuracy.

Figure 1

The source memory task. (A) During encoding participants are instructed to memorize words paired with locations, indicating the location after each trial to confirm attention. (B) During retrieval, participants are shown previously presented words and are asked to recall the position using the mouse. (C) Source accuracy is measured by calculating (in degrees) between true and responded locations.

Figure 2

Illustration of the effect of different latency variabilities. The far left panel illustrates increased amplitude with low latency jitter, the middle panel shows low amplitude and low jitter and the far right illustrates increased amplitude and high jitter. Note that the average waveform in the far right panel mimics the average waveform observed in the middle panel (i.e., low amplitude).

Figure 3

Schematic illustration of conventional stimulus-locked average Event Related Potential (ERP) (left) from single trials with latency jitter, decomposition byResidual Iteration Decomposition Analysis (RIDE; middle) and reconstructed ERP after correcting for latency variability (right). The S component is shown as a blue waveform and the C cluster as a red waveform. Figure is adapted from Ouyang et al. (2016).

Figure 4

The observed error distribution clearly shows a mixture of thresholded recollection with responses clustering around the target and sub-thresholded guessing that is uniformly distributed above zero. The dashed lines represent the division of ERP bins. Inset is the wrapped distribution of errors from both the left and right side of the target.

Figure 5

Grand average ERPs for High Accuracy responses (1°–10°) shown as a red dashed line, Low Accuracy responses (11°–35°) shown as a blue dashed line, Guess responses (36°–90°) shown as a Green dotted line, and Baseline (over 90°)—shown as a solid black line. Representative electrodes are shown over the left Parietal Region (PR; top row: PR6, PR3, PR1) and right PR (bottom row: PR6, PR4, PR2). Inset is the schematic map of the 62 recording electrodes—with those representative electrodes represented as black dots.

Figure 6

Scalp maps illustrating old/baseline distributions for High Accuracy and Low Accuracy responses during the 500–800 ms time window. Each map is shown as if looking down on the head with frontal sites pointing towards the top of the page. The scale bar indicates the voltage range (μV).

Figure 7

Grand average Reconstructed ERPs for High Accuracy responses (1°–10°) shown as a red dashed line, Low Accuracy responses (11°–35°) shown as a blue dashed line, Guess responses (36°–90°) shown as a Green dotted line and Baseline (over 90°)—shown as a solid black line. Representative electrodes are shown over the left PR (top row: PR5, PR3, PR1) and right PR (bottom row: PR6, PR4, PR2). Inset is the schematic map of the 62 recording electrodes—with those representative electrodes represented as black dots.

Figure 8

Scalp maps illustrating Reconstructed ERP old/baseline distributions for High Accuracy and Low Accuracy responses during the 500–800 ms time window. Each map is shown as if looking down on the head with frontal sites pointing towards the top of the page. The scale bar indicates the voltage range (μV).

Figure 9

Figure illustrating the effect of RIDE: on the left is the original ERP data (waveforms shown at representative electrode PR4) with the topographies for High and Low Accuracy responses directly below. On the right side is the Reconstructed ERP data (waveforms shown at electrode P5) with topographies for High and Low Accuracy shown below.