Motion-onset visual evoked potentials predict performance during a global direction discrimination task

Abstract

The relationship between cognitive processing stages and event-related potential components has been extensively researched for single components, but even the simplest task comprises multiple electrophysiological and cognitive components. Here we examined the relationship between behavioral measures and several visual evoked potentials (VEPs) related to global motion onset during a visual motion discrimination task. In addition to reaction time and accuracy, the EZ diffusion model was used to characterize elements of the decision process. Results showed that latencies, but not amplitudes, from three VEP components reliably predicted about 40% of the variance in reaction times for motion discrimination. These included the latency from stimulus motion onset to N2 onset, the latency from N2 onset to N2 peak, and the latency from the N2 peak to the peak of a late positive potential. These latencies were also able to predict the rate of information accumulation during the decision process and the duration of non-decision processes, but not the observer's threshold (boundary) for making a response. This pattern of results is consistent with an interpretation of these three latencies as reflecting a non-specific visual perceptual process, a motion-specific process, and a decision process, respectively. The relationship between the earliest interval and drift rate estimated with the EZ model also supports the notion that early perceptual processing might be a constituent part of the decision process itself.

Copyright © 2010 Elsevier Ltd. All rights reserved.

Figures

Figure 1

Schematic illustration of the diffusion model first proposed by Ratcliff (1978). Following appearance of a visual stimulus in a two-alternative forced-choice task, sensory encoding processes take some amount of time. Following sensory encoding, evidence for one of the possible responses (E) accumulates subject to random drift (grey lines). Once this accumulation process passes a threshold for one of the possible responses (correct or incorrect), that response is initiated. Performance is therefore a function of the rate of information accumulation (drift rate), the location of the threshold (boundary) for each possible response (correct and incorrect), and perceptual and responses factors that are combined into the non-decision time (NDT).

Figure 2

Schematic illustration of global motion discrimination task and stimuli. A: On each trial, after a uniformly random inter-trial interval of 1-2 s, a fixation spot appeared for 1 s, followed by an array of stationary random dots. After a uniformly random interval between 1-2 s, the randomly positioned dots began to move to the right or left. B: On coherent motion (DR0) trials, all dot moved in the same direction, while on noise (DR320) trials, the motion of each dot varied randomly at each time frame within a range of 320 degrees about the mean leftward or rightward direction.

Figure 3

A. Example of evoked responses to DR0 (red lines) and DR320 (blue lines) motion onsets at different scalp locations. More anterior scalp locations are situated at the top of the figure, Posterior scalp locations are situated at the bottom of the figure. Left and right are veridical. B. Motion-onset VEP from a single, typical participant, collected from the Oz electrode (see A for relative scalp location). Waveform components P1, N2, and LP are labeled. The grey bar illustrates the region within 2 standard deviations of the mean of the baseline epoch. Below the waveform, the horizontal brackets and dotted lines illustrate the three intervals used in the regression analysis: 1 = motion onset to onset of the N2, defined as the point at which the N2 component crosses the threshold of 2 standard deviations below the mean of the baseline epoch, tracking backward from the N2 peak. 2 = the interval between the onset and peak of the N2. 3 = the interval between the peak of the N2 and the peak of the late positive potential LP. Scalp topographies from the same participant are presented below the waveform, corresponding to the P1 peak, N2 peak, P2 peak (which is absent in most participants, including the example) and LP peak.

Figure 4

Waveforms from central electrodes Oz, POz, Pz, CPz, Cz, and Fz, averaged across all 23 participants. Thick continuous lines represent the DR0 condition, dashed lines the DR320 condition, and the vertical line at 0 ms represents motion stimulus onset. The P1, N2 and LP peaks are indicated where present.

Figure 5

Scatter plots showing correlations between reaction time residuals and different VEP epochs. Black diamonds represent the DR0 condition, open circles represent the DR320 condition, black lines represent model fits for the DR0 condition and grey lines represent model fits for the DR320 condition. In order to approximately represent the multiple regression fit (which is a hyperplane), the ordinate plots unstandardized residuals of the regression of RT on the two predictors that are not included in each graph (i.e., the variance not accounted for by those predictors), and fit lines are then based on the regression of these residuals on the predictor indicated on the abscissa. A. RT (residuals after removing variance accounted for by N2 onset – N2 peak and N2 peak – LP intervals) as a function of the interval from motion onset to N2 onset (interval 1 in Figure 3). B. RT residuals as a function of the interval from N2 onset to N2 peak (interval 2 in Figure 3). C. RT residuals as a function of the duration between N2 and LP peaks (interval 3 in Figure 3).