ERP Indices of Stimulus Prediction in Letter Sequences

Abstract

Given the current focus on anticipation in perception, action and cognition, including language processing, there is a need for a method to tap into predictive processing in situations in which cue and feedback stimuli are not explicitly marked as such. To this aim, event related potentials (ERPs) were obtained while participants viewed alphabetic letter sequences ("A", "B", "C", "D", "E", …), in which the letters were highly predictable, and random sequences ("S", "B", "A", "I", "F", "M", …), without feedback. Occasionally, the presentation of a letter in a sequence was delayed by 300 ms. During this delay period, an increased negativity was observed for predictive versus random sequences. In addition, the early positivity following the delay was larger for predictive compared with random sequences. These results suggest that expectation-sensitive ERP modulations can be elicited in anticipation of stimuli that are not explicit targets, rewards, feedback or instructions, and that a delay can strengthen the prediction for a particular stimulus. Applications to language processing will be discussed.

Figure 1

Event-related potentials (ERPs) time-locked to the onset of the pre-delay letter in the delay conditions for 25 selected electrodes (left hemisphere (F3, F5, FC3, FC5, C3, C5, CP3, CP5, P3, P5); midline (Fz, FCz, Cz, CPz, Pz); right hemisphere (F4, F6, FC4, FC6, C4, C6, CP4, CP6, P4, P6). The delay period (500–800 ms) is indicated with the black bar above the time scale; the letter following the delay starts at 800 ms. Blue line: predictive (alphabetic) condition; red line: random condition. Gray vertical bar: 600–900-ms interval used for the analysis of the effects during the delay; vertical dotted lines: analysis window for the P2/P3 effects. ERPs are baselined to the 200 ms following the onset of the second letter before the potential delay (−500 to −300 ms, see the green bar above the time scale). In this and the following figures, negative is plotted up.

Figure 2

ERPs for the no-delay conditions, time-locked to the onset of the letter equivalent in position to the pre-delay letter in Figure 1. The next letter starts at 500 ms. Blue line: predictive (alphabetic) condition; red line: random condition. ERPs are baselined to the −500 to −300 ms interval (green bar). Gray vertical bar: 600–900 ms interval used in the analysis of the delay effects; vertical dotted lines: analysis window for the P2/P3 effects.

Figure 3

Mean amplitude (µV) in the 600–900-ms window for the lateral regions, collapsed over hemisphere (error bars are standard errors). Blue lines: predictive sequences (Pred); red lines: random sequences; solid lines: delay conditions; dotted lines: conditions without a delay. F: frontal; FC: fronto-central; C: central; CP: central-parietal; P: parietal.

Figure 4

Mean amplitude (µV) between 180 and 280 ms from the onset of the post-delay letter, for the lateral positions, collapsed over hemisphere (error bars are standard errors). Blue lines: predictive sequences (Pred); red lines: random sequences; solid lines: delay conditions; dotted lines: conditions without a delay. F: frontal; FC: fronto-central; C: central; CP: central-parietal; P: parietal.

Figure 5

ERPs time-locked to the onset of the first letter in the sequence. ERPs are baselined to 100 ms preceding the onset of the first letter. Letters are presented for 300 ms, starting at 0 ms, 500 ms and 1000 ms. Blue line: predictive (alphabetic) condition; red line: random condition. Dotted lines: window for the analysis of the P2; shaded intervals: windows used for the analysis of the negativity (dark bar: 800–1000 ms; light bar: 1300–1500 ms).

Figure 6

Mean amplitude (µV) between 180 and 280 ms from the onset of the second letter in the trial, for the lateral positions, collapsed over hemisphere and delay (error bars are standard errors). Blue lines: predictive sequences; red lines: random sequences. F: frontal; FC: fronto-central; C: central; CP: central-parietal; P: parietal.