Dissociating N400 effects of prediction from association in single-word contexts

Abstract

When a word is preceded by a supportive context such as a semantically associated word or a strongly constraining sentence frame, the N400 component of the ERP is reduced in amplitude. An ongoing debate is the degree to which this reduction reflects a passive spread of activation across long-term semantic memory representations as opposed to specific predictions about upcoming input. We addressed this question by embedding semantically associated prime-target pairs within an experimental context that encouraged prediction to a greater or lesser degree. The proportion of related items was used to manipulate the predictive validity of the prime for the target while holding semantic association constant. A semantic category probe detection task was used to encourage semantic processing and to preclude the need for a motor response on the trials of interest. A larger N400 reduction to associated targets was observed in the high than the low relatedness proportion condition, consistent with the hypothesis that predictions about upcoming stimuli make a substantial contribution to the N400 effect. We also observed an earlier priming effect (205-240 msec) in the high-proportion condition, which may reflect facilitation because of form-based prediction. In summary, the results suggest that predictability modulates N400 amplitude to a greater degree than the semantic content of the context.

Figure 1

Grand-averaged waveforms to target words following related and unrelated primes under conditions of low and high relatedness proportion at site Cz. Voltage maps comparing ERPs evoked by the target between 300 and 500 ms (unrelated – related) for each level of relatedness proportion. See Supplementary Figures 1–2 for full 32-electrode waveform maps at each level of relatedness proportion.

Figure 2

Grand-averaged waveforms to target words following related and unrelated primes under conditions of low and high relatedness proportion at site Cz.

Figure 3

Quadrant analysis of N400 priming effect (amplitude difference between unrelated and related targets during the 300–500 ms time window). Bar plots comparing grand-average amplitude differences in each of four quadrants indicated on voltage maps, for each level of relatedness proportion. Voltage maps comparing average ERP amplitude difference between unrelated and related targets between 300–500 ms for each level of relatedness proportion.

Figure 4

Grand-averaged difference waves reflecting the priming effect (unrelated – related) at site Cz for each level of relatedness proportion. Time-windows showing a significant priming effect (p < .05) in the latency onset analysis are indicated. Voltage map comparing ERPs evoked by the target between 200–250 ms (unrelated – related) in high relatedness proportion condition.

Figure 5

Grand-averaged waveform to target words following related and unrelated primes in the high relatedness proportion condition at site FPz. Voltage map comparing ERPs evoked by the high relatedness proportion targets between 500 and 800 ms (unrelated – related), the time-window expected to show costs of prediction violation. See Supplementary Figures 3–4 for full 32-electrode waveform maps at each level of relatedness.

Figure 6

Grand-averaged waveforms to probe words (following unrelated primes) under conditions of low and high relatedness proportion at two sites that showed a significant difference between conditions. Voltage maps comparing ERPs evoked by probe words under conditions of low and high relatedness proportion. See Supplementary Figure 5 for full 32-electrode waveform map.

Figure 7

Grand-averaged waveforms to prime words under conditions of low and high relatedness proportion at sites showing a significant difference between conditions. Voltage maps comparing ERPs evoked by prime words under conditions of low and high relatedness proportion (high – low). See Supplementary Figure 6 for full 32-electrode waveform map.