Multimodal effects of local context on target detection: evidence from P3b

Abstract

We used the P300 component to investigate how changes in local context influenced the ability to detect target stimuli. Local context was defined as the occurrence of a short predictive series of stimuli before delivery of a target event. EEG was recorded in 12 subjects during auditory and visual sessions. Stimuli were presented in the center of the auditory and visual field and consisted of 15% targets (1000 Hz tone or downward facing triangle) and 85% of equal amounts of three types of standards (1500, 2000, and 2500 Hz tones or triangles facing left, upward, and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Peak target P300 (P3b) amplitude and latency were evaluated for targets after predictive and nonpredictive sequences using conventional averaging and a novel single-trial analysis procedure. Reaction times were shorter for predictable targets than for nonpredicted targets. P3b latency was shorter for predicted targets than for nonpredictive targets, and there were no significant P3b amplitude differences between predicted and random targets, as determined by both conventional averaging and single-trial analysis. Comparable effects on amplitude and latency were observed in both the auditory and visual modalities. The results indicate that local context has differential effects on P3b amplitude and latency, and exerts modality-independent effects on cognitive processing.

Figure 1

Auditory (A) and visual (B) sessions. Sequences of standards S1, S2 and S3 with a predicted sequence (top) and in randomized order (bottom) preceding the target (T). Stimuli presented centrally. Inter-trial intervals, including duration of stimulus presentation (100ms) are displayed.

Figure 2

Reaction times for predicted and random targets in the auditory and visual modalities. Bars = standard errors of mean.

Figure 3

Grand average (n=12) at Pz for the 6 conditions: targets after non predictive (random) and predictive sequences (predicted), the three standards comprising the predicting sequence (n-3, n-2, n-1) and random preceding standards (standards) for auditory (A) and visual (B) modalities. Topographical maps for the peak P3b are shown for random, predicted and n-1 conditions.

Figure 4

P3b peak latency at Pz for predicted and random targets in the auditory and visual modalities. Bars = standard errors of mean.

Figure 5

P3b peak amplitude for the six conditions: targets after non predictive (random) and predictive sequences (predicted), the three standards comprising the predicting sequence (n-3, n-2, n-1) and random preceding standards (standards) in the auditory (A) and visual (B) modalities. Bars = standard errors of mean.

Figure 6

Grand average (n=12) for the difference wave (DW) between predicted and random targets at electrode sites Fz, Cz and Pz in the auditory (A) and visual (B) modalities. Topographical maps for the peak DW at Pz are displayed.

Figure 7

Grand average (n=12) for predicted and random targets at Fz in the auditory modality (A) and at PO8 in the visual modality (B), illustrating that N1 is not different in the two target conditions.

Figure 8

Grand average (n=12) for standards containing predictive information (n-1 and n-2, informative condition) and for standards containing no predictive information (n-3,n-4, non-informative condition) regarding the subsequent target at CPz in the auditory modality (A) and at Cz in the visual modality (B). These electrodes showed the greatest change in mean CNV amplitude between 900 and 1100 ms post-stimulus presentation, with significance difference (p<0.05, indicated by the star) in the auditory modality.

Figure 9

Single-trial P3b amplitude and latencies for predicted and random targets are plotted for auditory (A) and visual (B) modalities (n=1). Normalized values (z-transformed) of P3b amplitude and latency of predicted and random targets in all 12 subjects plotted for the auditory (C) and visual (D) modalities. Note that in all the plots latency is shorter, while amplitude is similar for predicted vs. random targets.