Sustained posterior contralateral activity indicates re-entrant target processing in visual change detection: an EEG study

Abstract

The present study investigated the neural mechanisms that contribute to the detection of visual feature changes between stimulus displays by means of event-related lateralizations of the electroencephalogram (EEG). Participants were instructed to respond to a luminance change in either of two lateralized stimuli that could randomly occur alone or together with an irrelevant orientation change of the same or contralateral stimulus. Task performance based on response times and accuracy was decreased compared to the remaining stimulus conditions when relevant and irrelevant feature changes were presented contralateral to each other (contralateral distractor condition). The sensory response to the feature changes was reflected in a posterior contralateral positivity at around 100 ms after change presentation and a posterior contralateral negativity in the N1 time window (N1pc). N2pc reflected a subsequent attentional bias in favor of the relevant luminance change. The continuation of the sustained posterior contralateral negativity (SPCN) following N2pc covaried with response times within feature change conditions and revealed a posterior topography comparable to the earlier components associated with sensory and attentional mechanisms. Therefore, this component might reflect the re-processing of information based on sustained short-term memory representations in the visual system until a stable target percept is created that can serve as the perceptual basis for response selection and the initiation of goal-directed behavior.

Keywords: N2pc; SPCN; attention; perception; re-entrant processing; short-term memory.

Figure 1

Experimental setup. Participants were instructed to localize a change of luminance within a fast sequence of two stimulus displays containing two lateralized bars. These luminance changes either occurred alone (LUM) or simultaneous with an orientation change of the same (LOU) or contralateral bar (LOB). Additionally, only the orientation of one bar changed in one out of four trials (ORI). Localization was accomplished by a button press at the side of the luminance change using the index finger.

Figure 2

Behavioral results. The contralateral distractor condition (LOB) revealed higher error rates (bars) and response times (diamond shapes) compared to the remaining stimulus conditions. While both error rates and RTs did not differ between the LUM and LOU conditions, lower error rates were revealed in the No-Go ORI condition compared to all conditions containing a relevant luminance change (LUM, LOB, LOU). The error bars depict the standard error of the mean.

Figure 3

Grand average and vincentized ERLs (PO7/PO8) for all change conditions. (A) Depicts the grand average ERL data for the LUM, LOB, LOU, and ORI conditions. The components of interest (i.e., change-related positivity, N1pc, N2pc, and SPCN) were marked for each change condition. Both N2pc and SPCN were only analyzed in the target change conditions. (B) Depicts the vincentized ERLs for the LUM, LOB, and LOU conditions based on a color scale (negativity = blue, positivity = red) and the same time window also used for the grand average data. The bold black sinusoid line represents the mean response time (RT) across each bin. Change-related positivity, N1pc and N2pc were shown across all RT bins and revealed no consistent co-variation with RT based on both latency and amplitude of the components. However, the continuation of posterior contralateral negativity in the later time window (SPCN) was prolonged with increasing RTs in the LUM and LOB condition. The posterior asymmetry subsequent to response likely represented the lateralized sensory response to the manual key presses.

Figure 4

Grand average posterior ERPs (PO7/PO8) contralateral and ipsilateral to the feature change. In case of the luminance change conditions (LUM, LOB, LOU), the black curve represents the activation contralateral to the luminance change. For the ORI condition, the black curve represents the activation contralateral to the orientation change. The ERL (contralateral ipsilateral difference function) is plotted as a red dashed curve. Although an overlap of the P2 related to the first stimulus display and early sensory components related to the target display were revealed in the contralateral and ipsilateral ERPs, the ERL revealed a clear pattern of posterior asymmetries associated with lateralized target processing.

Figure 5

Scalp topographies of posterior asymmetries for all change conditions. For N1pc, N2pc, and SPCN, the higher negativity contralateral to the relevant luminance change compared to the negativity contralateral to the irrelevant stimulus is plotted on the left hemisphere, while an asymmetry toward the irrelevant stimulus is plotted on the right hemisphere (see LOB condition). The same logic applies to the change-related positivity. The depicted 20 ms time windows correspond to those used for statistical analyses and were oriented at the grand average ERL peaks. Because the LOU condition revealed no distinct ERL peak in the N2 range, a 200–220 ms time window was used to illustrate N2pc scalp topography.

Figure 6

Statistical analyses for the vincentized ERLs (PO7/PO8) based on bootstrap t-tests against zero (FDR corrected p-values). The reliable occurrence of posterior asymmetries was tested in 10 bins within an overall time window from 160 to 510 ms subsequent to the feature change(s). While a blue square represents negativity in the ERL that differed significantly from zero, a red square indicates a significant positivity in the posterior ERLs. Non-significant values are depicted in green. In the LUM and LOB conditions, a longer lasting negative difference from zero was shown for slower compared to faster responses. No comparable co-variation of RT and the continuation of posterior contralateral negativity was revealed for the LOU condition.