Isolating objective and subjective filling-in using the Drift Diffusion Model

Abstract

Spatial context is known to influence the behavioral sensitivity (d) and the decision criterion (c) for detection of low-contrast targets. Of interest here is the effect on the decision criterion. Polat and Sagi (2007) demonstrated that for a Gabor target positioned between two similar co-aligned high contrast flankers, observers reports of seeing the target (Hit and False-Alarm) decreased with increasing target-flankers distance. This effect was more pronounced when distance was randomized within testing blocks compared to when it was fixed. According to Signal-Detection-Theory (SDT), the latter result suggests that the decision-criterion is adjusted to a specific, distance-dependent combination of signal (S) and Noise (N) when the S and N statistics are fixed, but not when they vary across trials. However, SDT cannot differentiate between changes in the decision-bias (criterion-shift) and changes introduced by variations in S and N (signal-shift). To circumvent this limitation of SDT, we analyze reaction-time data (RT) within the framework of the Drift-Diffusion-Model (DDM). We performed an RT analysis of target-flanker interactions using data from Polat & Sagi (2007) and Zomet et al. (2008; 2016). The analysis revealed a stronger dependence on flankers for faster RTs and a weaker dependence for slower RTs. The results are explained by DDM, where an evidence accumulation process depends on the flankers via a change in the rate of the evidence (signal-shift), and on observers prior via a change in the starting point (criterion-shift), leading to RT-independent and RT-dependent effects, respectively. The RT-independent distance-dependent response-bias is attributed to the observers inability to learn multiple internal distributions required to accommodate the distance-dependent effects of the flankers on both the Signal and Noise.