Qualitative differences in response bias from spatial cueing.

Two response bias parameters were examined in a spatial (peripheral) cueing paradigm: (i) "cued location bias": the relative probability with which a signal is incorrectly assigned to the cued location; and (ii) "likelihood ratio (beta)": the amount of evidence required to decide that a signal appeared at a particular, cued or uncued, location. The two parameters were found to be differentially sensitive to the stimulus onset asynchrony (SOA) between peripheral cue and target. Cued location bias was strongly affected by SOA, decreasing within 300 ms after peripheral cue onset. In contrast, the difference in beta between cued and uncued locations (cued < uncued) was independent of SOA. This suggests that cued location bias reflects strong but transitory pathway pre-activation following a peripheral cue, consistent with accounts of response bias effects in terms of relatively "early" processes of stimulus coding (e.g., Hawkins, Shafto, & Richardson, 1988; Shulman & Posner, 1988). In contrast, beta effects predominantly reflect "late" decision making processes that differentially weight the sensory evidence from cued and uncued locations according to their a-priori signal probabilities (e.g., Müller & Findlay, 1987; Shaw, 1982).