Abstract
A number of studies have examined whether the statistical imbalance of target frequency in the two-rectangle paradigm drives prioritization in object-based attention (OBA). While results suggest that OBA effects can be driven by spatial probability, behavioral measures are limited to the inclusion of invalid trials and it is yet unknown whether OBA would be observed in a condition where the cue is presented at 100% validity. To investigate this, we leveraged the spatial specificity of fMRI and the retinotopic organization of early visual cortex to identify potential neural correlates of OBA in the complete absence of invalid trials. Using fMRI, we had nine participants perform an exogenous version of the classic two-rectangle OBA paradigm while simultaneously measuring changes in BOLD signals arising from retinotopically organized cortical areas V1, V2 and V3. In the first half of the experiment, the exogenous cue was 100% valid. In the second half, the cue was 83.3% valid, more closely matching standard OBA paradigms. We then sorted BOLD signals arising from quadrant-based regions of interest according to whether they corresponded to cued, uncued same object or uncued other object. Results: in both the 100% and 83.3% runs we compared cued and uncued responses and as expected observed effects of spatial attention (cued versus uncued) in V2 and V3 but not V1. We also observed small but statistically significant effects of OBA in V3 (but not V2 and V1) in both the 100% and 83.3% valid runs. Moreover, the effects of OBA were no smaller in the 100% compared to 83.3% valid runs. Conclusions: Neural correlates of OBA can be observed using exogenous cueing and in the absence of invalid trials. This suggests that while target-location frequency may influence the distribution of cued attention, at least some OBA enhancement can arise independently of target uncertainty.