The impact of phantom decoys on the neural processing of valuation

Rational decision theories posit that good choices should be based solely on information that is relevant to the choice at hand. However, introducing an inferior option that would never be chosen can influence choices among other relevant options, known as decoy effect. We used functional magnetic resonance imaging (fMRI) combined with a simple gambling task to investigate the neural signature of decision-making in or against the influence of the decoy effect in inferior and superior phantom decoy conditions. The fMRI results show that compared with choosing against the influence of the dominated phantom inferior option, choosing in the influence of the same option was associated with stronger activation in bilateral caudate and weaker functional connectivity between the left ventral anterior cingulate cortex (vACC) and the left caudate. Phantom inferior effect selectively enhanced the connectivity from the caudate to the vACC but not vice versa. Choosing in the influence of the dominated phantom superior option engaged greater activity in the left dorsal ACC and stronger functional connectivity between the left dACC and bilateral anterior insula. Furthermore, the direction of the phantom superior effect was restricted from left dACC to the anterior insula, but not vice versa. Our findings suggest that a phantom inferior decoy may boost the value of the target via the reward network, whereas a phantom superior decoy may diminish the value of the target option via the aversion network. Our study provides neural evidence to support that valuation is context dependent and delineates differential neural networks underlying the influence of unavailable inferior and superior decoy options on our decision-making.