Individual differences in response precision correlate with adaptation bias

The internal representation of stimuli is imperfect and subject to bias. Noise introduced at initial encoding and during maintenance degrades the precision of representation. Stimulus estimation is also biased away from recently encountered stimuli, a phenomenon known as adaptation. Within a Bayesian framework, greater biases are predicted to result from poor precision. We tested for this effect in individual difference measures. 202 subjects contributed data through an on-line experiment ( https://cfn.upenn.edu/iadapt ). During separate face and color blocks, subjects performed three different tasks: an immediate stimulus-match (15 trials), a 5 seconds delayed match (30 trials), and 5 seconds of adaptation followed by a delayed match (30 trials). The stimulus spaces were circular and subjects entered their responses using a color/face wheel. Bias and precision of responses were extracted by fitting a mixture of von Mises distributions to account for random guesses. Two blocks of each measure were obtained, allowing for tests of measure reliability. We found that reliable differences between individuals in precision were as great as those between tasks or materials. The adaptation manipulation induced the expected bias in responses (colors: 7.8°; faces: 5.0°), and the magnitude of this bias reliably and substantially varied between subjects. Across subjects, there was a negative correlation between mean precision and bias (color: ρ = −0.26; faces: ρ = −0.13). This relationship was replicated in a new experiment with 192 subjects (color: ρ = −0.22; faces: ρ = −0.19). This result is consistent with a Bayesian observer model, in which individual differences in the precision of perceptual representation influences the magnitude of adaptation bias