stability important for ego / allocentric weighting 1 1 2 3

29 30 It is not known how egocentric visual information (location of a target relative to the self) and 31 allocentric visual information (location of a target relative to external landmarks) are integrated 32 to form reach plans. Based on behavioural data from rodents and humans we hypothesized that 33 the degree of stability in visual landmarks would influence the relative weighting. Furthermore, 34 based on numerous cue-combination studies we hypothesized that the reach system would act 35 like a maximum-likelihood estimator (MLE), where the reliability of both cues determines their 36 relative weighting. To predict how these factors might interact we developed an MLE model that 37 weighs egocentric and allocentric information based on their respective reliabilities, and also on 38 an additional stability heuristic. We tested the predictions of this model in 10 human subjects by 39 manipulating landmark stability and reliability (via variable amplitude vibration of the landmarks 40 and variable amplitude gaze-shifts) in three reach-to-touch tasks: an egocentric control (reaching 41 without landmarks), an allocentric control (reaching relative to landmarks), and a cue-conflict 42 task (involving a subtle landmark ‘shift’ during the memory interval). Variability from all three 43 experiments was used to derive parameters for the MLE model, which was then used to simulate 44 egocentric-allocentric weighting in the cue-conflict experiment. As predicted by the model, 45 landmark vibration –despite its lack of influence on pointing variability (and hence allocentric 46 reliability) in the control experiment— had a strong influence on egocentric-allocentric 47 weighting. A reduced model without the stability heuristic was unable to reproduce this effect. 48 These results suggest heuristics for extrinsic cue stability are at least as important as reliability 49 for determining cue weighting in memory-guided reaching. 50