Action ability modulates time-to-collision judgments

Abstract

Time-to-collision (TTC) underestimation has been interpreted as an adaptive response that allows observers to have more time to engage in a defensive behaviour. This bias seems, therefore, strongly linked to action preparation. There is evidence that the observer's physical fitness modulates the underestimation effect so that people who need more time to react (i.e. those with less physical fitness) show a stronger underestimation effect. Here we investigated whether this bias is influenced by the momentary action capability of the observers. In the first experiment, participants estimated the time-to-collision of threatening or non-threatening stimuli while being mildly immobilized (with a chin rest) or while standing freely. Having reduced the possibility of movement led participants to show more underestimation of the approaching stimuli. However, this effect was not stronger for threatening relative to non-threatening stimuli. The effect of the action capability found in the first experiment could be interpreted as an expansion of peripersonal space (PPS). In the second experiment, we thus investigated the generality of this effect using an established paradigm to measure the size of peripersonal space. Participants bisected lines from different distances while in the chin rest or standing freely. The results replicated the classic left-to-right gradient in lateral spatial attention with increasing viewing distance, but no effect of immobilization was found. The manipulation of the momentary action capability of the observers influenced the participants' performance in the TTC task but not in the line bisection task. These results are discussed in relation to the different functions of PPS.

Keywords: Action ability; Emotion; Looming; Motor ability; Peripersonal space representation; Time-to-collision.

Fig. 1

Judged TTC as a function of actual TTC in all the different conditions. Judgments increased monotonically as a function of actual TTC for non-threatening (butterflies and rabbits) and threatening (snakes and spiders) stimuli. There was a clear bias to underestimate TTC for threatening compared to non-threatening stimuli (grey and pink lines). Moreover, the black and grey lines appear slightly below the red and pink ones suggesting that the judgments were reduced in the chin rest condition relative to no chin rest condition. The grey dotted line indicates veridical judgments

Fig. 2

Scatterplot showing relation of TTC judgments and fear. For both TTC judgments and fear ratings, variance specifically related to the threatening stimuli was isolated by calculating the residuals regressing scores for threatening on those for non-threatening stimuli. These residuals were significantly negatively correlated, indicating that greater fear was associated with increased tendency to underestimate TTC. The grey line represents the least-squares regression line, regressing fear on TTC judgments

Fig. 3

Mean (and SE) rightward bisection bias for chin rest and no chin rest conditions. Negative values indicate leftward bias while positive values rightward bias