Influence of Different Types of Prior Knowledge on Haptic Exploration of Soft Objects

When estimating the softness of an object by active touch, humans typically indent the object’s surface several times with their finger, applying higher peak indentation forces when they expect to explore harder as compared to softer stimuli [1]. Here, we compared how different types of prior knowledge differentially influence exploratory forces in softness discrimination. On each trial, participants successively explored two silicone rubber stimuli which were either both relatively soft or both relatively hard, and judged which of the two were softer. We measured peak forces of the first indentation. In the control condition, participants obtained no information about whether the upcoming stimulus pair would be from the hard or the soft category. In three test conditions, participants received implicit (pairs from the same category were blocked), semantic (the words soft and hard), or visual prior knowledge about the softness category. Visual information was provided by displaying the rendering of a compliant object deformed by a probe. Given implicit information, participants again used significantly more force in their first touch when exploring harder as compared to softer objects. Surprisingly, when given visual information, participants used significantly less force in the first touch when exploring harder objects. There was no effect when participants were given semantic information. We conclude that different types of prior knowledge influence the exploration behavior in very different ways. Thus, the mechanisms through which prior knowledge is integrated in the exploration process might be more complex than expected.