Abstract. We demonstrate the influence of picture size on haptic recognition and exploratory behaviour. The stimuli were raised-line drawings of everyday objects. Participants were instructed to think aloud during haptic exploration of the pictures. We measured the delay between initial correct speculation and final correct response. The results indicate that picture size influences accuracy but not response latency: large drawings are recognised more often but not faster. By analysing video recordings of the experiment we found that two-handed exploration increases when picture size increases and that, on average, 83% of the exploration time involves the use of two hands. The thinking-aloud data showed that the average time difference between the initial correct speculation and final correct response amounted to 23% of the total reaction time. We discuss our results with respect to the design of tactile aids and the ecological validity of single-finger exploration.

The difficulty that observers experience when trying to identify a raised line drawing by touch is still largely unexplained. In this article, we show that observers who are unable to haptically identify a raised line drawing are suddenly able to do so after they have sketched on paper what they have in their mind. We conducted three experiments: first of all we show that this effect is robust; in the second experiment, we show that identification-after-sketching is caused by visual inspection of the sketch, and not caused by feedback in general; and in the third we show that sketches which were identified by the observers who produced them, were also identified by completely naive viewers. These experiments demonstrate that during raised line drawing identification the mental capacities required to interpret the stimulus seem to be inadequate: although enough pictorial information was present to produce a sketch which could even be identified by naive viewers, the stimulus could not be identified by haptic and mental processing alone. Furthermore, we investigated whether increasing the haptic perceptual field by using two hands instead of one hand had an influence on identification performance. We did indeed find that using two hands significantly increased identification. We use both results to discuss the underlying mechanisms of haptic raised line drawing identification.