Cross-modal transfer in visual and haptic object categorization

When humans have to categorize objects they often rely on shape as a deterministic feature. However, shape is not exclusive to the visual modality: the haptic system is also an expert in identifying shapes. This raises the question whether humans store separate modality-dependent shape representations or if one multimodal representation is formed. To better understand how humans categorize objects based on shape we created a set of computer-generated amoeba-like objects varing in defined shape steps. These objects were then printed using a 3D printer to generate tangible stimuli. In a discrimination task and a categorization task, participants either visually or haptically explored the objects. We found that both modalities lead to highly similar categorization behavior indicating that the processes underlying categorization are highly similar in both modalities. Next, participants were trained on special shape categories by using the visual modality alone or by using the haptic modality alone. As expected, visual training increased visual performance and haptic training increased haptic performance. Moreover, we found that visual training on shape categories greatly improved haptic performance and vice versa. Our results point to a shared representation underlying both modalities, which accounts for the surprisingly strong transfer of training across the senses