Laterality effects in the haptic discrimination of verbal and non-verbal shapes

The left hemisphere is known to be generally predominant in verbal processing and the right hemisphere in non-verbal processing. We studied whether verbal and non-verbal lateralization is present in haptics by comparing discrimination performance between letters and nonsense shapes. We addressed stimulus complexity by introducing lower case letters, which are verbally identical with upper case letters but have a more complex shape. The participants performed a same-different haptic discrimination task for upper and lower case letters and nonsense shapes with the left and right hand separately. We used signal detection theory to determine discriminability (d'), criterion (c) and we measured reaction times. Discrimination was better for the left hand for nonsense shapes, close to significantly better for the right hand for upper case letters and with no difference between the hands for lower case letters. For lower case letters, right hand showed a strong bias to respond "different", while the left hand showed faster reaction times. Our results are in agreement with the right lateralization for non-verbal material. Complexity of the verbal shape is important in haptics as the lower case letters seem to be processed as less verbal and more as spatial shapes than the upper case letters.