Refutation of Poffenberger's inference: an existence proof and a countermodel.

Interhemispheric relay time (IHRT) is inferred from simple (visual) reaction time (SRT) by subtracting SRTs in ipsilateral hand-field conditions from SRTs in contralateral hand-field conditions. This index, the "crossed-uncrossed differential" (CUD) is a synonym of IHRT in the current literature. The CUD has been shown to be significantly moderated by experimental variables, including attentional. One reason the CUD = IHRT hypostasy persists today is that no experimental manipulation has yet been able to generate a significantly negative CUD in a subject manifesting a significant positive CUD in classical conditions. We implemented an experimental manipulation, in SRT, consisting of increasing complexity of the response after the initial key press required at detection. In a very large scale single-case experiment, we found that the CUD thus went from significantly positive in classical SRT to significantly negative in the increased response load condition.