Judging the laterality of a hand vs of a letter.
For ordinary subjects, the tasks differ: they are less accurate
when the hand's position is biomechanically awkward.
But Fiori et al. (2013) show that the tasks do not so differ for subjects suffering Amyotrophic
Lateral Sclerosis (ALS), which impairs motor representation (Parsons, Gabrieli, Phelps, & Gazzaniga, 1998).
It may be objected that performance differences such as these can be explained without appealing to
a difference in format. After all, rotating a ball involves an action whereas a ball rotating does
not; consequently, imagining the former may be thought to differ from imagining the latter with
respect to the contents of the representations involved. Supposing that there are differences in
content here and in other cases, could these fully explain differences in performance profile? To
see why not, consider two tasks involving mental rotation. Judging the laterality of a rotated
letter is thought to involve phenomenologically vision-like imagination
(Jordan, Heinze, Lutz, Kanowski, & Jäncke, 2001), whereas judging the laterality of a rotated hand is thought to involve
phenomenologically action-like imagination \citep{parsons:1987_imagined, gentilucci:1998_right}.
Ordinary subjects who are asked to judge the laterality of a hand rotated to various degrees are
less accurate when the hand's position is biomechanically awkward. By contrast, no such effect
occurs for comparable tasks involving letters rather than hands. How could this difference in
performance in imagining hands and letters be explained? Consider the claim that the difference in
performance can be fully explained by a difference in the content of the representations involved.
Initially this might seem plausible because one task involves hands whereas the other involves
letters. However, there are subjects who can perform both tasks but whose performance is not
different for hands and letters (Fiori et al., 2013). These are subjects suffering Amyotrophic
Lateral Sclerosis (ALS), which impairs motor representation (Parsons et al., 1998). Since
ALS and ordinary subjects encounter the same stimuli and perform the same tasks, there seems to be
no reason (other than our hypothesis about a difference in format) to suppose that the two groups'
performance involves representations with different contents. So if the hand-letter difference in
performance were entirely explained by a difference in content, we would expect ALS and ordinary
subjects to exhibit the same difference in performance. But this is not the case. This is an
obstacle to supposing that the hand-letter difference in performance in ordinary subjects could be
explained by appeal to content.