A post by Jorge Morales

“I just learned something about you and it is blowing my [expletive] mind. This is not a joke. [...] It is, I think, as close to an honest-to-goodness revelation as I will ever live in the flesh. Here it is: You can visualize things in your mind.” 

Blake Ross, Mozilla Firefox Co-Founder, April 2016

Late one night, Blake Ross discovered something incredible (as in, almost impossible to believe): other people had visual imagery! Like Ross, a surprisingly large number of people (about 3-5% of the population) have aphantasia, which is characterized as the lack of mental imagery.

Seeing things in the mind’s eye is an important faculty during development, in art and science, and it has a tight relation to cognitive skills such as mental simulation, short-term memory and mental rotation, among many others (Pearson et al., 2015). Despite this, aphantasics do not seem to suffer from any noticeable deficit. Blake Ross founded Mozilla without even realizing he was aphantasic (or those close to him noticing anything was amiss). Millions more live oblivious of how different their mental imagery skills are (Lupyan et al., 2023).

But how can this be? One possible implication is that imagination—and perhaps even consciousness in general—is not as useful as we might think: people do not even notice when this entire faculty is absent! This, of course, goes against common sense and millennia of research. Philosophers have discussed the importance of imagination at least since Plato (Bundy, 1922; Schwartz, 2020) and Aristotle (1993), scientists have studied it and its effects for more than a hundred years (e.g., Kosslyn, 1980, 1996; Perky, 1910; Shepard & Cooper, 1982), and most of us seem to use it all the time. Could mental imagery be more epiphenomenal than we thought?

To address this question, I will focus on mental rotation and aphantasia as a case study. I will pose more questions than answers, but in the end, we may see that whatever the function of mental imagery during mental rotation is, it must be an incredibly subtle one.

The Science of Mental Rotation

Cognitive psychologists have long suggested that mental imagery supports our ability to rotate objects in our minds (for example, when you are trying to decide whether a new armchair will fit in your living room). Roger Shepard and his students first established the aforementioned connection, and it is now a foundational result in psychological science. Shepard and Jacqueline Metzler (1971) showed subjects pairs of 3D line drawings of connected cubes arranged in different configurations (Figures 1 and 2). The objects had different orientations and subjects’ task was simply to determine whether the two objects were exactly the same (Fig. 1) or not (a mirror image of each other; Fig. 2).

The now-classic result is that people’s response times increased with the rotation angle between shapes. In other words, an almost perfect linear relationship was found between how long subjects took to make similarity judgments and the angle of rotation that separated both items (Figure 3).

Shepard and Metzler’s explanation was that subjects created mental images of the objects and rotated them in their heads, in the same manner they would in physical space. To determine if the images are the same or not, participants rotate them at a constant speed until their orientation is matched and it becomes easy to “see” in their mind’s eye whether the shapes overlap or not. Like in the physical world, it takes longer in the mind to turn a more rotated object, which elegantly explains these results.

This classic finding, backed by several follow-ups and replications, established that mental rotation involves mental imagery, generally considered to have an iconic format (Kosslyn, 1980, 1996). This does not mean there have not been objections and proposed alternative explanations (e.g., Just & Carpenter, 1976; Marks, 1999; Pylyshyn, 2003; Stewart et al., 2022; see also Morales & Firestone, 2022). Nevertheless, the view that mental rotation is enabled by mental imagery and that mental imagery is a quasi-perceptual, depictive capacity has largely prevailed (Pearson et al., 2015).

Enter aphantasia.

Aphantasia and Mental Rotation

Reports of people unable to visualize date back to at least the 19th century, but the modern interest in aphantasia was recently triggered by a case of an architect who suddenly lost the ability to visualize (Zeman et al., 2010). This patient—MX—does not have any other cognitive deficits or measurable brain lesions. With the popularization of MX’s case, it became evident that aphantasia could also occur congenitally (Zeman et al., 2015).

A natural prediction emerged: aphantasics should not be able to perform normally in mental rotation tasks. The reality that has emerged in the last decade or so is more complicated. In a traditional mental rotation task, MX performed with comparable accuracy to the controls (in fact, he was a bit better). MX, however, was almost 40% slower! Importantly, the relationship between response time and angle of rotation was not perfectly linear for MX, and it differed from controls (Zeman et al., 2010). These results indicate that MX must have used a different strategy. Yet, he can solve mental rotation tasks relatively quickly with increasing (albeit non-linear) difficulty as angle of rotation increases. This should not be possible if the received theory of mental rotation is taken at face value.

MX is, after all, a patient with presumably some (hard-to-detect) brain damage. Congenital aphantasics, however, offer a similar picture (pun intended). They have displayed somewhat lower performance in some tasks that require mental imagery (Jacobs et al., 2018), but the differences are quite minimal (Milton et al., 2021; Pounder et al., 2022). In mental rotation tasks specifically, a recent study showed that they performed like controls, and they even displayed the classic increasing linear pattern with angle of rotation. However, just like MX, aphantasics were much slower than controls (up to ~25%) (Pounder et al., 2022).

In summary, aphantasics seem to perform mental rotation without any problem, but they must do so differently. Most strikingly, they perform these tasks more slowly than individuals who appear to rely on mental imagery.

In Search of a Function for Mental Imagery

Evidence contributing to our understanding of aphantasia is rapidly mounting, but it is still quite scarce. So, at the moment, any conclusion must be tentative. What is clear is that—contrary to the classic view—conscious mental imagery is not the only way to perform mental rotation. The alternatives, however, are less clear.

One option is that Pylyshyn (2003) was right all along, and mental rotation is a propositional, non-imagistic phenomenon. This, however, leaves unexplained why aphantasics are slower. Perhaps we have access to two different strategies: one imagistic and one propositional. If so, most people use an imagistic, fast approach, while aphantasics only have the propositional, slower one available. Recall that MX passed mental rotation tasks shortly after his episode. If he was using a propositional strategy, it would suggest that this was available to him since before the incident or that it was easy to develop.

Yet another option is that aphantasics create mental representations that are just as imagistic as those of visualizers, except they are unconscious (Nanay, 2021). This route seems tempting. It would explain aphantasics’ accurate performance and perhaps why they are slower too. After all, even though aphantasics’ visual cortex activity is lower during attempted imagery (Milton et al., 2021; Zeman et al., 2010), it is not zero, which could be consistent with the unconscious imagery hypothesis. Can this explain why aphantasics appear to be slower? To borrow a phrase from Ned Block, consciousness may be what “greases the wheels” of unconscious visual representations, making them more readily available to us and thereby rendering us faster in mental rotation tasks. Without consciousness, the task is doable—even by means of the same underlying representations—but slower. Problematically for this hypothesis, aphantasic subjects in priming experiments do not show any kind of priming effects (Kay et al., 2022; Keogh & Pearson, 2018). They also do not show skin conductance response to fearful stimuli (Wicken et al., 2021) even though it has been demonstrated to arise after unconscious images (Öhman & Soares, 1994).

It could be that the lack of imagery in aphantasia is a symptom of a different underlying issue. The one consistent deficit found in aphantasics is their poor/non-existent episodic memory (but people also fail to realize it). Perhaps the order of explanation is inverted: the lack of conscious mental images is a result of deficits in episodic memory (Blomkvist, 2023; Dawes et al., 2020, 2022). This theory is consistent with the good performance in mental rotation tasks, but the slower reaction times would remain unexplained.

As anticipated, I offered more questions than answers. The case of mental rotation in aphantasia is perplexing because it stress-tests what have become fundamental assumptions in the field. It is entirely possible that our theories of mental rotation were completely wrong from the outset. More preoccupying though is what aphantasia says about our understanding of imagination and its function, or about the function of consciousness. Back to my original concern: just how important can conscious mental imagery be if its absence often goes completely unnoticed? Mental imagery may make us a bit faster, which is not bad, but it is nothing to blow one’s mind.

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