Discover covers a really cool Neuropsychologia article about a man, MX, who lost his ability to experience mental imagery.
Mental imagery is a divisive topic in psychology. Some (most notably Kosslyn) argue that mental images are essential to many types of cognition. According to this camp, mental images are functionally similar (but not identical to) like-modality perception (Kosslyn, 2004 summarises this view nicely). Imagining an apple and seeing an apple involve similar mechanisms. Furthermore, I can use my mental image of an apple to answer questions about its properties – is it red? is it heavier than a plum? But, many other people argue that, although we might feel like we’re using pictures in our imagination to solve various problems, the real work is done by non-depictive representations (see Pylyshyn, 2003 for a good review). When we’re asked to answer questions about an apple’s properties, we can think about what it would be like to see the apple, but this doesn’t entail that the representation is depictive, in this case, pictorial.
To be clear: The first argument depends on mental images being the medium through which we solve various problems. Mental images play a causal role in cognition. The second argument asserts that mental images may exist as a sensation (we experience feeling like we’re seeing something in our mind’s eye), but they are not the medium through which we solve problems. Both are representational accounts of cognition, so they’re both wrong (see this also). But they’re differently wrong.
So, what’s the evidence for mental images? Consider property verification tasks, where a participant is asked to decide whether a feature is true of a concept (for example: Are ears a feature of rabbits?). The stimuli for these tasks are words, not pictures. But, participants take longer to respond if they are asked to imagine a small rabbit compared to a large rabbit. The argument is that participants use visual imagery to solve this task. The difference in reaction time occurs because it’s easier to “see” a large visual image of a rabbit than a small visual image.
Other evidence comes from the mental rotation task (Shepard & Metzler, 1971). In this task, participants see two shapes, and they must decide whether the shapes are the same or different. On some of the trials, the two shapes are the same, but appear in different orientations (like and > and v). The time it takes participants to perform this task depends on the size of the difference in orientation. They are pretty fast to recognize that the shapes are the same given a difference of, say, 15 degrees. But, they take much longer to respond if the difference is, say, 70 degrees. The argument is that during the task participants imagine spinning one of the shapes around, so that its orientation matches the other one.
This evidence for mental imagery is indirect (no one is measuring or detecting mental images) and relies on the premise that it’s pretty difficult to think of another explanation for these results. This brings us back to MX, who lost his ability to form mental images. This patient offers a rare opportunity to see how cognition changes when this subjective feeling of mental imagery is wiped out. After undergoing surgery to unclog blocked arteries, MX reported losing his “mind’s eye.” Whereas previously he felt like he could conjure up images of people and things that were not present, now he felt like this visual element of thinking about things was completely gone.
Researchers from the Peninsula Medical School in Exeter and the University of Edinburgh ran several experiments comparing MX to matched controls on a battery of tests (Zeman, Dela Salla, Torrens, et al., 2010). First, they made sure that MX performed similarly to the controls on tests of memory and executive function. They also made sure that his visual system worked normally (e.g., MX was able to match celebrity names to faces). Finally, the researchers tested MX on tasks thought to require mental imagery, like the mental rotation task described above.
MX performed the mental rotation task with perfect accuracy. But, unlike the matched controls, the degree of rotation did not influence the time it took MX to decide if two pictures were the same. He also performed as well as control subjects on other tasks, like being able to describe visual properties of objects from memory.
So, the subjective feeling of mental imagery is not necessary for a number of tasks that many people believe require visual representation. I’d like to know how Kosslyn reacts to this. I think he might argue that MX simply lost the subjective experience of mental imagery, but the processes underlying his ability to solve these tasks remained intact. However, this view would predict that MX should perform like control subjects on all implicit imagery tasks. Thus, it wouldn’t explain MX’s unusual performance on the mental rotation task. If mental imagery does not play a causal role in cognition, then many questions remain. Primarily, we’re left wondering how to explain reaction time effects on things like property verification and mental rotation. However, difficulty accounting for these results is no reason to hang onto the idea that mental images explain anything about cognition.
Kosslyn, S. T. (2004). Mental images and the brain. Cognitive Neuropsychology, 22(3), 333-347.
Pylyshyn, Z. (2003). Return of the mental image. Are there really pictures in the brain? Trends in Cognitive Science, 7(3), 113-118.
Zeman, A. Z. J., Della Sala, S., Torrens L. A., et al (2010). Loss of imagery phenomenology with intact visuo-spatial task performance: A case of ‘blind imagination.’ Neuropsychologia, 48, 145-155.
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