Summary: The strength of a person’s mental imagery is linked to the excitability of brain regions, particularly the prefrontal and visual cortices. High excitability in the visual cortex appears to reduce the clarity of mental images, which may help explain conditions such as aphantasia, where people cannot form visual images.
Source: University of New South Wales
The strength of mental imagery — how clearly someone can picture things “in their mind’s eye” — is associated with the excitability of different brain areas, according to a study led by researchers at UNSW Sydney.
The study found that a more excitable prefrontal cortex was associated with stronger mental images, while greater excitability in the early visual cortex (V1–V3) was associated with weaker imagery. Brain excitability refers to how likely neurons are to fire, and it differs between individuals; for example, people who experience migraine with aura tend to show higher visual cortex excitability.
“Surprisingly, participants with less excitable visual cortex produced stronger mental images,” says Dr Rebecca Keogh, postdoctoral fellow in the School of Psychology and lead author. The results were published in the journal eLife.
The researchers suggest that highly excitable neurons in the visual cortex may introduce background neural “noise” that interferes with the sensory signal generated by imagery. “Think of the visual cortex like a chalkboard,” Dr Keogh explains. “Drawing on a dusty, more excitable chalkboard makes the picture hard to see; on a cleaner, less excitable board the image is clearer.”
The team used a multi-method approach to establish the link between cortical excitability and imagery strength. They analyzed fMRI data, measured responses to magnetically induced visual sensations (phosphenes) via Transcranial Magnetic Stimulation (TMS), and directly manipulated excitability with Transcranial Direct Current Stimulation (tDCS). Individual experimental phases involved between 16 and 37 participants, adding up to more than 150 people across the study; the researchers plan further, larger studies.
“People vary widely in their capacity for visual imagery,” Dr Keogh notes. “For some the image is nearly as vivid as actual sight, for others it is faint, and some report no imagery at all. Our findings offer a potential neural explanation for these individual differences.”
The observation that imagery strength varies among people dates back to Francis Galton’s 1883 work. This study provides one of the first physiological mechanisms that could explain why mental imagery quality differs so much from person to person.
Peering into the imagination
To quantify imagery vividness objectively, the researchers used a laboratory technique based on binocular rivalry, an established visual illusion that provides a sensory measure of imagery strength. This approach avoids relying on subjective self-reports, which can be biased.
In each trial, participants saw a prompt — the letter “R” or “G” — indicating which pattern to imagine: a horizontal red pattern for “R” or a vertical green pattern for “G.” They imagined the cued pattern for six to seven seconds. Immediately afterwards, a brief binocular rivalry display (750 milliseconds) presented both patterns simultaneously. Participants then reported which pattern appeared dominant: red, green, or a mixture.
Imagery strength was defined as the percentage of trials in which the imagined pattern dominated the rivalry display. The stronger the mental image, the more likely it was to bias perception during the brief visual presentation.
“This method captures the sensory trace left by imagery in the brain,” says Professor Joel Pearson, director of UNSW’s Future Minds Lab. “It offers a more reliable measure than asking people how vivid they think their images are.”
Modulating neural activity
The researchers also used tDCS to test causation: if visual cortex excitability influences imagery strength, altering excitability should change imagery. tDCS places a small positive (anode) and negative (cathode) electrode on the scalp to increase or decrease neuronal excitability beneath the electrodes. Participants might feel a mild tingling, but the procedure is non-invasive and painless.
“Placing the anode over a brain region tends to increase the probability that neurons will fire, while the cathode tends to decrease excitability,” Dr Keogh explains. Manipulating excitability with tDCS produced changes in imagery strength, supporting a causal role for visual cortex excitability in controlling mental images.
“Manipulating brain excitability caused image strength to change, indicating the relationship is not merely correlational,” Dr Keogh says.
The findings point to possible clinical applications: modulating cortical excitability with non-invasive stimulation could open routes to adjust imagery strength in conditions where imagery is problematic or weak.
Looking ahead
Further research is needed to understand longer-term effects of tDCS and why individuals respond differently to electrical stimulation. If repeatable and safe, controlled modulation of cortical excitability might offer therapeutic options for people with intrusive or traumatic imagery, or for those with underactive imagery.
Professor Pearson and Dr Keogh are also interested in whether these physiological mechanisms relate to aphantasia (absence of visual imagery) and hyperphantasia (exceptionally vivid imagery). Although this study did not specifically recruit people with those conditions, the results provide an initial clue to a brain mechanism that could underlie them.
“Any cognitive process that relies on visual imagery will be influenced by an individual’s imagery strength,” Dr Keogh says. “By understanding the neural drivers of these differences, we may be able to enhance imagery and related cognitive abilities in the future.”
Mental imagery contributes to memory, imagination, reading, and many everyday mental tasks. “Imagery is a keystone mental process,” Professor Pearson observes. “It helps shape how we think, feel, remember, and decide.”
About this neuroscience research article
Source: University of New South Wales
Original Research: Open access. Title: “Cortical excitability controls the strength of mental imagery” by Rebecca Keogh (corresponding author), Johanna Bergmann, and Joel Pearson. Published in eLife. DOI: 10.7554/eLife.50232
Image Source: Image in the public domain.