Overview: Researchers identified a new brain network that includes the fronto-parietal networks and spindle-shaped gyrus, which help encode visual mental images.
Source: Paris Brain Institute
Every day we call upon a unique ability of our brain, visual mental images, which enable us to visualize images, objects or people ‘in our heads’. Based on the recent case of a patient with a specific brain lesion, Paolo Bartolomeo’s (Inserm) group in the PICNIC Lab of the Paris Brain Institute has identified a region that may hold key in mental visualization.
A patient was admitted to the emergency room after a stroke that had spread to the occipitotemporal region of the left hemisphere. Although his life was saved, the patient woke up with multiple defects: hemianopia – the loss of vision on the right side – alexia – an inability to read – and an inability to name colors.
These multiple impairments and the presence of the lesion in the left temporal lobe prompted clinicians and researchers at the Paris Brain Institute to evaluate another brain function: visual mental images.
The brain networks of mental images
Currently, the predominant brain base model of mental images proposes that it engages the primary visual area at the back of our brain, which is also involved in processing what we actually see with our eyes. However, over the past two decades, evidence has accumulated from patient records that contradict this dogma.
In a recent meta-analysis, Paolo Bartolomeo’s team suggested that mental images are instead encoded in the fronto-parietal networks of attention and working memory, as well as in a small region of the spindle-shaped gyrus of the left temporal lobe.
The case of this new patient with a left occipitotemporal lesion was therefore an opportunity for the researchers at the Paris Brain Institute to reexamine their hypothesis.
Intact visual mental images, despite the lesions
To test the patient’s mental imaging, the doctors gave him a series of tests. These consisted of several questions about the appearance of objects: What is redder between a strawberry and a cherry? Which city is most right on a map of France between Bordeaux and Strasbourg?
To answer correctly, the patient had to use his mental images and visualize in his head a strawberry, a cherry or a map of France.
“To our great surprise, our patient’s visual mental images were well preserved,” explains Paolo Bartolomeo (Inserm), the study’s final author.
“Then came a new question: Why wasn’t he having problems, despite his lesion that would have affected important networks for performing this function in our brain?”
Halfway between language and semantic networks
MRI tractography, which makes it possible to visualize the parts of neurons in the brain — the wiring, so to speak — allowed the researchers to identify some key elements that explain why the patient’s mental images were intact despite his lesion. They found that the mental image node, which is located in the spindle-shaped gyrus in the left temporal lobe, was spared by the lesion.
The team of scientists then showed that two connecting channels ran through this node: the arcuate fasciculus, associated with the language system, and the inferior longitudinal fasciculus, linked to the semantic system, ie our knowledge of the world, objects and concepts.
Because of his lesion, the patient no longer received direct visual information in his left hemisphere. The spindle-shaped image node therefore no longer received this kind of information, but continued to be fed by the semantic network.
“These results support our hypothesis that visual mental images originate from a top-down activation of the language and semantic networks. This runs counter to the dominant model of mental imagery, according to which the primary visual regions are required for the implementation of this ability. ”, concludes Paolo Bartolomeo (Inserm).
About this visual neuroscience research news
Original research: Closed access.
†The connection anatomy of visual mental images : evidence from a patient with left occipito-temporal damage” by Paolo Bartolomeo et al. Brain structure and function
The connection anatomy of visual mental images : evidence from a patient with left occipito-temporal damage
Most of us can use our “mind’s eye” to mentally visualize things that are not in our direct line of sight, an ability known as visual mental images. Extensive temporary damage to the left side may impair patients’ visual mental imaging, but the critical site of the lesion is unknown.
Our recent meta-analysis of 27 fMRI studies of visual mental images highlighted a well-delineated region in the left lateral midfusiform gyrus, which was consistently activated during visual mental images, and which we termed the Fusiform Imagery Node (FIN).
Here we describe the junctional anatomy of FIN in neurotypic participants and in RDS, a right-handed patient with extensive occipito-temporal stroke in the left hemisphere. The stroke caused right homonym hemianopia, alexia without agraphia, and color anomy.
Despite these shortcomings, RDS had a normal subjective experience of visual mental images and fairly preserved behavioral performance on tests of visual mental images of object shape, object color, letters, faces, and spatial relationships.
We found that the FIN was spared by the lesion. We then assessed the connecting anatomy of the FIN in the MNI space and in the patient’s own space by visualizing the fibers of the inferior longitudinal fasciculus (ILF) and of the arcuate fasciculus (AF) passing through the FIN.
In both spaces, the ILF connected the FIN to the anterior temporal lobe and the AF connected it to frontal regions. Our evidence is consistent with the hypothesis that the FIN is a junction of a brain network devoted to voluntary visual mental images.
The FIN could act as a bridge between visual information and semantic knowledge that is processed in the anterior temporal lobe and in the language circuits.