How Do the Blind See?

Post by Elisa Guma

What's the science?

At rest — independent of external stimulation — the brain generates spontaneous activity. Our knowledge about the functional role of this brain activity is limited. However, scientists hypothesize that it generally underlies unprompted, internally-generated behaviours, including hallucinatory experiences. This week in Brain, Hahamy and colleagues investigated whether spontaneous brain activity evokes unprompted cognitive behaviours by observing the neural correlates of visual hallucinations in the visually impaired. 

How did they do it?

To investigate the relationship between spontaneous activity in the brain’s visual cortex and participants’ visual hallucinations, the authors recruited five individuals with Charles Bonnet syndrome (CBS), a rare condition characterized by the development of complex visual hallucinations following the onset of visual impairments or vision loss.

During functional magnetic resonance imaging (fMRI), CBS participants were instructed to provide verbal reports of their hallucinations. The authors then used these reports, along with reported post-hoc details about the content of the hallucinations, to create movies simulating the hallucinatory streams. To compare the neural activity associated with the internally-generated hallucinations, the authors showed these videos (external stimulation) to thirteen sighted controls during fMRI scanning. Finally, the 5 CBS participants, 13 sighted controls, and an additional group of 11 late-onset blind individuals not experiencing visual hallucinations underwent a cued visual imagery task during fMRI scanning where they were asked to imagine faces, houses, objects, and patterns, to serve as cued, internally-generated vision.

The authors extracted the blood oxygenation level-dependent (BOLD) brain activity from participants’ fMRI scans and used nonparametric statistics to identify the brain regions associated with a) hallucinations in the CBS group, b) the external stimulation (video of hallucinations) in the sighted controls, and c) the internally-generated, cued visual imagery task in all groups. Additionally, the authors compared the temporal dynamics of brain activity (as measured by BOLD) during hallucinatory events and visual stimulation to identify whether any changes in BOLD preceded the onset of visual experiences.

What did they find?

CBS participants’ brains showed significant activation across the entire visual cortex during hallucinations. Similar effects were observed in sighted controls during simulated hallucinations (i.e., watching the videos). Next, the authors compared brain activity during hallucinations to that of cued imagery, both of which are internally generated, but only hallucinations were unprompted. During the visual imagery task, sighted controls tended to activate high-order visual areas while deactivating mid-level areas, whereas CBS and blind control groups showed activations across the entire visual system, with very similar spatial patterns as those found during hallucinations.

Investigation of the temporal dynamics of the BOLD signal revealed BOLD signal increases during hallucinations in the CBS group and during simulated hallucinations in the sighted controls. Interestingly, the signal in the CBS group increased before the onset of hallucinations in early visual areas and then spread to higher-order areas, providing some evidence that the hallucinations may be a result of a buildup in spontaneous activity in the visual cortex. Importantly, no differences in dynamics were observed in the cue-driven visual imagery scans.

What's the impact?

The findings presented here show that, unlike other visual experiences, visual hallucinations may arise due to a buildup of neural activity in the early visual cortex, which then spreads to higher-order visual areas. This provides a plausible mechanism for the emergence of visual hallucinations in CBS. More broadly, these findings highlight the possible role of spontaneous brain activity in evoking visual hallucinations following visual deprivation. Future work may further investigate the relationship between spontaneous brain activity and other internally-generated behaviours, such as dreaming.     

Hahamy et al. How do the blind ‘see’? The role of spontaneous brain activity in self-generated perception. Brain (2020). Access the original scientific publication here.