Once you see it, you can’t stop seeing it. A frowny face on your outlet. A look of shock on your bowling ball. Even a colon and the letter D, “:D” – all these things were not designed after human faces, and yet, we regularly perceive human expressions in them anyway. This phenomenon is called face pareidolia, or the tendency to see faces in inanimate objects. But while most of us experiencing pareidolia are perfectly healthy, those who suffer from neurodegenerative diseases like Parkinson’s can experience this phenomenon more than the average individual [1]. To understand why, we must first look at which parts of the brain are responsible for face perception, and what allows us to derive a face from nearly nothing.

Face pareidolia has long been associated with increased activity in the fusiform face area (FFA) of the brain, a region in the temporal lobe named after its role in recognizing faces [2]. This area is thought to facilitate pareidolia by bridging the brain’s bottom-up processing, where the brain builds its perception block by block using only the given information, with its top-down processing, where the brain fills in gaps in its perception using limited information in combination with pre-existing knowledge. Specifically, research indicates the FFA works to merge information from the prefrontal cortex, linked to top-down processing, and the occipital and visual areas of the brain, linked to bottom-up processing [2]. The right FFA, in particular, has been found to be more specialized in perceiving faces, while the left FFA is generally less precise, but still responsive even when a participant is shown a non-face object [4]. In addition to the FFA, a different study saw a link between increased face pareidolia and decreased activity in the medial frontal cortex, indicating that this region may also play a role in sorting through and regulating top-down processing signals that contribute to the phenomenon [5]. 

Given this, we arguably know quite a bit about the neural activity correlated with face pareidolia in healthy patients. But recently, researchers have found that some patients with Parkinson’s Disease experience greater face pareidolia than healthy participants [1]. Using PET scans, which approximate brain activity by detecting the metabolism of glucose, the same study saw decreased metabolic activity in patients’ left temporal and occipital cortices [1], as we might expect knowing the correlation between the left FFA, occipital lobe, and low-level facial recognition. However, the question remains: can we more precisely determine the neurological functions impaired in patients experiencing increased face pareidolia as a symptom of Parkinson’s disease?

Turns out, to do so we may have to look at the connectivity between regions, rather than the activity of the regions themselves. A new study finds that this increased face pareidolia in Parkinson’s disease patients may be due to impaired regulation of top-down processing abilities in the FFA and associated areas by the medial prefrontal cortex, or mPFC [3]. Specifically, the study found that connectivity between the patients’ mPFC, and their left temporal fusiform cortex (TFusC), a region involved in visual processing that includes the FFA, was decreased compared to a healthy participant’s. Their findings indicate a correlation between the decreased connectivity between these areas and increased face pareidolia, which fits in with these two regions’ known involvement with face pareidolia [3].

 But there’s still more to know, including whether these results are specific to just Parkinson’s disease. Further research might center around the mPFC and TFusC’s connectivity in patients with other dementias. Similar results would provide evidence for pathologies across similar dementias [3]. Such research can eventually make for better targets for therapies like deep brain stimulation that work to reduce the side effects of neurodegeneration.

While this experiment opens exciting new doors for the field of neurodegenerative diseases and pareidolias, it also comes with limitations. To fully contextualize the results of the study, it is important to note that researchers selected from a limited range of participants who did not experience hallucinations or dementia, indicating its results may not be applicable to patients with certain types or severities of Parkinson’s  [3]. While there are few ethical concerns to this study, the accuracy of its characterization of all patients with Parkinson’s and potential universalizability is questionable. Still, it provides invaluable insight into the neurology of the mind and helps link how a relatively common phenomenon can easily go wrong. 

As we learn more about Parkinson’s disease and its many side effects, we gain a better and better understanding of the disease and how it changes our everyday perception. Face pareidolia, after all, is a phenomenon we each experience constantly without even thinking about it. The more we know about the effect of Parkinson’s on such neurological phenomena, the closer we can get to understanding the disease – and someday, to treating it as well.

References:

  1. Uchiyama, M., Nishio, Y., Yokoi, K., Hosokai, Y., Takeda, A., & Mori, E. (2015). Pareidolia in parkinson’s disease without dementia: A positron emission tomography study. Parkinsonism & Related Disorders, 21(6), 603–609. https://doi.org/10.1016/j.parkreldis.2015.03.020 
  2. Liu, J., Li, J., Feng, L., Li, L., Tian, J., & Lee, K. (2014). Seeing Jesus in toast: Neural and behavioral correlates of face Pareidolia. Cortex, 53, 60–77. https://doi.org/10.1016/j.cortex.2014.01.013 
  3. Kajiyama, Y., Hattori, N., Nakano, T., Revankar, G. S., Otomune, H., Hashimoto, R., Mori, E., Ikeda, M., Mihara, M., & Mochizuki, H. (2021). Decreased frontotemporal connectivity in patients with parkinson’s disease experiencing face Pareidolia. Npj Parkinson’s Disease, 7(1). https://doi.org/10.1038/s41531-021-00237-z  
  4. Meng, M., Cherian, T., Singal, G., & Sinha, P. (2012). Lateralization of face processing in the human brain. Proceedings. Biological sciences, 279(1735), 2052–2061. https://doi.org/10.1098/rspb.2011.1784 
  5. Center for Human Development, Department of Psychology. (n.d.). Detecting faces in pure noise images: A functional MRI... : NeuroReport. LWW. https://journals.lww.com/neuroreport/fulltext/2008/01220/detecting_faces_in_pure_noise_images__a_functional.19.aspx