What is my brain tasting?

A persistent and unpleasant taste – like metal, rust, or blood in your mouth. No amount of rinsing can get rid of it. That is the unsettling and eerie sensation that people with gustatory hallucinations often face.

Gustatory hallucination is a type of false perception that makes you experience tastes that are not caused by any external stimulus and are often strange or unpleasant. It is a haunting presence that urges us to delve deeper into its origins, and also to seek medical guidance. Gustatory hallucinations can be a sign of a serious condition that affects your brain or nervous system. They can be an underlying symptom of epilepsy, and associated with other disorders like schizophrenia, dementia, brain tumors or migraines.

Gustatory hallucinations can also be caused by hunger, being deprived of senses or sleep, feeling very tired, having sleep disorders, facing death or danger, losing someone close, feeling isolated for a long time, being sexually abused, participating in religious ceremonies or entering a trance state. Patients might report hallucinatory experiences during situations of increased stimulation (e.g. when in a crowd), decreased external stimulation (such as when alone at night) or when there is repetitive, background noise (e.g., being close to air fans or washing machines) (Chaudhury, 2010).

The capacity to deceive our senses can have a plethora of psychological and behavioral repercussions, including lack of sleep, depression, anxiety and diet changes (Waters et al., 2015). A person may lose their appetite and develop a food aversion due to a lack of satisfaction when eating. The importance of taste for our general wellbeing is made clear when our sense of taste is impaired.

Prevalence

The constant perception of unpleasant tastes can lead to feelings of distress and anxiety (Jafari et al., 2021). But it's a two-way relationship between gustatory hallucinations and wellbeing – for example, they can be affected by work related stress, and exacerbate stress in the workplace. Capampangan et al. (2010) report the case of a 48-year-old man with untreated work-related stress, anxiety, and depression. He reported to the emergency department that he had woken up at 4am experiencing gustatory hallucinations (metallic taste) and olfactory hallucinations (smell of burning tyres). Symptoms of aphasia, numbness, tingling in all extremities, nausea, and imbalance lasted for several minutes.

Gustatory hallucinations are rare in primary psychoses, and more common in secondary psychoses. They can occur in different diagnoses, rather than being specific to any disorder (Lewandowski et al., 2009). People with schizophrenia, depression, and multiple sclerosis may experience gustatory hallucinations as a result of medications such as bupropion, which can produce metallic, bitter, or salty taste perceptions.

How common are gustatory hallucinations? Research has examined the frequency and prevalence of various hallucination modalities over the course of a year, in a large sample of schizophrenia patients from countries including Austria, Lithuania, Poland, Georgia, Ghana, Nigeria, and Pakistan. The most frequently reported type of hallucination were auditory, followed by visual and cenesthetic – less frequent (1.3 per cent – 6.6 per cent) were tactile, olfactory, and gustatory hallucinations (Bauer et al., 2011). A study in the UK found that out of 82 people (aged 30-40 years) who had psychosis for the first time, a third reported having hallucinations of smell or taste (Dudley et al., 2023).

There are also some intriguing findings around a possible cultural influence on the experience and reporting of different types of hallucinations. Thomas et al. (2007 study) compared hallucination modalities in patients with schizophrenia from India (n=807) and USA (n=480), aged 30-38. Visual and auditory hallucinations were the most frequent, while gustatory hallucinations were much less reported in India (8.5 per cent) than in the USA (14.4 per cent).

Hallucinations can also be experienced in non-psychiatric populations, such as people with Parkinson's disease. An Italian study (Solla et al., 2021) of 141 patients (mean age 70 years) with Parkinson's disease found that 2.8 per cent had gustatory hallucinations. Occasionally these patients describe gustatory, olfactory, or tactile hallucinations as co-occurring with visual hallucinations (Lewandowski et al., 2009).

In addition, research involving a large non-clinical sample from the UK, Germany and Italy found that gustatory and olfactory hallucinations were the most frequent daytime hallucinations among 158 out of 13,057 participants (Ohayon, 2000). Two-thirds of those participants who had gustatory hallucinations every week also had a mental disorder or organic pathology or used psychoactive substances. Compared to those who had gustatory hallucinations once a month, those who had them every week were more likely to have bipolar disorder (5.8 per cent), anxiety (15.5 per cent), or sleep problems (13 per cent).

Tactile, Olfactory, Gustatory

So, it's clear our mental health is linked to our sense of taste, which can be affected by various factors. Neuroscience can help us understand how the brain creates and changes the taste sensations, especially when we experience gustatory hallucinations.

Gustatory hallucinations have been examined at the neural level, and research supports the implication of the temporal lobe. People with temporal lobe epilepsy may have focal aware seizures causing gustatory hallucinations. These hallucinations are rare compared to hearing or seeing things that are not there, but they may have similar underlying brain mechanisms. By exploring how the brain produces more common hallucinations, we may also learn more about the causes of less common ones, such as, Tactile, Olfactory, as well as Gustatory Hallucinations (TOGHs) (Weil & Reeves, 2020).

What the research seems to suggest (e.g. Lewandowski et al., 2009) is that TOGHs may be symptoms of the same neurological processes that cause auditory and visual hallucinations – they just affect the parts of the brain that process tactile, olfactory, and gustatory information. Those with TOGHs may exhibit identical abnormalities in their somatosensory, olfactory, and gustatory cortices as patients who experience auditory hallucinations, according to postmortem and neuroimaging results. Such abnormalities might have been related to poor sensory cortex filtering of irrelevant information.

As well as the temporal lobe, research examining the neural basis of gustatory and olfactory hallucinations has implicated the limbic system. We know that taste receptor cells are connected to afferent cranial nerves, which help relay gustatory signals to the brain for processing taste-related information which reaches the gustatory cortex via two pathways: a thalamic and a limbic pathway (Samuelsen et al., 2013). The thalamic pathway involves the thalamus, a relay station for sensory information, such as taste, prior to reaching the gustatory cortex. Furthermore, the thalamus, receives taste information from taste cells that release neurotransmitters, such as those related to the pleasantness of taste, and may also be involved in gustatory hallucinations (Leung & Covasa, 2021). Dysfunctional afferent cranial nerves and changes in the processing of taste information may be implicated in gustatory hallucinations and the thalamus could also be involved in the gustatory route. The limbic pathway consists of regions like the amygdala, hippocampus, and parahippocampal gyrus: all these regions play a role in processing emotions and memory associated with certain foods. 

Gustatory hallucinations have also been linked to stimulation of regions such as the amygdala and hippocampus (Henkin et al., 2013). These parts of the brain are responsible for processing information related to taste, including memories and emotions associated with certain foods or flavours. Stimulation of these regions can lead to hallucinatory experiences of taste. A childhood memory and a cherished meal often go hand in hand. The food we enjoyed then evokes the emotion we experienced in that moment and place. These findings suggest that the pathway for taste processing is intricately linked to emotional and memory processing in the brain, highlighting the role of higher-order cognitive functions in the frontal lobe network.

Therefore, there is a close association between taste emotion and memory processing in the limbic, temporal, and frontal part of the brain. One study has found lesions in the temporal lobe and the uncinate gyrus relate to such hallucinatory experiences (Chaudhury, 2010). The uncinate gyrus is a substructure of the anterior-most part of the parahippocampal gyrus, which may be a bi-directional pathway between the temporal lobe and frontal lobe (Granger et al., 2021). Thus, problems in connectivity within this neural network could lead to the misinterpretation of taste signals in the brain. The uncinate fasciculus is a white matter association tract that joins regions of the limbic system with the frontal lobe, potentially playing a crucial role in taste processing in the brain. The uncinate faciculus provides a link between emotional and memory processing networks (limbic system) and higher order decision making parts of the brain like the prefrontal cortex. Dysfunction or damage to the uncinate fasciculus could lead to gustatory hallucinations, by adversely affecting the interaction between the limbic/temporal lobe and frontal network in taste processing.

A unique connection with emotions and memories

Gustatory hallucinations are relatively rare, but they can be distressing and impairing for those who experience them. The neural network responsible appears to comprise the temporal lobe and limbic system. Anatomical correlates of this system include several key brain regions including the amygdala, hippocampus, thalamus and parahippocampal gyri, which process taste related sensory information as well as emotion and memory. This suggests that stimulation in those regions may result in hallucinations, and that the pathway to processing taste is heavily connected with emotional and memory processing in the brain.

Further research may help to elucidate the neural underpinnings of gustatory hallucinations, taste processing and the unique connection with our emotions and memories. What is clear now is that gustatory hallucinations have an effect on general wellbeing and mental health and could even be a sign of a serious condition that affects your brain or nervous system. If you experience gustatory hallucinations, you should see a doctor to find out the cause and get the appropriate treatment.

• Jigar Jogia is a British Psychological Society Chartered Psychologist and associate professor of psychology at Zayed university, Dubai.
• Khawla Almaktoum is a research assistant working with Dr Jogia in the cognition and neuroscience laboratory at Zayed University.

References  

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