When Psychedelics Linger: Understanding HPPD and Its Effects on Thinking

For most people, psychedelic experiences end when the drug wears off. But for a small percentage of users, certain visual disturbances can persist long after the trip is over. This condition is called Hallucinogen Persisting Perception Disorder (HPPD).A new study has shed light on how HPPD affects not just what people see, but potentially how they think. This research offers important insights for anyone considering psychedelic use, as well as for healthcare providers.

What Is HPPD?

HPPD is a condition where someone experiences ongoing visual disturbances similar to those they had during a psychedelic experience, but these continue long after the substance has left their system. People with HPPD know that these perceptions aren't real – they're aware they're experiencing something unusual. According to medical guidelines, HPPD involves "re-experiencing one or more perceptual symptoms originally encountered during intoxication with hallucinogens." These visual disturbances can cause distress and interfere with daily life.

How Common Is HPPD?

While exact numbers are hard to determine, research suggests that about 4.2% of people who use psychedelics may develop HPPD. The condition appears to be more common in women (10.5%) and typically begins around age 21.It's worth noting that these estimates come from web-based surveys with certain limitations. The true number might be different, as many cases may go unreported or misdiagnosed.

The Visual World of HPPD

The new research provides a detailed look at the visual symptoms experienced by people with HPPD. The most commonly reported disturbances include:

• Visual snow (a TV-like static that covers the field of vision)
• Trails (afterimages that follow moving objects)
• Halos (circles of light around bright sources or objects)
• Movement (stationary objects appearing to grow, shrink, or breathe)
• Patterns (seeing geometric patterns on surfaces)
• Irritation from flickering lights
• Heightened awareness of grid or net-like patterns

For most participants in the study, these symptoms were constant or nearly constant. Imagine trying to read, drive, or simply enjoy a sunset while experiencing these persistent visual distortions. It's not hard to understand why HPPD can significantly impact quality of life.

Beyond Visual Disturbances: Thinking Effects

While the visual symptoms of HPPD have been relatively well-documented, less attention has been paid to how the condition might affect thinking and cognitive abilities. The new research aimed to fill this gap by conducting comprehensive brain testing on eight individuals with HPPD, comparing them to matched control groups of psychedelic users without HPPD and people who had never used psychedelics. The researchers found that while most thinking functions remained intact, there were some concerning patterns:

Visual Memory Challenges

Five of the eight HPPD participants showed below-average performance on tests requiring them to remember and reproduce complex visual information. Specifically, they struggled with a test that asks people to copy a detailed geometric figure and then reproduce it from memory both immediately and after a delay. These difficulties suggest potential problems with visual-spatial memory – the ability to remember what you've seen and where things were located. This makes sense given the visual disturbances that characterize HPPD.

Mental Flexibility Differences

Four participants with HPPD performed below average on a test that requires connecting alternating numbers and letters in sequence. This test measures cognitive flexibility – essentially, the brain's ability to switch between different tasks or thought patterns. Additionally, three participants showed poor performance on a test where they needed to figure out and apply hidden rules for matching cards. The HPPD group made significantly more errors (10.6 on average) compared to the psychedelic-using control group (3.6 errors).These findings hint at potential difficulties with mental flexibility, problem-solving, and the ability to adapt to changing rules or situations.

What Causes HPPD?

While the exact causes of HPPD remain unclear, several theories have been proposed:

Brain Pathway Theories

Some researchers believe HPPD may involve disturbances in visual pathways in the brain. One specific theory suggests a chronic disinhibition possibly through reduced GABA output (GABA is a brain chemical that helps "calm down" brain activity).Others point to potential impairment in crucial visual processing areas, such as the lateral geniculate nucleus, which relays visual information from the eyes to the brain's visual cortex.

Functional Considerations

Some experts have suggested that HPPD could be a functional disorder – meaning it involves changes in how the brain works rather than physical damage to brain structures. However, the visual symptoms of HPPD share similarities with neurological conditions like epileptic auras, migraines, or certain types of encephalitis, suggesting possible neurological underpinnings.

Substance Associations

While HPPD is most commonly associated with classic psychedelics like LSD and psilocybin, it has also been reported after use of other substances including MDMA, ketamine, and cannabis. Interestingly, there have even been rare reports of HPPD-like symptoms following use of non-hallucinogenic substances like amphetamines and even certain medications. This wide range of potential triggers suggests that HPPD might involve complex interactions between individual vulnerability factors and various substances that affect brain chemistry.

Limitations of the Research

The researchers acknowledge several important limitations to their study:

1. The small sample size (only eight HPPD participants) makes it difficult to draw definitive conclusions.
2. The extensive battery of tests required statistical adjustments for multiple comparisons, which reduced the ability to detect subtle differences between groups.
3. After these statistical adjustments, none of the cognitive differences remained significant, though the patterns observed at the individual level and in unadjusted comparisons remain intriguing.
4. The HPPD group had a higher history of using other substances beyond psychedelics and cannabis, which could potentially influence cognitive performance.

What This Means for Psychedelic Users and Therapists

The findings from this research have several important implications:

For Potential Psychedelic Users

While HPPD appears to be relatively rare, it represents a real risk that anyone considering psychedelic use should be aware of. The condition can cause not just persistent visual disturbances but potentially subtle cognitive effects that might impact daily functioning. Risk factors for developing HPPD aren't well understood, but the condition has been reported after just a single use of a psychedelic substance in some cases. This unpredictability makes informed decision-making particularly important.

For Healthcare Providers

Clinicians working with patients who have used psychedelics should be aware of both the visual and potential cognitive aspects of HPPD. The condition is often misdiagnosed or dismissed, leaving patients without proper support. The research suggests that neuropsychological testing focusing on visual memory and executive function might be particularly valuable for patients reporting HPPD symptoms, as these areas may show subtle impairments even when other cognitive functions remain intact.

For Psychedelic Therapy

As psychedelic therapy becomes increasingly accepted for conditions like depression, PTSD, and addiction, understanding rare but serious side effects like HPPD becomes crucial. Proper screening, informed consent, and follow-up protocols should include awareness of both the perceptual and potential cognitive aspects of HPPD.

Living with HPPD

For those experiencing HPPD, the condition can be challenging and isolating. Many report that healthcare providers don't take their symptoms seriously or don't know how to help. The visual disturbances can make everyday activities difficult, and the potential cognitive effects may add another layer of challenge. Currently, there is no established treatment for HPPD, though some medications like clonazepam (a benzodiazepine) have shown promise in reducing symptoms for some individuals. Avoiding all psychoactive substances, including alcohol and caffeine, is generally recommended, as these can sometimes worsen symptoms.

The Road Ahead

This research represents an important step forward in understanding HPPD, but much more work is needed. Future studies with larger sample sizes and focused testing of visual memory and executive function could help clarify whether the patterns observed in this small study represent genuine cognitive effects of HPPD. For now, the findings serve as a reminder that psychedelics, while showing therapeutic potential, are powerful substances that can have unexpected and lasting effects on perception and possibly cognition. As research into psychedelic therapy continues to expand, understanding these risks becomes increasingly important for ensuring safe and effective treatment.

Source: Leistenschneider, G., Majić, T., Reiche, S., & Riemer, T. G. (2024). Neuropsychological profiles of patients suffering from hallucinogen persisting perception disorder (HPPD): A comparative analysis with psychedelic-using and non-using controls. Scientific Reports, 14(1), 3968. https://www.nature.com/articles/s41598-024-82216-x