This summary, written by Ilaria Tare, is based on a paper published from our lab and available on our website: Van Hedger, Kathryne, et al. “Effects of Methamphetamine on Neural Responses to Visual Stimuli.” Psychopharmacology, 236:1741–1748, 2019.
When we say methamphetamine, you may think of something like this—commonly known as crystal meth. However, methamphetamine itself is a man-made stimulant similar to amphetamine (a more common drug name you know) that is approved for treatment for ADHD (Desoxyn) and some treatment for obesity. We administer the drug (acquired from professional pharmacology labs after receiving IRB approval) orally in a pill form and under supervised conditions.
In the past, studies have shown links between stimulants and behavioral and reward-seeking effects, some of which appear to be related to the abuse potential of stimulants. These effects include enhanced cognitive performance, reduced inattention and impulsive behavior and improved memory. Previous brain scans have even shown a single dose, 20 mg, of methamphetamine (MA.) increases neural response during cognitive and attentional processing. However, very little is known about the effects of stimulants, like MA, on sensory processes, or information received from our 5 senses. Do stimulants affect the neurons that process sensory stimuli, and do these effects depends on the images shown? The Human Behavioral Pharmacology lab recently conducted a study in which a group of 18 healthy, adult participants received a single dose of MA, and then viewed a series of visual images.
Sensory processing is the process that organizes sensation from one’s own body and the environment, thus making it possible to use the body effectively within the environment.
Participants came in for two sessions, one where the drug was administered and one when a placebo was administered. They were shown a series of images of nature scenes that varied in complexity while being monitored by an fMRI machine. The complexity of the images was categorized by the degree of “non-straight edges” (NSE), which reflect the proportion of curves present in an image. Images with a high NSE are more complex than images with a low NSE. For the sake of this study, examples of high NSE images were more complex and included plants, like a mountain with flowers. Low NSE images were less complex and consisted of simpler scenes, like a desert with sand dunes. Drug effects on sensory perception were evaluated by the brain activation of participant fMRI scans as well as self-reported drug effects (e.g., “how high are you.”) Originally, researchers hypothesized that high NSE images would cause more activation in areas of the brain dealing with sensory processing, like the visual cortex, than low NSE images, and these effects would increase with the administration of MA. However, researchers found that, overall, high NSE images showed more activation in the primary visual cortex than low NSE images, but that stimulus-induced activation in the brain did not differ when administered MA versus placebo. They also found a significant interaction between the drug and the more complex images in areas of the brain that deal with visual processing-- specifically the left fusiform, the right cingulate/precuneus, and the posterior right middle temporal gyrus. Whereas activation in these areas was greater when shown high NSE images during the placebo sessions, the drug had a greater effect on activation in these areas when subjects viewed low NSE images.
Under placebo conditions, activation in the areas of the brain that deal with visual processing was greater when shown high NSE images. However, once drug was administered, the drug had a greater effect on activation in these areas when subjects viewed low NSE images.
So what does this all mean? Like previously predicted, neural processing of sensory stimuli was greater with the more complex images than with the less complex images, as one might intuitively guess. However, when MA was introduced, not only did sensory processing for the more complex images increase, which suggests a stronger conditioned response in the primary visual cortex, it also increased for the less complex images. This tells us that MA indeed does enhance sensory processing in the primary visual cortex, but less complex images may be more susceptible to drug influence. The authors conclude that more research is needed to better understand the exact effects MA has in sensory processing and its neural mechanisms.