How Does Thalamic Brain Connectivity Differ in Youth with ADHD?

What you need to know

• Youth with ADHD showed decreased thalamic connectivity with certain brain networks compared to youth without ADHD
• Different patterns were seen depending on how thalamic regions were defined - functionally or anatomically
• Some thalamic connectivity patterns correlated with ADHD symptom severity in youth with ADHD
• The findings suggest thalamic connectivity based on functional brain networks may be clinically relevant in ADHD

Background on ADHD and Brain Connectivity

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder that often persists into adulthood. It is characterized by symptoms of inattention, hyperactivity and impulsivity. Previous research has implicated differences in brain connectivity, particularly in circuits involving the thalamus, as playing a role in ADHD.

The thalamus is a structure deep in the brain that acts as a relay station, processing and transmitting information between different brain regions. Studies have shown that different parts of the thalamus connect with different areas of the cerebral cortex (the outer layer of the brain). However, few studies have looked specifically at how thalamic connectivity may differ in youth with ADHD.

This study aimed to investigate thalamic connectivity in youth with ADHD using two different approaches:

1. Defining thalamic regions based on their functional connections to large-scale brain networks
2. Defining thalamic regions based on their anatomical structure

The researchers hypothesized they would see different patterns of altered connectivity depending on which method was used to define the thalamic regions.

How the Study Was Conducted

The study analyzed brain imaging data from the publicly available ADHD-200 database. Specifically, they used resting-state functional MRI scans from 53 youth with ADHD and 53 youth without ADHD, matched for age, sex and IQ.

The researchers defined thalamic regions of interest in two ways:

1. Functionally - using an atlas that parcellates the thalamus based on its connections to 7 large-scale brain networks
2. Anatomically - using an atlas that defines thalamic nuclei based on anatomical structure

They then compared thalamic connectivity patterns between the ADHD and non-ADHD groups, and looked at correlations between connectivity and ADHD symptom severity.

Key Findings on Thalamic Connectivity in ADHD

Using the functionally-defined thalamic regions, the study found:

• Youth with ADHD showed decreased connectivity between the thalamus and the dorsal attention network compared to youth without ADHD
• In youth with ADHD, thalamic connectivity with the somatomotor network negatively correlated with inattention symptoms
• Thalamic connectivity with the default mode network negatively correlated with hyperactivity/impulsivity symptoms

Using the anatomically-defined thalamic regions, the study found:

• Youth with ADHD showed decreased connectivity between the mediodorsal thalamus and visual/default mode networks
• In youth with ADHD, mediodorsal thalamic connectivity with the dorsal attention network positively correlated with inattention symptoms
• Lateral geniculate thalamic connectivity with the frontoparietal network positively correlated with hyperactivity/impulsivity symptoms

Importantly, the patterns of altered connectivity were different depending on how the thalamic regions were defined. With the functional approach, differences were seen within the boundaries of the predefined brain networks. With the anatomical approach, differences were seen outside the expected anatomical projection areas.

Implications of the Findings

The fact that different patterns emerged based on how thalamic regions were defined has interesting implications. The findings suggest that thalamic connectivity based on functional brain networks may be more clinically relevant in ADHD than connectivity based purely on anatomical structure.

The decreased thalamic connectivity seen in youth with ADHD aligns with previous research implicating altered brain connectivity in the disorder. The correlations between connectivity and symptom severity provide further evidence that these connectivity differences may be meaningful.

Interestingly, opposite directions of correlation were seen with the functional versus anatomical approaches. The researchers suggest this could potentially reflect compensatory brain changes in ADHD. For example, strengthened connectivity in some circuits may develop over time to compensate for decreased connectivity in others.

Limitations and Future Directions

Some limitations of this study include the small sample size, particularly the low number of female participants. The cross-sectional design also limits the ability to draw conclusions about how connectivity may change over time.

Future studies with larger, more diverse samples and longitudinal designs could help clarify how thalamic connectivity patterns in ADHD may differ across development. More research is also needed to better understand the functional significance of the connectivity differences observed.

Conclusions

• Youth with ADHD show altered patterns of thalamic brain connectivity compared to youth without ADHD
• Different patterns emerge when defining thalamic regions functionally versus anatomically
• Thalamic connectivity based on functional brain networks appears clinically relevant in ADHD
• Some connectivity differences correlate with ADHD symptom severity
• Further research on thalamic connectivity could provide insights into the neurobiology of ADHD