Downstream effects may make it difficult to determine:

• A. Double dissociations
• B. Causality
• C. The direction of causation
• D. The presence of modules

Answer: (A)

Downstream effects in brain networks can smear lesion–behavior relationships, making it difficult to determine double dissociations. When a lesion hits one region, its influence can cascade to connected areas, producing deficits in multiple functions rather than a clean, isolated impairment. To show a double dissociation, you need a pattern where damage to one region selectively impairs one function while sparing another, and a separate lesion does the opposite. But network propagation can blur this selectivity, creating shared deficits or masking true independence, so observed patterns may not reflect distinct neural substrates. The other options address causal relationships or modular organization, which are separate issues; downstream network effects mainly complicate how cleanly we can map specific functions to specific brain regions.