In which condition is the EEG likely to show a decrease in amplitude across all channels?

A decrease in amplitude across all channels on an EEG is indicative of a significant alteration in brain function, often reflecting more global changes rather than localized issues. In the case of coma, there is widespread depression of cortical activity due to severe impairment of brain function. This results in the EEG displaying low amplitude, which signifies the loss of the brain's normal electrical activity and coherence between different areas.

In contrast, during sleep, particularly in non-REM stages, the EEG typically shows distinct patterns such as sleep spindles and slow waves which may vary by amplitude but don’t universally decrease. Seizures would generally produce spikes and sharp waves, leading to increased amplitude in affected channels rather than a decrease. Hyperventilation can induce changes in the EEG, but they do not typically present as a decrease in amplitude across all channels; rather, they might induce specific patterns like a response in amplitude or frequency shifts.

Therefore, coma leads to the most consistent presentation of decreased amplitude across all channels on the EEG, serving as a key characteristic for identifying the profound alteration in brain activity associated with this condition.