Which of the following represents an abnormal EEG pattern?

The correct choice indicates an abnormal EEG pattern due to the presence of alpha frequencies that vary significantly from the typical range.

Alpha waves are generally expected to be between 8 to 12 Hz, and while a frequency of 9 Hz in one region (O1) falls within the normal range, the 11.5 Hz in O2 can signal subtle irregularities, especially if the alpha patterns are expected to be synchronized and symmetric across homologous areas. In this case, the asymmetry or variation in frequency can suggest an abnormality, particularly if this discrepancy corresponds with clinical symptoms or conditions.

Rhythmic sinusoidal waves at 10 Hz may represent normal background activity and, depending on the context, could be considered benign. Sharp spikes at 4 Hz can often indicate some forms of pathological activity, but they are not inherently abnormal without additional context. Similarly, delta waves at 2 Hz, while potentially concerning, are generally indicative of sleep or certain conditions but do not necessarily constitute an abnormal pattern without additional clinical correlation.

The focus on the variation of alpha frequency makes the first choice the most indicative of an abnormality in this scenario.