In neocortical epilepsy, we showed that the seizure onset defined by ictal high-frequency oscillations (HFO: ≥ 70 Hz) with subsequent evolution into slower frequency activity (i.e., HFOs+) was smaller in spatial distribution than that defined by conventional frequency activity (1-70 Hz), and that resection of HFO+ areas resulted in favorable seizure outcome. This study further investigates ictal broadband EEG in the same cohort of patients by examining the infraslow activity, including ictal baseline ("direct current") shifts (IBS) and peri-ictal infraslow activity (0.02 to 0.2 Hz). The seizure onset zone had been defined and resected based on HFO+ by a prospectively defined protocol. We reviewed 11 representative seizures from 6 patients by visual and spectral analyses using appropriate filters and timescales. The HFO seizure onset, in the high gamma or ripple frequency, preceded or followed the IBS closely (<300 ms). The IBS were negative or positive, ∼1 mV in amplitude and 2 to 3 seconds long. Although the HFO+ were always ipsilateral to the surgical hemisphere, the IBS could be ipsilateral or contralateral. Compared with conventional frequency activity, the HFO+ and IBS were significantly smaller in spatial distribution and likely to be concordant. The peri-ictal infraslow activity consisted of distinct periodic or rhythmic (0.12 to 0.16 Hz) patterns, poorly concordant with IBS or HFO+. Although not statistically significant, better seizure outcome tended to correlate with smaller seizure onset zones and more complete resection of the HFO+ and IBS contacts. We conclude that IBS, like HFO+, define a smaller seizure onset zone and probably a more accurate epileptogenic zone in neocortical epilepsy.