Highlights•We studied habituation and novelty detection via EEG and fNIRS at 1, 5 & 18 months.•fNIRS and EEG responses were correlated for habituation (1&5 m) and novelty (5&18 m).•Findings represent the first converging longitudinal infant fNIRS and EEG responses.•Responses were associated across a wide age band despite using different stimuli.•Cross-modality correlations may be strongest at times of great developmental change.