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Abstract Neural rhythms of the infant brain are not well understood. Testing the rhythmic properties of the adult visual system with periodic or broadband visual stimulation elicited neural resonance phenomena at ∼10Hz alpha rhythm. To investigate the rhythmic properties of the infant brain upon visual stimulation, eight-month-olds ( N = 42) were presented with visual stimuli flickering at discrete frequencies (2, 4, 6, 8, 10, 15, 20, and 30Hz) and broadband (i.e., aperiodic) stimulation, while recording a high-density electroencephalogram (EEG). Infants’ visual system entrained to the harmonics of the periodic stimulation frequencies (first to third). Crucially, a 4Hz rhythm emerged independent of stimulation frequency and the impulse response function (IRF) of the broadband sequence revealed a perceptual echo, reverberating visual information at 4Hz. This echo lasted for about 1 second, extended into frontal sensors, and selectively resonated the 4Hz component of the input signal. In a complementary adult sample, we confirm an alpha response upon periodic and broadband stimulation. To conclude, perturbing the infant visual system elicited a neural response and resonant activity at the 4Hz theta rhythm, which contrasts with the 10Hz alpha rhythm found in the adult visual system. Neural processing dynamics are thus essential to understand early brain development in full. Significance Statement This study tested resonance properties of the infant visual system upon perturbation. For the visual stimulation at discrete frequencies between 2 and 30Hz, the infant brain responded at 4Hz independent of the stimulation frequency. Following broadband stimulation, a perceptual echo at 4Hz emerged – resonating the 4Hz component of visual information over several cycles. These findings reveal that the 4Hz theta rhythm is an inherent processing dynamic of the infant visual system, contrasting with the 10Hz rhythm found in adults. The processing dynamics of the human visual system undergo fundamental changes from infancy into adulthood.