Here, we show how developing thalamic and cortical pathways might temporarily enable spindle-bursts. Pathways linking the retina (pink), thalamus (green, T), reticular thalamic nucleus (purple, R), and cortex (grey) are shown at three different times during development based on data from rats and mice. The thickness of each line represents the strength of the pathway; arrows mark pathways that increase in strength at each developmental timepoint. Transmission speed and the strength of the connection between the thalamus and its synaptic partners help to set the frequency and level of synchronization of oscillations (Bal et al., 1995). During the early and middle stages of the spindle-burst window (left), an ascending pathway transmits retinal waves to thalamus and cortex. Early in the spindle-burst window, thalamic circuitry produces brief, low frequency spindle-bursts (indicated by sine wave), likely through connections to the reticular nucleus, which have just become functional at this time. A modest corticothalamic projection (grey) amplifies this oscillation. Later in the spindle-burst window (middle), a strengthened corticothalamic projection (arrow) amplifies, speeds, and prolongs thalamic spindle-bursts. After the spindle-burst window (right), reciprocal connections between thalamus and the reticular nucleus continue to develop (lower arrows), as may the projection from the cortex to the reticular nucleus (upper arrow). At the same time, both the thalamic and cortical components of spindle-bursts disappear: this enables the thalamus to respond to transient stimulation from the retina, before the thalamic response is quickly suppressed by the cortex.