The laminar development of direction selectivity in ferret visual cortex.

Sensory experience plays a critical role in the development of cortical circuits. At the time of eye opening, visual cortical neurons in the ferret exhibit orientation selectivity, but lack direction selectivity, which is a feature of mature cortical neurons in this species. Direction selectivity emerges in the days and weeks following eye opening via a process that requires visual experience. However, the circuit mechanisms that underlie the development of direction selectivity remain unclear. Here, we used microelectrodes to examine the laminar chronology of the development of direction selectivity around the time of eye opening to identify the locations within the cortical circuit that are altered during this process. We found that neurons in layers 4 and 2/3 exhibited weak direction selectivity just before natural eye opening. Layer 4 neurons in animals that had opened their eyes but were younger than postnatal day 35 (PND 35) exhibited modestly increased direction selectivity, but layer 2/3 cells remained as weakly tuned as before eye opening. Animals that had opened their eyes and were PND 35 or older exhibited increased direction selectivity in both layers 4 and 2/3. On average, initial increases in direction selectivity in animals younger than PND 35 were explained by increases in responses to the preferred direction, while subsequent increases in direction selectivity in animals PND 35 or older were explained by decreases in responses to the null direction. These results suggest that all cortical layers are influenced by sensory stimulation during early stages of experience-dependent development.