Maturational sequence of the visual system: serial measurements of visual evoked potential and electroretinogram in the healthy neonatal lamb.

The value of measurements of visual evoked potential (VEP) for neurological assessment of the adult is widely accepted. Its use for neonatal evaluation following birth asphyxia has been limited by our knowledge of developmental changes occurring in the newborn brain. VEPs and electroretinograms (ERGs) were simultaneously recorded from five healthy newborn lambs from birth until 30 days of life. Newborn age was then compared with amplitude (height of the signal) and latency (time from stimulus to a specific wave deflection) measurements from these neuroelectrical signals. Latencies to A wave (photoreceptor activity) and B wave (bipolar and ganglion cells) of the ERG declined in a small but significant manner, while the latency interval from A to B wave did not change with age. Latency to wave N1 of the VEP remained stable while latency to P2 declined from birth to 30 days. The ERG and VEP signals exhibited linear increases in amplitude as the lambs became older. We conclude from these observations that ganglion and bipolar cells (B wave of the ERG) within the retina and primary nerve tracts (N1 of the VEP) along the visual system are mature at birth. In contrast, photoreceptor activity within the retina (A wave of the ERG) and nerve conduction through the visual cortex (P2 of the VEP) exhibit changes which are consistent with ongoing maturation of these more specialized areas through the early newborn period.