Imprinting. An electron microscopic study of chick hyperstriatum ventrale.

Fourteen chicks were hatched and reared in darkness to approximately equal to 21 h when they were exposed to overhead illumination for 0.5 h and then to an imprinting stimulus (a pulsing red light) for 20 min. The chicks were then matched in pairs on the basis of their activity. One member of each pair was returned to the dark and the other was trained for a further 120 min. All chicks were killed approximately equal to 6.5 h after the onset of training and perfused with fixative. Blocks were removed bilaterally from a restricted part of the hyperstriatum ventrale and prepared for electron-microscopy. Various synaptic features were measured, all counts being performed 'blind'. In undertrained chicks the length of the synaptic apposition zone in this part of the left hemisphere was shorter than that in the right by a mean value of 35 +/- 11.4 (SEM) nm. Further training eliminated this difference. No other synaptic measurements were affected by prolonging the period of training.