Changes in the structure of synapses associated with learning

Abstract

Two experiments were conducted to investigate the effects of training on a region of the chick brain known to be critically involved in imprinting, the intermediate and medial extent of the hyperstriatum ventrale (IMHV). In the first experiment, three groups of chicks were used: (i) dark-reared (n = 9), (ii) trained for 20 min (n = 17), and (iii) trained for 140 min (n = 7). Chicks were trained by exposing them when they were approximately 21 hr old to a flashing red light. Chicks were killed when they were approximately 30 hr old and blocks of tissue were removed from the right and left IMHV. Stereological techniques were used to measure from electron micrographs the numerical density of dendritic spine and shaft synapses and the length of the postsynaptic density of these synaptic junctions. There was a significant effect of training only in the left IMHV and on only one measure, the overall mean length of the postsynaptic density of spine synapses, S̄P̄(L). This measure was significantly greater by 17.2% in chicks trained for 140 min than in dark-reared controls. There was no significant effect of training for 20 min. In the second experiment one group of chicks (n = 15) was exposed to a rotating red box for a total of 3 hr. Another group of chicks was dark-reared (n = 15). The chicks were killed when they were approximately 46 hr old. Samples from the hyperstriatum accessorium and IMHV of the right and left sides were analyzed. Training was associated with a significant change, an increase, only of S̄P̄(L) in the left IMHV. There was a significant correlation between the extent to which chicks ran toward the training object and values of S̄P̄(L) in this region of the brain. The results of the two experiments are discussed in relation to the role of IMHV in the learning process of imprinting.