Fig. 4.
Formation of imprinting memory in chicks. Imprinting memory is first stored in an initial store (IMM) and then probably redistributed to a long-term store (S′). Long-term storage depends on sleep. (A) Experimentally, imprinting memory is induced by exposing chicks during the first 48 h after hatching to a moving stimulus (red box). During training the chick is in a running wheel. (B) After training, neurons in the left IMM (also termed S) selectively respond to the imprinting stimulus. Thus behavioral imprinting response and IMM firing activity are correlated. About 4.5 h after imprinting training IMM neurons cease to fire in response to the imprinting stimulus, yet the animal continues to respond behaviorally to the imprinting stimulus. Bilateral lesion to the IMM does not impair the behavioral imprinting response if performed 6 h or more after training. Also, suppression of slow oscillatory (0.5–2.5 Hz) EEG activity during sleep impairs the formation of imprinting memory, when applied within the first 9 h after training. During this time interval the redistribution of the memory trace from the IMM toward the unknown locus S′ is assumed to occur. Black and red lines indicate the strength of the putative memory traces formed respectively in the IMM and S′, as derived from the experimental findings. Solid line indicates that behavioral expression of the imprinting response during the first ∼5 h critically relies on the initial storage system of the left IMM. Although later on responsiveness of left IMM neurons to the imprinting stimulus increases again and remains elevated, activation of the IMM trace is not anymore critical for behavioral expression of imprinting (dashed line), as during this time behavior essentially relies on the long-term memory trace formed in S′ (solid line). Modified from Horn (2004). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)