Brain functions, such as language, are shaped by experience during windows of heightened plasticity in early life. For example, children raised in an English- speaking environment will easily distinguish the phonemes /la/ and /ra/, whereas those growing up in Japan find it increasingly difficult. The underlying sites of plasticity along the auditory pathway, as well as potential mechanisms engaged during such ‘critical periods’ remain largely unknown.
Passive exposure to sound stimuli (tones) powerfully enhances response strength and expands topography within primary auditory cortex (Fig 1). Tania Rinaldi Barkat, a Harvard Fellow in the lab of Takao Hensch, has now isolated the refinement of connections specifically linking the mouse auditory thalamus to cortex. Such plasticity was strikingly restricted to a brief three-day critical period shortly after hearing onset in mice. Systematic, voltage-sensitive dye imaging in acute brain slices was confirmed (in collaboration with D. Polley, MEEI) by microelectrode recordings from intact animals.
Importantly, sound frequency maps were unaltered in the thalamus itself, suggesting this auditory plasticity originates in the cortex. Direct examination of dendritic spines, the sites of synaptic contact upon cortical neurons, revealed a preferential maturation of thalamo-cortical circuits over the timecourse of the critical period. Moreover, genetic deletion of a cortical cell-adhesion molecule (Icam5) induced precocious stubby spine maturation and accelerated the rate of auditory plasticity.
Early acoustic experience thus impacts the evolving postnatal connectivity between thalamus and cortex in the days following hearing onset. The ability to trace systems level plasticity to individual spines in the auditory pathway may offer novel therapeutic insight and educational strategies for language development and its disorders.
Read more in Nature Neuroscience
[July 31st, 2011]
Figure 1. Sound frequency maps in auditory cortex (ACx) shaped by early acoustic experience. Neuronal activity recorded in the intact animal (a, upper panel) or in acute brain slices that preserve connectivity from auditory thalamus (MGv) to cortex (a, lower panel) exhibits a balanced sound spectrum in ACx (color). Passive tone exposure (7kHz) during a brief developmental critical period distorts the map so as to over-represent the stimulus (b, green).
Black and grey lines (upper panels) indicate reference blood vessel patterns on the cortical surface. Colored dots and bars represent best frequency in vivo (color scale) or MGv input in vitro (colored arrows), respectively, to that cortical site.