Sound exposure accelerates reflex emergence and development in young rats

Early sensory experience affects brain development. In rats, most somatic reflexes are not expressed at birth but may take as long as 2 weeks to emerge. Whether sensory enrichment during this early period affects reflex maturation remains unknown. Here, we exposed rat pups to a pure tone (4 kHz, 65 dB SPL, 8 h/day) with their nursing mother during the first 3 postnatal weeks and measured the times when reflexes appeared on the basis of video recordings. Sound exposure accelerated by about 15% the appearance of all reflexes assessed (righting, cliff avoidance, vibrissa placing, negative geotaxis and auditory startle, p < 0.001). In addition, sound exposure accelerated the appearance of developmental characteristics: incisor eruption, ear unfolding and eye opening. These changes occurred concomitantly with an increase in pups’ body and brain weights, together with a dramatic increase in fluid intake of the nursing mother. These findings are the first evidence that early sound exposure, even before opening of ear canals, accelerates reflex development. We speculate that the observed changes could involve the nursing mother.