Hemispheric Asymmetry in New Neurons in Adulthood Is Associated with Vocal Learning and Auditory Memory

Abstract

Many brain regions exhibit lateral differences in structure and function, and also incorporate new neurons in adulthood, thought to function in learning and in the formation of new memories. However, the contribution of new neurons to hemispheric differences in processing is unknown. The present study combines cellular, behavioral, and physiological methods to address whether 1) new neuron incorporation differs between the brain hemispheres, and 2) the degree to which hemispheric lateralization of new neurons correlates with behavioral and physiological measures of learning and memory. The songbird provides a model system for assessing the contribution of new neurons to hemispheric specialization because songbird brain areas for vocal processing are functionally lateralized and receive a continuous influx of new neurons in adulthood. In adult male zebra finches, we quantified new neurons in the caudomedial nidopallium (NCM), a forebrain area involved in discrimination and memory for the complex vocalizations of individual conspecifics. We assessed song learning and recorded neural responses to song in NCM. We found significantly more new neurons labeled in left than in right NCM; moreover, the degree of asymmetry in new neuron numbers was correlated with the quality of song learning and strength of neuronal memory for recently heard songs. In birds with experimentally impaired song quality, the hemispheric difference in new neurons was diminished. These results suggest that new neurons may contribute to an allocation of function between the hemispheres that underlies the learning and processing of complex signals.

Unilateral vocal nerve resection alters neurogenesis in the avian song system in a region-specific manner

“…Earlier work showed that in unmanipulated controls, there were more new neurons 299 in the left NCM than the right, and that this asymmetry was lost after a unilateral 300 tracheosyringeal nerve cut (Tsoi et al, 2014). Likewise, we found more BrdU + /Hu + 301 neurons in the left than the right NCM (paired sample t-test, t (11) = 2.26, p = 0.045) in 302 controls.…”

Mirrored patterns of lateralized neuronal activation reflect old and new memories in the avian auditory cortex

“…Zebra finches whose song was most similar to the second tutor showed greater left-lateralization in NCM in response to second tutor song, which suggests that the left NCM may be more flexible in acquiring a representation of memory for a newly learned song. Perhaps this flexibility is supported by the addition of new neurons, as the degree of left-hemispheric dominance in adult neurogenesis correlates with the similarity between the bird’s song and the tutor song to which it was exposed during development (Tsoi et al, 2014). Left hemispheric dominance in good learners has also been reported during sleep: birds with high similarity to tutor song demonstrate greater neuronal activation in the left NCM, while poor learners show higher expression in the right NCM (Moorman et al, 2015).…”

The rodent hippocampus as a bilateral structure: A review of hemispheric lateralization

“…Lateralization in nonhuman animals was not reported until long after the initial reports on humans. Lateralization with respect to various forms of memory has since been reported not only in humans (e.g., Spiers et al, 2001), but in macaques (visual memory in Macaca nemestrina; Doty, Fei, Hu, & Kavcic, 1999), fruit flies (olfactory cued fear memory in Drosophila melanogaster ;Pascual, Huang, Neveu, & Préat, 2004), honeybees (olfactory cued reward memory in Apis mellifera; Rogers & Vallortigara, 2008), snails (food aversion memory in Helix lucorum; Kharchenko, Grinkevich, Vorobiova, & Grinkevich, 2010) and zebra finches (conspecific song memory in Taeniopygia guttata; Tsoi et al, 2014). Further, hippocampal lateralization of spatial memory in particular has also been reported in humans (Spiers et al, 2001) Shipton et al, 2014;El-Gaby et al, 2016), and rats (Rattus norvegicus; Klur et al, 2009).…”

“…Further, hippocampal lateralization of spatial memory in particular has also been reported in humans (Spiers et al, ) as well as homing pigeons ( Columba livia ; Kahn & Bingman, ), mice ( Mus musculus ; Shinohara et al, ; Shipton et al, ; El‐Gaby et al, ), and rats ( Rattus norvegicus ; Klur et al, ). In fact, flies and mice with asymmetric neural circuitry have superior memory compared to those with symmetric brains (flies: Pascual et al, ; mice: Kawakami, Dobi, Shigemoto, & Ito, ; Goto et al, ) and the degree of left‐dominance in the songbird auditory memory system positively correlates with song memory (Tsoi et al, ). These data indicate that lateralization in the neural systems underlying various forms of memory may be more of a rule across animals than a set of exceptions.…”