Behavioral and Neural Lateralization of Vision in Courtship Singing of the Zebra Finch

Much like humans, zebra finches must learn to produce their own vocalizations. This ability may come from lateralization of both behavior and brain function. Visual processing is highly lateralized in birds. This study proposed that lateralization of central brain processing would cause a behavioral bias towards use of one eye during courtship. The basis for this comes from the fact that less than 1% of axons from each retina project to the ipsilateral hemisphere and therefore must be concentrated elsewhere. To examine lateralization of brain function two activity dependent immediate early genes (IEGs), were monitored within the optic tectum (OTe). The OTe is the first relay of the avian visual pathway. In addition, preferential use of one eye over another was tested to see if it contributed to the effort with which the males courted the females.

A pair of caged female and male zebra finches was placed in a sound attenuation chamber. To test if one eye was preferred over the other during courtship, the left and right eyes were covered with opaque tape during separate trials. In a third trial, both eyes were left uncovered. After a one hour period of singing, IEG levels were significantly higher in the left OTe (which receives input from the right eye). In addition, levels of courtship singing increased when males could view females with only their right eye. In fact, 7/10 males produced 20-67% more song motifs when solely using their right eye during courtship.

Even though courtship singing was favored when the males had use of their right eye, normal levels of song could only be achieved with use of both eyes. With only the right eye uncovered, they produced about ½ the number of song motifs, took 4x longer to initiate singing and only initiated song about ½ of the time.

During normal courtship singing with the use of both eyes, neural activity is higher in the left OTe. Due to the fact that input from the eyes to the brain is almost 100% contralateral, males must favor viewing females from their right eye. This was supported by findings that male zebra finches sang significantly more when they had use of their right eye only. Increased levels in IEGs in the left OTe when the right eye was uncovered supports initial findings of increased levels in IEGs with binocular vision. The use of the right eye over the left to focus on a female during courtship suggests that the left hemisphere of the brain plays a predominant role in categorizing visual stimuli. Preferential use of the right eye could also be the cause of stronger input to the left OTe and an increase in levels of IEGs.

This study relates to class discussion about the critical role of visual input during behaviors such as courtship. Perhaps the circuitry of the right retina is more finely tuned to subtle differences in color in females. Or the lenses of the two eyes could differ in angles; maybe the right eye focuses specifically on objects that are closer to the body. Another thought is that lateral inhibition may play a larger role in the right eye, allowing the finch to better distinguish between edges and identify females. Although we have not yet discussed how the brain receives and processes information from the eyes, it would be interesting to see if the left visual cortex is responsible for categorizing visual stimuli.

George, I., Hara, E., & Hessler, N.A. (2006) Behavioral and neural lateralization of vision in courtship singing of the zebra finch. Journal of Neurobiology, 66, 1164-1173