Genesis Award 2003

Investigating the Potential Lateralization of the Search Image in the Avian Brain

Joyce M. Christensen & Jennifer J. Templeton, Knox College

          Many species have evolved cryptic colorations that allow them to blend into their natural environments and escape detection. A classic example is the peppered moth, at right.   One solution that some birds have evolved is the development of a search image.   Search images allow a predator to detect a cryptic prey type or species more easily as it gains more experiencewith that prey.   The ecological effects of this food-finding mechanism are well studied, but the underlying neurological mechanisms have not yet been examined.

 

Visual processing in birds is interesting because of the avian eye-brain system.   Birds lack a corpus callosum, a nerve structure which transmits information between the two brain hemispheres in mammals including humans.   Because birds lack this connection; all of the information entering the left eye goes through the optic nerve to the right brain hemisphere, but not the left hemisphere, and vice versa.   Because of this “split” brain system, determining which hemisphere is responsible for a particular task is relatively easy – just cover one eye.   Determining which hemisphere is responsible for a particular task is a broad, non-invasive method of studying the task’s neurological basis.

 

           

            In this study, twelve wild-caught adult European starlings were tested on a search image task with either their right eye (N=6) or their left eye (N=6) uncovered.   All birds, regardless of which eye-hemisphere system was available, were able to learn search images equally well over time, suggesting that the search image is not lateralized to one hemisphere or the other.

Although lateralization is one way to learn something about the neurological basis of a task, another way is to see if the learned task can subsequently be performed with the naïve eye-hemisphere system.   In order to test this possibility, I allowed the twelve birds to have access to their previously naïve (covered) eye and not the learning (uncovered) eye following the previous experiment, and found that they could not find the cryptic prey anymore.   Because the birds could perform the task with the learning eye-hemisphere system and not the naïve system, this suggests that search images become “trapped” in the learning hemisphere.   The results of the first experiment suggest that search images can be learned by either hemisphere, and is not lateralized in that sense.   The results of the second experiment suggest that the search image is lateralized in another sense, that the learned search image is restricted to a single hemisphere when learning occurred in only that hemisphere.   Therefore, the search image seems to have a neurological basis in the brain, although the ability is not restricted to one particular hemisphere.

Future studies in this area should focus on identifying the particular neurological structures and processes that are responsible for search image formation and use.   Similar research should also study other species, namely blue jays ( Cyanocitta cristata ) and pigeons ( Columba livia ), which have a long history in the search image and lateralization literature.