A lateralized brain, in which processing of cognitive information is evenly distributed between two hemispheres, was thought by researchers to be a unique human characteristic and the cause of superior cognition. However, recent research shows that lateralization of the brain is not only present in humans but it is also widespread in vertebrates. In this paper, I will present how brain lateralization functions in birds, paying close attention to the pigeon and domestic chick to draw inferences about human brain lateralization. Furthermore, I will show the current interpretations how a lateralized brain might have evolved, and how it affects cognitive abilities of birds.
The brain hemispheres are divided in such a way that particular functions are distributed evenly between the hemispheres. It is widely known that the left hemisphere specializes in responding to situations that require consideration of several alternatives, whereas the right hemisphere is used to respond instinctively to unexpected stimuli (Vallortigara, 2000). In addition, the hemispheres of the brain control the opposite sides of the body. For instance, the left eye is controlled mostly by the right hemisphere while the right eye is controlled by the left hemisphere, a pivotal observation to studying avian lateralization. When needed, the two hemispheres communicate with one another through neural pathways. In humans, this communication occurs through a particularly thick set of neural fibers referred to as the corpus callosum.
In order to understand how brain lateralization works in animals, researchers have focused on the avian bird, especially of the domestic chick (Gallus gallus) and the Altricial pigeon (Columba livia). One advantage of studying these species is that brain lateralization can be experimentally manipulated with well known methods, such as altering the pre-hatching environment, or chirurgically altering the brain. Furthermore, based on the bird’s behavior, the effects of lateralization can be easily inferred, without requiring any brain imagining techniques. Finally, the observations can easily be interpolated to the human brain, since brain lateralization in humans has been shown to be very similar to that of chicks.
Until recently, there was no conclusive evidence that the division of functions between the hemispheres actually improved cognition. To examine this, a series of studies were developed to test the cognitive functions of pigeons and chicks. These focused on the animal’s visual ability to distinguish between grain and pebbles in various experimental conditions manipulating the presence of lateralization and the degree of stimulation to a particular hemisphere of the brain. To asses the bird’s cognitive ability, success was measured by counting the number of consumed grains as opposed to the total number of pecks. It is important to note that in all the studies, the probability of picking out food was significantly smaller (around .1) than the probability of picking the pebbles, which means that cognitive abilities must be involved, and the results are not simply the result of randomness.
The first studies about improved cognition as a result of lateralization were carried out in pigeons (Güntürkün, 1985, 1987). For these, brain lateralization was manipulated through a surgical procedure where a metal block was inserted in the brain of ten pigeons, preventing communication between the hemispheres of each animal, a procedure similar to the one used in the past on "split-brain" humans (Vallortigara, 2000). This isolates the processes that are carried out in each hemisphere giving the flexibility of studying the effects of stimulating only one hemisphere. To accomplish this, an eye cap was placed on one eye of the pigeon, so that only one eye receives information about the outer world. For instance, if the left eye is covered, then right eye is active, stimulating only the left hemisphere. The experiment consisted of placing a food-deprived pigeon (having 80% of its original weight) in a container with thirty randomly dispersed grains with thirty grams of pebbles (around 10,000) of similar size, shape and color to the grains. Thirty seconds after the pigeons’ first peck, the animal was removed from the container. This procedure was then repeated for ten sessions with each of the eyes being covered. It was found that when the pigeon uses the left eye, it pecked faster and picked the grain with higher discrimination rates. Interestingly, the greater success is not related with higher pecking activity, since there was no significant difference among the two conditions.
A similar procedure was carried out in an experiment by Rogers (2004) with domestic chicks. The purpose of this study was to measure if a lateralized brain did in fact provide some evolutionary advantage in terms of visual cognition while performing two simultaneous tasks. This time, instead of using medical procedures, brain lateralization was manipulated by changing the pre-hatching environment of the chicks by controlling the amount of light received by the eggs. It has been found that just days before hatching, the avian embryo has a tendency to turn to the one side, exposing the left eye to external light that penetrates the translucent shell of the egg, but keeping the right eye occluded by his body. This stimulus directly stimulates the right hemisphere of the brain, and creates visual lateralization of visual cognition: there is a marked division in how visual stimuli are processed in the brain. By this process, only visual lateralization develops through this procedure and not of other functions such as olfaction and audition (Rogers, 1997). In contrast, those chicks that are raised in the dark do not have visual lateralization. Thus, in order to manipulate this variable, researchers simply control the light stimulation near the final stage of incubation. The physiological explanation for this process seems to be related to sex hormone levels in the embryo: if they are artificially elevated, no visual perception asymmetry develops after light simulation (Vallortigara, 1999).
Two experiments were carried out, one where the chicks attention was strictly directed towards pecking, and another where the chicks had a threat of predation. Discriminating pebbles from grain is known to be associated with the left hemisphere, while monitoring for novel stimuli, like a predator, is related to the right hemisphere (Rogers, 2004). For both experiments, the chicks were food-deprived for a period of four hours. The sample chick population was divided into two: those that were exposed to light and had brain lateralization (lateralized chicks), and those that were incubated in the dark and thus had no lateralization. A similar procedure to the one used with pigeons was used, except that the experiment concluded after the bird had pecked 60 times, instead of based on a time period. This procedure was used to assess the level of learning by contrasting the success rates in three blocks of 20 pecks. It was found that when the chick’s attention was directed only towards the pecking, the lateralized chicks performed better at the task. Furthermore, both conditions showed good memory retention when tested the next day, but the non-lateralized chicks did not show significant learning within the blocks. For the second experiment (n=30), a model predator swung on a cord above the chick every 18 seconds, simulating a flying predator. The rest of the procedure was exactly the same. It was found that the chicks with a lateralized brain performed significantly better than those raised in a dark pre-hatching environment. Lateralized chicks stopped pecking when the predator appeared 79% of the times, while the non-lateralized chick did so 63% of the times, which implies that even though the lateralized chicks were more vigilant and stopped the pecking, the non-lateralized chicks were even more affected in their cognitive performance. It is very important to note that the lateralized chicks usually tilted their heads to view the predator with the left eye, which confirms that monitoring the predator is associated with the right hemisphere. Once again, lateralized chicks showed memory retention, but the non-lateralized chicks did not. As an overview, lateralized chicks performed better in both tasks, which corroborate the findings in pigeons (Güntürkün, 1987, Vallortigara, 1999), but also confirms the initial hypothesis that lateralization does in fact increase cognitive abilities when two simultaneous tasks are being performed. It is worth noting that this significant difference in cognition does not necessarily put the non-lateralized chicks in a clear disadvantage. For instance, another study shows that when competing for access to a food bowl, non-lateralized chicks are actually more successfully than lateralized ones (Rogers & Workman 1989 as cited in Rogers, 2004). Therefore, the increased cognitive abilities in lateralized chicks are very specific to the task being performed, and the difference between lateralized and non-lateralized chicks is seldom apparent in normal behavior.
The previous studies in pigeons and chicks suggest that brain lateralization does in fact provide cognitive advantages in certain specific tasks. However, a pivotal question to consider is, are there situations were brain lateralization is disadvantageous? Research in chicks and other species shows that this is the case. For instance, when chicks faced a physical barrier while approaching a familiar object, they have a tendency to detour to the right, keeping visual contact with the object through the left eye. Contrastingly, when there is novelty involved in the stimulus, such as a change in color, chicks are biased to detour the barrier on the right in order to view the object with the left eye (Vallortigara et al, 1999 as cited in Vallortigara, 2000). This pattern suggests that when facing a predator, the chick will have the tendency to monitor it with left eye in order to stimulate the right hemisphere, a behavior that could be potentially exploited by predators.
A more quantitative experiment about behaviors that result from lateralization was not carried out in birds but in various species of toads. It was found that when prey appeared in the right side of their visual field, the toad’s predatory behavior was evoked. Contrastingly, when the animal appeared on the right side the toad only responded after the prey entered the binocular region in which both eyes receive stimuli. These findings were also in accordance with prey animals, such as fish, who detected the prey on one side better than the other (Vallortigara, 2000).
From an evolutionary perspective, these studies suggest that it is contradictory that visual lateralization developed in the first place. Knowing that a predator can appear on either side of the animal with equal probability, it would be disadvantageous for survival that when an animal is being approached by a predator on the left side it is not as responsive as if it were being approached on the right side (Rogers, 2004). With this in mind, it has been suggested that perceptual asymmetries due to brain lateralization should have developed at the level of the individual, instead of at the population level. In the case that half the population responds better when the predator appears on the right side, while the other half when the predator appears on the left side, perceptual asymmetries could not be readily exploited by predators, since the behavior of a particular animal would not be predictable. It has been found that this is the case with species that are not highly sociable. However, in highly sociable species, including primates, brain lateralization manifests at the population level (Bisazza, 1999 as quoted in Vallortigara, 2000), so that most animals within the specie show a predisposition towards one side being more sensitive to stimuli. This finding suggests that survival of a group of individuals would increase if all the individuals have the same perceptual asymmetries, thus showing a similar behavior when in threat. Considering the case of a school of fish, if one individual detours to the side opposite to their group, it is very likely that it will be predated.
So why did lateralization developed in so many organisms? In general, there seems to be a consensus that the disadvantages of lateralization are undermined by the advantages it provides, including faster response rates and better cognitive performance. However, the reasons behind how the physiological change emerged are not entirely clear. Two main lines of thought are predominant. On one hand, some researchers believe that lateralization developed due to incompatibility between the strategies used in different experiences, mainly with novel and familiar stimulus, as well as limitations in brain size (Andrew, 1982 as cited in Rogers 2000; Vallortigara, 1999). Another approach has focused mostly on efficiency: without lateralization the same task could be performed, but at slower response rates. This approach suggests that having a specialized hemisphere for each of the two problem-solving strategies allows for parallel processing greatly enhances the animal’s ability to perform two tasks simultaneously (Halpern, 2005). It has also been proposed that lateralization would decrease redundancy of computations in both hemispheres, and avoiding the slow communication between the hemispheres (Deacon, 1997, Levy 1969, Ringo 1994 as cited in Rogers 2000). Both of these lines of thought support that in general brain lateralization was a result of pressures to adapt to new conditions in the external environment.
After extensively studying the bird’s brain, a greater understanding about how the human brain works has developed. Even though the domestic chick does not have corpus callosum, but smaller neural pathways, research shows that in terms of motor behavior, cognitive processes, and emotional behavior, lateralization in chicks closely parallels with that of humans (Andrew, 1991 as cited in Vallortigara & Rogers, 2000). In humans, a particular eye projects to both hemispheres of the brain, but still stimulates the contralateral hemisphere more, as in the case of birds (Rogers, 1997).
Drawing inferences from the chick, an interesting hypothesis was developed to explain handedness in humans. Just the differences in pigeon’s cognitive ability do not manifest in competition (Rogers & Workman 1989 as cited in Rogers, 2004), perceptual asymmetries in humans are seldom evident in normal everyday behavior (Vallortigara, 1999). However, one asymmetry that is widely spread in humans is the tendency to be right-handed, which implies that in general he left hemisphere of the human brain is more active. To explain this asymmetry, the hypothesis suggests that the predominant position of the fetus during the last trimester of pregnancy controls the amount of light stimulation of the fetus in the similar way that chick’s lateralization is affected by light during pre-hatching (Previc 1999, as cited in Rogers, 2004).
To understand the processes involved in higher cognitive functions, it is essential to fully understand the evolution of lateralization. Studies of the avian brain suggest that human higher cognitive functions are in fact related to a higher degree of brain lateralization than in other species, which could have developed due to increased social interactions within primate family. As the environment changes, new pressures are exerted to increase their cognitive abilities of all species in order to ensure survival. Human brain lateralization takes advantage of the new possibilities inherent to a larger brain. Through evolutionary selection of brain lateralization, problem-solving has surpassed the basic need of survival to extend to creative and community-oriented exploration of the complexities of our own universe.
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