Prejudice in the brain

Despite the great strides that have been made toward a more egalitarian society in the United States over the past 50 years, events like what occurred in Ferguson last month are a bleak reminder of the racial tensions that still exist here. Of course, the United States is not alone in this respect; throughout the world we can see abundant examples of strain between different races, as well as between any groups with dissimilar characteristics. In fact, it seems that the quickness with which we form a negative opinion about those who are not members of the same group as us may be characteristic of human nature in general, as its effects have been pervasive throughout history, and it persists even when we attempt fastidiously to stamp it out.

Indeed, it may be that our inclination towards prejudicial thinking has its roots in what was once an adaptive behavior. Some argue that our ancient hominid ancestors may have benefited from living in small groups, as this allowed for joint efforts in gathering and protecting resources. A logical offshoot of the development of group living would have been the emergence of skill in being able to tell members of your group apart from those who were not. It might have paid off to be wary of those who were not part of your group, as they would have been more likely to pose a threat. According to this evolutionary hypothesis, prejudice--which can be defined as an opinion of someone that is formed based on their group membership--may be the result of this strategy being so effective in the past. In essence, we may be saddled with the mindset of our evolutionary ancestors, which makes us more skeptical at first of anyone whom we see as "different" than us.

Prejudice and the amygdala

If prejudice is a deep-seated human behavior, it would not be surprising to find networks in the brain that are selectively activated when someone has xenophobic thoughts. One area of the brain that has been investigated in this context is the amygdala.

The amygdala is often associated with emotion, and is perhaps best known for its role in fear and the recognition of threats. If you were walking in the woods and saw a bear, your amygdalae would immediately become activated, helping to bring about a fear response that would encourage you to run away (or maybe cause you to freeze in place).

Several neuroimaging studies have looked at what happens in the brains of people when they see images of others outside of their racial group (e.g. white people looking at images of black faces). Some findings from these studies include: the amygdala is activated upon seeing such images, amygdala activation is correlated with xenophobic attitudes of the viewer, and amygdala activity in white people is higher when viewing black faces with darker skin tone.

Thus, the amygdala may serve as a threat-detection mechanism that is reflexively activated when we see an outsider. Perhaps because this has been adaptive in the past, it may act to put our brain on alert when someone outside of our racial group is near. In many societies today, however, where we are attempting to make racial divisions less distinct, this knee-jerk reaction seems to be counterproductive.

Prejudice and the insula

Another area of the brain that has been associated with prejudice in neuroimaging studies is the insula. The insula is also involved in processing emotional states, and has been linked to mediating feelings of social disapproval. For example, one study found that the insula and amygdala were activated in individuals while they viewed pictures of of people deemed to be social outcasts, such as homeless people or drug addicts. Because the insula is also activated when viewing pictures of people outside one's racial group, it has been hypothesized that the insula is involved in feelings of distaste that may arise when experiencing prejudicial thoughts.

Prejudice and the striatum

The striatum, a subcortical area thought to play an important role in reward processing, also has been implicated in prejudice--albeit in a very different way than the amygdala and insula. Activity in the striatum correlates with rewarding experiences, and neuroimaging studies have found that the striatum is also activated when looking at pictures of individuals from one's own racial group. When white participants were tested for implicit preferences (i.e. preferences they may not state or even be aware of, but that they still seem to possess) for people of their own race, activity in the striatum was stronger in response to white faces in those who scored higher on the test for implicit preferences.

Thus, there may be activity in the brain that reinforces our tendency toward prejudice in at least two ways: 1) we may be more likely to feel fear and aversion when seeing someone of another race, and 2) we may be more likely to experience positive emotions in response to seeing someone of our own race.

So, if there are structures in our brains that promote prejudice, does it mean attempts to reduce our prejudices--both individually and societally--are a lost cause? Of course not. Just as there are brain structures that may make us more likely to recognize differences, there are also structures (e.g. areas of the frontal cortex) that allow us to exert control over those potentially reflexive reactions.

It's possible that the recognition of deep-seated mechanisms for prejudice could help us to understand racism a little better. It could, for example, provide insight into why people in high-stress situations may be more likely to see things as divided down racial lines. For, if their brains are already inclined to see people of another race as more threatening and they are in a stressful situation, they may be quicker to identify someone of a different race as the threat.

However, the extent to which such innate responses to outsiders affects our behavior is still somewhat unclear, and the hypothesis that such responses are remnants of once-adaptive behavior is just that: a hypothesis. For practical purposes, it may not matter exactly what the basis of prejudicial thinking is, as we are certain it's a thought pattern that doesn't have much remaining value in today's world. However, being open to the idea that we have some inclinations toward prejudicial thinking may help us to be able to train people to more mindfully deal with high-stress interactions with people of another race. For, instead of pretending these prejudicial thoughts don't (or shouldn't) happen, it would allow us to focus more on ways to mitigate the damage that might occur when they do.

Amodio DM (2014). The neuroscience of prejudice and stereotyping. Nature reviews. Neuroscience PMID: 25186236

The neuroscience of self-control

In the 1960s, a psychologist at Stanford named Walter Mischel began a series of experiments exploring the dynamics of self-control in children. In one such experiment, Mischel gave preschoolers the choice between two outcomes, one of which was clearly preferable. For example, they were able to choose between 2 marshmallows and 1 marshmallow (the experiments became known as the Stanford marshmallow experiments for this reason).

But there was a catch. The experimenter would tell the children that he had to leave the room for a short period of time. In the best-known version of the experiment, the child was forced to sit in the room with the less appealing prize (e.g. just 1 marshmallow). However, the only way the child could get the two marshmallows is if she waited until the experimenter returned (about a 15-minute period) and did not eat the one marshmallow before that point.

The experiment was designed to measure delay of gratification. Would the child wait 15 minutes or would she give in and eat the marshmallow, knowing it meant she had to forego the ultimately more rewarding outcome of receiving two marshmallows? Mischel found, as would be expected, that there was a lot of variability in the capacity of children to delay their gratification to obtain a more valuable prize. Some ate the one marshmallow right away, not being able to subdue their desire for even a few minutes. About 1/3 of participants waited the entire 15 minutes to get the second marshmallow.

But the really interesting part about this experiment came when Mischel et al. followed up with these kids about 10 years later. They found that the kids who showed the most self-control as preschoolers were, in adolescence, rated by their parents to be more verbally fluent, attentive, competent, skillful, academically successful, socially adept, and better at dealing with frustration. What's even more interesting is that the amount of time the children were able to delay their gratification was correlated with their SAT scores. A number of other studies have since found associations between this early ability to delay gratification and later measures of intelligence, academic success, and even body mass index.

Neuroscience of self-control

It has been hypothesized that the ability to delay gratification is dependent on a push-pull relationship between the frontal cortex and the limbic system. The frontal cortex (and especially the prefrontal cortex) is frequently associated with planning and decision-making. Thus, it may be that this is the area of our brain that allows us to realize the value of being patient and waiting for a less immediate, but overall more satisfying, reward. Interestingly, in people who are addicted to drugs like methamphetamine or heroin, we tend to see reduced activity in the prefrontal cortex, suggesting that part of their difficulty in achieving abstinence might be due to a decreased ability to appreciate the value of a long-term reward like being drug-free.

When we consider a short- vs. long-term reward, however, another part of our brain becomes active as well. The limbic system, which contains several structures and is known for its involvement in emotional processing, is also activated. The limbic system is often implicated in "gut" responses to things, whether aversive or pleasurable. Thus, when we see or think about a valuable reward, the limbic system responds by pushing us to get it. The limbic system takes more of a primitive approach, telling us to chase after those things that feel good and avoid those that feel bad. It may be responsible for the impatience associated with short-term reward seeking.

Improving self-control

The ability to delay gratification is an important part of a healthy and satisfying life. It allows us to skip the fatty food to have a healthy snack, lets us stop after the first drink instead of having the second (and third, fourth, etc.), and encourages us to accomplish what we need to at work before opening up the web browser to peruse Facebook. Because it is such a valuable skill, researchers are interested in figuring out how we can improve it.

The research suggests that one important part of improving self-control is setting specific and realistic goals. Goals should be designed based on your internal motivation (in other words it should be something you--not somebody else--wants you to do), otherwise they tend to be less effective. It is most effective to set goals that are meant to be achieved within a certain time frame, as this allows you to monitor progress at specific intervals. Research suggests that just the act of setting a specific and attainable goal improves self-control.

The next step after setting a goal is to monitor your performance. It's important to pay attention to actions that conflict with achieving your goal. However, it's equally important to accept any deviations from the intended course of action as learning opportunities instead of looking at them as failures. Being compassionate about your slip-ups increases the probability that you will eventually reach your goal; this has been seen, for example, in studies of smokers and dieters.

Along the way, it can be helpful to develop specific behavior plans relating to your goal. Creating a schedule that determines when, where, and how you will exercise the behavior needed to reach your goal can help you actually follow through on that behavior. For example, deciding that you will run on the treadmill for 30 minutes right after work on Monday, Wednesday, and Friday is more effective than deciding you will use the treadmill a few times a week, but not determining when and for how long.

Although a propensity toward stronger or weaker self-control can be seen at a young age, research suggests that self-control is a skill that can be improved with practice. So, regardless of how inactive your prefrontal cortex might be in relation to your limbic system, and even if at preschool age you would have been more likely to eat the one marshmallow than wait 15 minutes for the second, with a little work and some good goal-setting anyone really can enact changes in their behavior.

Inzlicht, M., Legault, L., & Teper, R. (2014). Exploring the Mechanisms of Self-Control Improvement Current Directions in Psychological Science, 23 (4), 302-307 DOI: 10.1177/0963721414534256

Know your brain: Retina

Where is the retina?

Retina of left eye. Image courtesy of Richard Masoner.

Retina of left eye. Image courtesy of Richard Masoner.

The retina is a collection of cells at the back of the eye where the processing of visual information begins. It is only about as thick as a razor blade, but contains several layers of cells that are made up of a number of different cell types. The retina forms from brain tissue during development, and is considered part of the central nervous system.

What is the retina and what does it do?

When light reaches the back of the eye, it then enters the cellular layers of the retina. The design of the retina is somewhat counter-intuitive as the cells that detect and respond to light, known as photoreceptors, are located at the very back of the retina. This means that light must travel through several layers of cells before reaching the point where visual processing begins.

There are two types of photoreceptors: rods and cones. Rods have poor resolution, but are very sensitive to light. They allow us to see in dim light, but don't allow for the perception of color. Cones, on the other hand, have high resolution but are not as sensitive; they allow us to perceive color under normal lighting conditions. Throughout most of the retina, rods outnumber cones. In one area called the fovea, however, the number of cones is increased 200x, and they significantly outnumber rods. The fovea represents the area of the retina that provides our highest acuity vision, and thus is at the center of our gaze.

When light hits photoreceptors, it interacts with a molecule known as a photopigment, which responds by undergoing a chemical change. In this case the chemical change begins a chain of events that serves to propagate the visual signal. Activation of the photopigment leads to a change in membrane potential and a modification in the amount of neurotransmitter released from photoreceptor cells. This process of translating visual information into a signal that can be passed through the nervous system is called phototransduction.

Phototransduction leads to a signal being transmitted to cells called bipolar cells, which connect photoreceptors to ganglion cells. Bipolar cells send the signal to ganglion cells, which travel out of the eye in a large cluster through an opening called the optic disc. After leaving the retina, the ganglion cell fibers are called the optic nerve. The optic nerve carries visual information toward the brain to be processed.

The optic disc doesn't contain any photoreceptors, and so represents an area on the retina that can't process visual information, creating a natural blind spot. The blind spot is actually relatively large; although the size varies from individual to individual, it can be upwards of 2mm in diameter and cover an area of your visual field equivalent to the width of your four fingers when held at arm's length. However, we usually don't ever notice our blind spot. The brain uses information from surrounding photoreceptors and the other eye to fill in the gaps in the images that are processed by the retina.

If you want proof that the blind spot is there, take a look at the X and O below. Keep your computer screen at about arm's length and cover your left eye. Keep your right eye on the O, then slowly move your face towards the screen, all the while continuing to look only at the O. At some point, the image of the X in your peripheral view should disappear. This is because at that point, the image of the X is falling on the blind spot of your right eye.


There are two other cell types in the retina that should be mentioned: horizontal and amacrine cells. Horizontal cells receive input from multiple photoreceptor cells. They use that input to make adjustments to the signal that will be sent to the bipolar cells, in the process increasing sensitivity to contrast under different lighting conditions. Amacrine cells acts as interneurons between bipolar and ganglion cells. Although their function is poorly understand, they are thought to modify signals being sent from bipolar cells, affecting sensitivity, contrast, etc.