Patients with lesions of amygdala shown no enhanced awareness of the presentation of emotionally evocative words
Humans are not passive perceivers, giving equal access for entry into consciousness to all stimuli; we can ignore some events, words, and stimuli, but not others. For instance, our perceptions are influenced by the emotional significance of an impinging stimulus. Determining how the brain "decides" to attend to some stimuli and ignore others is emerging as a critical issue in neuroscience.
Now, new Yale/NYU neuroscientific research - published in an article in the May 17th edition of the journal Nature - has made progress in uncovering the brain mechanisms by which emotional significance influences perceptual experience. This research shows for the first time that the brain structure the amygdala is responsible for emotional influences on perception, altering the ease by which events with increased emotional value reach awareness.
Previous research has shown that the amygdala is critically involved in computing the emotional significance of stimulus events. In addition, the amygdala's connections with other brain structures suggest it is also strategically placed in the brain to modulate perceptual experience itself. Previous researchers have hypothesized that the substantial feedback connections from the amygdala to sensory cortical regions represent the potential for emotional value to influence early perceptual processing, biasing perception towards certain significant stimulus events. However, until now there has been no direct evidence in humans that the amygdala plays a meaningful role in how emotional value shapes our perceptual experiences.
Neuroscientists Adam K. Anderson of Yale and Elizabeth A. Phelps of NYU examined whether the amygdala is responsible for emotional influences on perception by using the attentional blink (AB) effect -- that is, an attentional refractory period that occurs after a person actively perceives a given stimulus. In the AB test, subjects were shown words flashed in rapid order. Subjects were instructed to be on the lookout for the appearance of two green target words occurring amongst a stream of black words. As expected, subjects showed a 500 milliseconds impairment in perceiving the second green target word following exposure to the first green target word. In particular, subjects were impaired in perceiving an emotionally neutral target word (EG, house or laundry) if it was shown within the refractory period. However, when the second target word was emotionally evocative in nature (e.g., cancer, rape), the impairment caused by the attentional blink was largely diminished.
Anderson and Phelps next administered the same test to subjects with lesions of the amygdala. Unlike neurologically intact subjects, patients with damage to the amygdala did not show enhanced awareness for aversive word targets during the AB test. Consistent with the notion of a left hemisphere basis for the processing of linguistic events, this enhanced identification of emotionally evocative words was found to be specifically impaired among patients with lesions of the left but not the right amygdala. This deficit was selective for the emotional modulation of visual awareness; nonemotional manipulations influencing awareness (such as enhancing the visual salience of neutral target words) were left intact.
These finding suggests that our perceptual systems are exquisitely tuned to the occurrence of emotionally significant stimulus events, requiring much less attention or effort to reach conscious awareness compared to events of neutral value. Even under conditions where stimulus events are often ignored and likely unseen due to other processing tasks (e.g. driving a car, or reading a newspaper on the subway), events of particular significance receive fuller processing, and thus are more likely to capture our attention. Dr. Anderson suggests that "the amygdala appears to be critical for the emotional tuning of perceptual experience, allowing perception of emotionally significant events to occur despite inattention."
This work is the first to directly examine the neural mechanisms supporting interactions between affective value and perceptual awareness, demonstrating that the human amygdala is critical for how emotional value influences conscious perceptual experience. In addition to the amygdala's known role in the emotional modulation of memory formation, the present findings suggest that the amygdala supports emotional influences directly on perception itself. Dr Phelps commented " it has been known for some time now that the amygdala is critical for why we tend to better remember events from our lives that are infused with emotional significance. We now know that the amygdala supports emotional influences very early on, beginning with perceptual processing." Thus, similar neural mechanisms appear to underlie emotional influences on both the ability to perceive and later remember experiences with emotional significance.
Adam K. Anderson is a post-doctoral fellow at Stanford University. Elizabeth A Phelps is an associate professor of psychology and neural science at NYU.