Evidence of Suppression
Linking suppression to widely accepted brain mechanisms involved in behavioral control moves this concept from the domain of the psychoanalyst’s couch to the physical realm of the brain.

A different form of suppression, known as visual perceptual suppression, occurs when an object—or part of one—is not consciously seen even though the image is always clearly visible. A common example is the bistable figure, such as the drawing of the cube, the vase-face or the duck-rabbit in the triptych illustration above. The eyes see the same lines and shapes on the page, but what you consciously see in your head changes from the duck to the rabbit and back again. When the image of the duck is being consciously seen in your mind’s eye, the image of the rabbit is “suppressed,” and vice versa.

Another example of visual suppression is binocular rivalry. Here two ifferent images are simultaneously present, one in each eye. Say a photograph of a smiling girl is projected into your left eye and an image of a car is projected into your right eye. Rather than appearing as the girl superimposed on the car, the two pictures rival for conscious access, and one will suppress the other briefly. For a few seconds you will see the girl’s face; suddenly, patches of the car begin to shine through until the face is entirely gone, and you’ll see only the car. Subsequently, the smiling eyes will break through the automobile, and it will disappear to be replaced by the girl’s face, and so on in a never-ending pas de deux.

So although the physical input to the eyes always remains the same, your conscious perception of it changes from one moment to the next and back again. Bistable percepts are ideal for tracking the footprints of consciousness in the human brain using functional brain ­imaging [see “Rendering the Visible Invisible,” by Christof Koch; Scientific American Mind, October/November 2008].

Provided the eyes don’t move or blink, this ceaseless dance is under only very limited voluntary control. Thus, from the point of view of psychoanalysis, it would be more proper to call this perceptual repression rather than perceptual suppression. Whether the neural mechanisms underlying visual perceptual suppression and repression are related to those underlying psychodynamic suppression or repression remains to be determined.

Emotions Apart
Dissociation is another controversial psychological state in which thoughts, emotions, sensations or memories are separated from the rest of the psyche. Originally championed by French psychiatrist Pierre Janet, dissociation can occur in healthy individuals such as when you blank out for a mile or two while driving along a freeway, become completely absorbed by a book or movie, or find yourself walking into a room in your house only to forget why you ventured there in the first place.

More extreme forms of dissociation manifest themselves in mental diseases such as dissociative identity disorder (DID)—formerly known as multiple personality disorder—which involves the presence of two or more distinct identity states. These states are characterized by different emotional responses, thoughts, moods and perceived self-images that recurrently and alternately take control of a patient’s behavior and consciousness. DID is considered to be a result of identity fragmentation rather than proliferation of separate personalities. So patients do not have more than one personality (a proliferation of selves), but rather they have less than one (a fragmented self).

Dissociative identity disorder is often associated with severe and prolonged childhood trauma (such as neglect or emotional or sexual abuse) and develops as a way to cope with an overwhelming situation that is too painful or violent to assimilate into one’s conscious self. The person literally “goes away” in his or her head to flee from the anxiety-producing experience from which there is no physical escape. This dissociative process allows traumatic feelings and memories to be psychologically separated off so that the person can function as if the trauma had not occurred. While in one mental state, the patient has access to traumatic autobiographical memories, say of a rape, and intense emotional responses to them. But when in her other state, she claims not to recall anything related to her rape. This defensive use of dissociation prevails long after the traumatic experiences have ended.