Overthinking Heidi Oh Overthinking Heidi Oh

When the Brain Fills in the Blanks

Overthinking and anxiety often come from the stories our brains create to explain uncertainty. Neuroscience shows how the brain fills in missing information—and why those interpretations can sometimes increase stress.

What Split-Brain Research Reveals About Human Explanation

The human brain continuously interprets and organizes information. In many situations, it produces explanations quickly and automatically. Research in split-brain neuroscience demonstrates that the brain can generate explanations even when it lacks access to the information that actually produced the behavior.

This phenomenon became visible through studies of patients who had undergone corpus callosotomy, a surgical procedure used in severe epilepsy. The procedure severs the corpus callosum, the bundle of neural fibers connecting the brain’s two hemispheres. Without this connection, information processed in one hemisphere cannot be directly shared with the other.¹

In the 1960s, neuroscientists Roger Sperry and Michael Gazzaniga used these patients to study hemispheric specialization. Participants were seated in front of a screen and asked to focus on a central point. Images were flashed briefly to either the left visual field or the right visual field. Because each visual field projects to the opposite hemisphere, this method allowed researchers to send information to one hemisphere without the other hemisphere receiving it.² The results demonstrated that the hemispheres could respond independently.

The right hemisphere could recognize images and guide the left hand to select matching objects. However, the right hemisphere has limited language capacity. When information was presented only to the right hemisphere, participants could select the correct object but could not verbally describe what they had seen.³

One widely cited experiment involved presenting different images to each hemisphere simultaneously. In one version, the left hemisphere (right visual field) was shown a chicken claw, while the right hemisphere (left visual field) was shown a snow scene. Participants were then asked to choose related images from several options using both hands.

The right hand selected a chicken, consistent with the chicken claw image. The left hand selected a snow shovel, consistent with the snow scene. The selections reflected the information each hemisphere had received. However, when participants were asked why they chose those objects, the verbal response came from the left hemisphere, which had not seen the snow scene. Instead of stating uncertainty, participants produced an explanation that incorporated both objects:

“The chicken claw goes with the chicken, and the shovel is for cleaning out the chicken coop.”

This explanation did not reflect the actual stimulus that guided the left hand. The brain produced a coherent narrative despite missing information. Gazzaniga later described this tendency as the “left-brain interpreter.”⁴ The interpreter refers to a process in which the left hemisphere constructs explanations for behavior even when it does not have access to the underlying cause. Rather than reporting uncertainty, the brain generates a plausible narrative that links actions together into a meaningful sequence.

Subsequent research has shown that this tendency is not limited to surgical split-brain cases. Cognitive neuroscience indicates that the human brain frequently organizes perception and behavior through post-hoc explanation, meaning explanations are created after the behavior has already occurred. These narratives help maintain a sense of continuity and coherence in human experience.⁵

This process also appears in everyday perception. Humans constantly interpret incomplete information from facial expressions, tone of voice, body posture, and environmental context. When information is ambiguous, the brain may still generate an explanation. For example, a person may glance across a room and see someone squinting with a tense facial expression. The brain may interpret the expression as anger or disapproval and quickly construct a narrative: “I must have done something wrong” or “they must not like me.” Behavior may then shift in response to that interpretation, such as avoiding the person or limiting interaction.

In reality, the expression may have been unrelated to the observer. The person could have been reacting to bright sunlight through a window or focusing on something at a distance. The original perception contained incomplete information, yet the brain still produced a coherent explanation.

Research in trauma neuroscience suggests that this interpretive process can become amplified when the nervous system is in a state of threat detection or activation. When the brain prioritizes safety and prediction, ambiguous signals may be interpreted more quickly as potential threats. Under these conditions, the brain may generate explanations that align with expectations of danger or rejection even when the available evidence is limited.⁶

Importantly, the brain’s explanation is not necessarily a deliberate deception. The process appears to arise from normal mechanisms involved in pattern recognition, meaning-making, and predictive processing. When information is incomplete, the brain tends to fill in missing pieces to produce a coherent interpretation of events. Split-brain studies therefore provide a clear demonstration of how brain narrative construction can occur independently of the information that originally produced behavior. The experiments show that the brain does not always report the source of a decision; instead, it can construct an explanation that fits the outcome.

Research in cognitive neuroscience continues to explore how the brain integrates perception, behavior, and narrative. The early split-brain experiments remain one of the clearest demonstrations that explanation and cause are not always the same process.

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