How the Brain Fills Memory Gaps:
The Phenomenon of Constructive Memory and the Mechanisms of Perceptual Filling

Human memory doesn’t function like a video recording device, storing an exact copy of events for later playback. It’s a reconstructive process, subject to constant modification, re-evaluation, and addition. The brain operates on fragmented data received from the senses. Missing elements are reconstructed based on previous experience, expectations, and logical connections. This mechanism ensures the integrity of perception of the world, but simultaneously creates the basis for distortions and false memories.

Understanding how neural networks fill in the gaps requires analyzing processes at multiple levels: from visual signal processing in the retina to complex cognitive functions in the prefrontal cortex. These processes occur automatically and often go unnoticed by the subject’s conscious awareness.

Physiological blind spot and visual filling

The most basic level of gap filling occurs in the visual system. Each human eye has an area of the retina lacking photoreceptors — rods and cones. This is the exit point of the optic nerve, known as the blind spot or scotoma. This area is approximately 1.5 millimeters in diameter on the retina, corresponding to a loss of approximately 5–7 degrees of visual field.

With binocular vision, the brain compensates for this defect by overlapping the visual fields of both eyes. However, even when looking with one eye, a person doesn’t see a black hole in space. The visual cortex actively interpolates the missing information using data from surrounding areas.

Mechanisms of neural interpolation

The process of perceptual filling-in is not a passive ignoring of the absence of a signal. Neurons in the primary visual cortex (V1) responsible for the blind spot receive feedback from neurons encoding the surrounding space. If a uniform color or texture surrounds the blind spot, the neurons "fill" the void with the same pattern.

Experiments presenting lines passing through the blind spot demonstrate the collinearity effect. The brain completes the line by connecting two separate segments if they are on the same axis. This process occurs early in visual processing and takes milliseconds. Filling in textures requires more complex processing and involves extrastriate cortical areas such as V2 and V3.

Completing patterns in the hippocampus

At the level of episodic memory, the hippocampus, a structure located in the medial temporal lobe, plays a key role. One of the fundamental operations of the hippocampus is pattern completion. This mechanism allows for the reconstruction of a complete memory from a partial or degraded input signal.

An external stimulus, such as a familiar smell or melody, activates a subset of neurons associated with a specific engram trace. Thanks to extensive recurrent connections in the CA3 region of the hippocampus, this excitation spreads to the rest of the neural network encoding the event. As a result, the entire circuit is activated, and the person recalls the event in its entirety, including visual imagery, emotions, and context.

The balance between separation and completion

The hippocampus constantly balances two processes: pattern completion and pattern separation. Pattern separation is necessary for similar events to be encoded as distinct memories. The dentate gyrus of the hippocampus is responsible for orthogonalizing input signals, making them as distinct as possible.

With age, the dentate gyrus’s efficiency may decline, leading to the dominance of pattern completion. This explains why older people are more likely to confuse similar events or mistakenly attribute details from one memory to another. The system begins to over-aggressively fill in gaps, relying on generalized patterns rather than capturing unique details.

Schema theory and cognitive scripts

In the first half of the 20th century, British psychologist Frederick Bartlett formulated a theory positing that memory is organized around mental schemas. Schemas are cognitive structures that represent generalized knowledge about the world, objects, and events. When a person encounters new information, they don’t memorize it verbatim, but rather integrate it into existing schemas.

Gaps in perception or memory are filled with elements that match the active schema. If a person hears a story about a restaurant visit, they automatically assume the presence of a menu, a waiter, and a bill, even if these details weren’t mentioned. This phenomenon is called "inferential filling."

Experiments with cultural distortions

Bartlett’s classic studies using the story "Ghost War" demonstrated how subjects transform unfamiliar narratives. Participants from British culture read a Native American legend. When retelling it, they unconsciously altered details they found illogical or incomprehensible, adapting them to familiar European norms.

Supernatural elements were omitted or rationalized. The characters’ unclear motives were replaced with more familiar ones. The brain filled in the semantic gaps, creating a coherent but distorted story. This proves that the brain prioritizes the pursuit of semantic coherence over accuracy.

Deese-Rodiger-McDermott paradigm (DRM)

One of the most reliable ways to demonstrate how the brain creates false memories to fill in gaps is the DRM paradigm. In this experiment, subjects are presented with a list of semantically related words, such as "bed," "rest," "wake," "tired," "blanket," and "night." The list is missing a crucial decoy word (in this case, "sleep").

When tested on memory, a significant percentage of participants confidently stated that the word "sleep" was present in the list. The frequency of such false memories is comparable to the frequency of remembering actual words from the middle of the list. The mechanism behind this phenomenon is linked to the spread of activation in the semantic network. Words from the list activate the node corresponding to the concept of "sleep." When the brain attempts to reconstruct the list, it finds this highly active node and misinterprets it as an external event.

Source Monitoring Errors

Filling in memory gaps is often associated with source attribution errors. The brain stores the content of information separately from metadata about where and when it was acquired. The process of linking content to context (source monitoring) requires the prefrontal cortex and is prone to failure.

Cryptomnesia is a prime example of such an error. A person may create an idea, a melody, or a solution to a problem, sincerely believing it to be their own invention. In reality, the brain is reproducing a forgotten memory of someone else’s work, filling the authorship gap with their own.

Another example is the illusion of truth effect. Information heard repeatedly begins to seem true, even if it was initially labeled as false. As the memory of the source (the refutation) fades, only a feeling of familiarity (fluency) remains, which the brain interprets as evidence of its truth.

The Phenomenon of the Left Hemisphere "Interpreter"

Studies of split-brain patients (after callosotomy — severing the corpus callosum) have revealed a fundamental mechanism for generating explanations. Neuropsychologist Michael Gazzaniga described a module in the left hemisphere called the "Interpreter."

In experiments, the right hemisphere (which lacks access to the speech center) was presented with a command, such as "stand up." The patient stood up. When asked why, the left hemisphere, unaware of the command, did not respond with "I don’t know." Instead, the Interpreter instantly fabricated a plausible explanation: "I wanted to stretch my legs" or "I wanted a Coke."

This mechanism also operates in healthy people. When faced with a gap in understanding one’s own motives or external events, the brain constructs a logical narrative that connects disparate facts. This provides a sense of self-continuity and control over the situation, but often leads to rationalization of random events.

Pathological confabulations

In clinical practice, the phenomenon of filling in memory gaps takes extreme forms, called confabulation. This is not intentional lying, but the unconscious production of false memories. Patients are absolutely certain of the truth of their statements, despite their obvious absurdity or contradiction with the facts.

Korsakoff’s syndrome

Alcoholic encephalopathy or thiamine deficiency can lead to the development of Korsakoff syndrome. Patients with this diagnosis suffer from severe anterograde amnesia — the inability to remember new events. To conceal these gaps (often from themselves), they fill them with fictitious stories. A patient hospitalized for several months may vividly recount yesterday’s theater visit or meeting with friends. The content of these confabulations is often derived from fragments of actual past memories, jumbled in time and space.

Anosognosia and Anton’s syndrome

In Anton-Babinski syndrome, patients with cortical blindness (damage to the occipital lobes) deny their blindness. The brain, deprived of visual input, continues to generate visual images. The patient "sees" the surroundings, describing the doctor’s clothing and objects in the room. These descriptions are pure confabulation, the brain’s attempt to fill a total sensory gap based on predictions and expectations.

Temporal illusions and postdiction

The brain fills not only spatial and semantic gaps, but also temporal ones. Processing sensory information takes time (tens and hundreds of milliseconds). Conscious perception always lags behind reality. To compensate for these delays, the brain uses prediction and postdiction mechanisms.

Stopped Clock Effect

The illusion of chronostasis occurs when a person quickly shifts their gaze (a saccade) to an object, such as the second hand of a clock. During the eye movement, visual information is suppressed (saccadic suppression) to avoid blurring. The brain fills this time gap with the image received immediately after the eyes stopped moving. Subjectively, this is perceived as the first second lasting longer than the rest. The brain "stretches" the perception of the current frame backward in time to cover the period of blindness during the saccade.

Flash lag effect

If a moving object flashes just as it approaches a stationary landmark, the observer perceives the moving object as being ahead of the landmark. The brain extrapolates the object’s trajectory into the future to compensate for neural delays. This is an example of predictive filling — the brain shows us not where the object is currently, but where it expects it to be.

The role of sleep in memory consolidation and modification

Memory transformation processes actively occur during sleep. During slow-wave sleep, the hippocampal neural ensembles encoding daytime events are reactivated. This information is transferred to the neocortex for long-term storage. However, this process is not simply a copying process.

During consolidation, the brain filters out insignificant details and emphasizes key, generalizing elements. This leads to the so-called "semanticization" of memory: episodic details are erased, leaving behind general facts. Distortions can occur during this process. If new information conflicts with old information, the brain may rewrite the original memory to resolve the discrepancy. Sleep facilitates the integration of new information into existing schemas, which sometimes leads to the formation of false associations and the filling of gaps with logically correct but incorrect details.

The influence of post-event information

Elizabeth Loftus conducted a series of fundamental studies demonstrating the suggestibility of memory. In a classic experiment, participants were shown a video of a car accident. They were then asked questions. The wording of the questions dramatically altered their memories.

When asked, "How fast were the cars traveling when they crashed into each other?", subjects estimated higher speeds and often "remembered" broken glass, which wasn’t present in the video. When the verb "touched" was used, speed estimates were lower, and no false memories of glass emerged.

This misinformation effect demonstrates that a memory isn’t a stable file. Every time we retrieve a memory, it enters a labile state and becomes vulnerable to modification. New information acquired after the event is woven into the original memory trace, overwriting it. This process is called reconsolidation.

Boundary Extension

The phenomenon of boundary expansion demonstrates the brain’s tendency to construct space beyond the visual field. If shown a close-up photograph and then asked to draw it from memory, they will typically depict the scene from a wider angle, adding details that would otherwise be outside the frame.

The brain automatically places the object in its assumed context. This memory bias is adaptive: it helps maintain the continuity of our perception of the world by predicting what lies beyond our current sensory field. We "remember" not the photograph itself, but the scene of which we imagined it as a part.

Predictive coding and the Bayesian brain

Modern neuroscience views the brain as a prediction machine. According to predictive coding theory (Karl Friston et al.), the brain constantly generates an internal model of the world and compares it with incoming sensory signals. Perception is not a bottom-up process of constructing images (from the retina to the cortex), but a top-down process of testing hypotheses. The brain predicts what it should see and fills the perceptual field with these predictions. Sensory signals are used primarily to correct prediction errors.

If the sensory signal is incomplete or noisy (twilight, fog, rapid speech), the weight of prior expectations increases. The brain aggressively fills in the gaps with the most probable data. This is why we can hear our name in the sound of the wind and see a person’s silhouette in the shadows of the trees. This phenomenon is called pareidolia — the illusory perception of a real object.

Neurobiology of imagination and memory

Functional magnetic resonance imaging (fMRI) shows that the same neural network (the Default Mode Network) is activated when remembering the past and imagining the future. This network includes the medial prefrontal cortex, the posterior cingulate cortex, and the hippocampus.

The fact that memory and imagination share the same neural substrate supports the constructive simulation hypothesis. Memory is a tool for simulating the future. The ability to fill in gaps in the past is necessary for playing out future scenarios. The brain combines fragments of past experience to construct possible future events. Memory inaccuracy is the price of flexible thinking and the ability to predict.

Social contagiousness of memory

The process of filling in gaps can occur collectively. Social contagion of memory occurs when a group of people discuss an event they witnessed. Erroneous details expressed by one confident participant can be integrated into the memories of others.

In experiments, pairs of subjects (one of whom was a decoy) recalled details of a scene. When the decoy confidently mentioned an object that wasn’t in the scene, the real subject often agreed and subsequently included the object in their individual report. This phenomenon demonstrates that the brain views social information as a valid source for filling its own mnemonic gaps.

The illusion of causality

The brain has a profound need for cause-and-effect relationships. When remembering a sequence of events, we tend to connect them causally, even if there is no such connection. If event B occurs after event A, the brain tends to interpret this as "B happened because of A."

When recalling events, we fill in the gaps with fictitious causal connections. This makes the story more logical and easier to remember, but less credible. In legal psychology, this leads to errors when jurors construct a coherent narrative from disparate evidence, filling in the missing links with their own conjectures about the defendant’s motives and actions.

Rewriting the emotional context

The emotional coloring of memories is also subject to reconstruction. Our current emotional state influences how we fill in the gaps in past experiences. A depressed person tends to recall past events in darker tones, interpreting neutral details as negative. Conversely, nostalgia can "whiten" the past, erasing unpleasant moments and filling the voids with idealized images.

This mechanism operates through interactions between the amygdala and the hippocampus. The amygdala modulates the strength of memory for emotionally significant events, but during memory retrieval, the current emotional environment can alter the valence of the memory trace.

Sensory deficits and hallucinations

Under conditions of sensory deprivation, the brain’s tendency to fill the void becomes pathological. The Ganzfeld effect, or exposure to sound in a chamber, can cause vivid hallucinations in healthy individuals after just 15 to 30 minutes. Deprived of external stimuli, the brain amplifies the internal noise of neural networks to the level of perception. Spontaneous activity in the visual cortex is interpreted as real objects. This is an extreme form of the filling mechanism, where the "canvas" of reality is created entirely from within.

A similar mechanism is observed in Charles Bonnet syndrome. Elderly people with progressive visual impairment begin to see complex, detailed hallucinations (of faces, animals, patterns). The brain, unable to receive sufficient signals from the retina, disinhibits the visual centers, which begin generating phantom images to fill the sensory void.

Linguistic filling

Speech perception also relies on powerful restoration mechanisms. The phonemic restoration effect demonstrates how the brain replaces a missing sound. If a cough or noise replaces one syllable in a recorded sentence, listeners typically claim to have heard the entire word. Moreover, they often cannot pinpoint the exact location of the noise.

The choice of the reconstructed phoneme depends on the context of the entire sentence. The brain stores auditory information in a buffer, waits for subsequent words, understands the meaning, and then retroactively "inserts" the desired sound into the perceptual image. This further confirms that we hear not what enters our ears, but what the brain has constructed based on expectations and context.

Reinforcing false memories through repetition

Repeatedly recalling a memory increases its subjective vividness, but not necessarily its veracity. Each repetition is an act of reconstruction and re-encoding. If the first time the gap was filled with a guess, the second time that guess is remembered as fact. The third time, it is enriched with additional details.

The neural connections that support a false detail are strengthened through long-term potentiation (LTP), just like the connections of real memories. At the physiological level, there is no marker that distinguishes a "true" synapse from a "false" one. The distinction exists only in comparison with external reality, to which the brain always has indirect access.

Transfer and interference

Interference plays a significant role in the formation and filling of gaps. Proactive interference occurs when old memories interfere with the acquisition of new ones. Retroactive interference occurs when new information distorts old information. The brain often resolves conflicts between two similar traces by merging them. Details from two different seaside trips can merge into a single, generalized image of a "vacation." Gaps in one trip are filled with details from another. This process conserves memory resources by creating compact, generalized models of experience instead of storing thousands of nearly identical records.

Neuroplasticity and adaptation

The brain’s ability to fill in gaps is a manifestation of its fundamental plasticity. Synaptic weights are constantly redistributed, adapting internal models to a changing environment. Memory errors are not system failures, but a side effect of its architecture, which is designed for rapid decision-making with insufficient information.

Evolutionarily, it was more advantageous to mistake a shadow in the bushes for a predator (a false alarm, filling the gap with a threat) and flee than to ignore real danger. The mechanisms of constructive memory ensure the integrity of perception, the continuity of personality, and the ability to predict the future, while sacrificing the absolute accuracy of recording the past. The brain is not an archive, but a creative studio, constantly rewriting the script of our lives to match current needs and knowledge.