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Pattern Separation Compliments Pattern Completion

By: , Posted on: January 19, 2016

Warren Carnival

Pattern Completion

Artificial and real neural networks exhibit a part-whole pattern completion property that I call Principle 6 in my book, Cognitive Neuroscience and Psychotherapy: Network Principles for a Unified Theory. This principle enables both artificial and real neural networks to generate a more complete memory or image from partial information using an autoassociative mechanism. For example, in Bidirectional Associative Models, stimuli are passed back and forth through a memory matrix. Each passage enhances recall up to an asymptotic result that may or may not be completely correct. This pattern completion mechanism also enables us to recognize people based upon a partial view of them such as when they are turning a corner or our view of them is otherwise obstructed. Pattern completion enables generalization to occur in so-called noisy environments; where perception is not entirely clear. Pattern completion facilitates accurate generalization in this case in that we can also recognize the person in a novel context. Marr (1971) was the first to suggest a computational model of pattern completion based upon the observation that recurrent collaterals can be found in the archicortex (old cortex). Recurrent collaterals are neurons whose axons lead back to a shared dendritic field thus creating a feedback loop.

The part-whole pattern completion mechanisms also explains how and why posttraumatic stress disorder (PTSD) works. I discuss this matter beginning on page 55 and again on page 515 of my book. When people with PTSD experience a partial cue that was associated with their traumatic experience, the part-whole pattern completion principle automatically and unconsciously reconstructs a more complete and often traumatic memory of their shocking experience. People with PTSD try not to think, talk about, or feel anything for fear that resulting partial cues will return a traumatic memory. Sad or negative emotions can serve as partial cues. People with PTSD are therefore tempted to use mood altering compounds such as alcohol and stimulant drugs (cocaine) to keep feeling happy. Their spouses perceive the not talking and use of alcohol and/or other substances as withdrawal from the marital relationship. The resulting arguments produce more partial cues that return more traumatic memories that motivate seeking greater distance from their spouse including divorce.

Pattern completion is problematic because it can make overlapping memories even more overlapping thereby leading to memory confusion. Where I put my car keys today can be confused with where I put them yesterday. A complementary pattern separation principle addresses this matter.

Pattern Separation

Pattern separation competes with pattern completion to keep memories separate. It enables memory for where you put your car keys today to be kept separate from where you put them yesterday or where you usually put them if you put them in a different place today. The ability to discriminate among similar experiences is a crucial feature of episodic memory. This ability has long been hypothesized to be mediated by the hippocampus. Yassa and Stark (2011) reviewed data from electrophysiological recordings, lesion studies, immediate-early gene imaging, transgenic mouse models, and human functional neuroimaging that provide convergent evidence that particular hippocampal subfields causally mediate the pattern separation process.

Whereas the CA1 and CA3 structures within the hippocampus appear to mediate pattern completion, the dentate gyrus appears to mediate pattern separation. Neurogenesis appears to be a primary pattern separation mechanism. Laplagne et al. (2006) reported that the dentate gyrus contains neural progenitor cells that generate new neurons throughout the life span. Yassa and Stark (2011) presented additional evidence that the new neurons generated daily in the dentate gyrus integrate into existing neural networks. Shors (2014) reported that integration of new neurons into existing neural networks only occurs as a result of effortful learning. Time stamping of new memories appears to be an important function of these newly generated neurons.

The pattern separation mechanism works remarkably well but is subject to notable event and timing problems that are reviewed by Rogler, Malgady and Tryon (1992). They reported a study in which 6% of employees over reported and 67% of employees under reported the sick leave that they actually took. Under reporting was the general tendency across multiple studies. These authors reported that employees were only 19% accurate on average when dating medical events to within 15 days of their actual occurrence. The following time dependency was reported. Employees were 37% accurate when quizzed one month after the target event occurred but only 8% accurate one year after the target event occurred.


The part-whole-pattern (memory) completion principle works cooperatively with the pattern (memory) separation principle to store memories in a way that they can be recalled. Our ability to remember varies across individuals and within individuals over time. These variations include notable errors when asked to recall specific events and even greater errors when asked to place them in time as structured diagnostic interviews require. These errors testify to the limitations of both the pattern completion and pattern separation neural network mechanisms. Never-the-less, these two principles enable us to better understand the mechanisms by which memory works.

Read more from Warren Tryon on SciTech Connect

Warren’s book, Cognitive Neuroscience and Psychotherapy: Network Principles for a Unified Theory is available for purchase on the Elsevier Store.

Cognitive Neuroscience and Psychotherapy cover

Use discount code “STC215” at checkout and save up to 30% on your very own copy.

About the Author

Warren ComputerWarren W. Tryon received his undergraduate degree from Ohio Northern University in 1966. He was enrolled in the APA approved Doctoral Program in Clinical Psychology at Kent State University from 1966 – 1970. Upon graduation from Kent State, Dr. Tryon joined the Psychology Department faculty at Fordham University in 1970 as an Assistant Professor. He was promoted to Associate Professor in 1977 and to Full Professor in 1983. Licensed as a psychologist in New York State in 1973, he joined the National Register of Health Service Providers in Psychology in 1976, became a Diplomate in Clinical Psychology from the American Board of Professional Psychology (ABPP) in 1984, was promoted to Fellow of Division 12 (Clinical) of the American Psychological Association in 1994 and a fellow of the American Association of Applied and Preventive Psychology in 1996. Also in 1996 he became a Founder of the Assembly of Behavior Analysis and Therapy.

In 2003 he joined The Academy of Clinical Psychology. He was Director of Clinical Psychology Training from 1997 to 2003, and presently is in the third and final year of phased retirement. He will become Emeritus Professor of Psychology in May 2015 after 45 years of service to Fordham University. Dr. Tryon has published 179 titles, including 3 books, 22 chapters, and 140 articles in peer reviewed journals covering statistics, neuropsychology, and clinical psychology. He has reviewed manuscripts for 45 journals and book publishers and has authored 145 papers/posters that were presented at major scientific meetings. Dr. Tryon has mentored 87 doctoral dissertations to completion. This is a record number of completed dissertations at the Fordham University Graduate School of Arts and Sciences and likely elsewhere.

His academic lineage is as follows. His mentor was V. Edwin Bixenstein who studied with O. Hobart Mowrer at the University of Illinois who studied with Knight Dunlap at Johns Hopkins University who studied with Hugo Munsterberg at Harvard University who studied with Wilhelm Wundt at the University of Leipzig.

Cognitive Neuroscience and Psychotherapy: Network Principles for a Unified Theory is Dr. Tryon’s capstone publication. It is the product of more than a quarter of a century of scholarship. Additional material added after this book was printed is available at This includes chapter supplements, a color version of Figure 5.6, and a thirteenth “Final Evaluation” chapter. He is on LinkedIn and Facebook. His email address is

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