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Mental synthesis is the conscious purposeful prefrontal cortex-driven process of synthesizing a novel mental image from parts stored in memory. Mental synthesis is neurologically different from the other key components of imagination: simple memory recall and dreaming. Unlike dreaming, which is spontaneous and not controlled by the prefrontal cortex (PFC), mental synthesis is controlled by and completely dependent on the intact lateral PFC. Unlike simple memory recall that involves activation of a single neuronal ensemble encoded at some point in the past, mental synthesis is a motor act that involves active combination of two or more neuronal ensembles. Mental synthesis is hypothesized to be organized by the lateral PFC acting in temporal domain to synchronize several independent neuronal ensembles. Once those neuronal ensembles are time-shifted by the lateral PFC to fire in-phase with each other, they are consciously experienced as one unified object or scene. In this process humans can manufacture an unlimited number of novel mental images and plan their future actions through mental simulation of the physical world.

History of the term
The earliest reference to mental synthesis is found in the doctoral dissertation of SJ Rowton written in 1864. Paraphrasing Cicero’s description of nature that can only be unified in someone’s mind, SJ Rowton writes: “... there cannot be one thing unless by a mental synthesis of many things or parts ...”

In the 20th century the term mental synthesis was often used in psychology to describe the experiments of combinatorial nature. In a common experimental setup, subjects are instructed to mentally assemble the verbally described shapes in various ways. For example, the shapes may have been the capital letters ‘J’ and ‘D’, and the subject would then be asked to combine them into as many objects as possible, with size being flexible. A suitable answer in this example would be: an umbrella. The performance in this task is then quantified by counting the number of legitimate patterns that participants construct using the presented shapes.

As the neurobiological study of imagination advanced in the 21st century, there was a need to distinguish the neurologically distinct components of imagination: fist in terms of their dependence on the lateral PFC and second in terms of the number of involved neuronal ensembles. As a result, “mental synthesis” was adapted to describe the lateral PFC-dependent process of assembling two or more independent neuronal ensembles from memory into novel combinations.