Try to picture a scene, or think about making a movement. If you’re musically inclined, compose a brief tune in your head. These are things most people do with little to no trouble at all. But what enables such behavior? How are the intricate structures and complex computations going on in your brain utilized to carry out these different forms of imagination? Does motor imagination require the motor system? Visual imagination the visual system? Auditory imagination the auditory system? Of the many capabilities the brain is endowed with, the ability to imagine is one that is as puzzling and complex as it is intriguing. This article will examine some of the research that has been done to reveal how a mass of cells bound by bone and blood can have the seemingly infinite capacity typical of human imagination.

The Role of Memory

When asking questions about how imagination works, one must consider the role of memory. Episodic memory, the ability to recollect autobiographical past experiences, is thought to play a crucial role in the ability to imagine. For the past several decades it has been widely viewed by memory researchers that engaging episodic memory is a constructive process rather than a reproductive one [1]. In other words, when one remembers they are pulling together pieces of information from multiple sources and reconstructing the memory for the conscious self instead of merely playing it back in full. Support for this notion comes from memory errors, which are thought to be reflective of the component-wise operations of this constructive process [1].

Memory researchers Shacter and Addis have hypothesized that imperfection in recalling memory fragments is actually conducive to imagination. Misconstruction of fragments allows the brain to piece together disparate bits of previous experiences in novel ways as a means to imagine and predict personalized future scenarios [1].  If this imaginative process, deemed episodic future thinking, occurs as Shacter and Addis propose, then one should be able to see neural correlates in similar areas to those where memory processes occur.

To investigate this, researchers have conducted several different neuroimaging studies where PET scans of subjects’ brains were taken to assess correspondences between imagined and remembered events. Participants were asked to remember past events from their personal life and to imagine personal future events at two different temporal scales: near (days/weeks) or distant (months/years) [2][3][4]. The studies showed that the same areas light up for both memory and imagination of such events. These areas include bilateral frontopolar cortex, involved in self reference; medial temporal lobe (MTL) which contains the hippocampus, a structure involved in memory formation; occipital cortex, involved in visual perception; and areas that process other things, such as semantics and emotional responses.

Interestingly, the variation in activation at different event time scales correlate positively between memory and imagination of events. More importantly though, a lesser level of activation in these areas was observed when subjects were asked to remember or imagine events not related to themselves, such as one involving Bill Clinton.

These findings suggest two things: 1) that episodic future thinking involves, to a greater extent, one’s personal event timeline and 2) that more generalized imagination likely utilizes more/other areas than those mentioned above. Further support for this comes from the fact that amnesic patients (with damage to MTL) have difficulty both remembering and imagining personal events but are more competent at remembering and imagining general events [1]. However, though these neuroimaging studies hint at physiological mechanisms for some kinds of imagination, the daunting process by which the imagination is actually perceived – that is seen, heard, felt, and experienced within the brain – is largely a mystery.

Brain Computer Interfaces and Motor Imagination

Though it is still unclear how the brain reacts to perceive different imagined events, it has been shown that certain kinds of imaginative processes do utilize distinct brain regions. Motor imagery, or the generation of an internal representation of a movement prior to and during its occurrence, is a process that happens constantly within the motor system [5]. Remarkably though, the execution of a movement is not necessary for such imagery to occur. That is, the signal recorded within the motor cortex, the precentral gyrus, during the imagination of a movement is identical spatially and temporally and nearly so in magnitude to that of realized movement [6].

Several groups of scientists have begun to utilize this phenomenon to create devices designed to aid those with motor deficits. By recoding electrical activity associated with intended movement in the motor cortex, researchers are opening doors on both motor deficit treatments as well as the mechanisms behind motor imagination.

For example, in 1995 Dr. Marc Jeannerod worked with and trained a tetraplegic patient, known as T.S., to control a robotic prosthetic hand via imagined movements in an effort to recover grasping function [6]. Over a period of several months T.S. was trained to imagine moving several parts of his body including his left and right hands and feet. Jeannerod used an electroencephalogram (EEG) based brain computer interface (BCI) to record electrical activity that occurred while T.S. imagined moving his limbs. At the conclusion of this training, the BCI was able to identify T.S.’s motor intent and engage the prosthetic with 100% accuracy.

More recently in 2006, Hochberg et al. continued this research by implanting a 96 microelectrode array in a tetraplegic patient known as M.N. [7]. Three years after M.N. suffered a spinal cord injury, Hochberg implanted the device in his primary motor cortex where neuronal activity associated with imagined movements was still robust. In this study, M.N. learned to modulate the firing rates of a small group of neurons via motor imagery in order to control different devices, including a computer cursor and several robotic arms. With one such robotic arm, M.N. was able pick up and move an object.

M.N.’s motor performance was not affected by engaging in other cognitive tasks such as conversing with Houchberg. Perhaps superficially this is unsurprising as M.N. is activating the same brain regions as able-bodied individuals while simultaneously performing motor and cognitive tasks.  However, such simultaneous cognitive function in M.N. is actually quite incredible. It suggests that M.N. may have imagined the movement of the prosthetic rather than the series of associated body movements. In a sense, M.N. had recruited the devices by imagination alone. Indeed, patients in other studies have reported such an experience. For example, patients in a study by Miller et al., who utilized an electrocorticography (ECoG) paradigm indicated that after only a few trials they merely had to imagine the movement of a cursor in the desired direction rather than the motor command trained for and assigned to it [8].

Motor imagery is clearly a very powerful form of imagination as well as a simpler concept to grasp than other imaginative processes. Although the underlying conscious decision process mediating this is still unknown, the physical output is far easier to understand and to design devices for compared to episodic future thinking and memory in general.

Visual and Auditory Imagination

Other imaginative processes related to memory are those of visual and autidory imagination. A suite of visual imagery paradigms have been carried out that indicate the visual system is involved in generating visual imaginations. One study of particular intrigue by Kosslyn et al. observed activation within Broddman area 17, the primary visual cortex, via PET while subjects were asked to imagine and describe a set of stripes, e.g. their width and orientation [9]. Intuitively, the activation was similar to that observed during actual visualization of a set of stripes. More notable though was the subjects’ inability to perform the imagery task while their visual cortical activity was disturbed via repetitive transcranial magnetic stimulation (rTMS).

In a similar vein, the act of imagining sound, related to but not constrained by memory of sound, requires the auditory system. An EEG study conducted by Brix demonstrated that both visual imagination potentials (VIP) and auditory imagination potentials (AIP) occur over the visual and auditory cortices, respectively, when subjects were asked to imagine certain sights and sounds [10]. Also shown was that the amplitude of the evoked potential correlated with the degree of concentration required by the subject to perform the imagination. For example, a particularly intense auditory imagination, such as the sound of a song, produces a greater AIP than that of a simple one, such as the sound of a horn. Taken together, these studies show that when one engages in sensory-motor imagery, they employ the associated modality to do so.

Mental Disorders

Discussed so far are the imaginative processes in psychologically intact individuals where conscious effort brought about the desired form of imagination. However, just as other cognitive functions can be impaired by neurological disorders, so too can one’s imagination.

Several mental disorders bring about a sense of delusion that strips people of the ability to distinguish between real and imaginary. Hallucinations are actually considerably common but most individuals who observe them have the cognitive faculty to know that what they are observing, be it sight, sound, or other, is not real. For example, a disorder known as Charles Bonnet Syndrome (CBS) occurs in a relatively large subpopulation of visually impaired individuals who report vivid hallucinations of random events that are strictly visual [11]. Very few of the individuals estimated to have CBS report their experiences for fear of being labeled as mentally unstable, but most of those who do report indicate they are fully aware the visions are not real.

Researchers have hypothesized that these hallucinations occur when deafferentiating neurons in visual association areas fire. It has been proposed that in such circumstances, these dying cells, which do not typically fire, behave like they would if they were responding to visual input. As a result, the brain creates a narrative around the activity [11]. Though bizarre, it is important to note that CBS individuals do not interact with these hallucinations in any way.

The faculty that enables individuals to recognize that imagined events are not real is known as reality-monitoring, and is thought to be a form of source memory, i.e. one’s ability to remember the origin of information. When reality-monitoring malfunctions, serious psychological harm can result. One particularly extreme example of such a loss in reality-monitoring occurs within schizophrenics [12]. Schizophrenic patients have a range of symptoms including the hallucination of voices and people. The difference between individuals with disorders like CBS and those with Schizophrenia is that the latter often report expressly interacting with their hallucinations, sometimes in frightful, harmful or threating ways; their inability to reality-monitor results in emotional responses to their hallucinations.

A study done by Brébion et al. demonstrated that schizophrenic patients have a reduced ability to reality-monitor related to an increased propensity to report events that had not explicitly occurred [12]. Patients were presented with a set of pictures and words and then tested, after a brief delay, for the form that each item took. Brébion et al. found that patients with schizophrenia were twice as likely as controls to misattribute the presentation of a word as an image. They also observed that patients did poorly at recognizing which items actually appeared as images in general. As such, Brébion et al. hypothesized that patients were engaging in excessive visual imagery which could have conflicted with internal representation of target images. They also contended that patients’ working memory, i.e. short term memory, could have been involved due to the relative timing of the tasks.

It has been observed that both source memory and working memory occur in the same area in the brain: the prefrontal cortex [13][14]. Furthermore, several studies have shown that cerebral blood flow to the prefrontal cortex is reduced in schizophrenic patients [14]. Postmortem studies of schizophrenic brains have shown a reduction in neuropil (dendrites or axons of neurons) that has been thought to result from this lack of blood flow to the prefrontal cortex, most notably in the supragranular layers, which are generally considered association areas of the brain [15]. Indeed, an electrophysiological study in monkeys has shown that there are ‘delay’ cells, within the supragranular layers of the prefrontal cortex, thought to connect items of information with one another [13]. Although the exact mechanisms behind mental disorders like schizophrenia are vastly unknown, it is clear that the brain sits delicately within its reality, and that minor disturbances in function can turn imagination from a beautiful and limitless tool into something frightening and confusing.

Final Thoughts

Through the observation of the various studies reviewed above, it is quite clear that imagination is a complex behavior that takes many forms via dispersed brain structures and is extremely powerful. Creativity, the act of generating new ideas or combining old ones in novel ways, is tightly related to imagination and an attribute that humans are particularly well-endowed with. Of course, like other human skills, imagination and creativity take time and practice, much of which begins when we are mere children. Countless psychological studies have shown the benefits of imaginative play throughout development from early childhood and into adolescence in producing creative and productive individuals [16]. The combinatorial process of ideas at work when children imagine fantastic scenes and figures paves a path for not only creative but also emotionally and socially adept lives in the future [17].

With an activity as widespread in the brain and as potent as imagination, one cannot help but think that the human brain evolved so as to encourage the creation of a boundless internal representation of the world for its own manipulation. And, though it currently remains perplexing how, as Charles Bonnet so eloquently put it, “the theater of the mind is generated by the machinery of the brain,” such understanding is not requisite to marvel at the limitless capability made possible by it.


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