Extraordinary Mnemonic Capabilities and Their Implications on Human Memory
Much of what is currently known about the processes of memory storage and retrieval has been defined within the context of studying patients with mild to severe memory impairments. The most famous of these studies (and among the earliest) concerns patient H.M., whose bilateral medial temporal lobectomy in attempt to cure his epilepsy led to an accidental discovery of the significance of these cortical regions in memory (Scoville & Milner, 1957). He suffered from a severe memory impairment, marked by both anterograde amnesia and retrograde amnesia, referring to his inability to form new memories as well as retrieve memories that fall within a certain timeline in his life. His case, along with that of other amnesic patients, led to a widespread interest in defining the role of the hippocampus in the process of memory formation and storage (Scoville & Milner, 1957).
The study of individuals on the other end of the spectrum (namely, those with extraordinary memory capabilities), however, has been comparatively sparse, which is partly due to their rarity in existence. Such individuals seem to possess almost superpower-like abilities: the famed mnemonist, S., for instance, was known to leverage his exceptional ability of memory retrieval to control his own physiology (Luria, 1987). He could arbitrarily regulate his heart activity and body temperature by simply retrieving a memory – in order to increase his heartbeat, for example, he simply imagines himself running after a train. He was famously quoted in 1994 as saying, “If I want something to happen, I simply picture it in my mind. I don’t have to exert any effort to accomplish it – it just happens” (Luria, 1987). S. is just one example of a cohort of individuals who possess the ability to recall abnormally vast amounts of information. These individuals derive their mnemonic capabilities not from an overall superior memory function but rather from their modality-specific visually-centered process of encoding information.
subjective recollection of one’s own past episodic events evokes a strong sense of affective arousal and triggers a heightened emotional response due to the self-relevant nature of the experience. Furthermore, researchers have shown that regardless of the emotional valence of a memory, the amygdala activity during encoding correlates with the vividness of the memory that is experienced upon retrieval. These data support the significance of the amygdala in the encoding and retrieval processes of AMs.
Nevertheless, despite heightened cortical connectivity in their temporal lobe, hyperthymesics actually experience somewhat impaired performance on tests of executive functioning localized to the prefrontal cortex, involving tasks of abstraction, self-generated organization, and mental control (Parker et al., 2006). This indication of disorganized memory corresponds with these individuals’ inability to turn off their episodic retrieval, resulting in the hyper-activation of semantic cues that lead to a vicious cycle of constant remembering. A.J., for instance, claimed to have trouble inhibiting recollections of her past at will and often experienced disruptions from her present state of being by her past memories. In fact, inhibition of episodic retrieval is found to be localized to the right inferior frontal cortex, which corroborates the findings of impaired performance on tests of executive functioning Literature shows that patients with prefrontal cortex lesions often have disorganized memory, affirming that hyperthymesia is not the result of superior logic and memory organization (i.e. they are not practiced mnemonists) but rather of an innately-driven heightened awareness of semantic cues associated with episodic recollection. Moreover, these deficits in executive functioning match the neurological profiles of many neurodevelopmental frontostriatal disorders, including autism, obsessive-compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), Tourette’s syndrome, and schizophrenia (Parker et al., 2006). Many hyperthymesics, A.J. in particular, also tend to exhibit repetitive habits,
both in their recollection of the past and in their rather myopic obsession over specific domains of knowledge such as dates (Parker et al., 2006). Thus, there is reason to believe that hyperthymesia can be partly explained by aberrations from normal neurodevelopmental processes.
Another class of individuals with superior memory abilities, some of which overlap in their classification under hyperthymesia, are synthesthetes, or individuals who have synesthesia.
Synesthesia is a condition marked by the activation of multiple modalities upon sensory input. For example, individuals possessing grapheme-color synesthesia are able to attribute colors to visual inputs of graphemes (words or letters) (Rothen & Meier, 2010). Studies have shown that synesthesia is closely tied to performance advantages in visual-based memory, most notably observed through individual S., a man who experienced synesthesia in all five senses (Luria, 1987) He was able to recall a virtually unlimited amount of words and retain them indefinitely, exhibiting an extraordinary temporal as well as spatial capacity for memory (Luria, 1987). S. noted that during sensory input, there was no distinct line between modalities; for example, he “[saw] puffs or splashes when [he] hear[d] certain sounds,” a phenomenon which was replicated during auditory input of tones or voices (Luria, 1987). Thus, every sensory experience elicits a multitude of cues that generate a synesthetic background for future memory retrieval. Among the various senses, his visual quality of recall was particularly important and fundamental to his capacity for recollection, and once a memory (such as a string of words) was visually encoded, he was able to produce it both forwards and backwards (Luria, 1987). Despite some researchers’ dismissal of S. as an extreme case of synesthesia, most other synesthetes report that their condition provides additional sensory cues that help them in their retrieval of material (Rothen & Meier, 2010).
Nevertheless, like hyperthymesics, synesthetes don’t exhibit superior performance on all tests of memory function. In fact, their advantage is only specific to the modalities for which they
show cross-activation. For example, Rothen and Meier showed that grapheme-color synesthetes only showed a memory advantage in episodic memory tests but did not perform any better than controls on normal short-term memory tests (2010). These results demonstrate that the superior recall performance is due to the synesthetic components of recall rather than to an overall higher memory capacity. Furthermore, these individuals performed better in visual compared to verbal episodic memory tests, which proves that synesthetes profit from modality-specific inputs. From these data, it can be tempting to presume that these individuals possess superior memory functions across the entire visual modality. However, Smilek et al. provided evidence against this claim by testing the long-term recall of a grapheme-color synesthete (2002). In this study, C. was shown to have superior recall of digits but normal recall of shapes; the former is associated in the individual with synesthetic color experiences while the latter is not. These findings demonstrate the integral role that synesthetic cues play in memory encoding by enhancing retrieval through relational information processing.
This heightened level of sensory cue interaction also explains why synesthetes exhibit a substantially reduced false memory effect (Radvansky et al., 2011). Because synesthetes are able to employ additional sensory cues, they have multiple layers of checkpoints to guide their retrieval of information; subsequently, the likelihood of producing inaccuracies in their memory recollection is highly reduced. S., in particular, scarcely ever produced erroneous information; more common were omissions in memory (although these were few and far between as well) (Luria, 1987). These omissions were not a result of defects in memory but rather of defects in perception, when the layout of a particular image made it difficult to discern the subject from the background due to a lack of clarity, contrast, or optimal lighting (Luria, 1987). Thus, S. primarily
relied on his visual modality to accurately and effectively encode information that was not necessarily presented visually in nature
Neuroimaging studies reveal strong structural similarities between hyperthymesics and synesthetes. Rouw and Scholte validated the hypothesis that synesthesia and its link to superior memory functions is caused by hyperconnectivity in the regions localized for memory (2007). In this study, grapheme-color synesthetes showed greater white matter coherence (demonstrated through greater anistropic diffusion) in the inferior temporal cortex as compared to controls. These results varied even among synesthetes themselves according to the subjective nature of synesthesia. Projectors, individuals who experienced synesthesia externally, showed greater levels of structural connectivity than associators, individuals who only experienced synesthesia internally or in their “mind’s eye” (Rouw & Scholte, 2007). This phenomenon further supports the notion that the coherence of white matter tract in the inferior temporal lobe is crucial for superior recall abilities. Interestingly, imaging results also demonstrate a possible cross-activation in graphemecolor synesthetes between the grapheme area and the nearby right fusiform gyrus (FG), which overlaps spatially with the V4 region of color processing (Weiss & Fink, 2009). Results show an increase in both the white matter and grey matter volumes of the parietal cortex and FG, reflecting hyperconnectivity at at the level of the FG as well as the underlying parietal mechanisms that further bind the synesthetic perceptions generated in the FG (Weiss & Fink, 2009). These imaging studies provide neuroanatomical evidence for the postulated cross-activation of brain regions otherwise localized for distinct sensory processing functions.
The final category of case studies involves individuals who possess eidetic memory or, more colloquially, photographic memory. These people are characterized by their deliberate ability to memorize impersonal lists of objects or words after only brief exposure without the use of
mnemonics. However, despite many historical claims of eidetic memory, only one individual has demonstrated experimentally the ability to display such remarkable innate capacities. Elizabeth was the subject of a Stromeyer study that tested the memory capabilities of random dot patterns (1970). The observer was shown 100x100 matrices of random dot patterns separately in each eye. These patterns were designed so that they appeared to be random texture images separately but, upon overlap, created a figure seen in three dimension. Given the largely objective nature of such a task, the hyperthymesic advantage of using episodic retrieval cues would not be beneficial. In fact, most observers (rather, everyone except for Elizabeth) were unable to retrieve the memories of the random dot patterns from each eye and thus were not able to visualize the 3D figure without the help of a stereoscope. Elizabeth, however, exhibited the remarkable ability to not only be able to superimpose the images but do so even after a fairly large temporal delay (more than 24 hours). Thus, she displayed an unprecedented ability for memory recall of knowledge that was impersonal and non-episodic. Unfortunately, no neuroanatomical analyses of Elizabeth were run, but observations were made regarding her encoding behavior. Her visual encoding process was slow and deliberate: she scrutinized the visual image section by section and frequently shut her eyes to generate a conjugate image in her mind (Stromeyer & Psotka, 1970). Such behavior parallels other memory types. S., for instance, uses the same strategy for visual encoding: he requires the slow and distinct input of sensory information (e.g. auditory, visual), often pausing between bouts of sensory input to encode and regenerate the information in his mind (Luria, 1987).
In general, these case studies point to the fact that the psychological basis for superior memory stems from a hyperactive encoding process that involves the use of additional relational cues. In the case of hyperthymesia, these individuals rely on a relatively uninhibited episodic retrieval process whose repetition and use of emotionally relevant semantic cues heighten the
recollection of AMs. Anatomically, this augmentation is evinced through the increased connectivity of white matter within the inferior medial temporal cortex and specifically within the amygdala-hippocampal complex, which is consistent with data demonstrating the temporal lobe as the region of localization for memory impairments. Synesthetes, on the other hand, experience encoding through several different modalities with any given input, resulting in a heightened level of relational connectivity. Due to the multiplied network of cues that trigger any given memory, the likelihood of false or inaccurate memories reduces substantially. Like hyperthymesia, synesthesia is characterized by increased connectivity in the white matter tract of the inferior temporal lobe, but the latter is also shown to be correlated with hyperconnectivity of the grey matter tract, localized particularly to the FG and the parietal cortex. The interplay between these two regions allow for the cross-activation of various sensory modalities (causing the synesthetic experience) and the strengthening of the synesthetic components of perception initiated in the FG. Even in the case of individuals with eidetic memory, such as Elizabeth, for whom we lack supporting neuroanatomical data, there is reason to believe that the underlying mechanism is rooted in the deliberate and careful visual encoding process exhibited in these individuals.
At first glance, these “superior” memory mechanisms seem to display only positive implications: a heightened sense of autobiographical introspection, in the case of hyperthymesia, and an ability to use multiple sensory modalities to retrieve memory cues, in the case of synesthesia. However, many of these individuals suffer from major behavioral issues, which tends to have an isolating effect in the context of social interaction. A.J., for example, spent the majority of her time reliving the past, which interfered with her ability to absorb the present or make plans for the future (Parker et al., 2006). S., despite his quite enviable ability of virtually unlimited recall, showed signs of impairment in tasks that were routine and simple for normal individuals. For
example, he showed difficulty in recognizing human faces due to the irregularity and variance within even just one face; his obsession and focus on the visual details of each face precluded him from internalizing the semantic, high-level nature of human recognition (Luria, 1987). Furthermore, his hyperactive imagination led to a blurring of the lines between reality and fantasy, which stunted his personal and social development. Even he himself admitted that he perceived everything “as though through a haze…indeed, one would be hard put to say which was more real for him: the world of imagination in which he lived, or the world of reality in which he was but a temporary guest” (Luria, 1987). Thus, the abnormal nature of these individuals should be treated as just that: abnormality, rather than enhancement. Although these “extraordinary individuals” possess innate mechanisms that shed light on the neuroanatomical and psychological origins of memory encoding and storage, more research needs to be done in the field of hyperactive memory recall before steps can be made to incorporate these findings into possible supplementary or curative therapeutics.
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