Synesthesia and Memory

Daniel Tammet
Image Unavailable
Daniel Tammet's pi landscape, taken from www.daneltammet.net

Synaesthesia has been linked to superior memory abilities in many anecdotal accounts. One very popularized case being Daniel Tammet’s description in his autobiography Born on a Blue Day, where he recounts his memorization of 22,000 digits of pi as walking through a landscape of numbers[1]. The link between synaesthesia and memory was not extensively studied in scientifically controlled settings until recently, and many new discoveries are still being made. Current evidence suggest memory abilities in synaesthetes is linked to the superior processing in the involved synaesthetic modalities (visual memory for grapheme-colour synaesthetes, positional memory and rotation for space-time synaesthetes etc[2]). The memory-modality linkages By studying the different sensory modalities and correlation with memory, scientists can investigate the border between perception and memory, the neurological basis of memory, and how different memory techniques aid the memory coding process.

1. Vision: Grapheme-colour Synesthesia and Visual Memory

Grapheme-colour Synaesthesia is the most often studied form of memory-linked synaesthesia due to the abundance of subjects and ease of testing. In terms of visual memory tests, many are available, and dependent variables are measurable in terms of reaction time, accuracy, recall, and memory retainment over long periods of rest [3]. With modern technology, images can be easily manipulated and pulled from the internet. Grapheme-colour synesthesia lends an opportune window into the world of synesthesia through which we can glimpse at the potential for superior visual memory, and any encoded relationship to meaning/lexical elements. The investigation also lends some insight to the relationship between perception and learning.

1.1 Neurological basis

For more on grapheme-colour please refer to the page on grapheme-colour synesthesia. The sections below is strictly concerned with memory.

Extensive neural imaging has been reported using grapheme-colour synesthetes as subjects. Grapheme-colour synesthetes show increased gray matter in the superior parietal cortex, commonly linked to the synesthetic experience [4, 5]. Synesthetes and neurotypicals also differ in their visual and spatial perception areas (fusiform gyrus (face recognition) and parietal cortices), along with mentions of an enhanced V4 region, which is associated with colour perception [6, 7]. This was further confirmed with TMS interference with superior parietal areas to either induce a synesthetic experience or to reduce the synesthetic experience in a previously synesthetic individual. No neurological basis for episodic memory enhancement (medial temporal)

1.2 Meaningful vs. Meaningless Memorization

Image Unavailable
Example of meaningful and meaningless images used in an achromatic visual memory test [8]

Increasingly, researchers are finding that perception and memory exist on a continuum, and that increased perceptions and coding abilities are linked with superior memory [6]. So what is the deal with synesthetes? How do they memorize so well? And how do they encode meaningful and meaningless stimuli? Rothen recently performed a scientifically controlled study to try to address the aforementioned questions [8]. They used achromatic stimuli of words vs. non-words, and scenes vs. abstract images (fractals) for the subjects to memorize. The subjects included neurotypical controls, lexical-gustatory synesthete controls, and grapheme-colour synesthetes. Researchers found that meaningful stimuli were generally remembered more accurately by all the subjects, noting that grapheme-colour synesthetes showed a general trend of improved memory on all visual memorization domains. The most difference was seen with achromatic fractal visuals - graphic-colour synesthetes significantly outperformed everyone (including the lexical-gustatory synesthete controls)

There has been many accounts of mnemic strategies being employed to help achieve the “superior memory” seen in popular media, and many critics sceptic of the superior memory abilities reported by synesthetes [9] however this study proved such accusations to be incorrect, or simply less likely. During this experiment, subjects had self-reported mnemic strategies with every memory test, and the graphic-colour synesthetes did not differ from anybody else. Furthermore, abstract, achromatic images are not very encodable and are not conducive to the more common mnemic strategies such as the method of loci.

1.3 Colour-associated


Many studies also investigated grapheme-colour synesthetes’ abilities to for visual recall, and consistently found that they showed better memory for colour than the shape or location of an object [6, 8]. This interesting divide lends some insight to the complex nature of visual perception, and the multifaceted nature of an encoded memory. This is a mere fact that was discovered and confirmed, however the spatial aspects of visual input is linked to a different brain region.

1.4 Episodic memory

Perhaps not surprisingly, since grapheme-colour synesthetes were shown to have an average medial-temporal area of the brain, no superior episodic memories were found when tested against controls. No superior episodic memories other than anecdotal accounts were found [10]. Even though both visual long-term memory and verbal long-term memory were enhanced in grapheme-colour synesthetes, visual memory was enhanced significantly more, and did not lead to superior episodic memory.

2. Space: Spatial-time Synesthesia and Spatial Memory

Spatial-Time Visualization
Image Unavailable
Drawings of space-time visualizations re-edited from drawing done by spatial-time synesthetes [5]

Perhaps less well-known, but equally impressive memory recall would be the spatial-time synesthete AJ. She could recall specific events of her past, dates, the days of the week they fell on, and whether any internationally significant events happened on that day [11]. Her case study merely described her calendars were drawn as a spatial visualization, but goes into extensive detail about her recall ability with regards to chronology.

2.1 Neurological basis

Previous imaging studies showed that parietal lobes were involved in spatial processing and mental rotation [5]. Interestingly, no mention of neurological significance in regions associated with episodic memory: the medial temporal lobe[4] and the left posterior cingulate [7]

2.2 Chronological and Autobiographical memories

Space-time synesthetes displayed superb episodic memory and memory for dates and times related to personal life. In AJ’s case, she cannot memorize historical dates and details that were of no personal significance. Her childhood move and her almost compulsive journalling might suggest that some of this ability is learned or acquired [11]. On the other hand in the study by Simmer et al (2009), The authors found that space-time synesthetes consistently performed better than controls in areas requiring date recall and chronology. Even though the synesthetes were significantly better, they remain inaccurate. The writers proposed that spatial visualizations provided a significant advantage for memory, however not absolutely flawless. [5]

2.3 Visual Memory and Mental Rotation

Under the Simmer study, the researchers tested for visual memory in space-time synesthetes as well as their rotation-chronology ability. The visual tasks are quite different from the grapheme-colour synesthesia memory tests. The Progressive Silhouettes tasks, though designed to assess visual memory, requires visualization and mental rotation of a 3-D object represented on paper, giving it a definite spatial component. So the specific sub-divide of visual memory is currently unclear.

3 Other Modalities

3.1 Auditory

Frequently these are either sound-induced colour or grapheme-colour synesthetes whose synesthesia can be elicited by either spoken or visually reading a word. FMRI scans have shown that in individuals with ‘colour-hearing’ synesthesia, the V4/V8 areas of the brain light up upon hearing a word [7]. Such neurological associations should logically lead to interesting studies of auditory-visual association and investigation of the incorporation of simultaneous input on memory. However auditory-linked synesthesia has not been extensively studied with regards to its effects on memory.

One interesting study pointed out that there is significant genetic overlap between absolute pitch and synesthesia [12]. This might arguably be presented as a memory advantage connected to the synesthetic experience, but case study accounts in Cytowic’s book (The Man Who Tasted Shapes) indicates that it’s more of an ability to perceive colours assigned to specific notes [13]

3.2 Olfactory

Like audition, our olfactory senses have been somewhat neglected in the search for memory association. Interestingly enough it is the sensory modality that is the most remarkably involved in memory. When you smell fresh home-baked bread or when you smell that unique smell in your grandparents’ backyard, the sensory perception is likely going to trigger some form of episodic memory, or a mental image. Some argue that this can be considered a form of synesthesia on its own [14]. Despite smells being a commonly accepted trigger of vivid memory, has not been extensively studied in synesthetes.

Bibliography
1. Tammet, D. Born on a Blue Day: Inside the Extraordinary Mind of an Autistic Savant. Hodder and Stoughton: Great Britain (2006)
2. Terhune, D. B., Wudarczyk, O. A., Kochuparampil, P., Kadosh, C. Enhanced Dimension-specific Visual Working Memory in Grapheme–color Synesthesia. Cognition 129, 123-137. (2013)
3. Rothen, N., Meier, B., Ward, J. Enhanced memory ability: Insights from synaesthesia. Neuroscience and Biobehavioural Reviews 36, 1952-1963 (2012)
4. Rouw, R., Scholte H.S., Neural Basis of Individual Differences in Synesthetic Experiences. Journal of Neuroscience 30(18): 6205-6213 (2010)
5. Simmer, J., Mayo, N., Spiller, M., A foundation for savantism? Visuo-spatial synaesthetes present with cognitive benefits. Cortex 45: 1246-1260 (2009)
6. Pritchard, J., Rothen, N., Coolbear, D., Ward, J., Enhanced associative memory for colour (but not shape or location) in synaesthesia. Cognition 127, 230-234 (2013)
7. Nunn, J. A., Gregory, L. J., Brammer M., Williams, S. C. R., Parslow, D. M., Morgan, M. J., Morris, R. G., Bullmore, E. T., Baron-Cohen, S., and Gray, J. A., Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words. Nature Neuroscience 5(4), 371-375 (2002)
8. Rothen, N., Ward. J., Hovard. P., Jones, A., Enhanced recognition memory in grapheme-color synaesthesia for different categories of visual stimuli. Frontiers in Psychology, 4 (article 762) (2013)
9. Foer, J., Moonwalking with Einstein: The Art and Science of Remembering Everything. Penguin Books: New York (2011)
10. Rothen, N., Meier, B., Do Synesthetes Have a General Advantage in Visual Search and Episodic Memory? A Case for Group Studies PLoS ONE 4(4) (2009)
11. Parker, E. S., Cahill, L., Mcgaugh J. L., A Case of Unusual Autobiographical Remembering. Neurocase 12, 35-49 (2006)
12. Gregersen, P.K., Kowalsky, E., Lee, A., Baron-Cohen, S., Fisher, S. E., Asher, J. E., Ballard, D., Freudenberg, J., and Li, W., Absolute pitch exhibits phenotypic and genetic overlap with synesthesia. Human Molecular Genetics 22(10), 2097-2104 (2013)
13. Cytowic, R., The Man Who Tasted Shapes. MIT Press: USA (2003)
14. Stevenson, R. J., Tomiczek Olfactory-Induced Synesthesias: A Review and Model. Psychological Bulletin 133(2), 294-309 (2007)

Add a New Comment
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License