An empirically driven guide on using Bayes factors for M/EEG decoding

Bayes factors can be used to provide quantifiable evidence for contrasting hypotheses and have thus become increasingly popular in cognitive science. However, Bayes factors are rarely used to statistically assess the results of neuroimaging experiments. Here, we provide an empirically driven guide on implementing Bayes factors for time-series neural decoding results. Using real and simulated magnetoencephalography (MEG) data, we examine how parameters such as the shape of the prior and data size affect Bayes factors. Additionally, we discuss the benefits Bayes factors bring to analysing multivariate pattern analysis data and show how using Bayes factors can be used instead or in addition to traditional frequentist approaches

Digit-colour synaesthesia only enhances memory for colours in a specific context:A new method of duration thresholds to measure serial recall

For digit-color synaesthetes, digits elicit vivid experiences of color that are highly consistent for each individual. The conscious experience of synaesthesia is typically unidirectional: Digits evoke colors but not vice versa. There is an ongoing debate about whether synaesthetes have a memory advantage over non-synaesthetes. One key question in this debate is whether synaesthetes have a general superiority or whether any benefit is specific to a certain type of material. Here, we focus on immediate serial recall and ask digit-color synaesthetes and controls to memorize digit and color sequences. We developed a sensitive staircase method manipulating presentation duration to measure participants' serial recall of both overlearned and novel sequences. Our results show that synaesthetes can activate digit information to enhance serial memory for color sequences. When color sequences corresponded to ascending or descending digit sequences, synaesthetes encoded these sequences at a faster rate than their non-synaesthetes counterparts and faster than non-structured color sequences. However, encoding color sequences is approximately 200 ms slower than encoding digit sequences directly, independent of group and condition, which shows that the translation process is time consuming. These results suggest memory advantages in synaesthesia require a modified dual-coding account, in which secondary (synaesthetically linked) information is useful only if it is more memorable than the primary information to be recalled. Our study further shows that duration thresholds are a sensitive method to measure subtle differences in serial recall performance

Temporal dissociation of neural activity underlying synesthetic and perceptual colors

Grapheme-color synesthetes experience color when seeing achromatic symbols. We examined whether similar neural mechanisms underlie color perception and synesthetic colors using magnetoencephalography. Classification models trained on neural activity from viewing colored stimuli could distinguish synesthetic color evoked by achromatic symbols after a delay of ∼100 ms. Our results provide an objective neural signature for synesthetic experience and temporal evidence consistent with higher-level processing in synesthesia

Associations, expectations and meaning: decoding the neural processes underlying conceptual representations

Thesis by publication."Department of Cognitive Science, ARC Centre of Excellence of Cognition and its Disorders, Perception in Action Research Centre, Faculty of Human Sciences, Macquarie University, Sydney, Australia" -- title page.Includes bibliographic references.Chapter 1. Introduction -- Chapter 2. Decoding digits and dice with magnetoencephalography : evidence for a shared representation of magnitude -- Chapter 3. Seeing versus knowing : the temporal dynamics of real and implied colour processing in the human brain -- Chapter 4. Yellow strawberries and red bananas : the influence of object-colour knowledge on emerging object representations in the brain -- Chapter 5. Predicting the temporal dynamics of synaesthetic associations with real colour classification models -- Chapter 6. Discussion -- Appendix.Integrating incoming visual information with prior knowledge is vital when interacting with the world around us. But how do internal concepts and expectations about the environment shape perception? The research presented in this thesis focuses on disentangling neural activity associated with conceptual representations from the visual information that activate them. Using time-resolved multivariate pattern analyses (MVPA), I present four Magnetoencephalography (MEG) studies that aim to increase our understanding of how the neural activity associated with conceptual representations unfolds over time.In the first study, I examine whether there is a shared magnitude representation that can be activated independently of numerical format. The results showed that a shared representation of magnitude can be accessed via different numerical symbols (i.e., digits and dice) but that this representation is accessed slightly earlier via digits than dice. These findings highlight that there is an internally generated magnitude representation that can be separated from incoming visual information.In the second study, I explore whether accessing representations of objects with a strong canonical colour results in typical object colour being activated. When participants viewed greyscale objects that are associated with a specific colour (e.g., a greyscalestrawberry), the typical colour of the object (e.g., red) was decodable suggestingactivation of the object representation includes colour information. Further, I showed thatcolour information accessed via object-colour activation resembles later stages of real colour perception. These findings provide novel insights into the time course of object feature representations that are based on prior knowledge.In the third study, I describe the use of a congruency paradigm to investigate what happens when our prior expectations about object features are violated. The results show that the typicality of feature binding (colour and form) influences the neural response over time and can be differentiated. Whether the conjunction of object and colour was typical had an effect on colour representations but not on object representations. By focusing on the temporal aspect of object processing, this study highlights the effect of typicality of feature binding on object and colour processing.In the fourth study, I discuss data of a special population, grapheme-colour synaesthetes, who experience colours when perceiving achromatic letters and digits. For grapheme-colour synaesthetes, colour information could be decoded from brain activation patterns associated with viewing achromatic symbols in a very brief time window. Colour representations accessed via synaesthetic inducers resembled later stages of colour representations activated via real colour perception. Focusing on the timecourse of the neural signal associated with synaesthetic colours suggests that synaesthesia might be similar to object-colour knowledge.Together, the findings of this show that MVPA applied to MEG data is a suitable method to measure conceptual representations independent from the visual information that activates them. These results give novel insights into the complexities of visual perception and its dependency on prior knowledge.Mode of access: World wide web1 online resource (xi, 256 pages) diagrams, table

Long term associations and serial recall: using synaesthesia to probe memory for sequences

Theoretical thesis.Bibliography: pages 53-61.1. Immediate serial recall : data, models and methodology -- 2. Synaesthesia and the effect of experience on serial recall -- 3. The generalisability of the effect of structure -- 4. General discussion : long term associations and serial recall.In our daily lives, we constantly have to remember sequences such as telephone numbers, license plates or pin codes. How do we hold and recall information that is presented in a sequential manner? Recent interest in the field of serial recall has highlighted that long-term memory seems to interact with our ability to accurately recall sequences. In this thesis, I examine this interaction using overlearnt sequences of digits and letters, and by testing an unusual group with extremely consistent long-term links between digits and colours : synaesthetes. Fror grapheme-colour synaesthetes, letters and digits elicit vivid perceptions of colours. As grapheme-colour pairings are different between synaesthetes but highly consistent for each individual, synaesthesia offers a unique opportunity to examine the effect of long-term associations on immediate serial recall. A recent study suggests that serial colour memory in synaesthetes is not generally enhanced but that synaesthetic associations can, under certain circumstances, be used strategically to improve colour memory. By further exploring this link, we may gain a more detailed insight into the time course of backward translation between colours and digits in synaesthetes and are at the same time able to examine the effect of long-term knowledge on Immediate Serial Recall (ISR) of colours.Mode of access: World wide web1 online resource (vii, 61 pages