Biology subjects

Carlson, T. A.

Publications and source records attributed to Carlson, T. A..

At least 19 recordsLinked to original sources

bioRxiv2025

The effect of hunger and state preferences on the neural processing of food images

Visual information plays a key role in guiding food-related decisions. While previous studies have shown that features such as calories and naturalness are encoded by the brain, upon simply seeing the stimuli, it remains unclear how this encoding is shaped by the observers current state. In this study, we explore the effect of 1) hunger state, 2) task relevance, and 3) current individual preference on the processing of visual food information. Participants (N = 23) underwent two EEG sessions: one after fasting overnight and another after eating normally. During each session, participants did two separate tasks, one where the stimuli were task-relevant and one where attention was distracted away. We used multivariate analysis methods to assess the impact of hunger on the representation of food-related features, and to determine the time-course of information related to food flavour, personal appeal, and arousal, across both tasks. Results showed that information about edibility (food versus non-food object), food identity (e.g. hamburger versus pizza), flavour profile, or personal appeal and arousal was not influenced by the hunger manipulation. Flavour was represented regardless of attentive state, whereas personal appeal and arousal information emerged later and were only observed when the food was task relevant. We found that food appeal and arousal encoding were more closely aligned with behavioural ratings within rather than between sessions, suggesting the nature of the encoding was driven by current state. The study provides insights into how personal preferences and physiological states influence the representation of food information in the brain.

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bioRxiv2025

Disentangling objects' contextual associations from perceptual and conceptual attributes using time-resolved neural decoding

Humans effortlessly relate what they see to what they know, drawing on existing knowledge of objects perceptual, conceptual, and contextual attributes while searching for and recognising objects. While prior studies have investigated the temporal dynamics of perceptual and conceptual object properties in the neural signal, it remains unclear whether and when contextual associations are uniquely represented. In this study, we used representational similarity analysis on electroencephalography (EEG) data to explore how the brain processes the perceptual, conceptual, and contextual dimensions of object knowledge over time. Using human similarity judgments of 190 naturalistic object concepts presented as either as images or words, we constructed separate behavioural models of objects perceptual, conceptual, and contextual properties. We correlated these models with neural patterns from two EEG datasets, one publicly available and one newly collected, both recorded while participants passively viewed the same object stimuli. Across both datasets, we found that perceptual features dominated the early EEG response to object images, while conceptual features emerged later. Contextual associations were also reflected in neural patterns, but their explanatory power largely overlapped with that of conceptual models, suggesting limited unique representation of objects contextual attributes under passive viewing conditions. These results highlight the brains integration of perceptual and conceptual information when processing visual objects. By combining high temporal resolution EEG with behaviourally derived models, this study advances our understanding of how distinct dimensions of object knowledge are encoded in the human brain.

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bioRxiv2024

Neural interference between real and imagined visual stimuli

Evidence suggests that mental imagery and veridical perception recruit similar components of the human visual system. If so, neural representations of imagined and real stimuli should interact with one another, combining constructively or competing antagonistically. To determine if and how real and imagined visual stimuli interact in the brain, we asked participants to mentally visualise white bars at specific orientations after a rhythmic countdown while their brain activity was recorded using electroencephalography. Stimuli were imagined in isolation, or while another stimulus at a highly or poorly congruent orientation appeared on-screen. Multivariate pattern analysis was used to assess whether overlap between imagined and real stimulus features enhanced or diminished stimulus-specific sensory information in the brain. Findings showed that imagined and real orientation could be decoded from brain activity, with real orientation decoding mildly amplified by highly congruent, but not poorly congruent, imagined orientations. Although interactions between real and imagined stimuli were observed, no evidence was detected to suggest that imagined and real stimuli use the same neural activity patterns to encode sensory information. Instead, congruent imagery seemed only to amplify activity which had already been induced by real percepts, targeting late- but not early-stage perceptual representations. Ultimately, this study suggests that imagined and real stimuli interact in a mildly constructive manner, with imagination mostly acting in a modulatory capacity.

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bioRxiv2023

The Time-Course of Food Representation in the Human Brain

Humans make decisions about food every day. The visual system provides important information that forms a basis for these food decisions. Although previous research has focused on visual object and category representations in the brain, it is still unclear how visually presented food is encoded by the brain. Here, we investigate the time-course of food representations in the brain. We used time-resolved multivariate analyses of electroencephalography (EEG) data, obtained from human participants (both sexes), to determine which food features are represented in the brain, and whether focused attention is needed for this. We recorded EEG while participants engaged in one of two tasks. In one task the stimuli were task relevant, whereas in the other task the stimuli were not task relevant. Our findings indicate that the brain can differentiate between food and non-food items from approximately 112 milliseconds after stimulus onset. The neural signal at later latencies contained information about food naturalness, how much the food was transformed, as well as the perceived caloric content. This information was present regardless of the task. Information about whether food is immediately ready to eat, however, was only present when the food was task relevant and presented at a slow presentation rate. Furthermore, the recorded brain activity correlated with the behavioural responses in an odd-item-out task. The fast representation of these food features, along with the finding that this information is used to guide food categorisation decision-making, suggests that these features are important dimensions along which the representation of foods is organised.

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bioRxiv2023

Comparing mental imagery experiences across visual, auditory, and other sensory modalities

Although mental imagery is often studied as a visual phenomenon, it can occur in any sensory modality. Given that mental images may recruit similar modality-specific neural systems to those which support veridical perception, the properties of mental images may be constrained by the modality in which they are experienced. Yet, little is known about how mental images are experienced at all, let alone how such experiences may vary depending on the modality in which they occur. Here we explored how mental images are experienced in different modalities using an extensive questionnaire. Mainly focusing on visual and auditory mental imagery, we surveyed participants on if and how they experienced their thought content in a sensory way when thinking about the appearance or sound of the letter "O". Specifically, we investigated temporal properties of imagined content (e.g. onset latency, duration), as well as spatial properties (e.g. apparent location), effort (e.g. ease, spontaneity, control), dependence on body movements (e.g. eye movements), interactions between real and imagined content (e.g. inner speech during reading), the perceived normality of imagery experiences, and how participants labeled their own experiences. Participants also ranked their mental imagery experiences in the five traditional sensory modalities and reported on the involvement of each modality during their thoughts, imagination, and dreams. Confidence ratings were taken for every answer recorded. Overall, visual and auditory experiences tended to dominate mental events relative to other sensory modalities. However, most people reported that auditory mental imagery was superior to visual mental imagery on almost every metric tested, except with respect to spatial properties. Our findings suggest that mental images are restrained in a similar matter to other modality-specific sensory processes in the brain. Broadly, our work also provides a wealth of insights and observations into how mental images are experienced by individuals, acting as a useful resource for future investigations.

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bioRxiv2022

Perception as a hierarchical competition: a model that differentiates imagined, veridical, and hallucinated percepts

Mental imagery is a process by which thoughts become experienced with sensory characteristics. Yet, it is not clear why mental images appear diminished compared to veridical images, nor how mental images are phenomenologically distinct from hallucinations, another type of non-veridical sensory experience. Current evidence suggests that imagination and veridical perception share neural resources. If so, we argue that considering how neural representations of externally-generated stimuli (i.e. sensory input) and internally-generated stimuli (i.e. thoughts) might interfere with one another can sufficiently differentiate veridical, imaginary, and hallucinatory perception. We here use a simple computational model of a serially-connected, hierarchical network with bidirectional information flow to emulate the primate visual system. We show that modelling even first-approximations of neural competition can more coherently explain imagery phenomenology than non-competitive models. Our simulations predict that, without competing sensory input, imagined stimuli should ubiquitously dominate hierarchical representations. However, with competition, imagination should dominate high-level representations but largely fail to outcompete sensory inputs at lower processing levels. To interpret our findings, we assume low-level stimulus information (e.g. in early visual cortex) contributes most to the sensory aspects of perceptual experience, while high-level stimulus information (e.g. towards temporal regions) contributes most to its abstract aspects. Our findings therefore suggest that ongoing bottom-up inputs during waking life may prevent imagination from overriding veridical sensory experience. In contrast, internally-generated stimuli may be hallucinated when sensory input is dampened or eradicated. Our approach can explain individual differences in imagery, along with aspects of daydreaming, hallucinations, and non-visual mental imagery.

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bioRxiv2021

THINGS-EEG: Human electroencephalography recordings for 1,854 concepts presented in rapid serial visual presentation streams

The neural basis of object recognition and semantic knowledge has been extensively studied but the high dimensionality of object space makes it challenging to develop overarching theories on how the brain organises object knowledge. To help understand how the brain allows us to recognise, categorise, and represent objects and object categories, there is a growing interest in using large-scale image databases for neuroimaging experiments. In the current paper, we present THINGS-EEG, a dataset containing human electroencephalography responses from 50 subjects to 1,854 object concepts and 22,248 images in the THINGS stimulus set, a manually curated and high-quality image database that was specifically designed for studying human vision. The THINGS-EEG dataset provides neuroimaging recordings to a systematic collection of objects and concepts and can therefore support a wide array of research to understand visual object processing in the human brain.

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bioRxiv2021

Undivided attention: The temporal effects of attention dissociated from decision, memory, and expectation

Selective attention prioritises relevant information amongst competing sensory input. Time-resolved electrophysiological studies have shown stronger representation of attended compared to unattended stimuli, which has been interpreted as an effect of attention on information coding. However, because attention is often manipulated by making only the attended stimulus a target to be remembered and/or responded to, many reported attention effects have been confounded with target-related processes such as visual short-term memory or decision-making. In addition, the effects of attention could be influenced by temporal expectation. The aim of this study was to investigate the dynamic effect of attention on visual processing using multivariate pattern analysis of electroencephalography (EEG) data, while 1) controlling for target-related confounds, and 2) directly investigating the influence of temporal expectation. Participants viewed rapid sequences of overlaid oriented grating pairs at fixation while detecting a "target" grating of a particular orientation. We manipulated attention, one grating was attended and the other ignored, and temporal expectation, with stimulus onset timing either predictable or not. We controlled for target-related processing confounds by only analysing non-target trials. Both attended and ignored gratings were initially coded equally in the pattern of responses across EEG sensors. An effect of attention, with preferential coding of the attended stimulus, emerged approximately 230ms after stimulus onset. This attention effect occurred even when controlling for target-related processing confounds, and regardless of stimulus onset predictability. These results provide insight into the effect of attention on the dynamic processing of competing visual information, presented at the same time and location.

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bioRxiv2020

The neural dynamics underlying prioritisation of task-relevant information

The human brain prioritises relevant sensory information to perform different tasks. Enhancement of task-relevant information requires flexible allocation of attentional resources, but it is still a mystery how this is operationalised in the brain. We investigated how attentional mechanisms operate in situations where multiple stimuli are presented in the same location and at the same time. In two experiments, participants performed a challenging two-back task on different types of visual stimuli that were presented simultaneously and superimposed over each other. Using electroencephalography and multivariate decoding, we analysed the effect of attention on the neural responses to each individual stimulus. Whole brain neural responses contained considerable information about both the attended and unattended stimuli, even though they were presented simultaneously and represented in overlapping receptive fields. As expected, attention increased the decodability of stimulus-related information contained in the neural responses, but this effect was evident earlier for stimuli that were presented at smaller sizes. Our results show that early neural responses to stimuli in fast-changing displays contain remarkable information about the sensory environment but are also modulated by attention in a manner dependent on perceptual characteristics of the relevant stimuli. Stimuli, code, and data for this study can be found at https://osf.io/7zhwp/.

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bioRxiv2020

Now you see it, now you don't: Overlapping neural representations for the position of visible and invisible objects

Humans can covertly track the position of an object, even if the object is temporarily occluded. What are the neural mechanisms underlying our capacity to track moving objects when there is no physical stimulus for the brain to track? One possibility is that the brain "fills-in" information about imagined objects using internally generated representations similar to those generated by feed-forward perceptual mechanisms. Alternatively, the brain might deploy a higher order mechanism, for example using an object tracking model that integrates visual signals and motion dynamics (Kwon et al., 2015). In the present study, we used electroencephalography (EEG) and time-resolved multivariate pattern analyses to investigate the spatial processing of visible and imagined objects. Participants tracked an object that moved in discrete steps around fixation, occupying six consecutive locations. They were asked to imagine that the object continued on the same trajectory after it disappeared and move their attention to the corresponding positions. Time-resolved decoding of EEG data revealed that the location of the visible stimuli could be decoded shortly after image onset, consistent with early retinotopic visual processes. For processing of unseen/imagined positions, the patterns of neural activity resembled stimulus-driven mid-level visual processes, but were detected earlier than perceptual mechanisms, implicating an anticipatory and more variable tracking mechanism. Encoding models revealed that spatial representations were much weaker for imagined than visible stimuli. Monitoring the position of imagined objects thus utilises similar perceptual and attentional processes as monitoring objects that are actually present, but with different temporal dynamics. These results indicate that internally generated representations rely on top-down processes, and their timing is influenced by the predictability of the stimulus. All data and analysis code for this study are available at https://osf.io/8v47t/.

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bioRxiv2019

Decoding images in the mind’s eye: The temporal dynamics of visual imagery

Mental imagery is the ability to generate images in the mind in the absence of sensory input. Both perceptual visual processing and internally generated imagery engage large, overlapping networks of brain regions. However, it is unclear whether they are characterized by similar temporal dynamics. Recent magnetoencephalography work has shown that object category information was decodable from brain activity during mental imagery, but the timing was delayed relative to perception. The current study builds on these findings, using electroencephalography to investigate the dynamics of mental imagery. Sixteen participants viewed two images of the Sydney Harbour Bridge and two images of Santa Claus. On each trial, they viewed a sequence of the four images and were asked to imagine one of them, which was cued retroactively by its temporal location in the sequence. Time-resolved multivariate pattern analysis was used to decode the viewed and imagined stimuli. Our results indicate that the dynamics of imagery processes are more variable across, and within, participants compared to perception of physical stimuli. Although category and exemplar information was decodable for viewed stimuli, there were no informative patterns of activity during mental imagery. The current findings suggest stimulus complexity, task design and individual differences may influence the ability to successfully decode imagined images. We discuss the implications of these results for our understanding of the neural processes underlying mental imagery.

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bioRxiv2019

Untangling featural and conceptual object representations

How are visual inputs transformed into conceptual representations by the human visual system? The contents of human perception, such as objects presented on a visual display, can reliably be decoded from voxel activation patterns in fMRI, and in evoked sensor activations in MEG and EEG. A prevailing question is the extent to which brain activation associated with object categories is due to statistical regularities of visual features within object categories. Here, we assessed the contribution of mid-level features to conceptual category decoding using EEG and a novel fast periodic decoding paradigm. Our study used a stimulus set consisting of intact objects from the animate (e.g., fish) and inanimate categories (e.g., chair) and scrambled versions of the same objects that were unrecognizable and preserved their visual features (Long, Yu, & Konkle, 2018). By presenting the images at different periodic rates, we biased processing to different levels of the visual hierarchy. We found that scrambled objects and their intact counterparts elicited similar patterns of activation, which could be used to decode the conceptual category (animate or inanimate), even for the unrecognizable scrambled objects. Animacy decoding for the scrambled objects, however, was only possible at the slowest periodic presentation rate. Animacy decoding for intact objects was faster, more robust, and could be achieved at faster presentation rates. Our results confirm that the mid-level visual features preserved in the scrambled objects contribute to animacy decoding, but also demonstrate that the dynamics vary markedly for intact versus scrambled objects. Our findings suggest a complex interplay between visual feature coding and categorical representations that is mediated by the visual systems capacity to use image features to resolve a recognisable object.

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bioRxiv2019

In Search of Consciousness: Examining the Temporal Dynamics of Conscious Visual Perception using MEG time-series data

The mere presence of information in the brain does not always mean that this information is available to consciousness (de-Wit, Alexander, Ekroll, & Wagemans, 2016). Experiments using paradigms such as binocular rivalry, visual masking, and the attentional blink have shown that visual information can be processed and represented by the visual system without reaching consciousness. Using multivariate pattern analysis (MVPA) and magneto-encephalography (MEG), we investigated the temporal dynamics of information processing for unconscious and conscious stimuli. We decoded stimulus information from the brain recordings while manipulating visual consciousness by presenting stimuli at threshold contrast in a backward masking paradigm. Participants consciousness was measured using both a forced-choice categorisation task and self-report. We show that brain activity during both conscious and non-conscious trials contained stimulus information, and that this information was enhanced in conscious trials. Overall, our results indicate that visual consciousness is characterised by enhanced neural activity representing the visual stimulus, and that this effect arises as early as 180 ms post-stimulus onset.

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bioRxiv2019

Spatial and feature-selective attention have distinct effects on population-level tuning.

Attention is a fundamental brain process by which we selectively prioritize relevant information in our environment. Cognitively, we can employ different methods for selecting visual information for further processing, but the extent to which these are implemented by similar or different neural processes remains unclear. Spatial and feature-selective attention both change the stimulus related information signaled by single-cells and neural populations, but relatively few studies have directly compared the effects of these distinct types of attention. We scanned participants (n=20) using MEG, while they covertly attended to an object on the left or the right of fixation (spatial attention manipulation) and reported the objects shape or color (feature-selective attention manipulation). We used multivariate pattern classification to measure population stimulus-coding in occipital and frontal areas, for attended and non-attended stimulus features, at attended and non-attended locations. In occipital cortex, we show that both spatial and feature-selective attention enhanced object representations, and the effects of these two attention types interacted multiplicatively. We also found that spatial and feature-selective attention induced qualitatively different patterns of enhancement in occipital cortex for the encoding of stimulus color. Specifically, feature-based attention primarily enhanced small color differences, while spatial attention produced greater enhancement for larger differences. We demonstrate that principles of response-gain and tuning curve sharpening that have been applied to describe the effects of attention at the level of a single neuron can account for these differences. An information flow analysis suggested that these attentional effects may be driven by feedback from frontal areas.

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bioRxiv2019

The influence of image masking on object representations during rapid serial visual presentation

Rapid image presentations combined with time-resolved multivariate analysis methods of EEG or MEG (rapid-MVPA) offer unique potential in assessing the temporal limitations of the human visual system. Recent work has shown that multiple visual objects presented sequentially can be simultaneously decoded from M/EEG recordings. Interestingly, object representations reached higher stages of processing for slower image presentation rates compared to fast rates. This fast rate attenuation is probably caused by forward and backward masking from the other images in the stream. Two factors that are likely to influence masking during rapid streams are stimulus duration and stimulus onset asynchrony (SOA). Here, we disentangle these effects by studying the emerging neural representation of visual objects using rapid-MVPA while independently manipulating stimulus duration and SOA. Our results show that longer SOAs enhance the decodability of neural representations, regardless of stimulus presentation duration, suggesting that subsequent images act as effective backward masks. In contrast, image duration does not appear to have a graded influence on object representations. Interestingly, however, decodability was improved when there was a gap between subsequent images, indicating that an abrupt onset or offset of an image enhances its representation. Our study yields insight into the dynamics of object processing in rapid streams, paving the way for future work using this promising approach.

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bioRxiv2018

The dynamic emergence of musical pitch structure in human cortex

Tonal music the world over is characterized by a hierarchical structuring of pitch, whereby certain tones appear stable and others unstable within their musical context. Despite its prevalence, the cortical mechanisms supporting such a percept remain poorly understood. The current study probed the neural processing dynamics underlying the representation of pitch in Western Tonal Music. Listeners were presented with tones comprising all twelve pitch-classes embedded within a musical context whilst having their magnetoencephalographic (MEG) activity recorded. Using multivariate pattern analysis (MVPA), decoders attempted to classify the identity of tones from their corresponding MEG activity at each peristimulus time sample, providing a dynamic measure of their cortical dissimilarity. Time-evolving dissimilarities between tones were then compared with the predictions of several acoustic and perceptual models. Following tone onset, we observed a temporal evolution in the brains representation. Dissimilarities between tones initially reflected their fundamental frequency separation, but beyond 200 ms reflected their status within the tonal hierarchy of perceived stability. Furthermore, when the dissimilarities corresponding to this latter period were transposed into different keys, cortical relations between keys correlated with the well-known circle of fifths. Convergent with fundamental principles of music-theory and perception, current results detail the dynamics with which the complex perceptual structure of Western tonal music emerges in human cortex within the timescale of an individual tone.\n\nSignificance statementIn music, pitch is organized along a hierarchy of perceived stability. Applying stimulus decoding techniques to the Magnetoencephalographic activity of subjects during music-listening, we examined the structure of this hierarchy in cortex and the dynamics with which it emerges at the timescale of an individual tone. Following its onset, we observed a temporal evolution in the brains representation of a tone. Activity initially reflected its pitch-value (fundamental frequency) before reflecting its status within the tonal hierarchy of perceived stability. Transposing this later period of activity into different musical keys, we found that inter-key distances reflected the well-known circle of fifths. Our results provide a link between the complex perceptual structure of tonal music and its dynamic emergence in cortex.

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bioRxiv2018

The representational dynamics of visual objects in rapid serial visual processing streams

In our daily lives, we are bombarded with a stream of rapidly changing visual input. Humans have the remarkable capacity to detect and identify objects in fast-changing scenes. Yet, when studying brain representations, stimuli are generally presented in isolation. Here, we studied the dynamics of human vision using a combination of fast stimulus presentation rates, electroencephalography and multivariate decoding analyses. Using a presentation rate of 5 images per second, we obtained the representational structure of a large number of stimuli, and showed the emerging abstract categorical organisation of this structure. Furthermore, we could separate the temporal dynamics of perceptual processing from higher-level target selection effects. In a second experiment, we used the same paradigm at 20Hz to show that shorter image presentation limits the categorical abstraction of object representations. Our results show that applying multivariate pattern analysis to every image in rapid serial visual processing streams has unprecedented potential for studying the temporal dynamics of the structure of representations in the human visual system.

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bioRxiv2017

Neural signatures of dynamic emotion constructs in the human brain

How is emotion represented in the brain: is it categorical or along dimensions? In the present study, we applied multivariate pattern analysis (MVPA) to magnetoencephalography (MEG) to study the brains temporally unfolding representations of different emotion constructs. First, participants rated 525 images on the dimensions of valence and arousal and by intensity of discrete emotion categories (happiness, sadness, fear, disgust, and sadness). Thirteen new participants then viewed subsets of these images within an MEG scanner. We used Representational Similarity Analysis (RSA) to compare behavioral ratings to the unfolding neural representation of the stimuli in the brain. Ratings of valence and arousal explained significant proportions of the MEG data, even after corrections for low-level image properties. Additionally, behavioral ratings of the discrete emotions fear, disgust, and happiness significantly predicted early neural representations, whereas rating models of anger and sadness did not. Different emotion constructs also showed unique temporal signatures. Fear and disgust - both highly arousing and negative - were rapidly discriminated by the brain, but disgust was represented for an extended period of time relative to fear. Overall, our findings suggest that 1) dimensions of valence and arousal are quickly represented by the brain, as are some discrete emotions, and 2) different emotion constructs exhibit unique temporal dynamics. We discuss implications of these findings for theoretical understanding of emotion and for the interplay of discrete and dimensional aspects of emotional experience.

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