Learning and experience are known to improve our ability to make perceptual decisions. Yet, our understanding of the brain mechanisms that support improved perceptual decisions through training remains limited. Here, we test the neurochemical and functional interactions that support learning for perceptual decisions in the context of an orientation identification task. Using magnetic resonance spectroscopy (MRS), we measure neurotransmitters (i.e., glutamate, GABA) that are known to be involved in visual processing and learning in sensory [early visual cortex (EV)] and decision-related [dorsolateral prefrontal cortex (DLPFC)] brain regions. Using resting-state functional magnetic resonance imaging (rs-fMRI), we test for functional interactions between these regions that relate to decision processes. We demonstrate that training improves perceptual judgments (i.e., orientation identification), as indicated by faster rates of evidence accumulation after training. These learning-dependent changes in decision processes relate to lower EV glutamate levels and EV-DLPFC connectivity, suggesting that glutamatergic excitation and functional interactions between visual and dorsolateral prefrontal cortex facilitate perceptual decisions. Further, anodal transcranial direct current stimulation (tDCS) in EV impairs learning, suggesting a direct link between visual cortex excitation and perceptual decisions. Our findings advance our understanding of the role of learning in perceptual decision making, suggesting that glutamatergic excitation for efficient sensory processing and functional interactions between sensory and decision-related regions support improved perceptual decisions.
Abstract Mitochondrial function declines with age, and many pathological processes in neurodegenerative diseases stem from this dysfunction when mitochondria fail to produce the necessary energy required. Photobiomodulation (PBM), long‐wavelength light therapy, has been shown to rescue mitochondrial function in animal models and improve human health, but clinical uptake is limited due to uncertainty around efficacy and the mechanisms responsible. Using 31 P magnetisation transfer magnetic resonance spectroscopy (MT‐MRS) we quantify, for the first time, the effects of 670 nm PBM treatment on healthy ageing human brains. We find a significant increase in the rate of ATP synthase flux in the brain after PBM in a cohort of older adults. Our study provides initial evidence of PBM therapeutic efficacy for improving mitochondrial function and restoring ATP flux with age, but recognises that wider studies are now required to confirm any resultant cognitive benefits.
It is often assumed that works of art have the ability to elicit emotion in their observers. An emotional response to a visual stimulus can occur as early as 120 ms after stimulus onset, before object categorisation can take place. This implies that emotions elicited by an artwork may depend in part on bottom-up processing of its visual features (e.g., shape, colour, composition) and not just on object recognition or understanding of artistic style. We predicted that participants are able to judge the emotion conveyed by an artwork in a manner that is consistent across observers. We tested this hypothesis using abstract paintings; these do not provide any reference to objects or narrative contexts, so that any perceived emotion must stem from basic visual characteristics. Nineteen participants with no background in art rated 340 abstract artworks from different artistic movements on valence and arousal on a Likert scale. An intra-class correlation model showed a high consistency in ratings across observers. Importantly, observers used the whole range of the rating scale. Artworks with a high number of edges (complex) and dark colours were rated as more arousing and more negative compared to paintings containing clear lines, bright colours and geometric shapes. These findings provide evidence that emotions can be captured in a meaningful way by the artist in a set of low-level visual characteristics, and that observers interpret this emotional message in a consistent, uniform manner.
Motivation: Exploring the cerebrum's functional organisation and processing is challenging. Functional Magnetic Resonance Imaging (fMRI) measures neuronal activity (NA) noninvasively, but relies on indirect signals related to cerebral haemodynamics. Goal(s): We rigorously investigate if NA in the human brain can be measured using diffusion-weighted fMRI and Direct Imaging of Neuronal Activity (DIANA). Approach: We utilise DW-fMRI and DIANA at 3 Tesla to record the responses in the somatosensory cortex following electric stimulation of the digits. Results: We confirm BOLD responses in somatosensory cortex. Both DW-fMRI and DIANA also show stimulus-locked responses. However, we express concerns regarding electrical stimulation noise artefacts and neuronal inhibition. Impact: This study advances our understanding of neuronal activity measurement using innovative fMRI techniques. It sheds light on the challenges, potential artefacts, and optimal strategies for precise human brain mapping, which is crucial for both basic research and clinical applications.
Abstract Identifying and segmenting objects in an image is generally achieved effortlessly and is facilitated by the presence of symmetry: a principle of perceptual organisation used to interpret sensory inputs from the retina into meaningful representations. However, while imaging studies show evidence of symmetry selective responses across extrastriate visual areas in the human brain, whether symmetry is processed automatically is still under debate. We used functional Magnetic Resonance Imaging (fMRI) to study the response to and representation of two types of symmetry: reflection and rotation. Dot pattern stimuli were presented to 15 human participants (10 female) under stimulus-relevant (symmetry) and stimulus-irrelevant (luminance) task conditions. Our results show that symmetry-selective responses emerge from area V3 and extend throughout extrastriate visual areas. This response is largely maintained when participants engage in the stimulus irrelevant task, suggesting an automaticity to processing visual symmetry. Our multi-voxel pattern analysis (MVPA) results extend these findings by suggesting that not only spatial organisation of responses to symmetrical patterns can be distinguished from that of non-symmetrical (random) patterns, but also that representation of reflection and rotation symmetry can be differentiated in extrastriate and object-selective visual areas. Moreover, task demands did not affect the neural representation of the symmetry information. Intriguingly, our MVPA results show an interesting dissociation: representation of luminance (stimulus irrelevant feature) is maintained in visual cortex only when task relevant, while information of the spatial configuration of the stimuli is available across task conditions. This speaks in favour of the automaticity for processing perceptual organisation: extrastriate visual areas compute and represent global, spatial properties irrespective of the task at hand. Key Points Symmetry selective responses observed in extrastriate visual cortex but not V1 whether stimulus spatial configuration is task relevant or irrelevant. Representation for reflection and rotation differ in extrastriate and object-selective areas, during both stimulus relevant and irrelevant tasks. Representation of luminance information is available only when task relevant.
Abstract Radial frequency patterns - created by sinusoidal modulations of a circle’s radius - are processed globally when radial frequency is low. These closed shapes therefore offer a useful way to interrogate the human visual system for global processing of curvature. Radial frequency patterns elicit greater responses than those to radial gratings in V4 and more anterior face selective regions of the ventral visual pathway. This is largely consistent with work on non-human primates showing curvature processing emerges in V4, but is evident also higher up the ventral visual stream. Rather than contrasting radial frequency patterns with other stimuli, we presented them at varied frequencies in a regimen that allowed tunings to radial frequency to be derived from 8 human participants (3 female). We found tuning to low radial frequency in lateral occipital areas and to some extent in V4. In a control experiment we added a high frequency ripple to the stimuli disrupting the local contour. Low frequency tuning to these stimuli remained in the ventral visual stream underscoring its role in global processing of shape curvature. We then used representational similarity analysis to show that in lateral occipital areas the neural representation was related to stimulus similarity, when it was computed with a model that captured how stimuli are perceived. We show therefore that global processing of shape curvature emerges in the ventral visual stream as early as V4, but is found more strongly in lateral occipital regions, which exhibit responses and representations that relate well to perception.
Ultra-high (7T) functional magnetic resonance imaging (fMRI) was used to determine the contribution of feedforward and feedback brain mechanisms to adaptive processing, i.e. a rapid form of plasticity critical for efficient processing of sensory information. Specifically, we acquired sub-millimetre (0.8mm isotropic) BOLD fMRI data while healthy participants were presented with blocks of gratings at the same or different orientations and engaged in an attentional demanding task at fixation. We then contrasted the fMRI BOLD responses for the two conditions (adaptation, i.e. repeated orientation, and non-adaptation, i.e. different orientations) at different cortical depths in different regions of interest: early visual cortex (V1), extrastriate areas (V2, V3, V4), and posterior parietal (IPS1, IPS2). This allowed us to inform our understanding of the circuit involved in adaptive processing: increased fMRI adaptation (i.e. decreased BOLD response) was observed for superficial and middle rather than deeper layers across the visual cortex. Moreover, functional connectivity analysis across cortical depths of visual and posterior parietal areas showed increased functional connectivity for adaptation condition across visual cortex: increased feedforward connectivity between V1-V2, V1-V3, and V1-V4 superficial-middle layers, increased feedback connectivity for adaptation condition between deeper layers of V2 and deeper layers of V1, as well as deeper layers of V1 and IPS1. Several pre-processing steps were performed to accurately account for potential biases and distortions in the fMRI images that could confound the BOLD signal. In particular, methods for distortion corrections using images acquired with inverted phase encoding were applied to reduce blurring and distortions due to non-zero off-resonance field; alignment between functional and structural images was validated using mcheck tools provided in this repository (i.e. computing the spatial correlation between images); borders between white matter and grey matter, as well as grey matter and cerebrospinal fluid in each participant's brain segmentation were inspected and manually adjusted. In order to account for superficial bias in the BOLD signal (i.e. increased BOLD signal towards the grey matter - pial surface), several steps were applied: (a) voxels with low temporal signal to noise ratio and high t-values in a stimulus vs fixation GLM contrast were removed from each region of interest; (b) across cortical depths, the same amount of voxels was kept to avoid biases in the signal; (c) BOLD signal was z-scored to control for differences in signal levels across cortical depths while preserving signal differences across conditions. The data presented in the dataset here are therefore the result of these steps, in particular: normalised fMRI responses for each condition (adaptation, non-adaptation) were averaged across the stimulus presentation (excluding responses; 32-34s after stimulus onset), blocks, and runs for each condition. For visual cortex ROIs we focussed on the 4 to 18s after stimulus onset, a time window capturing the peak of the haemodynamic response. These values are reported for each participant, each cortical depth (deeper ,middle, superficial) and each ROI (V1, V2, V3, V4, IPS1, and IPS2). We computed functional connectivity within visual cortex and between visual and posterior parietal cortex. We used ICA-based and Finite Impulse Response (FIR) functions to denoise and deconvolve the fMRI time course data per cortical depth, controlling for noise and potential task-timing confounds. We then conducted Pearson correlations between the fMIR eigenvariate time courses across cortical depths. Feedforward functional connectivity is computed between superficial and middle layers, feedback connectivity is computed between deeper layers. The data presented in the dataset correspond to the difference in Fisher's z-transformed R values between adaptation and non-adaptation, i.e. positive value corresponds to increased functional connectivity for adaptation compared to non-adaptation, for the pathway (feedforward / feedback) of interest. Please see also the file 'Description of uploaded data' for a detailed description of the dataset.
Radial frequency (RF) patterns, created by sinusoidal modulations of a circle's radius, are processed globally when RF is low. These closed shapes therefore offer a useful way to interrogate the human visual system for global processing of curvature. RF patterns elicit greater responses than those to radial gratings in V4 and more anterior face-selective regions of the ventral visual pathway. This is largely consistent with work on nonhuman primates showing curvature processing emerges in V4, but is evident also higher up the ventral visual stream. Rather than contrasting RF patterns with other stimuli, we presented them at varied frequencies in a regimen that allowed tunings to RF to be derived from 8 human participants (3 female). We found tuning to low RF in lateral occipital areas and to some extent in V4. In a control experiment, we added a high-frequency ripple to the stimuli disrupting the local contour. Low-frequency tuning to these stimuli remained in the ventral visual stream, underscoring its role in global processing of shape curvature. We then used representational similarity analysis to show that, in lateral occipital areas, the neural representation was related to stimulus similarity, when it was computed with a model that captured how stimuli are perceived. We therefore show that global processing of shape curvature emerges in the ventral visual stream as early as V4, but is found more strongly in lateral occipital regions, which exhibit responses and representations that relate well to perception. SIGNIFICANCE STATEMENT We show that tuning to low radial frequencies, known to engage global shape processing mechanisms, was localized to lateral occipital regions. When low-level stimulus properties were accounted for such tuning emerged in V4 and LO2 in addition to the object-selective region LO. We also documented representations of global shape properties in lateral occipital regions, and these representations were predicted well by a proxy of the perceptual difference between the stimuli.
Abstract Learning and experience are critical for translating ambiguous sensory information from our environments to perceptual decisions. Yet, evidence on how training molds the adult human brain remains controversial, as fMRI at standard resolution does not allow us to discern the finer-scale mechanisms that underlie sensory plasticity. Here, we combine ultra-high field (7T) functional imaging at sub-millimetre resolution with orientation discrimination training to interrogate experience-dependent plasticity across cortical depths. Our results provide evidence for recurrent plasticity, by contrast to sensory encoding vs. feedback mechanisms. We demonstrate that learning alters orientation-specific representations in superficial rather than middle V1 layers, suggesting changes in read-out rather than input signals. Further, learning increases feedforward rather than feedback layer-to-layer connectivity in occipito-parietal regions, suggesting that sensory plasticity gates perceptual decisions. Our findings propose finer-scale plasticity mechanisms that re-weight sensory signals to inform improved decisions, bridging the gap between micro- and macro-circuits of experience-dependent plasticity.
The intermediate processing steps in human vision are not well characterised. We show however that the high specificity of VASO fMRI permits investigation functional organisation of curvature responses in human visual area V4, which is an intermediate region in the visual system. Understanding how the functional architecture and hierarchical integration of local contours (curvature) contributes to formation of shapes can inform computational models of object recognition. The emergence of inter-individual differences in these organisations can explain individual differences in healthy and impaired visual perception.
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