Biology subjects

Ekman, M.

Publications and source records attributed to Ekman, M..

6 recordsLinked to original sources

bioRxiv2022

Successor-like representation guides the prediction of future events in human visual cortex and hippocampus

Human agents build models of their environment, which enable them to anticipate and plan upcoming events. However, little is known about the properties of such predictive models. Recently, it has been proposed that hippocampal representations take the form of a predictive map-like structure, the so-called successor representation. Here we used human fMRI to probe whether activity in the early visual cortex (V1) and hippocampus adhere to the postulated properties of the successor representation after visual sequence learning. Participants were exposed to an arbitrary spatiotemporal sequence consisting of four items (A-B-C-D). We found that after repeated exposure to the sequence, merely presenting single sequence items (e.g., - B - -) resulted in V1 activation at the successor locations of the full sequence (e.g., C-D), but not at the predecessor locations (e.g., A). This highlights that visual representations are skewed toward future states, in line with the successor representation. Similar results were also found in the hippocampus. Moreover, the hippocampus developed a tuning profile that showed sensitivity to the temporal distance in sequence-space, with fading representations for sequence events in the more distant past and future. V1, in contrast, showed a tuning profile that was only sensitive to spatial distance in stimulus-space. Together, these results provide empirical evidence for the proposition that both visual and hippocampal cortex represent a predictive map of the visual world akin to the successor representation.

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bioRxiv2022

Probabilistic forward replay of anticipated stimulus sequences in human primary visual cortex and hippocampus

The ability to recognize and predict future spatiotemporal sequences is vital for perception. It has been proposed that the brain makes intelligent guesses about future inputs by forward replaying these events. However, it is unknown whether and how this mechanism incorporates the probabilistic structure that is inherent to naturalistic environments. Here we tested forward replay in human V1 and hippocampus using a probabilistic cueing paradigm. Participants were exposed to two visual moving dot sequences (A and B) that shared the same starting point. Each stimulus sequence was paired with either a high or a low tone that predicted which sequence would follow with 80% cue validity (probabilistic context) or 50% cue validity (random context). We found that after exposure, the auditory cue together with the starting point triggered simultaneous forward replay of both the likely (A) and the less likely (B) stimulus sequence. Crucially, forward replay preserved the probabilistic relationship of the environment, such that the likely sequence was associated with greater anticipatory V1 activity compared to the less likely stimulus sequence. Analogous to V1, forward replay in hippocampus was also found to preserve the probabilistic cue-sequence relationship. Further, the anterior hippocampus was found to represent the predicted stimulus sequence, irrespective of the input, while the posterior hippocampus revealed a prediction error-like signal that was only observed when predictions were violated. These findings show how mnemonic and sensory areas coordinate predictive representations in probabilistic contexts to improve perceptual processing.

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bioRxiv2020

Object selection by automatic spreading of top-down attentional signals in V1

What is selected when attention is directed to a specific location of the visual field? Theories of object-based attention have suggested that when spatial attention is directed to part of an object, attention does not simply enhance the attended location but automatically spreads to enhance all locations that comprise the object. Here, we tested this hypothesis by reconstructing the distribution of attention from population neuronal activity patterns in V1 using functional magnetic resonance imaging (fMRI) and population-based receptive field mapping. We find that attention spreads from a spatially cued location to the underlying object - and enhances all spatial locations that comprise the object. Importantly, this spreading was also evident when the object was not task-relevant. These data suggest that attentional selection automatically operates at an object level, facilitating the reconstruction of coherent objects from fragmented representations in early visual cortex.

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bioRxiv2019

Visual working memory representations in visual and parietal cortex do not remap after eye movements

It has been suggested that our visual system does not only process stimuli that are directly available to our eyes, but also has a role in maintaining information in VWM over a period of seconds. It remains unclear however what happens to VWM representations in the visual system when we make saccades. Here, we tested the hypothesis that VWM representations are remapped within the visual system after making saccades. We directly compared the content of VWM for saccade and no-saccade conditions using MVPA of delay-related activity measured with fMRI. We found that when participants did not make a saccade, VWM representations were robustly present in contralateral early visual cortex. When making a saccade, VWM representations degraded in contralateral V1-V3 after the saccade shifted the location of the remembered grating to the opposite visual field. However, contrary to our hypothesis we found no evidence for the representations of the remembered grating at the saccadic target location in the opposite visual field, suggesting that there is no evidence for remapping of VWM in early visual cortex. Interestingly, IPS showed persistent VWM representations in both the saccade and no-saccade condition. Together, our results indicate that VWM representations in early visual cortex are not remapped across eye movements, potentially limiting the role of early visual cortex in VWM storage. HighlightsO_LIVisual working memory (VWM) representations do not remap after making saccades C_LIO_LIEye movement degrade VWM representations in early visual cortex, limiting the role of early visual cortex in VWM storage C_LIO_LIParietal cortex shows persistent VWM representations across saccades C_LI

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bioRxiv2017

Suppressed sensory response to predictable object stimuli throughout the ventral visual stream

Prediction plays a crucial role in perception, as prominently suggested by predictive coding theories. However, the exact form and mechanism of predictive modulations of sensory processing remain unclear, with some studies reporting a downregulation of the sensory response for predictable input, while others observed an enhanced response. In a similar vein, downregulation of the sensory response for predictable input has been linked to either sharpening or dampening of the sensory representation, which are opposite in nature. In the present study we set out to investigate the neural consequences of perceptual expectation of object stimuli throughout the visual hierarchy, using fMRI in human volunteers. Participants (n=24) were exposed to pairs of sequentially presented object images in a statistical learning paradigm, in which the first object predicted the identity of the second object. Image transitions were not task relevant; thus all learning of statistical regularities was incidental. We found strong suppression of neural responses to expected compared to unexpected stimuli throughout the ventral visual stream, including primary visual cortex (V1), lateral occipital complex (LOC), and anterior ventral visual areas. Expectation suppression in LOC, but not V1, scaled positively with image preference, lending support to the dampening account of expectation suppression in object perception.\n\nSignificance StatementStatistical regularities permeate our world and help us to perceive and understand our surroundings. It has been suggested that the brain fundamentally relies on predictions and constructs models of the world in order to make sense of sensory information. Previous research on the neural basis of prediction has documented expectation suppression, i.e. suppressed responses to expected compared to unexpected stimuli. In the present study we queried the presence and characteristics of expectation suppression throughout the ventral visual stream. We demonstrate robust expectation suppression in the entire ventral visual pathway, and underlying this suppression a dampening of the sensory representation in object-selective visual cortex, but not in primary visual cortex. Taken together, our results provide novel evidence in support of theories conceptualizing perception as an active inference process, which selectively dampens cortical representations of predictable objects. This dampening may support our ability to automatically filter out irrelevant, predictable objects.

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bioRxiv2017

Cued reactivation during slow-wave sleep induces connectivity changes related to memory stabilization.

Memory reprocessing following acquisition enhances memory consolidation. Specifically, neural activity during encoding is thought to be replayed during subsequent slow-wave sleep (SWS). This natural tendency of memory replay can be induced by external cueing, known as \"targeted memory reactivation\". Here, we analyzed data from a published study (van Dongen, Takashima, et al. 2012), where auditory cues reactivated learned visual object-location memories during SWS. Memory replay during sleep presumably involves a shift in connectivity across the brain. Therefore, we characterized the effects of memory reactivation on brain network connectivity using graph-theory. We found that cue presentation during SWS introduced increased network integration of the occipital cortex, a visual region that was also active during the object retrieval task. Importantly, enhanced network integration of the occipital cortex showed a behavioural benefit and predicted overnight memory stabilization. Furthermore, occipital cortex displayed enhanced connectivity with mnemonic regions, namely the hippocampus, parahippocampal gyrus, thalamus and medial prefrontal cortex during cue versus control sound presentation. Finally, network integration of early occipital cortex during cueing in SWS was related to increased activation of the bilateral parahippocampal gyrus, a region involved in coding for spatial associative information, at the post-sleep test. Together, these results support a neural mechanism where cue-induced replay during sleep promotes memory consolidation by increased integration of task-relevant perceptual regions with mnemonic regions.

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