48 research outputs found
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Temporal Dynamics of GABA and Glx in the Visual Cortex.
Magnetic resonance spectroscopy (MRS) can be used in vivo to quantify neurometabolite concentration and provide evidence for the involvement of different neurotransmitter systems (e.g., inhibitory and excitatory) in sensory and cognitive processes. The relatively low signal-to-noise ratio of MRS measurements has shaped the types of questions that it has been used to address. In particular, temporal resolution is often sacrificed in MRS studies to achieve a signal sufficient to produce a reliable estimate of neurometabolite concentration. Here we apply novel analyses with large datasets from human participants (both sexes) to reveal the dynamics of GABA+ and Glx in visual cortex while participants are at rest (with eyes closed) and compare this with changes in posterior cingulate cortex from a previously collected dataset (under different conditions). We find that the dynamic concentration of GABA+ and Glx in visual cortex drifts in opposite directions; that is, GABA+ decreases while Glx increases over time. Further, we find that in visual, but not posterior cingulate cortex, the concentration of GABA+ predicts that of Glx 120 s later, such that a change in GABA+ is correlated with a subsequent opposite change in Glx. Together, these results expose novel temporal trends and interdependencies of primary neurotransmitters in visual cortex. More broadly, we demonstrate the feasibility of using MRS to investigate in vivo dynamic changes of neurometabolites
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Exploring and explaining properties of motion processing in biological brains using a neural network
Visual motion perception underpins behaviours ranging from navigation to depth perception and grasping. Our limited access to biological systems constrain our understanding of how motion is processed within the brain. Here we explore properties of motion perception in biological systems by training a neural network to estimate the velocity of image sequences. The network recapitulates key characteristics of motion processing in biological brains, and we use our access to its structure to explore and understand motion (mis)perception. We find that the network captures the biological response to reverse-phi motion in terms of direction. We further find that it overestimates and underestimates the speed of slow and fast reverse-phi motion, respectively, because of the correlation between reverse-phi motion and the spatiotemporal receptive fields tuned to motion in opposite directions. Second, we find that the distribution of spatiotemporal tuning properties in the V1 and MT layers of the network are similar to those observed in biological systems. We then show that compared to MT units tuned to fast speeds, those tuned to slow speeds primarily receive input from V1 units tuned to high spatial frequency and low temporal frequency. Next, we find that there is a positive correlation between the pattern-motion and speed selectivity of MT units. Finally, we show that the network captures human underestimation of low coherence motion stimuli, and that this is due to pooling of noise and signal motion. These findings provide biologically plausible explanations for well-known phenomena, and produce concrete predictions for future psychophysical and neurophysiological experiments
The cost of parallel consolidation into visual working memory
A growing body of evidence indicates that information can be consolidated into visual working memory in parallel. Initially, it was suggested that color information could be consolidated in parallel while orientation was strictly limited to serial consolidation (Liu & Becker, 2013). However, we recently found evidence suggesting that both orientation and motion direction items can be consolidated in parallel, with different levels of accuracy (Rideaux, Apthorp, & Edwards, 2015). Here we examine whether there is a cost associated with parallel consolidation of orientation and direction information by comparing performance, in terms of precision and guess rate, on a target recall task where items are presented either sequentially or simultaneously. The results compellingly indicate that motion direction can be consolidated in parallel, but the evidence for orientation is less conclusive. Further, we find that there is a twofold cost associated with parallel consolidation of direction: Both the probability of failing to consolidate one (or both) item/s increases and the precision at which representations are encoded is reduced. Additionally, we find evidence indicating that the increased consolidation failure may be due to interference between items presented simultaneously, and is moderated by item similarity. These findings suggest that a biased competition model may explain differences in parallel consolidation between features
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Voluntary control of illusory contour formation.
The extent to which visual inference is shaped by attentional goals is unclear. Voluntary attention may simply modulate the priority with which information is accessed by the higher cognitive functions involved in perceptual decision making. Alternatively, voluntary attention may influence fundamental visual processes, such as those involved in segmenting an incoming retinal signal into a structured scene of coherent objects, thereby determining perceptual organization. Here we tested whether the segmentation and integration of visual form can be determined by an observer's goals, by exploiting a novel variant of the classical Kanizsa figure. We generated predictions about the influence of attention with a machine classifier and tested these predictions with a psychophysical response classification technique. Despite seeing the same image on each trial, observers' perception of illusory spatial structure depended on their attentional goals. These attention-contingent illusory contours directly conflicted with other, equally plausible visual forms implied by the geometry of the stimulus, revealing that attentional selection can determine the perceived layout of a fragmented scene. Attentional goals, therefore, not only select precomputed features or regions of space for prioritized processing, but under certain conditions also greatly influence perceptual organization, and thus visual appearance
Evidence for parallel consolidation of motion direction and orientation into visual short-term memory
Recent findings have indicated the capacity to consolidate multiple items into visual short-term memory in parallel varies as a function of the type of information. That is, while color can be consolidated in parallel, evidence suggests that orientation cannot. Here we investigated the capacity to consolidate multiple motion directions in parallel and reexamined this capacity using orientation. This was achieved by determining the shortest exposure duration necessary to consolidate a single item, then examining whether two items, presented simultaneously, could be consolidated in that time. The results show that parallel consolidation of direction and orientation information is possible, and that parallel consolidation of direction appears to be limited to two. Additionally, we demonstrate the importance of adequate separation between feature intervals used to define items when attempting to consolidate in parallel, suggesting that when multiple items are consolidated in parallel, as opposed to serially, the resolution of representations suffer. Finally, we used facilitation of spatial attention to show that the deterioration of item resolution occurs during parallel consolidation, as opposed to storage.This work was supported by an Australian Postgraduate
Award to R. R., an NHMRC Early Career
Fellowship (1054726) to D. A., and an Australian
research Council Grant (DP110104553) to M. E
The cost of parallel processing in the human visual system
Our environment is visually rich, containing a multitude of
objects that can be
defined by many different features, e.g. shape, colour, and
motion. To navigate and
interact with the environment, we must process this information
efficiently. The human
visual system can process information either serially or in
parallel. While there is a clear
timesaving benefit of parallel processing, its cost is less well
understood. Consequently,
the aim of this thesis is to address three key theoretical
questions underlying the cost of
parallel processing.
The first aim was to determine how the capacity of parallel
processing varies as a
function of the detail of information extraction. Previous
research has demonstrated that
brief presentations of five and six motion signals can be
differentiated; this suggests that
up to five signals can be simultaneously processed. However, it
is unclear how much
information is being extracted, i.e. whether observers are
extracting direction information
from all five signals. To examine this we presented observers
with multiple moving
objects and evaluated their parallel processing capacity as a
function of the information
required to perform the task. We found that the resolution of
parallel motion processing
varies as a function of the information that is extracted;
specifically, as information
extraction becomes more detailed, the capacity to process
multiple signals is reduced.
The second aim was to investigate whether there is a cost to the
fidelity of
information that is processed in parallel. Previous research
suggests that there may not be
a cost associated with parallel consolidation of information from
sensory to visual shortterm
memory (VSTM). Here we examined this by first determining that
motion direction,
and possibly orientation, can be consolidated in parallel, then
explicitly evaluating the
cost to the fidelity of information consolidated in parallel,
compared to serially. We found
that there is a twofold cost associated with parallel
consolidation: a reduction in resolution
of encoded items due to spreading of spatial attention, and an
increase in the likelihood
of consolidation failure due to interference between items.
The third aim was to examine whether the cost associated with
parallel processing
can ultimately explain its capacity. We extended our previous
findings regarding the cost
associated with parallel consolidation to examine whether the
capacity of parallel
consolidation results from biased competition, the same mechanism
proposed to account
for spatial attention and VSTM storage, as evidenced from the
interference between items
presented simultaneously. This was achieved by demonstrating that
parallel consolidation
performance is influenced by factors predicted by a biased
competition model.
Furthermore, we found evidence suggesting that the capacity may
be as high as three,
with increasingly poorer resolution and higher consolidation
failure-rates.
Together, these results demonstrate that a) parallel processing
is limited by the
complexity of information to be processed, b) there is a twofold
cost of processing
information in parallel, and c) that increasing the amount of
information processed in
parallel also increases this cost to the fidelity of the
information and ultimately leads to
the capacity of this process
Adaptation to binocular anticorrelation results in increased neural excitability
Throughout the brain, information from individual sources converges onto higher order neurons. For example, information from the two eyes first converges in binocular neurons in area V1. Some neurons appear tuned to similarities between sources of information, which makes intuitive sense in a system striving to match multiple sensory signals to a single external cause, i.e., establish causal inference. However, there are also neurons that are tuned to dissimilar information. In particular, some binocular neurons respond maximally to a dark feature in one eye and a light feature in the other. Despite compelling neurophysiological and behavioural evidence supporting the existence of these neurons (Cumming & Parker, 1997; Janssen, Vogels, Liu, & Orban, 2003; Katyal, Vergeer, He, He, & Engel, 2018; Kingdom, Jennings, & Georgeson, 2018; Tsao, Conway, & Livingstone, 2003), their function has remained opaque. To determine how neural mechanisms tuned to dissimilarities support perception, here we use electroencephalography to measure human observers’ steady-state visually evoked potentials (SSVEPs) in response to change in depth after prolonged viewing of anticorrelated and correlated random-dot stereograms (RDS). We find that adaptation to anticorrelated RDS results in larger SSVEPs, while adaptation to correlated RDS has no effect. These results are consistent with recent theoretical work suggesting ‘what not’ neurons play a suppressive role in supporting stereopsis (Goncalves & Welchman, 2017); that is, selective adaptation of neurons tuned to binocular mismatches reduces suppression resulting in increased neural excitability.This work was supported by the Leverhulme Trust (ECF-2017-573 to R. R.), the Isaac Newton Trust (17.08(o) to R. R.), and the Wellcome Trust (095183/Z/10/Z to A. E. W. and 206495/Z/17/Z to E. M.)
Proscription supports robust perceptual integration by suppression in human visual cortex.
Perception relies on integrating information within and between the senses, but how does the brain decide which pieces of information should be integrated and which kept separate? Here we demonstrate how proscription can be used to solve this problem: certain neurons respond best to unrealistic combinations of features to provide 'what not' information that drives suppression of unlikely perceptual interpretations. First, we present a model that captures both improved perception when signals are consistent (and thus should be integrated) and robust estimation when signals are conflicting. Second, we test for signatures of proscription in the human brain. We show that concentrations of inhibitory neurotransmitter GABA in a brain region intricately involved in integrating cues (V3B/KO) correlate with robust integration. Finally, we show that perturbing excitation/inhibition impairs integration. These results highlight the role of proscription in robust perception and demonstrate the functional purpose of 'what not' sensors in supporting sensory estimation
Temporal synchrony is an effective cue for grouping and segmentation in the absence of form cues
The synchronous change of a feature across multiple discrete elements, i.e., temporal synchrony, has been shown to be a powerful cue for grouping and segmentation. This has been demonstrated with both static and dynamic stimuli for a range of tasks. However, in addition to temporal synchrony, stimuli in previous research have included other cues which can also facilitate grouping and segmentation, such as good continuation and coherent spatial configuration. To evaluate the effectiveness of temporal synchrony for grouping and segmentation in isolation, here we measure signal detection thresholds using a global-Gabor stimulus in the presence/absence of a synchronous event. We also examine the impact of the spatial proximity of the to-begrouped elements on the effectiveness of temporal synchrony, and the duration for which elements are bound together following a synchronous event in the absence of further segmentation cues. The results show that temporal synchrony (in isolation) is an effective cue for grouping local elements together to extract a global signal. Further, we find that the effectiveness of temporal synchrony as a cue for segmentation is modulated by the spatial proximity of signal elements. Finally, we demonstrate that following a synchronous event, elements are perceptually bound together for an average duration of 200 ms
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Mixed-polarity random-dot stereograms alter GABA and Glx concentration in the early visual cortex.
The offset between images projected onto the left and right retina (binocular disparity) provides a powerful cue to the three-dimensional structure of the environment. It was previously shown that depth judgements are better when images comprise both light and dark features, rather than only light or only dark elements. Since Harris and Parker (Nature 374: 808-811, 1995) discovered the "mixed-polarity benefit," there has been limited evidence supporting their hypothesis that the benefit is due to separate bright and dark channels. Goncalves and Welchman (Curr Biol 27: 1403-1412, 2017) observed that single- and mixed-polarity stereograms evoke different levels of positive and negative activity in a deep neural network trained on natural images to make depth judgements, which also showed the mixed-polarity benefit. Motivated by this discovery, we seek to test the potential for changes in the balance of excitation and inhibition that are produced by viewing these stimuli. In particular, we use magnetic resonance spectroscopy to measure Glx and GABA concentrations in the early visual cortex of adult humans during viewing of single- and mixed-polarity random-dot stereograms (RDS). We find that participants' Glx concentration is significantly higher, whereas GABA concentration is significantly lower, when mixed-polarity RDS are viewed than when single-polarity RDS are viewed. These results indicate that excitation and inhibition facilitate processing of single- and mixed-polarity stereograms in the early visual cortex to different extents, consistent with recent theoretical work (Goncalves NR, Welchman AE. Curr Biol 27: 1403-1412, 2017).NEW & NOTEWORTHY Depth judgements are better when images comprise both light and dark features, rather than only light or only dark elements. Using magnetic resonance spectroscopy, we show that adult human participants' Glx concentration is significantly higher whereas GABA concentration is significantly lower in the early visual cortex when participants view mixed-polarity random-dot stereograms (RDS) compared with single-polarity RDS. These results indicate that excitation and inhibition facilitate processing of single- and mixed-polarity stereograms in the early visual cortex to different extents.This work was supported by the Leverhulme Trust (ECF-2017-573), the Issac Newton Trust (17.08(o)), and the Wellcome Trust (095183/Z/10/Z)