4,403 research outputs found
Local field potentials reflect multiple spatial scales in V4
Local field potentials (LFP) reflect the properties of neuronal circuits or columns recorded in a volume around a microelectrode (Buzsáki et al., 2012). The extent of this integration volume has been a subject of some debate, with estimates ranging from a few hundred microns (Katzner et al., 2009; Xing et al., 2009) to several millimeters (Kreiman et al., 2006). We estimated receptive fields (RFs) of multi-unit activity (MUA) and LFPs at an intermediate level of visual processing, in area V4 of two macaques. The spatial structure of LFP receptive fields varied greatly as a function of time lag following stimulus onset, with the retinotopy of LFPs matching that of MUAs at a restricted set of time lags. A model-based analysis of the LFPs allowed us to recover two distinct stimulus-triggered components: an MUA-like retinotopic component that originated in a small volume around the microelectrodes (~350 μm), and a second component that was shared across the entire V4 region; this second component had tuning properties unrelated to those of the MUAs. Our results suggest that the LFP reflects neural activity across multiple spatial scales, which both complicates its interpretation and offers new opportunities for investigating the large-scale structure of network processing
Sensitivity to Timing and Order in Human Visual Cortex
Visual recognition takes a small fraction of a second and relies on the
cascade of signals along the ventral visual stream. Given the rapid path
through multiple processing steps between photoreceptors and higher visual
areas, information must progress from stage to stage very quickly. This rapid
progression of information suggests that fine temporal details of the neural
response may be important to the how the brain encodes visual signals. We
investigated how changes in the relative timing of incoming visual stimulation
affect the representation of object information by recording intracranial field
potentials along the human ventral visual stream while subjects recognized
objects whose parts were presented with varying asynchrony. Visual responses
along the ventral stream were sensitive to timing differences between parts as
small as 17 ms. In particular, there was a strong dependency on the temporal
order of stimulus presentation, even at short asynchronies. This sensitivity to
the order of stimulus presentation provides evidence that the brain may use
differences in relative timing as a means of representing information.Comment: 10 figures, 1 tabl
Top-down effects on early visual processing in humans: a predictive coding framework
An increasing number of human electroencephalography (EEG) studies examining the earliest component of the visual evoked potential, the so-called C1, have cast doubts on the previously prevalent notion that this component is impermeable to top-down effects. This article reviews the original studies that (i) described the C1, (ii) linked it to primary visual cortex (V1) activity, and (iii) suggested that its electrophysiological characteristics are exclusively determined by low-level stimulus attributes, particularly the spatial position of the stimulus within the visual field. We then describe conflicting evidence from animal studies and human neuroimaging experiments and provide an overview of recent EEG and magnetoencephalography (MEG) work showing that initial V1 activity in humans may be strongly modulated by higher-level cognitive factors. Finally, we formulate a theoretical framework for understanding top-down effects on early visual processing in terms of predictive coding
Inhibitory synchrony as a mechanism for attentional gain modulation
Recordings from area V4 of monkeys have revealed that when the focus of
attention is on a visual stimulus within the receptive field of a cortical
neuron, two distinct changes can occur: The firing rate of the neuron can
change and there can be an increase in the coherence between spikes and the
local field potential in the gamma-frequency range (30-50 Hz). The hypothesis
explored here is that these observed effects of attention could be a
consequence of changes in the synchrony of local interneuron networks. We
performed computer simulations of a Hodgkin-Huxley type neuron driven by a
constant depolarizing current, I, representing visual stimulation and a
modulatory inhibitory input representing the effects of attention via local
interneuron networks. We observed that the neuron's firing rate and the
coherence of its output spike train with the synaptic inputs was modulated by
the degree of synchrony of the inhibitory inputs. The model suggest that the
observed changes in firing rate and coherence of neurons in the visual cortex
could be controlled by top-down inputs that regulated the coherence in the
activity of a local inhibitory network discharging at gamma frequencies.Comment: J.Physiology (Paris) in press, 11 figure
Towards an interpretation of MOND as a modification of inertia
We explore the possibility that Milgrom's Modified Newtonian Dynamics (MOND)
is a manifestation of the modification of inertia at small accelerations.
Consistent with the Tully-Fisher relation, dynamics in the small acceleration
domain may originate from a quartic (cubic) velocity-dependence of energy
(momentum) whereas gravitational potentials remain linear with respect to mass.
The natural framework for this interpretation is Finsler geometry. The simplest
static isotropic Finsler metric of a gravitating mass that incorporates the
Tully-Fisher relation at small acceleration is associated with a spacetime
interval that is either a homogeneous quartic root of polynomials of local
displacements or a simple root of a rational fraction thereof. We determine the
low energy gravitational equation and find that Finsler spacetimes that produce
a Tully-Fisher relation require that the gravitational potential be modified.
For an isolated mass, Newton's potential is replaced by where is MOND's acceleration scale and is a yet
undetermined distance scale. Orbital energy is linear with respect to mass but
angular momentum is proportional to . Asymptotic light deflection
resulting from time curvature is similar to that of a singular isothermal
sphere implying that space curvature must be the main source of deflection in
static Finsler spacetimes possibly through the presence of the distance scale
that appears in the asymptotic form of the gravitational potential. The
quartic nature of the Finsler metric hints at the existence of an underlying
area-metric that describes the effective structure of spacetime.Comment: Revised version, 9 pages, 1 figure. Accepted for publication in
Monthly Notices of the Royal Astronomical Societ
Spatial Resolution of Local Field Potential Signals in Macaque V4
A main challenge for the development of cortical visual prostheses is to
spatially localize individual spots of light, called phosphenes, by assigning
appropriate stimulating parameters to implanted electrodes. Imitating the
natural responses to phosphene-like stimuli at different positions can help in
designing a systematic procedure to determine these parameters. The key
characteristic of such a system is the ability to discriminate between
responses to different positions in the visual field. While most previous
prosthetic devices have targeted the primary visual cortex, the extrastriate
cortex has the advantage of covering a large part of the visual field with a
smaller amount of cortical tissue, providing the possibility of a more compact
implant. Here, we studied how well ensembles of Multiunit activity (MUA) and
Local Field Potentials (LFPs) responses from extrastriate cortical visual area
V4 of a behaving macaque monkey can discriminate between two-dimensional
spatial positions. We found that despite the large receptive field sizes in V4,
the combined responses from multiple sites, whether MUA or LFP, has the
capability for fine and coarse discrimination of positions. We identified a
selection procedure that could significantly increase the discrimination
performance while reducing the required number of electrodes. Analysis of noise
correlation in MUA and LFP responses showed that noise correlations in LFP
responses carry more information about the spatial positions. Overall, these
findings suggest that spatial positions could be localized with patterned
stimulation in extrastriate area V4
fMRI adaptation revisited
Adaptation has been widely used in functional magnetic imaging (fMRI) studies to infer neuronal response properties in human cortex. fMRI adaptation has been criticised because of the complex relationship between fMRI adaptation effects and the multiple neuronal effects that could underlie them. Many of the longstanding concerns about fMRI adaptation have received empirical support from neurophysiological studies over the last decade. We review these studies here, and also consider neuroimaging studies that have investigated how fMRI adaptation effects are influenced by high-level perceptual processes. The results of these studies further emphasize the need to interpret fMRI adaptation results with caution, but they also provide helpful guidance for more accurate interpretation and better experimental design. In addition, we argue that rather than being used as a proxy for measurements of neuronal stimulus selectivity, fMRI adaptation may be most useful for studying population-level adaptation effects across cortical processing hierarchies
Probing early-universe phase transitions with CMB spectral distortions
Global, symmetry-breaking phase transitions in the early universe can
generate scaling seed networks which lead to metric perturbations. The acoustic
waves in the photon-baryon plasma sourced by these metric perturbations, when
Silk damped, generate spectral distortions of the cosmic microwave background
(CMB). In this work, the chemical potential distortion () due to scaling
seed networks is computed and the accompanying Compton -type distortion is
estimated. The specific model of choice is the nonlinear -model
for , but the results remain the same order of magnitude for other
scaling seeds. If CMB anisotropy constraints to the model are saturated,
the resulting chemical potential distortion .Comment: 17 pages, 6 figures, v2: References added, submitted to Phys. Rev.
Neural population coding: combining insights from microscopic and mass signals
Behavior relies on the distributed and coordinated activity of neural populations. Population activity can be measured using multi-neuron recordings and neuroimaging. Neural recordings reveal how the heterogeneity, sparseness, timing, and correlation of population activity shape information processing in local networks, whereas neuroimaging shows how long-range coupling and brain states impact on local activity and perception. To obtain an integrated perspective on neural information processing we need to combine knowledge from both levels of investigation. We review recent progress of how neural recordings, neuroimaging, and computational approaches begin to elucidate how interactions between local neural population activity and large-scale dynamics shape the structure and coding capacity of local information representations, make them state-dependent, and control distributed populations that collectively shape behavior
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