Stefan Herlitzea, Lynn T Landmesser
Current Opinion in Neurobiology, Volume 17, Issue 1 , February 2007, Pages 87-94
A major challenge in understanding the relationship between neural activity and development, and ultimately behavior, is to control simultaneously the activity of either many neurons belonging to specific subsets or specific regions within individual neurons. Optimally, such a technique should be capable of both switching nerve cells on and off within milliseconds in a non-invasive manner, and inducing depolarizations or hyperpolarizations for periods lasting from milliseconds to many seconds. Specific ion conductances in subcellular compartments must also be controlled to bypass signaling cascades in order to regulate precisely cellular events such as synaptic transmission. Light-activated G-protein-coupled receptors and ion channels, which can be genetically manipulated and targeted to neuronal circuits, have the greatest potential to fulfill these requirements.
Fulltext: Science Direct
Tuesday, February 27, 2007
Stefan Herlitzea, Lynn T Landmesser
Posted by Ali at 1:57 PM
Monday, February 26, 2007
Marianne Fyhn, Torkel Hafting1, Alessandro Treves, May-Britt Moser & Edvard I. Moser
A fundamental property of many associative memory networks is the ability to decorrelate overlapping input patterns before information is stored. In the hippocampus, this neuronal pattern separation is expressed as the tendency of ensembles of place cells to undergo extensive ‘remapping’ in response to changes in the sensory or motivational inputs to the hippocampus. Remapping is expressed under some conditions as a change of firing rates in the presence of a stable place code (‘rate remapping’), and under other conditions as a complete reorganization of the hippocampal place code in which both place and rate of firing take statistically independent values (‘global remapping’). Here we show that the nature of hippocampal remapping can be predicted by ensemble dynamics in place-selective grid cells in the medial entorhinal cortex, one synapse upstream of the hippocampus. Whereas rate remapping is associated with stable grid fields, global remapping is always accompanied by a coordinate shift in the firing vertices of the grid cells. Grid fields of co-localized medial entorhinal cortex cells move and rotate in concert during this realignment. In contrast to the multiple environment-specific representations coded by place cells in the hippocampus, local ensembles of grid cells thus maintain a constant spatial phase structure, allowing position to be represented and updated by the same translation mechanism in all environments encountered by the animal.
Sunday, February 25, 2007
Saturday, February 24, 2007
Spencer L Smith & Joshua T Trachtenberg
Nature Neuroscience - 10, 370 - 375 (2007)
Visual experience begins at eye opening, but current models consider cortical circuitry to be resistant to experience-dependent competitive modification until the activation of a later critical period. Here we examine this idea using optical imaging to map the time course of receptive field refinement in normal mice, mice in which the contralateral eye never opens and mice in which the contralateral eye is silenced. We found that the refinement of ipsilateral eye retinotopy is retarded by contralateral deprivation, but accelerated by silencing of the contralateral eye. Patterned visual experience through the ipsilateral eye is required for this acceleration. These differences are most obvious at postnatal day 15, long before the start of the critical period, indicating that experience-dependent binocular plasticity occurs much earlier than was previously thought. Furthermore, these results suggest that the quality of activity, in terms of signal to noise, and not the quantity, determines robust receptive field development.
Fangtu T Qiu & Rüdiger von der Heydt
Nature Neuroscience - 10, 283 - 284 (2007)
Perceptual transparency is a surprising phenomenon in which a number of regions of different shades organize into overlaying transparent objects. We recorded single neuron responses from Macaca mulatta area V2 to a display of two bright and two dark squares that appeared as two overlaying bars. We found that neurons assign border ownership according to the transparent interpretation, representing the shapes of the bars rather than the squares.
Kayser C, Petkov CI, Augath M, Logothetis NK
J Neurosci. 2007 Feb 21;27(8):1824-35
Merging the information from different senses is essential for successful interaction with real-life situations. Indeed, sensory integration can reduce perceptual ambiguity, speed reactions, or change the qualitative sensory experience. It is widely held that integration occurs at later processing stages and mostly in higher association cortices; however, recent studies suggest that sensory convergence can occur in primary sensory cortex. A good model for early convergence proved to be the auditory cortex, which can be modulated by visual and tactile stimulation; however, given the large number and small size of auditory fields, neither human imaging nor microelectrode recordings have systematically identified which fields are susceptible to multisensory influences. To reconcile findings from human imaging with anatomical knowledge from nonhuman primates, we exploited high-resolution imaging (functional magnetic resonance imaging) of the macaque monkey to study the modulation of auditory processing by visual stimulation. Using a functional parcellation of auditory cortex, we localized modulations to individual fields. Our results demonstrate that both primary (core) and nonprimary (belt) auditory fields can be activated by the mere presentation of visual scenes. Audiovisual convergence was restricted to caudal fields [prominently the core field (primary auditory cortex) and belt fields (caudomedial field, caudolateral field, and mediomedial field)] and continued in the auditory parabelt and the superior temporal sulcus. The same fields exhibited enhancement of auditory activation by visual stimulation and showed stronger enhancement for less effective stimuli, two characteristics of sensory integration. Together, these findings reveal multisensory modulation of auditory processing prominently in caudal fields but also at the lowest stages of auditory cortical processing.
Tuesday, February 20, 2007
Shmuel A, Yacoub E, Chaimow D, Logothetis NK, Ugurbil K.
Neuroimage. 2007 Jan 4;
This study investigated the spatio-temporal properties of blood-oxygenation level-dependent (BOLD) functional MRI (fMRI) signals in gray matter, excluding the confounding, inaccurate contributions of large blood vessels. Specifically, we quantified the spatial specificity of the BOLD response, and we investigated whether this specificity varies as a function of time from stimulus onset. fMRI was performed at 7 Tesla (T), where mapping signals of parenchymal origin are easily detected. Two abutting visual stimuli were adjusted to elicit responses centered on a flat gray matter region in V1. fMRI signals were sampled at high-resolution orthogonal to the retinotopic boundary between the representations of the stimuli. Signals from macro-vessels were masked out. Principal component analysis revealed that the first component in space accounted for 96.2+/-1.6% of the variance over time. The spatial profile of this time-invariant response was fitted with a model consisting of the convolution of a step function and a Gaussian point-spread-function (PSF). The mean full-width at half-maximal-height of the fitted PSF was 2.34+/-0.20 mm. Based on simulations of confounding effects, we estimate that BOLD PSF in human gray matter is smaller than 2 mm. A detailed time-point to time-point analysis revealed that the estimated PSF obtained during the 3rd (1.52 mm) and 4th (1.99 mm) seconds of stimulation were narrower than the mean estimated PSF obtained from the 5th second on (2.42+/-0.15 mm, mean+/-SD). The position of the edge of the responding region was offset (1.72+/-0.07 mm) from the boundary of the stimulated region, indicating a spatial non-linearity. Simulations showed that the effective contrast between active and non-active columns is reduced 25-fold when imaged using a PSF whose width is equal to the cycle of the imaged columnar organization. Thus, the PSF of the hyper-oxygenated BOLD response in human gray matter is narrower than that reported at 1.5 T, where macro-vessels dominate the mapping signals. The initial phase of this response is more spatially specific than later phases. Data acquisition methods that suppress macro-vascular signals should increase the spatial specificity of BOLD fMRI. The choice of optimal stimulus duration represents a trade-off between the spatial specificity and the overhead associated with short stimulus duration.
Free fulltext: Science Direct
Sunday, February 18, 2007
Horwitz GD, Chichilnisky EJ, Albright TD
J Neurophysiol. 2007 Feb 15;
The rules by which V1 neurons combine signals originating in the cone photoreceptors are poorly understood. We measured cone inputs to V1 neurons in awake, fixating monkeys with white noise analysis techniques that reveal properties of light responses not revealed by purely linear models used in previous studies. Simple cells were studied by spike-triggered averaging that is robust to static nonlinearities in spike generation. This analysis revealed, among heterogeneously tuned neurons, two relatively discrete categories: one with opponent L- and M-cone weights and another with non-opponent cone weights. Complex cells were studied by spike-triggered covariance, which identifies features in the stimulus sequence that trigger spikes in neurons whose receptive fields have multiple linear subunits that combine nonlinearly. All complex cells responded to non-opponent stimulus modulations. Although some complex cells responded to cone-opponent stimulus modulations too, none exhibited the pure opponent sensitivity observed in many simple cells. These results extend the findings on distinctions between simple and complex cell chromatic tuning observed in previous studies in anesthetized monkeys.
Neural coding of reward prediction error signals during classical conditioning with attractive faces
Bray SL, O'doherty JP.
J Neurophysiol. 2007 Feb 15;
Attractive faces can be considered to be a form of visual reward. Previous imaging studies have reported activity in reward structures including orbitofrontal cortex and nucleus accumbens during presentation of attractive faces. Given that these stimuli appear to act as rewards, we set out to explore whether it was possible to establish conditioning in human subjects by pairing presentation of arbitrary affectively neutral stimuli with subsequent presentation of attractive and unattractive faces. Furthermore, we scanned human subjects with fMRI while they underwent this conditioning procedure in order to determine whether a reward prediction error signal is engaged during learning with attractive faces, as is known to be the case for learning with other types of reward such as juice and money. Subjects showed conditioning-related changes in behavioral ratings to the CS stimuli, notably for those CSs paired with attractive female faces. We used a Rescorla-Wagner learning rule to generate a reward prediction error signal, entered as a regressor in our fMRI analysis. We found significant prediction error-related activity in the ventral striatum during conditioning with attractive compared to unattractive faces. These findings suggest that an arbitrary stimulus can acquire conditioned value by being paired with pleasant visual stimuli just as with other types of reward such as money or juice. The findings we describe here may provide insights into the neural mechanisms tapped into by advertisers seeking to influence behavioral preferences by repeatedly exposing consumers to simple associations between products and rewarding visual stimuli such as pretty faces.
Thursday, February 8, 2007
Rotshtein P, Vuilleumier P, Winston J, Driver J, Dolan R.
Cereb Cortex. 2007 Feb 5
We tested for differential brain response to distinct spatial frequency (SF) components in faces. During a functional magnetic resonance imaging experiment, participants were presented with "hybrid" faces containing superimposed low and high SF information from different identities. We used a repetition paradigm where faces at either SF range were independently repeated or changed across consecutive trials. In addition, we manipulated which SF band was attended. Our results suggest that repetition and attention affected partly overlapping occipitotemporal regions but did not interact. Changes of high SF faces increased responses of the right inferior occipital gyrus (IOG) and left inferior temporal gyrus (ITG), with the latter response being also modulated additively by attention. In contrast, the bilateral middle occipital gyrus (MOG) responded to repetition and attention manipulations of low SF. A common effect of high and low SF repetition was observed in the right fusiform gyrus (FFG). Follow-up connectivity analyses suggested direct influence of the MOG (low SF), IOG, and ITG (high SF) on the FFG responses. Our results reveal that different regions within occipitotemporal cortex extract distinct visual cues at different SF ranges in faces and that the outputs from these separate processes project forward to the right FFG, where the different visual cues may converge.
Free Fulltext: http://cercor.oxfordjournals.org/cgi/reprint/bhl180v1
Friday, February 2, 2007
Top-Down Control-Signal Dynamics in Anterior Cingulate and Prefrontal Cortex Neurons following Task Switching
Kevin Johnston, Helen M. Levin, Michael J. Koval, Stefan Everling
Neuron, Vol 53, 453-462, 01 February 2007
The prefrontal cortex (PFC) and anterior cingulate cortex (ACC) have both been implicated in cognitive control, but their relative roles remain unclear. Here we recorded the activity of single neurons in both areas while monkeys performed a task that required them to switch between trials in which they had to look toward a flashed stimulus (prosaccades) and trials in which they had to look away from the stimulus (antisaccades). We found that ACC neurons had a higher level of task selectivity than PFC neurons during the preparatory period on trials immediately following a task switch. In ACC neurons, task selectivity was strongest after the task switch and declined throughout the task block, whereas task selectivity remained constant in the PFC. These results demonstrate that the ACC is recruited when cognitive demands increase and suggest a role for both areas in task maintenance and the implementation of top-down control.
Trends in Cognitive Sciences, Volume 11, Issue 2 , February 2007, Pages 58-64
How do we learn to recognize visual categories, such as dogs and cats? Somehow, the brain uses limited variable examples to extract the essential characteristics of new visual categories. Here, I describe an approach to category learning and recognition that is based on recent computational advances. In this approach, objects are represented by a hierarchy of fragments that are extracted during learning from observed examples. The fragments are class-specific features and are selected to deliver a high amount of information for categorization. The same fragments hierarchy is then used for general categorization, individual object recognition and object-parts identification. Recognition is also combined with object segmentation, using stored fragments, to provide a top-down process that delineates object boundaries in complex cluttered scenes. The approach is computationally effective and provides a possible framework for categorization, recognition and segmentation in human vision.
Fulltext: @ Science Direct