David S Greenberg, Arthur R Houweling, Jason N D Kerr
Nature Neuroscience 11, 749 - 751 (2008), doi:10.1038/nn.2140
It is unclear how the complex spatiotemporal organization of ongoing cortical neuronal activity recorded in anesthetized animals relates to the awake animal. We therefore used two-photon population calcium imaging in awake and subsequently anesthetized rats to follow action potential firing in populations of neurons across brain states, and examined how single neurons contributed to population activity. Firing rates and spike bursting in awake rats were higher, and pair-wise correlations were lower, compared with anesthetized rats. Anesthesia modulated population-wide synchronization and the relationship between firing rate and correlation. Overall, brain activity during wakefulness cannot be inferred using anesthesia.
Sunday, June 29, 2008
David S Greenberg, Arthur R Houweling, Jason N D Kerr
Posted by Ali at 8:10 AM
Joshua M Susskind, Daniel H Lee, Andrée Cusi, Roman Feiman, Wojtek Grabski, Adam K Anderson
Nature Neuroscience 11, 843 - 850 (2008), doi:10.1038/nn.2138
It has been proposed that facial expression production originates in sensory regulation. Here we demonstrate that facial expressions of fear are configured to enhance sensory acquisition. A statistical model of expression appearance revealed that fear and disgust expressions have opposite shape and surface reflectance features. We hypothesized that this reflects a fundamental antagonism serving to augment versus diminish sensory exposure. In keeping with this hypothesis, when subjects posed expressions of fear, they had a subjectively larger visual field, faster eye movements during target localization and an increase in nasal volume and air velocity during inspiration. The opposite pattern was found for disgust. Fear may therefore work to enhance perception, whereas disgust dampens it. These convergent results provide support for the Darwinian hypothesis that facial expressions are not arbitrary configurations for social communication, but rather, expressions may have originated in altering the sensory interface with the physical world.
Posted by Ali at 8:06 AM
Saturday, June 28, 2008
Haushofer J, Baker CI, Livingstone MS, Kanwisher N.
J Neurophysiol. 2008 Jun 25.
What is the neural code for object shape? Despite intensive research, the precise nature of object representations in high-level visual cortex remains elusive. Here we use functional magnetic resonance imaging (fMRI) to show that convex shapes are encoded in a privileged fashion by human lateral occipital complex (LOC), a region which has been implicated in object recognition. On each trial, two convex or two concave shapes that were either identical or different were presented sequentially. Critically, the convex and concave stimuli were the same except for a binocular disparity change that reversed the figure-ground assignment. The fMRI response in LOC for convex stimuli was higher for different than for identical shape pairs, indicating sensitivity to differences in convex shape. However, when the same stimuli were seen as concave, the response for different and identical pairs was the same, indicating lower sensitivity to changes in concave shape than convex shape. This pattern was more pronounced in the anterior than in the posterior portion of LOC. These results suggest that convex contours could be basic building blocks of cortical object representations.
Posted by Ali at 7:27 AM
Saturday, June 21, 2008
Meyers EM, Freedman DJ, Kreiman G, Miller EK, Poggio TA.
J Neurophysiol. 2008 Jun 18.
Most electrophysiology studies analyze the activity of each neuron separately. While such studies have given much insight into properties of the visual system, they have also potentially overlooked important aspects of information coded in changing patterns of activity that are distributed over larger populations of neurons. In this work, we apply a population decoding method, to better estimate what information is available in neuronal ensembles, and how this information is coded in dynamic patterns of neural activity in data recorded from inferior temporal cortex (ITC) and prefrontal cortex (PFC) as macaque monkeys engaged in a delayed match-to-category task (Freedman et al. 2003). Analyses of activity patterns in ITC and PFC revealed that both areas contain 'abstract' category information (i.e., category information that is not directly correlated with properties of the stimuli); however, in general, PFC has more task-relevant information, and ITC has more detailed visual information. Analyses examining how information coded in these areas show that almost all category information is available in a small fraction of the neurons in the population. Most remarkably, our results also show that category information is coded by a non-stationary pattern of activity that changes over the course of a trial, with individual neurons containing information on much shorter time scales than the population as a whole.
Posted by Ali at 9:17 AM
Antonio Rangel, Colin Camerer, P. Read Montague
Nature Reviews Neuroscience 9, 545-556 (July 2008) | doi:10.1038/nrn2357
Neuroeconomics is the study of the neurobiological and computational basis of value-based decision making. Its goal is to provide a biologically based account of human behaviour that can be applied in both the natural and the social sciences. This Review proposes a framework to investigate different aspects of the neurobiology of decision making. The framework allows us to bring together recent findings in the field, highlight some of the most important outstanding problems, define a common lexicon that bridges the different disciplines that inform neuroeconomics, and point the way to future applications.
Posted by Ali at 7:34 AM
Friday, June 20, 2008
Tsao DY, Livingstone MS.
Annu Rev Neurosci. 2008;31:411-37.
Faces are among the most informative stimuli we ever perceive: Even a split-second glimpse of a person's face tells us his identity, sex, mood, age, race, and direction of attention. The specialness of face processing is acknowledged in the artificial vision community, where contests for face-recognition algorithms abound. Neurological evidence strongly implicates a dedicated machinery for face processing in the human brain to explain the double dissociability of face- and object-recognition deficits. Furthermore, recent evidence shows that macaques too have specialized neural machinery for processing faces. Here we propose a unifying hypothesis, deduced from computational, neurological, fMRI, and single-unit experiments: that what makes face processing special is that it is gated by an obligatory detection process. We clarify this idea in concrete algorithmic terms and show how it can explain a variety of phenomena associated with face processing.
Posted by Ali at 10:50 AM
Thursday, June 19, 2008
Sunday, June 15, 2008
PLoS Comput Biol. 2007 Nov;3(11):e199.Click here to read
Posted by Ali at 7:49 AM
Saturday, June 14, 2008
Nature. 2008 Jun 12;453(7197):869-78
Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.
Posted by Ali at 7:30 AM
Tuesday, June 10, 2008
Moeller S, Freiwald WA, Tsao DY.
Science. 2008 Jun 6;320(5881):1355-9.
The brain processes objects through a series of regions along the ventral visual pathway, but the circuitry subserving the analysis of specific complex forms remains unknown. One complex form category, faces, selectively activates six patches of cortex in the macaque ventral pathway. To identify the connectivity of these face patches, we used electrical microstimulation combined with simultaneous functional magnetic resonance imaging. Stimulation of each of four targeted face patches produced strong activation, specifically within a subset of the other face patches. Stimulation outside the face patches produced an activation pattern that spared the face patches. These results suggest that the face patches form a strongly and specifically interconnected hierarchical network.
Posted by Ali at 9:10 AM
Sunday, June 1, 2008
In vivo two-photon voltage-sensitive dye imaging reveals top-down control of cortical layers 1 and 2 during wakefulness
B. Kuhn, W. Denk, R. M. Bruno
PNAS | May 27, 2008 | vol. 105 | no. 21 | 7588-7593
Conventional methods of imaging membrane potential changes have limited spatial resolution, particularly along the axis perpendicular to the cortical surface. The laminar organization of the cortex suggests, however, that the distribution of activity in depth is not uniform. We developed a technique to resolve network activity of different cortical layers in vivo using two-photon microscopy of the voltage-sensitive dye (VSD) ANNINE-6. We imaged spontaneous voltage changes in the barrel field of the somatosensory cortex of head-restrained mice and analyzed their spatiotemporal correlations during anesthesia and wakefulness. EEG recordings always correlated more strongly with VSD signals in layer (L) 2 than in L1. Nearby (<200 µm) cortical areas were correlated with one another during anesthesia. Waking the mouse strongly desynchronized neighboring cortical areas in L1 in the 4- to 10-Hz frequency band. Wakefulness also slightly increased synchrony of neighboring territories in L2 in the 0.5- to 4.0-Hz range. Our observations are consistent with the idea that, in the awake animal, long-range inputs to L1 of the sensory cortex from various cortical and thalamic areas exert top-down control on sensory processing.
Posted by Ali at 11:23 AM