Monday, March 19, 2007

Saturday, March 17, 2007

Time course of precision in smooth-pursuit eye movements of monkeys

Osborne LC, Hohl SS, Bialek W, Lisberger SG
J Neurosci. 2007 Mar 14;27(11):2987-98

To evaluate the nature and possible sources of variation in sensory-motor behavior, we measured the signal-to-noise ratio for the initiation of smooth-pursuit eye movements as a function of time and computed thresholds that indicate how well the pursuit system discriminates small differences in the direction, speed, or time of onset of target motion. Thresholds improved rapidly as a function of time and came close to their minima during the interval when smooth eye movement is driven only by visual motion inputs. Many features of the data argued that motor output and sensory discrimination are limited by the same noise source. Pursuit thresholds reached magnitudes similar to those for perception: <2-3 degrees of direction, approximately 11-15% of target speed, and 8 ms of change in the time of onset of target motion. Pursuit and perceptual thresholds had similar dependencies on the duration of the motion stimulus and showed similar effects of target speed. The evolution of information about direction of target motion followed the same time course in pursuit behavior and in a previously reported sample of neuronal responses from extrastriate area MT. Changing the form of the sensory input while keeping the motor response fixed had significant effects on the signal-to-noise ratio in pursuit for direction discrimination, whereas holding the sensory input constant while changing the combination of muscles used for the motor output did not. We conclude that noise in sensory processing of visual motion provides the major source of variation in the initiation of pursuit.

PMID: 17360922


Stimulus-specific competitive selection in macaque extrastriate visual area V4.

Fallah M, Stoner GR, Reynolds JH
Proc Natl Acad Sci U S A. 2007 Mar 6;104(10):4165-9

Macaque visual area V4 has been implicated in the selective processing of stimuli. Prior studies of selection in area V4 have used spatially separate stimuli, thus confounding selection of retinotopic location with selection of the stimulus at that location. We asked whether V4 neurons can selectively respond to one of two differently colored stimuli even when they are spatially superimposed. We find that delaying one of the two stimuli leads to selective processing of the delayed stimulus by area V4 neurons. This selective processing persists when the stimuli move together across the visual field, thereby successively activating different populations of neurons. We also find that this effect is not a spatially global form of feature-based selection. We conclude that selective processing in area V4 is neither exclusively spatial nor feature-based and may thus be surface- or object-based.

PMID: 17360494

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In the Eye of the Beholder: Visual Experience and Categories in the Human Brain

Johannes Haushofer, Nancy Kanwisher
Neuron 53, March 15, 2007

How does experience change representations of visual objects in the brain? Do cortical object representations reflect category membership? In this issue of Neuron, Jiang et al. show that category training leads to sharpening of neural responses in high-level visual cortex; in contrast, category boundaries may be represented only in prefrontal cortex.

Categorization Training Results in Shapeand Category-Selective Human Neural Plasticity

Xiong Jiang, Evan Bradley, Regina A. Rini, Thomas Zeffiro, John VanMeter, Maximilian Riesenhuber
Neuron 53, 891–903, March 15, 2007

Object category learning is a fundamental ability, requiring the combination of ‘‘bottom-up’’ stimulus-driven with ‘‘top-down’’ task-specific information. It thereforemay be a fruitful domain for study of the general neural mechanisms
underlying cortical plasticity. A simple model predicts that category learning involves the formation of a task-independent shape-selective representation that provides input to circuits learning the categorization task, with the computationally
appealing prediction of facilitated learning of additional, novel tasks over the
same stimuli. Using fMRI rapid-adaptation techniques, we find that categorization training (on morphed ‘‘cars’’) induced a significant release from adaptation for small shape changes in lateral occipital cortex irrespective of category membership, compatible with the sharpening of a representation coding for physical appearance.
In contrast, an area in lateral prefrontal cortex, selectively activated during ategorization, showed sensitivity posttraining to explicit changes in category membership. Further supporting the model, categorization training also improved discrimination performance on the trained stimuli.

Thursday, March 15, 2007

Activity of Inferior Temporal Cortical Neurons Predicts Recognition Choice Behavior and Recognition Time during Visual Search

Ryan E. B. Mruczek and David L. Sheinberg
The Journal of Neuroscience, March 14, 2007, 27(11):2825-2836; doi:10.1523/JNEUROSCI.4102-06.2007

Although the selectivity for complex stimuli exhibited by neurons in inferior temporal cortex is often taken as evidence of their role in visual perception, few studies have directly tested this hypothesis. Here, we sought to create a relatively natural task with few behavioral constraints to test whether activity in inferior temporal cortex neurons predicts whether or not a monkey will recognize and respond to a complex visual object. Monkeys were trained to freely view an array of images and report the presence of one of many possible target images previously associated with a hand response. On certain trials, the identity of the target was swapped during the monkeys' targeting saccade. Furthermore, the response association of the preswap target and the postswap target differed (e.g., right-to-left target swap). Neural activity in cells selective for the preswap target was significantly higher when the monkeys' response matched the hand association of the preswap target. Furthermore, the monkeys' response time was predicted by the magnitude of the presaccadic firing rate on nonswap trials. Our results provide additional support for the role of inferior temporal cortex in object recognition during natural behavior.


Synchronization of Neural Activity across Cortical Areas Correlates with Conscious Perception

Lucia Melloni, Carlos Molina, Marcela Pena, David Torres, Wolf Singer, Eugenio Rodriguez
The Journal of Neuroscience, March 14, 2007, 27(11):2858-2865; doi:10.1523/JNEUROSCI.4623-06.2007

Subliminal stimuli can be deeply processed and activate similar brain areas as consciously perceived stimuli. This raises the question which signatures of neural activity critically differentiate conscious from unconscious processing. Transient synchronization of neural activity has been proposed as a neural correlate of conscious perception. Here we test this proposal by comparing the electrophysiological responses related to the processing of visible and invisible words in a delayed matching to sample task. Both perceived and nonperceived words caused a similar increase of local (gamma) oscillations in the EEG, but only perceived words induced a transient long-distance synchronization of gamma oscillations across widely separated regions of the brain. After this transient period of temporal coordination, the electrographic signatures of conscious and unconscious processes continue to diverge. Only words reported as perceived induced (1) enhanced theta oscillations over frontal regions during the maintenance interval, (2) an increase of the P300 component of the event-related potential, and (3) an increase in power and phase synchrony of gamma oscillations before the anticipated presentation of the test word. We propose that the critical process mediating the access to conscious perception is the early transient global increase of phase synchrony of oscillatory activity in the gamma frequency range.


Wednesday, March 14, 2007

Paradoxical influence of hippocampal neurogenesis on working memory

Michael D. Saxe, Gaël Malleret, Svetlana Vronskaya, Indira Mendez, A. Denise Garcia, Michael V. Sofroniew, Eric R. Kandel, and René Hen
PNAS | March 13, 2007 | vol. 104 | no. 11 | 4642-4646

To explore the function of adult hippocampal neurogenesis, we ablated cell proliferation by using two independent and complementary methods: (i) a focal hippocampal irradiation and (ii) an inducible and reversible genetic elimination of neural progenitor cells. Previous studies using these methods found a weakening of contextual fear conditioning but no change in spatial reference memory, suggesting a supportive role for neurogenesis in some, but not all, hippocampal-dependent memory tasks. In the present study, we examined hippocampal-dependent and -independent working memory using different radial maze tasks. Surprisingly, ablating neurogenesis caused an improvement of hippocampal-dependent working memory when repetitive information was presented in a single day. These findings suggest that adult-born cells in the dentate gyrus have different, and in some cases, opposite roles in distinct types of memory.

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Sunday, March 11, 2007

Efficient signal processing of multineuronal activities for neural interface and prosthesis

Kaneko H, Tamura H, Kawashima T, Suzuki SS, Fujita I
Methods Inf Med. 2007;46(2):147-50.

OBJECTIVES: Multineuronal spike trains must be efficiently decoded in order to utilize them for controlling artificial limbs and organs. Here we evaluated the efficiency of pooling (averaging) and combining (vectorizing) activities of multiple neurons for decoding neuronal information. METHODS: Multineuronal activities in the monkey inferior temporal (IT) cortex were obtained by classifying spikes of constituent neurons from multichannel data recorded with a multisite microelectrode. We compared pooling and combining procedures for the amount of visual information transferred by neurons, and for the success rate of stimulus estimation based on neuronal activities in each trial. RESULTS: Both pooling and combining activities of multiple neurons increased the amount of information and the success rate with the number of neurons. However, the degree of improvement obtained by increasing the number of neurons was higher when combining activities as opposed to pooling them. CONCLUSION: Combining the activities of multiple neurons is more efficient than pooling them for obtaining a precise interpretation of neuronal signals.

PMID: 17347745

Saturday, March 10, 2007

Precise alignment of micro-machined electrode arrays with V1 functional maps

Nauhaus IM, Ringach DL
J Neurophysiol. 2007 Mar 7;

Recent theoretical models of primary visual cortex predict a relationship between receptive field properties and the location of the neuron within the orientation maps. Testing these predictions requires the development of new methods that allow the recording of single units at various locations across the orientation map. Here we present a novel technique for the precise alignment of functional maps and array recordings. Our strategy consists of first measuring the orientation maps in V1 using intrinsic optical imaging. A micro-machined electrode array is subsequently implanted in the same patch of cortex for electrophysiological recordings, including the measurement of orientation tuning curves. The location of the array within the map is then obtained by finding the position that maximizes the agreement between the preferred orientations measured electrically and optically. Experimental results of the alignment procedure from two implementations in monkey V1 are presented. The estimated accuracy of the procedure is evaluated using computer simulations. The methodology should prove useful in studying how signals from the local neighborhood of a neuron, thought to provide a dominant feedback signal, shape the receptive field properties in V1.

PMID: 17344376

Repetition Suppression in Monkey Inferotemporal Cortex: Relation to Behavioral Priming

McMahon DB, Olson CR
J Neurophysiol. 2007 Mar 7;

In tasks requiring judgments about visual stimuli, humans exhibit repetition priming, responding with increased speed when a stimulus is repeated. Repetition priming might depend on repetition suppression, a phenomenon, first observed in monkey inferotemporal cortex (IT), whereby, when a stimulus is repeated, the strength of the neuronal visual response is reduced. If the reduction resulted in sharpening of the cortical representation of the stimulus, and did not just scale it down, then speeded processing might result. To explore the relation between repetition priming and repetition suppression, we monitored neuronal activity in IT while monkeys performed a symmetry decision task. We found (1) that monkeys exhibit repetition priming, (2) that IT neurons simultaneously exhibit repetition suppression, (3) that repetition priming and repetition suppression do not vary in a significantly correlated fashion across trials and (4) that repetition suppression scales down the representation of the stimulus without sharpening it. We conclude that repetition suppression accompanies repetition priming but is unlikely to be its cause.

PMID: 17344370

Friday, March 9, 2007

Orientation-Selective Adaptation to Illusory Contours in Human Visual Cortex

Leila Montaser-Kouhsari, Michael S. Landy, David J. Heeger, Jonas Larsson
The Journal of Neuroscience, February 28, 2007, 27(9):2186-2195; doi:10.1523/JNEUROSCI.4173-06.2007

Humans can perceive illusory or subjective contours in the absence of any real physical boundaries. We used an adaptation protocol to look for orientation-selective neural responses to illusory contours defined by phase-shifted abutting line gratings in the human visual cortex. We measured functional magnetic resonance imaging (fMRI) responses to illusory-contour test stimuli after adapting to an illusory-contour adapter stimulus that was oriented parallel or orthogonal to the test stimulus. We found orientation-selective adaptation to illusory contours in early (V1 and V2) and higher-tier visual areas (V3, hV4, VO1, V3A/B, V7, LO1, and LO2). That is, fMRI responses were smaller for test stimuli parallel to the adapter than for test stimuli orthogonal to the adapter. In two control experiments using spatially jittered and phase-randomized stimuli, we demonstrated that this adaptation was not just in response to differences in the distribution of spectral power in the stimuli. Orientation-selective adaptation to illusory contours increased from early to higher-tier visual areas. Thus, both early and higher-tier visual areas contain neurons selective for the orientation of this type of illusory contour.


Adaptive Surround Modulation in Cortical Area MT

Xin Huang, Thomas D. Albright, Gene R. Stoner
Neuron, Vol 53, 761-770, 01 March 2007

Visual motion perception relies on two opposing operations: integration and segmentation. Integration overcomes motion ambiguity in the visual image by spatial pooling of motion signals, whereas segmentation identifies differences between adjacent moving objects. For visual motion area MT, previous investigations have reported that stimuli in the receptive field surround, which do not elicit a response when presented alone, can nevertheless modulate responses to stimuli in the receptive field center. The directional tuning of this “surround modulation” has been found to be mainly antagonistic and hence consistent with segmentation. Here, we report that surround modulation in area MT can be either antagonistic or integrative depending upon the visual stimulus. Both types of modulation were delayed relative to response onset. Our results suggest that the dominance of antagonistic modulation in previous MT studies was due to stimulus choice and that segmentation and integration are achieved, in part, via adaptive surround modulation.


Modeling the Role of Mid-Wavelength Cones in Circadian Responses to Light

Ouria Dkhissi-Benyahya, Claude Gronfier, Wena De Vanssay, Frederic Flamant, Howard M. Cooper
Neuron, Vol 53, 677-687, 01 March 2007

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.


Stimulus-specific competitive selection in macaque extrastriate visual area V4

Mazyar Fallah, Gene R. Stoner, John H. Reynolds
PNAS | March 6, 2007 | vol. 104 | no. 10 | 4165-4169

Macaque visual area V4 has been implicated in the selective processing of stimuli. Prior studies of selection in area V4 have used spatially separate stimuli, thus confounding selection of retinotopic location with selection of the stimulus at that location. We asked whether V4 neurons can selectively respond to one of two differently colored stimuli even when they are spatially superimposed. We find that delaying one of the two stimuli leads to selective processing of the delayed stimulus by area V4 neurons. This selective processing persists when the stimuli move together across the visual field, thereby successively activating different populations of neurons. We also find that this effect is not a spatially global form of feature-based selection. We conclude that selective processing in area V4 is neither exclusively spatial nor feature-based and may thus be surface- or object-based.

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Monday, March 5, 2007

Vocal-Tract Resonances as Indexical Cues in Rhesus Monkeys

Ghazanfar AA, Turesson HK, Maier JX, van Dinther R, Patterson RD, Logothetis NK
Curr Biol. 2007 Feb 21;

Vocal-tract resonances (or formants) are acoustic signatures in the voice and are related to the shape and length of the vocal tract. Formants play an important role in human communication, helping us not only to distinguish several different speech sounds , but also to extract important information related to the physical characteristics of the speaker, so-called indexical cues. How did formants come to play such an important role in human vocal communication? One hypothesis suggests that the ancestral role of formant perception-a role that might be present in extant nonhuman primates-was to provide indexical cues . Although formants are present in the acoustic structure of vowel-like calls of monkeys and implicated in the discrimination of call types , it is not known whether they use this feature to extract indexical cues. Here, we investigate whether rhesus monkeys can use the formant structure in their "coo" calls to assess the age-related body size of conspecifics. Using a preferential-looking paradigm and synthetic coo calls in which formant structure simulated an adult/large- or juvenile/small-sounding individual, we demonstrate that untrained monkeys attend to formant cues and link large-sounding coos to large faces and small-sounding coos to small faces-in essence, they can, like humans , use formants as indicators of age-related body size.

PMID: 17320389
Full text: Science Direct