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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

Fulltext: http://www.jneurosci.org/cgi/reprint/27/11/2987

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

Free Fulltext: http://www.pnas.org/cgi/reprint/104/10/4165.pdf

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.

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.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/11/2825

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.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/11/2858

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.

Free Fulltext: http://www.pnas.org/cgi/reprint/104/11/4642

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

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
Fulltext: http://jn.physiology.org/cgi/reprint/00120.2007v1