Effects of contrast and contrast adaptation on static receptive field features in macaque area V1.

The spatiotemporal features of the "static" receptive field (RF), as revealed with flashing bars or spots, determine other RF properties. We examined how some of these static RF features vary with contrast and contrast adaptation in area V1 of the anesthetized macaque monkey. RFs were mapped with light and dark flashing bars presented at three different contrasts, with the low and medium contrasts eliciting approximately 1/3 and 2/3 of the high-contrast response amplitude.

Timing of interactions across the visual field in the human cortex.

While it is generally believed that interactions across long distances in the visual field occur only in the higher-order cortical areas, other results suggest that such interactions are processed very early. In the preceding paper, we identified the latencies within a subset of cortical areas in the human visual system. In the present study, we test in which areas and at which latencies the responses to two visual patterns start interacting.

Sequence of pattern onset responses in the human visual areas: an fMRI constrained VEP source analysis.

We measured the timing of activity in distinct functional areas of the human visual cortex after onset of a visual pattern. This is not possible with visual evoked potentials (VEPs) or magnetic fields alone, and direct combination of functional magnetic resonance imaging (fMRI) with electromagnetic data has turned out to be difficult. We tested a relatively new approach, where both position and orientation of the active cortex was given to the VEP source model.

Reaching beyond the classical receptive field of V1 neurons: horizontal or feedback axons?

It is commonly assumed that the orientation-selective surround field of neurons in primary visual cortex (V1) is due to interactions provided solely by intrinsic long-range horizontal connections. We review evidence for and against this proposition and conclude that horizontal connections are too slow and cover too little visual field to subserve all the functions of suppressive surrounds of V1 neurons in the macaque monkey. We show that the extent of visual space covered by horizontal connections corresponds to the region of low contrast summation of the receptive field center mechanism.

Single-unit analysis of the spinal dorsal horn in patients with neuropathic pain.

Despite the key role played by the dorsal horn of the spinal cord in pain modulation, single-unit recordings have only been performed very rarely in this structure in humans. The authors report the results of a statistical analysis of 64 unit recordings from the human dorsal horn. The recordings were done in three groups of patients: patients with deafferentation pain resulting from brachial plexus avulsion, patients with neuropathic pain resulting from peripheral nerve injury, and patients with pain resulting from disabling spasticity.

Clinical and electrophysiological expression of deafferentation pain alleviated by dorsal root entry zone lesions in rats.

Object: The aims of this study were to construct an animal model of deafferentation of the spinal cord by brachial plexus avulsion and to analyze the effects of subsequent dorsal root entry zone (DREZ) lesions in this model. To this end, the authors measured the clinical and electrophysiological effects of total deafferentation of the cervical dorsal horn in rats and evaluated the clinical efficacy of cervical DREZ lesioning.

Methods: Forty-three Sprague-Dawley rats were subjected to total deafferentation of the right cervical dorsal horn by performing a posterior rhizotomy from C-5 to T-1.

Shape discrimination deficits during reversible deactivation of area V4 in the macaque monkey.

The role of area V4 in the primate extrastriate cortex has received much attention in recent years. However, the deficit specificity following area V4 ablations has been difficult to determine due to the ablations including area V4 and additional adjacent areas, deficit attenuation and the numerous variations in the results of different research teams. In order to address these issues, we examined the role of area V4 during reversible deactivation of the lower visual field representation within this area while macaque monkeys performed simple pattern discriminations and their eye position was monitored.

Circuits for local and global signal integration in primary visual cortex.

Contrast-dependent changes in spatial summation and contextual modulation of primary visual cortex (V1) neuron responses to stimulation of their receptive field reveal long-distance integration of visual signals within V1, well beyond the classical receptive field (cRF) of single neurons. To identify the cortical circuits mediating these long-distance computations, we have used a combination of anatomical and physiological recording methods to determine the spatial scale and retinotopic logic of intra-areal V1 horizontal connections and inter-areal feedback connections to V1. We have then compared the spatial scales of these connectional systems to the spatial dimensions of the cRF, spatial summation field (SF), and modulatory surround field of macaque V1 neurons.

Cortical mapping of gamma oscillations in areas V1 and V4 of the macaque monkey.

To characterize the temporal and spatial parameters of gamma activity evoked by visual stimuli in areas V1 and V4 of the monkey cortex, we recorded the electrocorticogram (ECoG) with an implanted array of 28 and 31 subdural electrodes placed over the surface of the operculum in two anesthetized monkeys. This intermediate level of recordings should help to bridge the gap between multiunit and scalp recordings. Both averaged and single-trial responses to small flashed stimuli, for which we varied the retinotopic position, the luminance and the color, were analyzed in the time-frequency domain using a wavelet-based decomposition of the signal.

The role of feedback connections in shaping the responses of visual cortical neurons.

The results of a previous study [Hupé et al. (1998) Nature, 394: 784-787] led us to conclude that feedback connections are important for differentiating a figure from the background, particularly in the case of low salience stimuli. This conclusion was principally based on the observation in area V3 neurons that inactivating MT by cooling led to a severe weakening of the center response and of the center-surround interactions, and that these effects were particularly strong for low salience stimuli.

Integrated model of visual processing.

Cortical processing of visual information requires that information be exchanged between neurons coding for distant regions in the visual field. It is argued that feedback connections are the best candidates for such rapid long-distance interconnections. In the integrated model, information arriving in the cortex from the magnocellular layers of the lateral geniculate nucleus is first sent and processed in the parietal cortex that is very rapidly activated by a visual stimulus.

Feedforward and feedback connections between areas V1 and V2 of the monkey have similar rapid conduction velocities.

It is often assumed that the action of cortical feedback connections is slow and modulatory, whereas feedforward connections carry a rapid drive to their target neurons. Recent results from our laboratory showed a very rapid effect of feedback connections on the visual responses of neurons in lower order areas. We wanted to determine whether such a rapid action is mediated by fast conducting axons.

Response modulations by static texture surround in area V1 of the macaque monkey do not depend on feedback connections from V2.

We analyzed the extracellular responses of 70 V1 neurons (recorded in 3 anesthetized macaque monkeys) to a single oriented line segment (or bar) placed within the cell classical receptive field (RF), or center of the RF. These responses could be modulated when rings of bars were placed entirely outside, but around the RF (the "near" surround region), as described in previous studies. Suppression was the main effect.

Feedback connections act on the early part of the responses in monkey visual cortex.

We previously showed that feedback connections from MT play a role in figure/ground segmentation. Figure/ground coding has been described at the V1 level in the late part of the neuronal responses to visual stimuli, and it has been suggested that these late modulations depend on feedback connections. In the present work we tested whether it actually takes time for this information to be fed back to lower order areas.

Moving illusory contours activate primary visual cortex: an fMRI study.

Identifying the cortical areas activated by illusory contours provides valuable information on the mechanisms of object perception. We applied functional magnetic resonance imaging to identify the visual areas of the human brain involved in the perception of a moving Kanizsa-type illusory contour. Our results indicate that, in addition to other cortical regions, areas V5 and V1 are activated.

A new type of microelectrode for obtaining unitary recordings in the human spinal cord.

Object: In this paper the authors report on the conception and adjustment of a microelectrode used to obtain unitary recordings in the human spinal cord.

Method: To overcome the difficulties related to intraoperative pulsations of the spinal cord, the authors opted to use a floating microelectrode. Because the recordings are obtained most often from spontaneous activities, it is difficult, with a single microelectrode, to separate spikes from electrical artifacts that are related to the switching of devices.

Cross-correlation study of the temporal interactions between areas V1 and V2 of the macaque monkey.

Cross-correlation studies performed in cat visual cortex have shown that neurons in different cortical areas of the same hemisphere or in corresponding areas of opposite hemispheres tend to synchronize their activities. The presence of synchronization may be related to the parallel organization of the cat visual system, in which different cortical areas can be activated in parallel from the lateral geniculate nucleus. We wanted to determine whether interareal synchronization of firing can also be observed in the monkey, in which cortical areas are thought to be organized in a hierarchy spanning different levels.

Spatial and temporal parameters of cortical inactivation by GABA.

Inactivation by GABA is a powerful tool for studying the function of specific cortical regions. It is especially useful in electrophysiology, because inactivation is reversible within short time periods, and because the extent of the inactivated region can be accurately controlled. Iontophoresis of GABA inactivates neurons up to 300 microm around the micropipette.

Cortical feedback improves discrimination between figure and background by V1, V2 and V3 neurons.

A single visual stimulus activates neurons in many different cortical areas. A major challenge in cortical physiology is to understand how the neural activity in these numerous active zones leads to a unified percept of the visual scene. The anatomical basis for these interactions is the dense network of connections that link the visual areas.

Axons, but not cell bodies, are activated by electrical stimulation in cortical gray matter. II. Evidence from selective inactivation of cell bodies and axon initial segments.

The results presented in the companion paper showed that extracellular electrical stimulation of the gray matter directly activates axons, but not cell bodies. The second set of experiments presented here was designed to separate the contribution of the axon initial segments and cell bodies from that of the axonal branches to the pool of presynaptic neuronal elements activated by electrical stimulation. For that purpose, N-methyl-D-aspartate (NMDA) iontophoresis was used to induce a selective inactivation of the cell body and of the adjoining portion of the axon by depolarization block, without affecting axonal branches that lack NMDA receptors.

Axons, but not cell bodies, are activated by electrical stimulation in cortical gray matter. I. Evidence from chronaxie measurements.

Extracellular electrical stimulation of the gray matter is often used to determine the function of a given cortical area or pathway. However, when it is used to elicit postsynaptic effects, the presynaptic neuronal elements activated by electrical stimulation have never been clearly identified: it could be the excitable dendrites, the cell body, the axon initial segment, or the axonal branches. To identify these elements, we performed two series of experiments on slices of rat visual cortex maintained in vitro.

Corticocortical connections between visual areas 17 and 18a of the rat studied in vitro: spatial and temporal organisation of functional synaptic responses.

Much is known about the anatomy of corticocortical connections, yet little is known concerning their physiology. In order to have access to the synaptic and temporal aspects of the activity elicited through corticocortical connections, we developed an in vitro approach on slices of rat visual cortex. We used extracellular recordings of field potentials combined with electrical stimulation to localise regions of areas 17 and 18a that are connected.

Reversible deactivation of cerebral network components.

Reversible deactivation techniques have shown that the cerebral network: (1) is dynamic, its functions depending on contemporaneous processing elsewhere in the network; (2) is composed of single nodes that contribute to several behaviors; (3) possesses an inherent plasticity that tends to minimize lesion-induced deficits; and (4) comprises feedforward and lateral connections that contribute in different ways to network operations. The next major advances in understanding network operations will probably be made by applying a combination of behavioral, neuron-recording and deactivation techniques. The greatest near-term gains are likely to be made in understanding the contributions that feedback projections make to cerebral network function.