The time windows of the sense of agency.

The sense of agency depends on some internal cues that derive from action control, as well as external cues like contextual information and prior information (degree of contingency between an action and is effect). We assessed whether external agency cues are combined with internal agency cues to affect the sense of agency. In two experiments participants performed a movement (button press) that elicited, after a varying delay, an effect (ball appearing on a screen), and reported their sense of agency over the effect (full, partial or no-agency) while internal cues (premotor information) and external cues (contextual and prior information) were manipulated. We assessed the effect of agency cues on the delays at which the sense of agency varied. The delays were increased with premotor signals but were decreased with contextual information. These findings favour a model of integration of internal and external agency cues over time.

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. In the present paper, we first show that the results extend to areas V1 and V2. In particular, the inhibitory center-surround interactions in areas V1, V2 and V3 disappear almost completely in the absence of feedback input from MT for low salience stimuli, whereas the effects are much more limited for stimuli of middle and high salience. We then compare the results obtained in studies of feedback connections from MT to those obtained in a study of the feedback action of area V2 onto V1 neurons [Hupé et al. (2001) J. Neurophysiol., 85: 146-163], in which the same effects were observed on the center mechanism (decrease in response), but no effects were seen on the center-surround interactions. We conclude that feedback connections act in a non-linear fashion to boost the gain of the center mechanism and that they combine with horizontal connections to generate the center-surround interactions.

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. Using electrical stimulation, we compared the conduction velocities along feedforward and feedback axons between areas V1 and V2 of the macaque monkey. We conclude that feedback and feedforward connections between V1 and V2 have comparable fast conduction velocities (around 3.5 m/s).

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. We analyzed the extracellular responses of 169 V1, V2, and V3 neurons that we recorded in two anesthetized macaque monkeys. MT was inactivated by cooling. We studied the time course of the responses of the neurons that were significantly affected by the inactivation of MT to see whether the effects were delayed relative to the onset of the response. We first measured the time course of the feedback influences from MT on V1, V2, and V3 neurons tested with moving stimuli. For the large majority of the 51 neurons for which the response decreased, the effect was present from the beginning of the response. In the responses averaged after normalization, the decrease of response was significant in the first 10-ms bin of response. A similar result was found for six neurons for which the response significantly increased when MT was inactivated. We then looked at the time course of the responses to flashed stimuli (95 neurons). We observed 15 significant decreases of response and 14 significant increases. In both populations, the effects were significant within the first 10 ms of response. For some neurons with increased responses we even observed a shorter latency when MT was inactivated. We measured the latency of the response to the flashed stimuli. We found that even the earliest responding neurons were affected early by the feedback from MT. This was true for the response to flashed and to moving stimuli. These results show that feedback connections are recruited very early for the treatment of visual information. It further indicates that the presence or absence of feedback effects cannot be deduced from the time course of the response modulations.

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. The response was enhanced for 12 neurons when orthogonal bars in the surround were presented instead of bars having the same orientation as the center bar. This orientation contrast property is possibly involved in the mediation of perceptual pop-out. The enhancement was delayed compared with the onset of the response by about 40 ms. We also observed a suppression originating specifically from the flanks of the surround. This "side-inhibition," significant for nine neurons, was delayed by about 20 ms. We tested whether these center/surround interactions in V1 depend on feedback connections from area V2. V2 was inactivated by GABA injections. We used devices made of six micropipettes to inactivate the convergent zone from V2 to V1. We could reliably inactivate a 2- to 4-mm-wide region of V2. Inactivation of V2 had no effect on the center/surround interactions of V1 neurons, even those that were delayed. Therefore the center/surround interactions of V1 neurons that might be involved in pop-out do not appear to depend on feedback connections from V2, at least in the anesthetized monkey. We conclude that these properties are probably shaped by long-range connections within V1 or depend on other feedback connections. The main effect of V2 inactivation was a decrease of the response to the single bar for about 10% of V1 neurons. The decrease was delayed by <20 ms after the response onset. Even the earliest neurons to respond could be affected by the feedback from V2. Together with the results on feedback connections from MT (previous paper), these findings show that feedback connections potentiate the responses to stimulation of the RF center and are recruited very early for the treatment of visual information.

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. Consequently, the authors designed a dual microelectrode made of two tungsten-in-glass-attached microelectrodes separated by 300 microm. Because the two electrodes cannot obtain recordings in the same neuron, it is possible to distinguish unambiguously spikes (recorded on one tip) from electrical artifacts (recorded simultaneously on the two tips). The dual microelectrode is 2 cm long, with a 20-microm tip length, and 800 to 1200-Ohms impedance. This microelectrode can be implanted "free hand," in the dorsal horn, by using a microsurgical forceps under a surgical microscope. The data analysis is performed off-line with spike sorter hardware. In the dorsal horns in 17 patients who were selected to undergo a dorsal root entry zone (DREZ) rhizotomy to treat various pathological conditions, unitary recordings were obtained using this double microelectrode. The authors recorded 57 neurons in good conditions of stability and isolation. CONCLUSIONS: The microelectrode described in this paper was successfully used to obtain recordings in neurons in more than 85% of the patients. This simplified, floating double microelectrode can therefore be considered for use in microsurgical DREZ rhizotomy to obtain unitary recordings in the human spinal dorsal horn.

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. Pressure injection of GABA inactivates neurons further away, but the spatial and temporal characteristics of inactivation by this method have been poorly studied. In order to address this question, we built devices made of micropipettes and microelectrodes glued at various distances. We experienced that repetition of small injections of 100 mM GABA inactivate cortex in a more homogenous way than bolus injections. Diffusion of GABA after pressure injection does not seem to follow a point spread diffusion model as in the case of iontophoresis: GABA probably goes up along the micropipette shaft, and the volume of inactivation has an ellipsoidal form. In order to precisely determine the extent of the inactivated region, we built a mathematical model to fit the experimental data of inactivations obtained above and below the pipette tip. The model provides estimates of the inactivated region for volumes smaller than 60 nl of GABA 100 mM. Limits of inactivation are between 250 and 500 microm lateral to the tip of the pipette. The geometry of inactivation is difficult to predict beyond 60 nl and it seems hazardous to try to inactivate neurons beyond 800 microm with pressure injections of GABA 100 mM.

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. Within this network, feedforward connections transmit signals from lower-order areas such as V1 or V2 to higher-order areas. In addition, there is a dense web of feedback connections which, despite their anatomical prominence, remain functionally mysterious. Here we show, using reversible inactivation of a higher-order area (monkey area V5/MT), that feedback connections serve to amplify and focus activity of neurons in lower-order areas, and that they are important in the differentiation of figure from ground, particularly in the case of stimuli of low visibility. More specifically, we show that feedback connections facilitate responses to objects moving within the classical receptive field; enhance suppression evoked by background stimuli in the surrounding region; and have the strongest effects for stimuli of low salience.

Functional interactions between areas V1 and V2 in the monkey.

The role of feedback connections from area V2 to V1 was studied by reversible inactivation. When V2 was inactivated the responses of some V1 neurons to stimulation of the surround region were increased while responses to center stimulation were unchanged or decreased. Latencies to small flashing stimuli were also compared in areas V1 and V2. The distributions in the two areas overlap largely, with a 10 ms shift between the two. Neurons of V1 and V2 that are driven by the magnocellular layers of the LGN are activated 20 ms earlier than neurons of the parvocellular stream.