Feedback connections and conscious vision.
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Journal Article↗
Biomedical subjects
J Bullier
Publications and source records attributed to J Bullier.
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.
Animals↗
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. Results from this first-pass computation are then sent back by feedback connections to areas V1 and V2 that act as 'active black-boards' for the rest of the visual cortical areas: information retroinjected from the parietal cortex is used to guide further processing of parvocellular and koniocellular information in the inferotemporal cortex.
Animals↗
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.
Animals↗
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.
Anesthesia↗
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).
Action Potentials↗
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. Activity in area V1 was particularly prominent.
Adult↗
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.
Algorithms↗
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. Cross-correlation histograms (CCHs) were calculated from pairs of single or pairs of multiunit activities simultaneously recorded in areas V1 and V2 of paralyzed and anesthetized macaque monkeys. Moving bars and flashed bars were used as stimuli. The shift predictor was calculated and subtracted from the raw CCH to reveal interactions of neuronal origin in isolation. Significant CCH peaks, indicating interactions of neuronal origin, were obtained in 11% of the dual single-unit recordings and 46% of the dual multiunit recordings with moving bars. The incidence of nonflat CCHs with flashed bars was 29 and 78%, respectively. For the pairs of recording sites where both flashed and moving stimuli were used, the incidences of significant CCHs were very similar. Three types of peaks were distinguished on the basis of their width at half-height: T (<16 ms), C (between 16 and 180 ms), and H peaks (>180 ms). T peaks were very rarely observed (<1% in single-unit recordings). H peaks were observed in 7-16% of the single-unit CCHs, and C peaks in 6-16%, depending on the stimulus used. C and H peaks were observed more often when the receptive fields were overlapping or distant by <2 degrees. To test for the presence of synchronization between neurons in areas V1 and V2, we measured the position of the CCH peak with respect to the origin of the time axis of the CCH. Only in the case of a few T peaks did we find displaced peaks, indicating a possible drive of the V2 neuron by the simultaneously recorded V1 cell. All the other peaks were either centered on the origin or overlapped the origin of time with their upper halves. Thus similarly to what has been reported for the cat, neurons belonging to different cortical areas in the monkey tend to synchronize the time of emission of their action potentials with three different levels of temporal precision. For peaks calculated from flashed stimuli, we compared the peak position with the difference between latencies of V1 and V2 neurons. There was a clear correlation for single-unit pairs in the case of C peaks. Thus the position of a C peak on the time axis appears to reflect the order of visual activation of the correlated neurons. The coupling strength for H peaks was smaller during visual drive compared with spontaneous activity. On the contrary, C peaks were seen more often and were stronger during visual stimulation than during spontaneous activity. This suggests that C-type synchronization is associated with the processing of visual information. The origin of synchronized activity in a serially organized system is discussed.
Animals↗
Human spinal microelectrode recordings
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Journal Article↗
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.
Artifacts↗
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.
Animals↗
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. The first series of experiments, reported in this paper, was aimed at determining the chronaxie, a temporal parameter related to the membrane properties of the neuronal elements. In order to identify the presynaptic elements that were activated by extracellular electrical stimulation, chronaxies corresponding to postsynaptic responses were measured and compared with those corresponding to the activation of axons (antidromic activation) and those corresponding to the activation of cell bodies (intracellular current injection in intracellularly recorded neurons). The chronaxie for orthodromic activation was similar to that for axonal activation, but was 40 times smaller than the chronaxie for direct cell body activation. This suggests that, whenever a postsynaptic response is elicited after electrical stimulation of the cortical gray matter, axons (either axonal branches or axon initial segments), but not cell bodies, are the neuronal elements activated.
Animals↗
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. After NMDA iontophoresis, the neurons located near the iontophoresis electrode became unable to generate action potentials in an irreversible manner. When the NMDA-induced depolarization block was performed at the site of electrical stimulation, an unexpected increase in the amplitude of the orthodromic responses was observed. Several control experiments suggested that the field potential increase was due to changes of the local environment in the vicinity of the iontophoresis pipette, which led to an increased excitability of the axons. After the period of superexcitability, the orthodromic responses displayed an amplitude that was 15-20% lower than that observed before the NMDA-induced depolarization block, even though cell bodies and axon initial segment at the site of stimulation could not be activated by electrical stimulation. This result shows a low contribution for axon initial segments to the pool of neuronal elements activated by the electrical stimulation. Altogether, these experiments demonstrate that the postsynaptic responses obtained after electrical stimulation of the cortical gray matter result almost exclusively from the activation of axonal branches. Since the neocortex is organised as a network of local and long-range reciprocal connections, great attention must be paid to the interpretation of data obtained with electrical stimulation.
Animals↗
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. We found that corticocortical connections between areas 17 and 18a can be preserved in 500 microm thick slices, with a focus of activity separated from the stimulating electrode by 1.5 mm to more than 3 mm. The potentials elicited in one area after stimulation of its neighbour displayed fast events, corresponding to action potentials, and slow events, corresponding to synaptic potentials. Intracellular recordings showed that the earliest synaptic responses consisted of monosynaptic excitatory potentials. Measurement of response latency showed that axons involved in both feedforward and feedback corticocortical connections are slowly conducting (0.3-0.8 m/s). Conduction velocity for antidromically activated cells was not significantly different for the two sets of connections. In an attempt to establish the spatial organisation of functional synaptic inputs, field potential recordings were performed in the different cortical layers and used to establish current source density (CSD) graphs along the depth axis. The CSD maps obtained were found to be somewhat variable from one case to another. It is suggested that this variability results from the use of electrical stimulation, which activates axons that are both afferent and efferent to a given cortical area. The field potentials are therefore likely to contain responses that correspond to the activity mediated by the intrinsic collaterals mixed in variable amount with responses produced by corticocortical synapses. With this restriction in mind, it is suggested that, after stimulation of the supragranular layers, the functional synaptic inputs of feedforward connections are concentrated in layer 4 and the bottom of layer 3, while those of feedback axons involve mainly the upper part of the supragranular layers. The intrinsic collaterals of the neurones participating in corticocortical connections seem also to provide the bulk of their inputs to the upper part of the supragranular layers. The laminar pattern of activity obtained after infragranular layer stimulation was comparable to that obtained after supragranular layer stimulation, except for the addition of a supplementary region of activated synapses in the infragranular layers.
Animals↗
Functional streams in occipito-frontal connections in the monkey.
It is known that the prestriate cortical regions that project to area LIP in parietal cortex and to areas TEO and TE in temporal cortex are mostly separated. Two separate streams of information transfer from occipital cortex can this be distinguished. We wished to determine whether the parietal and temporal streams remain segregated in their projections to frontal cortex. Paired injections of retrograde fluorescent tracers were placed in parietal and temporal cortex, or in the lateral and medial parts of the frontal eye field (FEF). The cortical regions containing retrogradely labeled cells were reconstructed in two-dimensional maps. The results show that temporal cortex mainly projects to lateral FEF (area 45). Parietal cortex sends projections to medial FEF (area 8a) and to lateral FEF, as well as to area 46. Thus, the parietal and temporal streams converge in lateral FEF. Most of the occipital regions projecting to medial FEF are the same as those projecting to parietal cortex, whereas lateral FEF receives afferents from the same occipital regions as those sending projections to temporal cortex. Thus, one can distinguish two interconnected networks. One is associated with the inferotemporal cortex and includes areas of the ventral bank and fundus of the superior temporal sulcus (STS), lateral FEF and ventral prestriate cortex. This network emphasizes central vision, small accades and form recognition. The other network is linked to cortex of the intraparietal sulcus. It consists of areas of the upper bank and fundus of STS, medial FEF and dorsal prestriate cortex. These areas encode peripheral visual field and are active during large saccades.
Animals↗
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.
Animals↗
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.
Animals↗