Daniel E. Shulz

Spike Timing-Dependent Synaptic Depression in the In Vivo Barrel Cortex of the Rat

Spike timing-dependent plasticity (STDP) is a computationally powerful form of plasticity in which synapses are strengthened or weakened according to the temporal order and precise millisecond-scale delay between presynaptic and postsynaptic spiking activity. STDP is readily observed in vitro, but evidence for STDP in vivo is scarce. Here, we studied spike timing-dependent synaptic depression in single putative pyramidal neurons of the rat primary somatosensory cortex (S1) in vivo, using two techniques. First, we recorded extracellularly from layer 2/3 (L2/3) and L5 neurons, and paired spontaneous action potentials (postsynaptic spikes) with subsequent subthreshold deflection of one whisker (to drive presynaptic afferents to the recorded neuron) to produce “post-leading-pre” spike pairings at known delays. Short delay pairings (<17 ms) resulted in a significant decrease of the extracellular spiking response specific to the paired whisker, consistent with spike timing-dependent synaptic depression. Second, in whole-cell recordings from neurons in L2/3, we paired postsynaptic spikes elicited by direct-current injection with subthreshold whisker deflection to drive presynaptic afferents to the recorded neuron at precise temporal delays. Post-leading-pre pairing (<33 ms delay) decreased the slope and amplitude of the PSP evoked by the paired whisker, whereas “pre-leading-post” delays failed to produce depression, and sometimes produced potentiation of whisker-evoked PSPs. These results demonstrate that spike timing-dependent synaptic depression occurs in S1 in vivo, and is therefore a plausible plasticity mechanism in the sensory cortex.

Synaptic origin and stimulus dependency of neuronal oscillatory activity in the primary visual cortex of the cat.

1. We have studied the oscillatory activity of single neurons (91 recorded extracellularly and 76 intracellularly) in the primary visual cortex of cats and kittens to characterize its origins and its stimulus dependency. A new method for the detection of oscillations was developed in order to maximize the range of detectable frequencies in both types of recordings. Three types of activity were examined: spontaneous background activity, responses to intracellular current steps and visual responses. 2. During spontaneous activity, persistent oscillatory activity was very rare in both types of recordings. However, when intracellular records were made using KCl‐filled micropipettes, spontaneous activity appeared rhythmic and contained repeated depolarizing events at a variety of frequencies, suggestive of tonic periodic inhibitory input normally masked at resting potential. 3. Patterns of firing activity in response to intracellular current steps allowed us to classify neurons as regular spiking, intrinsically bursting, and fast‐spiking types, as described in vitro. In the case of rhythmically firing cells, the spike frequency increased with the amount of injected current. Subthreshold current‐induced oscillations were rarely observed (2 out of 76 cells). 4. Visual stimulation elicited oscillations in one‐third of the neurons (55 out of 167), predominantly in the 7‐20 Hz frequency range in 93% of the cases. Rhythmicity was observed in both simple and complex cells, and appeared to be more prominent at 5 and 6 weeks of age. 5. Intracellular recordings in bridge mode and voltage clamp revealed that visually evoked oscillations were driven by synaptic activity and did not depend primarily on the intrinsic properties of recorded neurons. Hyperpolarizing the membrane led to an increase in the size of the rhythmic depolarizing events without a change in frequency. In voltage‐clamped cells, current responses showed large oscillations at the same frequency as in bridge mode, independently of the actual value of the holding potential. 6. In fourteen intracellularly recorded neurons, oscillations consisted of excitatory events that could be superimposed on a depolarizing or a hyperpolarizing slow wave. In two other neurons, visual responses consisted of excitatory and inhibitory events, alternating with a constant phase shift. 7. Drifting bars were much more efficient in evoking oscillatory responses than flashed bars. Except in three cells, the frequency of the oscillation did not depend on the physical characteristics of the stimulus that were tested (contrast, orientation, direction, ocularity and position in the receptive field). No significant correlation was found between the intensity of the visual response and the strength of the rhythmic component. 8. Although it cannot be excluded that the dominant frequency of oscillations might be related to the type of anaesthetics used, no correlation was found between local EEG and the oscillatory activity elicited by visual stimulation. 9. We conclude that the oscillations observed in the present work are generated by synaptic activity. It is likely that they represent an important mode of transmission in sensory processing, resulting from periodic packets of synchronized activity propagated across recurrent circuits. Their relevance to perceptual binding is further discussed.

Activity-Dependent Regulation of Receptive Field Properties of Cat Area 17 by Supervised Hebbian Learning

Most algorithms currently used to model synaptic plasticity in self-organizing cortical networks suppose that the change in synaptic efficacy is governed by the same structuring factor, i.e., the temporal correlation of activity between pre- and postsynaptic neurons. Functional predictions generated by such algorithms have been tested electrophysiologically in the visual cortex of anesthetized and paralyzed cats. Supervised learning procedures were applied at the cellular level to change receptive field (RF) properties during the time of recording of an individual functionally identified cell. The protocols were devised as cellular analogs of the plasticity of RF properties, which is normally expressed during a critical period of postnatal development. We summarize here evidence demonstrating that changes in covariance between afferent input and postsynaptic response imposed during extracellular and intracellular conditioning can acutely induce selective long-lasting up- and down-regulations of visual responses. The functional properties that could be modified in 40% of cells submitted to differential pairing protocols include ocular dominance, orientation selectivity and orientation preference, interocular orientation disparity, and the relative dominance of ON and OFF responses. Since changes in RF properties can be induced in the adult as well, our findings also suggest that similar activity-dependent processes may occur during development and during active phases of learning under the supervision of behavioral attention or contextual signals. Such potential for plasticity in primary visual cortical neurons suggests the existence of a hidden connectivity expressing a wider functional competence than the one revealed at the spiking level. In particular, in the spatial domain the sensory synaptic integration field is larger than the classical discharge field. It can be shaped by supervised learning and its subthreshold extent can be unmasked by the pharmacological blockade of intracortical inhibition.

A multi-electrode array for combined microiontophoresis and multiple single-unit recordings.

A remotely controlled multi-electrode array, equipped with a combined electrode (CE) and 3 regular tungsten-in-glass electrodes (TEs) is described. The CE enables ejection of different neuroactive substances from 6 barrels and recording of single-unit activity from the etched tungsten rod placed in the central glass capillary. The CE is prepared with standard tungsten rod, glass-capillaries, and regular micropipette pullers. Such CEs possess a good stiffness-flexibility balance, length, easy cell isolation, high stability of recordings, effective ejection properties, and ability to survive penetration of dura. The efficiency of a 4-electrode array, including the CE, was tested by recording the effects of extracellularly ejected drugs (glutamate, acetylcholine and atropine) on single neurons in the auditory cortex of anesthetized guinea pigs. Induced modifications of single-neuron firing patterns and evoked responses were in agreement with the known effects of individual and combined applications of these drugs. Using this multi-electrode array and spike sorting techniques, the pharmacological environment of up to 12 simultaneously recorded cells can be modulated, and its effect on single neurons and on their interactions can be monitored at distances of up to 900 microns from the CEs tip.

Emergent properties of tactile scenes selectively activate barrel cortex neurons.

Rats discriminate objects by scanning their surface with the facial vibrissae, producing spatiotemporally complex sequences of tactile contacts. The way in which the somatosensory cortex responds to these complex multivibrissal stimuli has not been explored. It is unclear yet whether contextual information from across the entire whisker pad influences cortical responses. Here, we delivered tactile stimuli to the rat vibrissae using a new 24 whisker stimulator. We tested sequences of rostrocaudal whisker deflections that generate multivibrissal motion patterns in different directions across the mystacial pad, allowing to disambiguate local from global sensory integration. Unitary electrophysiological recordings from different layers of the barrel cortex showed that a majority of neurons has direction selectivity for the multivibrissal stimulus. The selectivity resulted from nonlinear integration of responses across the mystacial pad. Our results indicate that the system extracts collective properties of a tactile scene.

Activity-dependent regulation of ‘on’ and ‘off’ responses in cat visual cortical receptive fields

1 A supervised learning procedure was applied to individual cat area 17 neurons to test the possible role of neuronal co‐activity in controlling the plasticity of the spatial ‘on‐off’ organization of visual cortical receptive fields (RFs). 2 Differential pairing between visual input evoked in a fixed position of the RF and preset levels of postsynaptic firing (imposed iontophoretically) were used alternately to boost the ‘on’ (or ‘off’) response to a ‘high’ level of firing (S+ pairing), and to reduce the opponent response (respectively ‘off’ or ‘on’) in the same position to a ‘low’ level (S− pairing). This associative procedure was repeated 50‐100 times at a low temporal frequency (0.1‐0.15 s−1). 3 Long‐lasting modifications of the ratio of ‘on‐off’ responses, measured in the paired position or integrated across the whole RF, were found in 44 % of the conditioned neurons (17/39), and in most cases this favoured the S+ paired characteristic. The amplitude change was on average half of that imposed during pairing. Comparable proportions of modified cells were obtained in ‘simple’ (13/27) and ‘complex’ (4/12) RFs, both in adult cats (4/11) and in kittens within the critical period (13/28). 4 The spatial selectivity of the pairing effects was studied by pseudorandomly stimulating both paired and spatially distinct unpaired positions within the RF. Most modifications were observed in the paired position (for 88 % of successful pairings). 5 In some cells (n= 13), a fixed delay pairing procedure was applied, in which the temporal phase of the onset of the current pulse was shifted by a few hundred milliseconds from the presentation or offset of the visual stimulus. Consecutive effects were observed in 4/13 cells, which retained the temporal pattern of activity imposed during pairing for 5‐40 min. They were expressed in the paired region only. 6 The demonstration of long‐lasting adaptive changes in the ratio of ‘on’ and ‘off’ responses, expressed in localized subregions of the RF, leads us to suggest that simple and complex RF organizations might be two stable functional states derived from a common connectivity scheme.

NORADRENERGIC MODULATION OF FUNCTIONAL SELECTIVITY IN THE CAT VISUAL CORTEX: AN IN VIVO EXTRACELLULAR AND INTRACELLULAR STUDY

In vitro intracellular studies have shown that norepinephrine modulates cellular excitability and synaptic transmission in the cortex. Based on these effects, norepinephrine has been proposed to enhance the signal-to-noise ratio and to improve functional selectivity by potentiating strong synaptic responses and reducing weak ones. Here we have studied the functional effects of iontophoretic applications of norepinephrine during in vivo extracellular and intracellular recordings from neurons of the primary visual cortex of kittens and adult cats. Analysis of extracellular data concentrated on norepinephrine-induced changes in spontaneous and evoked activities, in signal-to-noise ratio, and in orientation and direction selectivity. Analysis of the intracellular data concentrated on actions of norepinephrine on spike firing accommodation, which has been shown to be reduced by norepinephrine in vitro, and on synaptic responses. Application of norepinephrine resulted in a depression of both spontaneous and evoked spiking activity. However, no systematic change in signal-to-noise ratio was observed. The suppressive effect of norepinephrine was exerted with no significant sharpening of direction or orientation selectivity tuning. The overall reduction in visual activity by norepinephrine affected the orientation tuning curves in a way compatible with a divisive effect, that is a normalization or gain control with no change in tuning width. Norepinephrine applied during intracellular recordings reduced the visually evoked depolarizing potentials whereas no change in the responsiveness of the cell to current-induced depolarizations was observed. In conditions of optimal visual stimulation which produced large depolarizations of several hundreds of milliseconds and sustained repetitive firing comparable to that obtained by direct current injection, we were unable to observe a facilitation of the evoked responses by norepinephrine as it would be expected from the well-documented increase in excitability induced by norepinephrine in vitro. In conclusion, from these results we suggest that norepinephrine released in the primary visual cortex primarily reduces the level of cortical activation by afferent signals, without affecting the cortical functional selectivity nor increasing the signal-to-noise ratio.

Possible involvement of neuromodulatory systems in cortical Hebbian-like plasticity

Plasticity of neuronal covariances (functional plasticity) is controlled by behavior (Ahissar et al (1992) Science 257, 1412-1415). Whether this behavioral control involves neuromodulatory systems was tested by examining the effect of acetylcholine (ACh) and noradrenaline (NE) on functional plasticity in anesthetized animals and by comparing the effects of these neuromodulators in an anesthetized preparation to that of behavior in awake animals. Local ionotophoretic applications of these drugs during manipulations of activity covariance in guinea pig auditory cortex did not mimic the behavioral control of functional plasticity that was previously observed in awake monkeys. Thus, the hypotheses according to which these neuromodulators control functional plasticity independent of their concentration and time of release were not supported by our data. The significant plasticity induced nevertheless, by some of the conditionings in the presence of ACh and NE, suggests that factors, other than those that were experimentally controlled, could regulate this plasticity. These factors could be among others the timing of drug(s) applications relative to the conditioning time, the local concentrations of the drug(s) and/or the site of application with respect to the relevant synapses.

Synaptic origin of rhythmic visually evoked activity in kitten area 17 neurones

Rhythmic patterns in neuronal activity in response to moving stimuli were observed in 28% of cells recorded extracellularly or intracellularly in area 17 of 4-16 week old anaesthetized and paralysed kittens. In both recording modes, oscillation frequencies ranged between 7 and 71 Hz, and were confined for 88% of cells in the 7-20 Hz band of the spectrum. A comparative study of firing autocorrelograms) and subthreshold activity (autocorrelation functions) indicates that the regularity of discharge stemmed from visually evoked oscillations of membrane potential at the same frequency. These oscillations are shown to result from extrinsic excitatory activity, since their amplitude, but not their frequency, depends on the resting membrane potential. The dependency on stimulus configuration supports the hypothesis that oscillations in neuronal output are dictated by periodic activity in afferent circuits selectively recruited by different attributes of the visual input which are not exclusively processed at the cortical level.

Correlated input reveals coexisting coding schemes in a sensory cortex

As in other sensory modalities, one function of the somatosensory system is to detect coherence and contrast in the environment. To investigate the neural bases of these computations, we applied different spatiotemporal patterns of stimuli to rat whiskers while recording multiple neurons in the barrel cortex. Model-based analysis of the responses revealed different coding schemes according to the level of input correlation. With uncorrelated stimuli on 24 whiskers, we identified two distinct functional categories of neurons, analogous in the temporal domain to simple and complex cells of the primary visual cortex. With correlated stimuli, however, a complementary coding scheme emerged: two distinct cell populations, similar to reinforcing and antagonist neurons described in the higher visual area MT, responded specifically to correlations. We suggest that similar context-dependent coexisting coding strategies may be present in other sensory systems to adapt sensory integration to specific stimulus statistics.