Comparison of recordings from microelectrode arrays and single electrodes in the visual cortex.
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Action Potentials↗
Biomedical subjects
Adam Kohn
Publications and source records attributed to Adam Kohn.
Measuring spike pattern reliability with the Lempel-Ziv-distance.
Spike train distance measures serve two purposes: to measure neuronal firing reliability, and to provide a metric with which spike trains can be classified. We introduce a novel spike train distance based on the Lempel-Ziv complexity that does not require the choice of arbitrary analysis parameters, is easy to implement, and computationally cheap. We determine firing reliability in vivo by calculating the deviation of the mean distance of spike trains obtained from multiple presentations of an identical stimulus from a Poisson reference. Using both the Lempel-Ziv-distance (LZ-distance) and a distance focussing on coincident firing, the pattern and timing reliability of neuronal firing is determined for spike data obtained along the visual information processing pathway of macaque monkey (LGN, simple and complex cells of V1, and area MT). In combination with the sequential superparamagnetic clustering algorithm, we show that the LZ-distance groups together spike trains with similar but not necessarily synchronized firing patterns. For both applications, we show how the LZ-distance gives additional insights, as it adds a new perspective on the problem of firing reliability determination and allows neuron classifications in cases, where other distance measures fail.
Algorithms↗
Stimulus dependence of neuronal correlation in primary visual cortex of the macaque.
Nearby cortical neurons often have correlated trial-to-trial response variability, and a significant fraction of their spikes occur synchronously. These two forms of correlation are both believed to arise from common synaptic input, but the origin of this input is unclear. We investigated the source of correlated responsivity by recording from pairs of single neurons in primary visual cortex of anesthetized macaque monkeys and comparing correlated variability and synchrony for spontaneous activity and activity evoked by stimuli of different orientations and contrasts. These two stimulus manipulations would be expected to have different effects on the cortical pool providing input to the recorded pair: changing stimulus orientation should recruit different populations of cells, whereas changing stimulus contrast affects primarily the relative strength of sensory drive and ongoing cortical activity. Consistent with this predicted difference, we found that correlation was affected by these stimulus manipulations in different ways. Synchrony was significantly stronger for orientations that drove both neurons well than for those that did not, but correlation on longer time scales was orientation independent. Reducing stimulus contrast resulted in a decrease in the temporal precision of synchronous firing and an enhancement of correlated response variability on longer time scales. Our results thus suggest that correlated responsivity arises from mechanisms operating at two distinct timescales: one that is orientation tuned and that determines the strength of temporally precise synchrony, and a second that is contrast sensitive, of low temporal frequency, and present in ongoing cortical activity.
Action Potentials↗
Adaptation changes the direction tuning of macaque MT neurons.
Prolonged exposure to a stimulus, called 'adaptation', reduces cortical responsiveness. Adaptation has been studied extensively in primary visual cortex (V1), where responsivity is usually reduced most when the adapting and test stimuli are well matched. Theories about the functional benefits of adaptation have relied on this specificity, but the resultant changes in neuronal tuning are of the wrong type to account for well-documented perceptual aftereffects. Here we have used moving sinusoidal gratings to study the effect of adaptation on the direction tuning of neurons in area MT in macaques. Responsivity in MT is maintained best in the adapted direction and is strongly reduced for nearby directions. Consequently, adaptation in the preferred direction reduces the direction-tuning bandwidth, whereas adaptation at near-preferred directions causes tuning to shift toward the adapted direction. This previously unknown effect of adaptation is consistent with perceptual aftereffects and indicates that different cortical regions may adjust to constant sensory input in distinct ways.
Action Potentials↗
Neuronal adaptation to visual motion in area MT of the macaque.
The responsivity of primary sensory cortical neurons is reduced following prolonged adaptation, but such adaptation has been little studied in higher sensory areas. Adaptation to visual motion has strong perceptual effects, so we studied the effect of prolonged stimulation on neuronal responsivity in the macaque's area MT, a cortical area whose importance to visual motion perception is well established. We adapted MT neurons with sinusoidal gratings drifting in the preferred or null direction. Preferred adaptation reduced the responsiveness of MT cells, primarily by changing their contrast gain, and this effect was spatially specific within the receptive field. Null adaptation reduced the ability of null gratings to inhibit the response to a simultaneously presented preferred stimulus. While both preferred and null adaptation alter MT responses, these effects probably do not occur in MT neurons but are likely to reflect adaptation-induced changes in contrast gain earlier in the visual pathway.
Adaptation, Physiological↗
Sensory cortical dynamics.
Sensory cortical networks are commonly regarded as stable, changing only in the face of prolonged alteration of sensory input. There is increasing evidence, however, that the functional connectivity of cortical networks changes significantly, but reversibly, in response to conditions of sensory stimulation similar to those encountered in everyday life. In this review, we provide examples of sensory cortical dynamics at the single neuron and neural population levels. The dynamics detected at both levels of experimental observation suggest that a brief exposure (tens of milliseconds to tens of seconds) to sensory stimulation is accompanied by changes in the capacity of cortical networks to process and represent environmental stimuli. Candidate cellular mechanisms and the potential benefits of such stimulus-driven, rapid, and fully reversible sensory cortical dynamics are discussed.
Animals↗
Stimulus-evoked modulation of sensorimotor pyramidal neuron EPSPs.
Sensory cortical neurons display substantial receptive field dynamics during and after persistent sensory drive. Because a cell's response properties are determined by the inputs it receives, receptive field dynamics are likely to involve changes in the relative efficacy of different inputs to the cell. To test this hypothesis, we have investigated if brief repetitive stimulus drive in vitro alters the efficacy of two types of corticocortical inputs to layer V pyramidal cells. Specifically, we have used whole cell recordings to measure the effect of repetitive electrical stimulation at the layer VI/white matter (WM) border on the synaptic response of layer V pyramidal cells to corticocortical input evoked by electrical stimulation of layer I or layer II/III and emulated by local application of glutamate. Repetitive stimulation (10 Hz for 3 s) at the layer VI/WM border transiently potentiated excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of layer II/III by 97 +/- 12% (mean +/- SE). The recovery of EPSP amplitude to its preconditioning value was well-described by a single-term decaying exponential with a time constant of 7.2 s. The same layer VI/WM conditioning train that evoked layer II/III EPSP potentiation frequently caused an attenuation of layer I EPSPs. Similarly, subthreshold postsynaptic responses to local glutamate application in layers II/III and I were potentiated and attenuated, respectively, by the conditioning stimulus. Potentiation and attenuation could be evoked in the same cell by repositioning the glutamate puffer pipette in the appropriate layer. The conditioning stimulus that led to the transient modification of upper layer EPSP efficacy also evoked a slow depolarization in glial cells. The membrane potential of glial cells recovered with a time course similar to the dissipation of the potentiation effect, suggesting that stimulus-evoked changes in extracellular potassium (ECK) play a role in layer II/III EPSP potentiation. Consistent with this proposal, increasing the bath concentration of ECK caused a substantial increase of layer II/III EPSP amplitude. EPSP potentiation was sensitive to postsynaptic membrane potential and, more importantly, was significantly weaker for synaptic currents than for synaptic potentials, suggesting that it involves the recruitment of a postsynaptic voltage-dependent mechanism. Two observations suggest that layer II/III EPSP potentiation may involve the recruitment of postsynaptic sodium channels: EPSP potentiation was strongly reduced by intracellular application of N-(2,6-dimethyl-phenylcarbamoylmethyl) triethylammonium bromide (QX-314) and responses to local glutamate application were potentiated by high ECK in the presence of cadmium but not in the presence of tetrodotoxin. The results demonstrate a novel way in which brief periods of repetitive stimulus drive are accompanied by rapid, transient, and specific alterations in the functional connectivity and information processing characteristics of sensorimotor cortex.
Animals↗