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C X Falk

Publications and source records attributed to C X Falk.

12 recordsLinked to original sources
PubMed2000

Imaging membrane potential with voltage-sensitive dyes.

Membrane potential can be measured optically using a variety of molecular probes. These measurements can be useful in studying function at the level of an individual cell, for determining how groups of neurons generate a behavior, and for studying the correlated behavior of populations of neurons. Examples of the three kinds of measurements are presented. The signals obtained from these measurements are generally small. Methodological considerations necessary to optimize the resulting signal-to-noise ratio are discussed.

Animals↗
PubMed2000

Preliminary results of a new method for locating auricular acupuncture points.

Auricular acupuncture is widely used for the treatment of cocaine addiction, and there is an urgent need to conduct controlled clinical research of this intervention. One impediment to this endeavor is the lack of an objective and reliable method for identifying the hypothesized active and control points. In order to address this issue, we conducted two studies employing a constant current electrical device and a novel probing technique. In the first study, we assessed the reliability of our technique for measuring electrical skin resistance points (acupuncture or non-acupuncture) on the body and auricles. In the second study, we analyzed and compared the measurements of skin resistance of auricular acupuncture and control zones in a group of cocaine abusing patients. Findings suggest that our measurement method produced reliable measurements, and that active acupuncture zones revealed a significantly different pattern of electrical skin resistance readings compared to control zones. This method may be useful for locating active and control points in controlled clinical trials of auricular acupuncture.

Acupuncture Points↗
PubMed1998

Voltage-sensitive dyes for monitoring multineuronal activity in the intact central nervous system.

Optical monitoring of activity provides new kinds of information about brain function. Two examples are discussed in this article. First, the spike activity of many individual neurons in small ganglia can be determined. Second, the spatiotemporal characteristics of coherent activity in the brain can be directly measured. This article discusses both general characteristics of optical measurements (sources of noise) as well as more methodological aspects related to voltage-sensitive dye measurements from the nervous system.

Action Potentials↗
PubMed1996

Effect of feedback from peripheral movements on neuron activity in the aplysia abdominal ganglion.

We have made reasonably comprehensive measurements of action potential activity in the Aplysia californica abdominal ganglion to determine the amount of feedback the central nervous system (CNS) receives from a movement which it initiates. Voltage-sensitive dye measurements of action potential activity of cells in the ganglion were made during the gill-withdrawal reflex elicited by siphon stimulation. We compared recordings in two situations which differed dramatically in the amount the gill moved. In the control sea water, the gill withdrawal was normal; in low-Ca2+, high-Mg2+ sea water, the gill movement was blocked. Both the timing and the number of spikes of the individual neurons were similar in the two situations. Histograms of the summed spike activity versus time and histograms of the number of active neurons versus time in the two conditions were also similar. Finally, two numerical measures of trial-to-trial differences, a paired t-test and a measure we named fractional similarity, did not indicate larger differences between two trials in the different sea waters than two trials in the same sea water. Feedback from sensory neurons activated by the gill movement itself does not make a large contribution to the spike activity in the abdominal ganglion. Apparently the Aplysia CNS issues the command for the withdrawal and does not make adjustments for the magnitude of the actual withdrawal. It may not even receive the information necessary for such adjustments to be made. A second motivation for these experiments was to test whether removing the feedback might simplify the neuronal activity that occurs during the gill-withdrawal reflex. This did not occur.

Abdomen↗
PubMed1994

Neuronal activity during different behaviors in Aplysia: a distributed organization?

The active neuronal populations in the Aplysia abdominal ganglion during spontaneous and evoked behaviors were compared with the use of multineuronal optical measurements. In some preparations, more than 90 percent of the neurons activated during the reflex withdrawal of the gill also were activated during respiratory pumping and during small spontaneous gill contractions. Although the same neurons made action potentials in all three behaviors, the activity patterns were different. There was a substantial interaction between the neural substrates underlying evoked and spontaneous behaviors when they were made to occur together. If a gill withdrawal reflex was elicited a few seconds after a respiratory pumping episode, the evoked neuronal activity in most neurons was clearly altered. These results suggest that a distributed organization involving a large number of neurons may be responsible for generating the two behaviors. Different behaviors appear to be generated by altered activities of a single, large distributed network rather than by small dedicated circuits.

Action Potentials↗
PubMed1994

Distributed aspects of the response to siphon touch in Aplysia: spread of stimulus information and cross-correlation analysis.

We examined two aspects of the response to siphon stimulation in an attempt to test the hypothesis that the Aplysia CNS functions as a distributed system. First, we estimated the number of central neurons that respond to a light touch to the siphon skin. We made voltage-sensitive dye recordings from the abdominal, pleural, pedal, and cerebral ganglia. From these recordings we estimated that 220 abdominal neurons, 110 pleural neurons, and 650 pedal neurons were affected by the light touch. Thus, the information about this mild and localized stimulus is very widely distributed within the Aplysia CNS. This result allows the possibility that the Aplysia CNS functions as a distributed system. If only a small number of neurons had responded to the touch, it would have supported the conclusion that the gill-withdrawal reflex could be generated by a small, dedicated circuit. Second, we searched for correlations between the spike times of the individual abdominal ganglion neurons. Two time scales were examined: a millisecond time scale corresponding to the duration of a fast synaptic potential and a seconds time scale corresponding to the duration of the gill-withdrawal movement. Neuron pairs with highly correlated spike activity on a millisecond time scale must be connected by (or have a common input that uses) relatively powerful, fast, excitatory synapses. We expected that this kind of synaptic interaction would be relatively rare in nervous systems that functioned in a distributed manner. Indeed, only 0.3% of the neuron pairs had correlation coefficients of 0.15 or greater. These correlations accounted for approximately 2% of the action potentials generated in response to siphon stimulation. Thus, large, fast excitatory synaptic interactions appear to be relatively unimportant. This result is consistent with the hypothesis that the abdominal ganglion functions as a distributed system. When the longer time scale was used for the cross-correlograms, a large fraction of the cell pairs had correlated activity because many neurons are activated by the stimulus. It was not possible to interpret the slow correlations in terms of actual synaptic interactions between individual neurons. Our results are consistent with the possibility that the abdominal ganglion functions in a distributed manner. However, this evaluation is indirect and thus only tentative conclusions can be drawn. Evidence from several sources suggests that the neuronal interactions for generating the Aplysia gill-withdrawal reflex are complex.

Animals↗
PubMed1994

Consistency in nervous systems: trial-to-trial and animal-to-animal variations in the responses to repeated applications of a sensory stimulus in Aplysia.

What is the internal noise in a nervous system? We studied this question by determining the trial-to-trial consistency of the neuronal response in the abdominal ganglion of Aplysia californica. Because our voltage-sensitive dye recordings detected the spike activity from a large fraction of the neurons in the ganglion, these results provide a reasonably complete characterization of the consistency of the response to a sensory stimulus. The consistency of each neuron was evaluated by the number and timing of spikes in the response. The variability in the spike count was described using the coefficient of variation. The spike count variations follow a Poisson distribution, indicating that most of these variations were the result of a random process. For each neuron the reliability of the response to touch was measured in two ways; both measures indicated a broad distribution of reliabilities within the neuron population. The time of the maximum response also varied substantially in some animals. These timing variations were in part due to random processes and in part due to systematic effects (changes in activity of many neurons that were highly correlated). The time course of the activity of individual neurons was compared with the time course of the gill withdrawal. In some animals the activity of individual neurons was only poorly correlated with the behavior; in contrast, the summed activity of groups of neurons matched the behavior quite well. This implies that the behavioral output of the system may be a distributed combination of the activity of many neurons. The differences between animals were substantially larger than the trial-to-trial differences in one animal. The responses made by different preparations differed along many dimensions.

Action Potentials↗
PubMed1993

Optical measurement of action potential activity in invertebrate ganglia.

With a voltage-sensitive dye and an array of silicon photodiodes we monitored the action potential activity of a large fraction of the cells in the Aplysia abdominal ganglion during the gill-withdrawal reflex. This capability allowed a direct determination of the spread of sensory information within the 1,000 neurons of the ganglion. Surprisingly, approximately 30% of the neurons are activated by a light touch to a small area of siphon skin. It is likely that many other neurons also receive large synaptic potentials, either excitatory or inhibitory, and thus even a very mild and restricted stimulus will have widely distributed effects. It seems to us that these results will force a more pessimistic view of the present understanding of the neuronal basis of apparently simple behaviors.

Action Potentials↗
PubMed1993

Nonuniform expression of habituation in the activity of distinct classes of neurons in the Aplysia abdominal ganglion.

Global observations of neuronal response in the Aplysia abdominal ganglion were made during habituation of the gill withdrawal reflex using voltage-sensitive dye recording. This technique makes it possible to measure the spike activity of 30-50% of the 1000 neurons present in the ganglion. Our experiments address the issue of how habituation is expressed in the activity of the population of neurons responding to siphon stimulation. Several classes of neurons exhibited characteristically distinct responses to the stimuli and to habituation training. One class of neurons (group I) responded to the onset and offset of the sensory stimulus although they are probably not primary sensory neurons. They habituate only partially when the behavioral reflex has already habituated completely. Two other classes (groups II and III) both have sustained responses to the touch, but habituate differently. Members of group III habituate completely while those in group II habituate only partially. Another class of neurons are inhibited by the stimulus (group IV). They become less inhibited after habituation. The response of both group I and group IV are new classes of response that have not been previously reported.

Action Potentials↗
PubMed1989

Hundreds of neurons in the Aplysia abdominal ganglion are active during the gill-withdrawal reflex.

A combination of optical and electrode recording methods was used to obtain an overview of the neuron activity in the Aplysia abdominal ganglion in response to a light touch to the siphon skin. Spike activity was detected in up to 150 different neurons. Habituation and sensitization of the gill-withdrawal reflex was accompanied by large changes in the number of activated neurons. It is likely that these recordings are incomplete; the actual number of activated neurons is estimated to be about 300 in the acutely sensitized preparation. While we presume that not all 300 of these neurons are involved in the gill-withdrawal reflex, the number of neurons is so large that it may be difficult to determine the role of each activated neuron with presently available experimental tools.

Abdomen↗
PubMedDate not supplied

Methodological investigations for a multisite trial of auricular acupuncture for cocaine addiction: a study of active and control auricular zones.

We evaluated objective criteria for defining points for needle insertion prior to conducting a multisite clinical trial of auricular acupuncture for cocaine addiction. Thirty-four cocaine-abusing subjects participated in a study in which the trial's active zones (Shenmen, Liver, Lung, and Sympathetic) and control zones (located on the ear helix) were divided into quadrants and assessed along four dimensions: electrical resistance, skin discoloration, skin topography, and tenderness. Acute effects of needles inserted into points of low electrical resistance in one ear and high electrical resistance in the other were also assessed. Results showed that the active zones had lower overall electrical resistance and more subcutaneous ridges than control zones. Zones did not possess significant variability along any single dimension. Acute effects of needling high and low resistance points were similar, differing only for "fullness." Based on these findings, and in view of the difficulty of accurately measuring electrical resistance at ear points, we do not recommend the use of electrical devices for point determination in the multisite study. At present, there seems to be little scientific basis for the preselection of specific points for needle insertion within auricular zones. Needle placement should be based upon clinical judgement.

Acupuncture Points↗