Theoretical analysis of parameters leading to frequency modulation along an inhomogeneous axonParnas, I.; Hochstein, S.; Parnas, H.
doi: N/Apmid: 966045
Abstract 1. Theoretical computations were conducted on a computer model of a segmented, nonhomogeneous axon to understand the mechanism of frequency block of conduction. 2. The model is based on the Hodgkin-Huxley equations modified in several ways to better describe the cockroach axon. We used cockroach parameters where available. 3. The increase in fiber radius was spread over a series of segments to approximate a taper. We found that a taper allows a larger overall increase in fiber diameter than a single step to be successfully passed. 4. We studied effects on a train of impulses. The modified equations included effects due to changes in extracellular potassium concentration resulting from the repetitive firing of the axon. 5. An increase in diameter which allows a single spike to pass blocks the subsequent impulses in a train at the taper if potassium concentration variability is introduced. This could explain the low-pass filter characteristics of axon constrictions. 6. Results of the model fit well with the experiemental spike shape and height. Data were computed for the refractory period and its dependence on the taper parameters. Copyright © 1976 the American Physiological Society
Enhancement of visual responses in monkey striate cortex and frontal eye fieldsWurtz, R. H.; Mohler, C. W.
doi: N/Apmid: 823304
Abstract 1. We have studied the visual enhancement effect in two areas of the cerebral cortex of monkeys. The response of the cells to a visual stimulus was determined both when the monkey used the visual stimulus as the target for a saccadic eye movement and when he did not. 2. In striate cortex cells with nonoriented, simple, complex, and hypercomplex receptive-field types were studied. Clear enhancement of the response to the appropriate visual stimulus was seldom seen when the monkey used the stimulus as a target for a saccade. In addition, any enhancement effect seen was nonselective; it occurred whether the monkey made a saccade to the receptive-field stimulus or some other stimulus at a point distant from the receptive field. The enhancement also occurred whether the monkey made a saccade to the stimulus or just released the bar when the stimulus dimmed. 3. This nonselective enhancement in striate cortex is in striking contrast to the selective enhancement of the visual response seen in the superior colliculus. The different characteristics of the enhancement in striate cortex and the observation of enhancement in the colliculus following ablation of striate cortex suggest that this cortical area is an unlikely source of the collicular enhancement. 4. These observations reinforce the distinction between striate cortex and superior colliculus. Striate cortex is an excellent analyzer of stimulus characteristics but a poor evaluator of stimulus significance. The superior colliculus is an excellent evaluator but a poor analyzer. 5. The area of frontal eye fields in which cells have clear visual responses has been better localized. Enhancement of the visual response of these cells also occurs and, at least for some cells, the response enhancement is selective. The response enhancement, like the visual properties of these frontal eye field cells, appears to be more closely related to the properties of superior colliculus cells than to striate cortex cells. Copyright © 1976 the American Physiological Society
Use of an extraretinal signal by monkey superior colliculus neurons to distinguish real from self-induced stimulus movementRobinson, D. L.; Wurtz, R. H.
doi: N/Apmid: 823306
Abstract 1. In order to see whether cells in the superficial layers of the monkey superior colliculus can differentiate between real stimulus movement and self-induced stimulus movement we compared the discharge of these cells to stimulus movement in front of the stationary eye with stimulus movement generated by eye movements across a stationary stimulus. 2. Most of the cells recorded (65% of 231 cells) responded to stimulus velocities in front of the stationary eye as fast as those occurring during the peak velocity of a saccadic eye movement. Those cells that do respond usually have weak inhibitory regions and tend to have receptive fields further from fovea. 3. Move (61% of 105 cells) of the cells that did respond to rapid stimulus movement did not respond when an eye movement swept the receptive field over a stationary stimulus. 4. About half of these cells differentiated between these stimulus conditions when we used stimuli at least 1 log unit above background illumination; the remaining cells differentiated for stimuli 2 and 3 log units above background. Many cells differentiated between the two stimulus conditions over a wide range of directions of movement and the effect appears with about equal frequency in receptive fields at all distances from the fovea. 5. The differentiation is present for most cells even when the background illumination is reduced, indicating that visual factors are not the cause of the effect on these cells but may modify the response of other cells. 6. The suppression of background activity accompanying eye movements in the light is present following eye movements made in total darkness; the suppression, therefore, must result from an extraretinal signal. 4. The failure of these cells to respond to visual stimulation during eye movements is due to the same extraretinal signal that produces the suppression since a) the cells that show this suppression tend to be those that fail to respond to stimuli during eye movements, b) the time course of the suppression matches the time at which the effects of visual stimulation during an eye movement would reach the colliculus, and c) the cells which differentiate also show a decreased responsiveness to visual stimulation during the time of background suppression. While this extraretinal signal has the characteristics one would expect of a corollary discharge, proprioception as a source of the signal cannot be excluded. 8. Cells which differentiate between the two stimulus conditions usually also show an enhanced response to a visual stimulus in their receptive field when it is to be the target for a saccadic eye movement. These cells in the superior colliculus receive an extraretinal input which permits them to differentiate betweent real stimulus movements and stimulus movements resulting from the monkey's own eye movements. This differentiation would provide an uncontaminated visual movement signal and facilitate the detection of real movement in the environment... Copyright © 1976 the American Physiological Society
Organization of monkey superior colliculus: enhanced visual response of superficial layer cellsWurtz, R. H.; Mohler, C. W.
doi: N/Apmid: 823303
Abstract 1. Cells in the superficial layers of monkey superior colliculus respond more vigorously to a spot of light falling in their receptive fields when the monkey uses that spot of light as the target for a saccadic eye movement. Our purpose in these experiments was to investigate the characteristics of this enhancement effect. While monkeys fixated, we determined the response of a cell to a stimulus falling in its receptive field. Then we determined the response of the cell to the same stimulus when the monkey made a saccade to the stimulus or near to it. 2. The enhancement of the visual response is spatially limited. The receptive field of a cell always shows enhancement throughout its extent and frequently shows a slight expansion. Saccades made near to a stimulus in the visual receptive field, but not to it, also lead to an enhancement of that visual stimulus; an area around the excitatory center of the receptive field where such enhancement occurs was referred to as the enhancement field of the cell. An enhanced response in one part of the visual field was not accompanied by depressed responses associated with saccades to other parts of the visual field. 3. The enhancement effect is temporally limited; it begins 200-300 ms before the eye movement, as determined by the increasing response to 50-ms light pulses presented at varying intervals before the eye movement. The degree of enhancement intensifies when the visual stimulus is turned on closer in time to the onset of the saccade. A buildup of the enhancement also occurs on successive trials as does the response of eye movement-related cells in the intermediate layers. 4. The enhancement response is not present in the upper quarter-millimeter of the superficial layers, suggesting that the effect is not present in retinal afferents which terminate primarily in this area of the superficial layers. The enhancement effect is seen throughout the visual field; the foveal area was not tested. 5. In order to determine the relation of the enhancement effect to the monkey's behavioral response, we required the monkey to make a hand response rather than an eye movement-response to the visual stimuli. Cells did not show a clear enhancement with such a hand response. Results of these experiments indicate that the enhancement effect is dependent on the type of response the monkey makes to the stimulus and is probably specifically related to eye movements. Since the enhancement of visual response seems likely to be related specifically to eye movements both on physiological and behavioral grounds, the response-free term "attention" is probably inappropriate for the phenomenon. 6. The hypothesis advanced in the preceding paper that eye movement-related activity from intermediate and deep colliculus layers is directed upward to converge with visually related activity in the superficial layers is extended to include an input from cells in these deeper layers (or their afferents) to the superficial layer cells... Copyright © 1976 the American Physiological Society
Modulation of spike frequency by regions of special axonal geometry and by synaptic inputsSpira, M. E.; Yarom, Y.; Parnas, I.
doi: N/Apmid: 966043
Abstract 1. Spike propagation across the nonhomogeneous section of the giant axon in ganglion T3 of the cockroach was analyzed by intracellular microelectrodes recording at the posterior and anterior ends of T3. Ascending and descending potentials were evoked by stimulation of A5-A6 and T2-T3 connectives. 2. At high frequencies, descending and ascending impulses exhibit the following: a) consecutive reduction in the spike amplitude, b) a decrease in the afterhyperpolarization; c) gradual appearance of a prepotential together with an increase in delay of spike initiation; d) failure of full spike invasion into the recording area, showing only a decremental potential. 3. The duration of a train required to block spike propagation when the whole connective is stimulated is much shorter (about 6 times) than that required when a single giant axon is stimulated. 4. The conduction block is associated with a marked decrease in effective membrane resistance, greater than that expected from depolarization and delayed rectification. 5. Synaptic potentials could be recorded in the giant axons in the caudal base of ganglion T3 after stimulation of either the ipsilateral or contralateral connectives at both ends of the ganglion. These synaptic potentials could be blocked by d-tubocurarine (d-TC) or low Ca2+-high Mg2+. 6. Activation of these synapses produces a marked increase in membrane conductance, blocking propagation of spike trains through the ganglion. 7. After these synapses are blocked by d-TC or low Ca2+-high Mg2+, high-frequency stimulation still produces a conduction block. 8. It seems that conduction of spike during repetitive stimulation is affected both by accumulation of extracellular potassium, which depolarizes the membrane and causes sodium inactivation, and by activation of synaptic inputs to shunt the membrane in this region. 8. Each of these two mechanisms by itself can produce conduction block along the giant axons in ganglion T3. Copyright © 1976 the American Physiological Society
Coding of mechanical stimulus velocity and indentation depth by squirrel monkey and raccoon glabrous skin mechanoreceptorsPubols, B. H.; Pubols, L. M.
doi: N/Apmid: 823305
Abstract 1. A sample of 113 large, myelinated first-order afferent fibers innervating the glabrous skin of the squirrel monkey's hand proved to consist primarily of two basic types. In a sample where the only known source of sampling bias is a greater likelihood to record from larger diameter fibers, 40% of the fibers were rapidly adapting (RA) and 60% were very slowly adapting (VSA). Two units were moderately slowly adapting (MSA), and one had the properties of a Pacinian afferent (Pc). 2. The RA and VSA resemble those in the glabrous skin of other mammalian species in terms of thresholds, receptive-field areas, conduction velocities, and the coding of velocity of mechanical displacement of the skin. Mean instantaneous frequency during ramp stimulation is a power function of ramp velocity for both RA and VSA, with exponents generally less than 1.00. However, ramp discharge patterns differ for RA and VSA. 3. The VSA exhibit a wide range of coefficients of variation (CV) of their interspike-interval distributions, but form a continuous distribution with respect to this statistic. In other respects the VSA are more similar to slowly adapting type I than to slowly adapting type II. They lack spontaneous activity, have restricted receptive fields, and are relatively insensitive to skin stretch. 4. Effects of mechanical stimulus velocity and static indentation depth on static discharge rate were examined in 23 squirrel monkey and 22 raccoon SA units having receptive fields on glabrous skin of the hand. 5. Discharge rate during static indentation is a monotonic, increasing function of identation depth. However, the nature of the best-fitting function (highest r) varies from unit to unit. Using a set of standard conditions (milliseconds 100-500 of static displacements up to 960 mum, following a ramp velocity of 100 mum/ms, interstimulus interval of at least 10 s), the ratio of units for which linear, as opposed to logarithmic, functions provided the best fit was 4:3 for squirrel monkeys and 1:3 for raccoons. Few units had power functions as best fits in either species. Differences between fits for different functions within the same unit, however, were often trivial and insignificant. 6. Response rate during static skin displacement is also strongly influenced by prior stimulus ramp velocity. For at least the first 500 ms, discharge rate is positively related to onset velocity but, in many units, within the first 1 s of static displacement, this relationship reverses itself, and the inverse relationship may persist for at least 5 s. Copyright © 1976 the American Physiological Society
Sound localization in anurans. I. Evidence of binaural interaction in dorsal medullary nucleus of bullfrogs (Rana catesbeiana)Feng, A. S.; Capranica, R. R.
doi: N/Apmid: 1085815
Abstract 1. The response patterns of single cells to monaural and binaural acoustic stimuli were studied in the dorsal medullary nucleus of the bullfrog (Rana catesbeiana). This nucleus represents the first ascending center in the anuran's central auditory nervous system. 2. Of the 142 cells isolated, 75 units responded only to monaural stimulation. Approximately 80% of these monaural cells could be excited by the ipsilateral ear, while the remaining 20% received their excitatory input from the contralateral ear. The other 67 units responded to binaural stimuli. Of these binaural cells, 14 could be excited by either contralateral or ipsilateral stimuli, and the threshold and best excitatory frequency were similar for each ear (EE). The other 53 binaural cells (EI) could be excited by stimulation of one ear and inhibited by stimulation of the other ear; for almost all of these cells the contralateral ear was excitatory and the ipsilateral ear was inhibitory. The best inhibitory frequency for one ear was approximately the same as the best excitatory frequency for the other ear, and the threshold for inhibition was near the threshold for excitation. 3. The tuning curves for all of the cells in the dorsal medullary nucleus were unimodal with "Q" values ranging from 0.4 to 4. The excitatory thresholds were widely scattered between 22 and 115 dB SPL. 4. The distribution of best excitatory frequencies for the monaural cells comprised three groups: 200-300, 500-800, and 900-1,600 Hz. The best excitatory frequencies of the binaural cells were scattered over this entire range, with a broad peak around 200-800 Hz. 5. Approximately 80% of the cells in the dorsal nucleus responded tonically throughout the duration of an excitatory tone burst. The remaining 20% of the cells responded phasically during the transient stages of a tone burst over a wide intensity range. 6. Response latencies were compared for the two types of monaural cells to tones at their best exciatatory frequencies at 10 dB above threshold. The latencies for the contralaterally excitable cells were just a few milliseconds longer than the latencies for the ipsilaterally excitable cells. For binaural cells the latency for contralateral stimulation was only 1-2 ms longer than for ipsilateral stimulation. It was concluded that the contralateral input to the dorsal medullary nucleus is not of efferent descending origin from higher auditory centers. 7. All of the binaural EI cells were sensitive to small interaural intensity differences and many were also sensitive to minute interaural time differences.These cells likely play a role in localization of sounds of significance to anurans. Copyright © 1976 the American Physiological Society
Kinematics of locomotion by cats with a single hindlimb deafferentedWetzel, M. C.; Atwater, A. E.; Wait, J. V.; Stuart, D. G.
doi: N/Apmid: 966036
Abstract 1. Cinematographic measurements were made of stepping by cats on a motor-driven treadmill, both normally and 2-3 wk after deafferentation of the LH (left hind) limb. 2. After surgery, rhythmic cycling of the LH limb was blurred whether the leg was dragged, as by some cats, or if it was lifted from the surface, as by others. 3. Interlimb coordination was also blurred with respect to normal, although distinct rhythms were still seen. The RH (right hind) limb descended prematurely and, in the walk, had a prolonged stance phase. The interval between touchdowns of hind- and forelimb on the left side no longer equaled that interval on the right side. 4. As is true for a normal cat, if the LH-deafferented animal stumbled, relatively normal single and interlimb cyclings were regained after several strides. 5. By kinematic analysis, force deficits were found in the deafferented LH limb both during the stance, when extensors should be most active, and the swing, when the limb failed to attain a normal position above the surface of the belt. Weight bearing by the LF (left fore) limb was altered in some animals. 6. At high speed, mean LH stance duration failed to decrease in the normal fashion. Inter-limb timings were reset to greater extent than in low-speed walking, as if the LH limb was being used only minimally. 7. It was concluded that both rhythm and force were impaired in the deafferented limb and also in the three intact limbs, whose weight bearing had to compensate for LH weakness. The changed mechanical demands after surgery were probably met by interactions between the remaining afferent input and central pattern generators so as to secure fairly effective and expedient locomotion. Copyright © 1976 the American Physiological Society