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S Molotchnikoff

Publications and source records attributed to S Molotchnikoff.

At least 37 records · Page 2Linked to original sources
PubMed1996

Stimulus-dependent oscillations in the cat visual cortex: differences between bar and grating stimuli.

We have investigated the dependence of cortical oscillations on the type of visual stimulus. Single unit recordings were performed in areas 17 and 18 of the cat visual cortex. Among 217 cortical neurons oscillations in the frequency range of 22-102 Hz were found in 29 cells (13%). The proportion of oscillating cells was higher (16%) if both bar and grating stimuli were used to stimulate cortical neurons. It was found that gratings are more effective than bars in triggering oscillatory patterns in cortical cells. Among 21 oscillating cells which were stimulated with both bar and grating stimuli, oscillations evoked with gratings were found in 17 neurons (81%) while oscillations evoked with bar stimuli were triggered in 7 cells (33%). The distributions of oscillation frequencies were statistically different for oscillations evoked with bars and gratings. Frequencies of oscillations evoked with bars were in the lower and higher range than frequencies of oscillations evoked with gratings. In 3 cells (14%), rhythmic patterns could be evoked with both bar and grating stimuli. However, the oscillations were of different frequencies. No significant correlation was found between the strength of oscillations and firing rate of cortical neurons. Both simple and complex cells manifested the same dependence on stimulus type. However, complex cells mostly exhibited oscillations in the lower frequency range while simple cells did so when neurons were stimulated with bars. The results suggest that various classes of visual stimuli can be coded by a temporal pattern of cortical responses.

Animals↗
PubMed1996

The lateral posterior-pulvinar complex modulation of stimulus-dependent oscillations in the cat visual cortex.

It has been suggested that binding coherent targets depends on the capacity of excited cortical cells to fire in synchrony at approximately 40 Hz. However, the origin of stimulus-related cortical oscillations is still not clear. We hypothesized that 40 Hz oscillations might propagate to the visual cortex from the lateral posterior-pulvinar complex (LP-P) whose cells send fibers to the visual cortex and have a tendency to exhibit oscillations. To test our hypothesis, we recorded single unit activity in areas 17 and 18 of anaesthetized cats. The activity of neurons which showed oscillations evoked by optimal visual stimuli was analysed before, during and after a reversible inactivation of the LP-P with GABA. Such inactivation was found to markedly modify the strength of oscillatory activity of cortical neurons whose visual responses were affected by LP-P blockade. In contrast, the oscillation frequencies of cortical neurons were not modified by such inactivation. However, in some cells (three of nine), oscillatory activity was found to be completely abolished by injection of GABA into the LP-P. Collectively, these findings demonstrate that inputs from the LP-P play a key role in modulating the oscillatory activity of visual cortex neurons. Assuming that cortical neurons utilize oscillatory activity to encode perceptual aspects of the visual stimulus, our findings underscore the contribution of the LP-P in this process.

Action Potentials↗
PubMed1995

Visually-triggered oscillations in the cat lateral posterior-pulvinar complex.

The so-called 40 Hz oscillations are found at almost all stages of visual processing are thought to play a critical role in perception. The goal of this investigation was to look at the presence of stimulus-specific oscillations in the lateral posterior-pulvinar complex of the thalampus (LP-P) for which the oscillations were still not described. Rhythmic patterns in multiunit LP-P activity of anaesthetized cats were revealed in 14% of recording sites. With the exception of one pool of LP-P cells that exhibited stimulus-dependent rhythmic activity approximately 130 Hz, 90% of autocorrelograms were modulated between 18 and 74 Hz with dominant frequencies of 20-33 Hz. Since the LP-P sends efferents to the visual cortex it seems possible that oscillations from the LP-P can propagate to cortical neurones, especially to complex cells, for which similar dominant frequencies were noted by previous investigators.

Anesthesia↗
PubMed1995

Lateral geniculate neuron responses to drifting sine-wave gratings in rabbits.

Neurons of the lateral geniculate body in rabbits were excited with drifting sine-wave gratings. Rabbits were anesthetized and paralyzed under conventional methods to record action potentials of single cells using tungsten in glass microelectrodes. All classes of geniculate cells responded in a modulatory pattern. It appears that the unmodulatory pattern typical of complex cell types of the cortex is extremely infrequent or absent. In the spatial domain most cells are low pass and bandpass. Only one unit was high pass. In the temporal domain low-pass and bandpass cells were the most frequently recorded. Four geniculate cells were high pass. It appears, therefore, that neurons of rabbits' geniculate are tuned over spatial and temporal frequencies of sine-wave gratings. The comparison with cortical recordings revealed that geniculate cells are more broadly tuned than cortical neurons. This study suggests that the rabbit's visual system is sensitive to gratings. However cells respond optimally to lower values, e.g., broader gratings, than neurons of frontalized eye animals.

Animals↗
PubMed1995

Evolution of spontaneous activity in the developing rat superior colliculus.

During the first 10 days after birth in the rat there are a succession of major developmental stages in the retinotectal pathway. During most of this time, the only recordable event in the superior colliculus is spontaneous activity. We studied and characterized this spontaneous activity, hypothesizing that it could play an important role in pathway development. The spontaneous discharges are detectable on postnatal day 5 (P5). After P5, the number of spontaneously active cells per penetration increases up to P10, after which they decrease to adult-like levels by P14-P15. Between P5 and P10, the spontaneous discharges exhibit several patterns of activity, from constant firing to intermittent bursts with periods of quiescence, without any bearing to age. We isolated the retina and superior colliculus by injecting xylocaine onto the optic nerve and found no change in collicular activity. While this suggests that the spontaneous activity in the colliculus is independent of the retina at the ages studied, the opposite experiment, i.e., electrically stimulating the optic nerve, resulted in increased firing by collicular neurons, perhaps via nonclassical synaptic transmission. Finally, we compared interval histograms for spontaneously active cells between P5 and P15. The histograms suggest that at certain ages, spontaneous firing is more regular; moreover, these ages precede major functional advances, e.g., onset of numerous spontaneously firing cells at P6, the first response to optic nerve stimulation at P10, and the first light-evoked response at P12-P13. Our results support the hypothesis that spontaneous activity in the neonatal superior colliculus has a role in development of the retinotectal pathway, but the data also indicate that classical synaptic transmission is not involved.

Animals↗
PubMed1995

Comparison of the responses to moving texture patterns of simple and complex cells in the cat's area 17.

1. Whether complex (C) cells are the only truly texture-sensitive units in the cat's primary visual cortex remains controversial. In view of the strong physiological significance of having putatively only one class of cells sensitive to visual noise in the striate cortex, we reinvestigated this issue. Sensitivities of simple (S) and C cells to noise were quantitatively studied and compared in order to clearly document the response properties of cells in the striate cortex to visual noise and to establish whether one can unequivocally segregate S from C cells on the basis of those specific properties. 2. Receptive fields were stimulated with all relevant stimuli, i.e., drifting sine-wave gratings, electronically generated noise pattern of 256 x 256 elements (ratio 1:1 of dark and light elements), and flashing and moving bars (both bright and dark). 3. A total of 60 S cells out of 85 (70.6%) and 90 C cells out of 101 (81.8%) responded to the motion of visual noise. Responses of most C cells were sustained, i.e., their discharge rate was maintained at a constant level throughout presentation of the stimulus. On the other hand, responses of the majority of S cells were characterized by several bursts of discharges. On average, optimal firing rates were greater for gratings than for noise. 4. For practically all cells, responses to noise varied as a function of direction of motion. The mean direction bandwidths were, respectively, 43 +/- 24 degrees and 48 +/- 23 degrees (mean +/- SD) for S and C cells. In both groups, neurons were more broadly tuned for the direction of noise than that of gratings (t-test, P < 0.001). We rarely observed bimodal tuning curves for noise, with each peak lying on either side of the orientation curve. These results could be expected if one considers texture stimuli not in the space domain (as dot patterns) but in the frequency domain, i.e., patterns containing all spatial frequencies and orientations. 5. In general, the direction indexes of S and C cells were similar whether they were stimulated by drifting noise or gratings. S cells had a slight tendency to be more direction selective for noise than for gratings. 6. For all S and C cells tested, responses to noise varied as a function of drift velocity. The mean optimal velocity was 12.9 and 10.2 degrees/s for S and C cells, respectively (t-test, P > 0.05). Most cells were band-pass with mean bandwidths of 2.2 and 2.7 octaves for S and C cells, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Influences of horizontal connections on visual responses in rabbit striate cortex.

The goal of this study was to examine the role of horizontal connections in rabbit striate neurons. Anaesthetized rabbits were prepared in the usual fashion for single-cell recordings in area 17 of the visual cortex. We compared responses evoked by moving and stationary stimuli prior to, during and after recovery from lateral microinjection of either lidocaine (n = 61), gamma-aminobutyric acid (GABA, n = 18) or bicuculline (n = 8) 2 mm from the recording site. This procedure allows evaluation of the contribution of neighbouring neurons in visual responses. Results showed that striate neurons are dependent on the adjacent cells' excitability. Modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli. Lateral inactivation preferentially influenced non-directional over direction-selective units. This influence usually resulted in the non-directional neuron becoming directional by attenuation of the visually driven response in one direction. Simple and complex cells tended to be influenced differently by lateral inactivation. Simple cells became less responsive, whereas complex cells became more responsive. This dichotomy among cellular types suggests that simple cells receive mainly excitatory horizontal influences, while complex cells are contacted mostly by lateral inhibitory inputs.

Action Potentials↗
PubMed1994

Visual cortical neuron responses to drifting sine-wave gratings in rabbits.

Neurons of area 17 (V1) in rabbits were stimulated with drifting sine-wave gratings. Animals were anesthetized and paralyzed under conventional methods to record action potentials of single cells using tungsten-in-glass micro-electrodes. Neurons responded in modulatory or unmodulatory patterns depending on whether cells were of the simple or complex type, respectively. In the spatial domain, most cells were Low-Pass and Band-Pass, with no recordings of High-Pass units. In the temporal domain, Low-Pass, Band-Pass and High-Pass behaviours were found. Rabbits appeared to exhibit all types of pattern responses found in other foveate mammals, including special-complex and end-stopping units. However, in the majority of cells, there was a shift towards the lower end of the spatial frequency spectrum. For instance, the optimal spatial frequency was in the vicinity of lc/d while in the temporal domain, some units responded optimally at 10 c/s. In conclusion, it appears that rabbit cortex contain neurons which are selective for spatial frequency in a manner comparable to that seen in cat and monkey.

Animals↗
PubMed1994

Electroretinal responses are modified by chronic exposure to trichloroethylene.

Using an inhalation chamber, New Zealand albino rabbits were exposed to 350 ppm (n = 6) and 700 ppm (n = 8) of trichloroethylene (TRI) 4 hrs/day, 4 days/week for 12 weeks. Electroretinograms (ERG) and oscillatory potentials (OPs) were recorded weekly under mesopic conditions. Blood samples were also collected weekly to determine the concentration of TRI and its main metabolites. Recordings from the 350 and 700 ppm exposed groups showed a significant (p < 0.01) increase in the amplitude of the a- and b-waves (ERG), while the amplitude of the OPs was significantly (p < 0.01) decreased at 350 ppm and increased at 700 ppm. These electroretinal changes were reversed to the baseline value within six weeks after the inhalation stopped. The observed variations in a-wave and OP amplitudes were related to plasmatic level of trichloroethanol, while the effects on the b-wave were related to the blood level of TRI. These results confirm the neuro-ophthalmotoxicity of TRI and support the hypothesis that trichloroethanol is the major neurotoxic metabolite of TRI.

Animals↗
PubMed1993

Functional development of the neonatal rat retinotectal pathway.

Electrophysiological activity in the neonatal rat superior colliculus was recorded to measure neuronal and synaptic activity, and, therefore, functional development. Neonatal rat pups were studied from five days to two weeks of age. The earliest activity in the superior colliculus were spontaneous discharges at a frequency of one unit per animal on postnatal day 6 (P6). Spontaneously discharging units were more numerous at P8, and the number peaked on P10. The first clear response to optic nerve stimulation was seen on P10, with relatively long and variable latencies. By P14, electrically evoked responses had much shorter latencies. The results are in line with the first response to light flash in the superior colliculus at P12/13. The evidence suggests that functional development of the rat retinotectal pathway begins at the end of the first week after birth, and that much of the functional maturation occurs mainly during the second week after birth.

Animals↗
PubMed1992

Influence of remote targets on directionality of striate neurons in rabbits.

The described investigations study the influence of additional targets located well outside the classical receptive field on responses to motion of cortical cells in rabbits. Animals are anesthetized and prepared for acute single cells recordings in a conventional manner. The interactions between remote targets and central stimuli are abolished with microinjections of lidocaine hydrochloride or GABA at the site excited by remote stimuli. Results show that responses to motion of cortical cells are particularly sensitive to these manipulations. Although supplementary targets fail to influence spontaneous activity of all cells, they do influence responses to motion. Overall, the directionality indices (DI) declined. (53 to 45.) This decline may express itself either by a decrease of responses in the preferred direction or an enhancement of responses in the non-preferred direction or both. By contrast, responses to stationary stimuli are unaffected by additional targets in the visual field. Globally, cells whose directionality index was superior to 50% were significantly more affected then cells whose DI was less than 50%. This result suggests that similarly to cats, the directionality of cells in the striate cortex rests on a very fragile convergence of excitatory and inhibitory influences.

Animals↗
PubMed1992

Responses from outside classical receptive fields of dorsal lateral geniculate cells in rabbits.

The effects of stimuli at locations remote from classic receptive fields (CRF) of lateral geniculate cells were examined in rabbits. In anesthetized rabbits, small targets positioned well outside the CRF either facilitated or decreased responses evoked by a stimulus positioned within the most active area of the CRF in 51% of the cells tested, in spite of the fact that when presented in isolation the remote target failed to modify the spontaneous activity of the recorded cell. Late components of the discharge pattern evoked by the central stimulus were mostly influenced by the peripheral target. Focal or ectopic areas surrounding the CRF are thus identified. These areas were not a direct extension of the CRF, since the normal evoked response was unchanged when the remote stimulus moved closer to the CRF. Cells whose CRF were centrally located reacted with an augmented response in the presence of the additional stimulus, whereas units whose CRF was more eccentric exhibited a weaker response when the peripheral target was introduced in the visual field. We also investigated whether superior colliculus afferents to the lateral geniculate nucleus could be associated with these ectopic areas (EA). Depressing superior colliculus activity produced two types of results: (a) often the late component of the response pattern was modified; and (b) the influence of the remote stimulus disappeared with collicular blockade in 80% of tested neurons. These results provide evidence that the CRF of geniculate cells may be surrounded by satellite zones, which modify the responses to the central target when invaded by circumscribed stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Generation of end-inhibition in striate neurons in rabbits.

Eighty-six rabbit striate neurons were tested with lateral microinjection of lidocaine, GABA or bicuculline. Seven of the neurons expressed different levels of end-inhibition. We examined these end-stopping units by injection of lidocaine or bicuculline in adjacent areas in order to determine if a lateral cortical mechanism is underlying end-inhibition in rabbits as it has been proposed in cats. Microinjection of lidocaine resulted in an attenuation of the end-inhibition strength. Application of bicuculline had the opposite effect; the end-inhibition was reinforced. We suggest that as observed in cats, in rabbits end-inhibition is mediated through horizontal cortical connections which implies a postsynaptic inhibitory input to the end-stopping cell.

Animals↗
PubMed1992

Visual responsiveness and direction selectivity of cells in area 18 during local reversible inactivation of area 17 in cats.

We have investigated the effects of inactivation of localized sites in area 17 on the visual responses of cells in visuotopically corresponding regions of area 18. Experiments were performed on adult normal cats. The striate cortex was inactivated by the injection of nanoliters of lidocaine hydrochloride or of gamma-aminobutyric acid (GABA) dissolved in a staining solution. Responses of the simple and complex cells of area 18 to optimally oriented light and dark bars moving in the two directions of motion were recorded before, during, and after the drug injection. Two main effects are described. First, for a substantial number of cells, the drug injection provoked an overall reduction of the cell's visual responses. This nonspecific effect largely predominated in the complex cell family (76% of the units affected). This effect is consistent with the presence of long-range excitatory connections in the visual cortex. Second, the inactivation of area 17 could affect specific receptive-field properties of cells in area 18. The main specific effect was a loss of direction selectivity of a number of cells in area 18, mainly in the simple family (more than 53% of the units affected). The change in direction selectivity comes either from a disinhibitory effect in the nonpreferred direction or from a reduction of response in the preferred direction. It is proposed that the disinhibitory effects were mediated by inhibitory interneurones within area 18. In a very few cases, the change of directional preference was associated with a modification of the cell's response profile. These results showed that the signals from area 17 are necessary to drive a number of units in area 18, and that area 17 can contribute to, or at least modulate, the receptive-field properties of a large number of cells in the parastriate area.

Animals↗
PubMed1992

Evoked potentials are modified by long term exposure to trichloroethylene.

Two groups of New Zealand albino rabbits were respectively exposed to 350 and 700 ppm of trichloroethylene (TRI) 4 hrs/day, 4 days/week for 12 weeks. Weekly, visual evoked potentials (VEP) recordings were obtained under mesopic condition. Blood samples were also collected weekly to determine the concentration of TRI and its metabolites. Recordings from the 350 ppm group showed a significant (p less than 0.001) decrease in the amplitude of VEPs, while a significant (p less than 0.001) increase was observed in the 700 ppm group. Both effects were reversed to baseline values within six weeks after the last exposure. The observed modifications in VEP amplitudes were related to blood level of trichloroethanol. These results thus confirm the neuro-ophthalmotoxicity of TRI and support the hypothesis that trichloroethanol is a reliable marker of the effective neurotoxic dose of this organic solvent.

Animals↗
PubMed1991

Temporal relationship between the ERG and geniculate unit activity in rabbit: influence of background luminance.

The comparative analysis of the retinal and lateral geniculate nucleus (LGN) intensity-response function revealed that the timing of the LGN unit response was highly correlated to that of one oscillatory potential (OP2). To examine if this OP2-LGN intensity-response function was retained irrespective of the state of retinal adaptation, we performed simultaneous recordings to the ERG and single-cell unit activity at the geniculate level evoked to a flash of constant energy while the level of background luminence was varied. For a stimulus of 6.7 cd m-2 sec, the shifts in latency induced by increasing the background luminance from 0 to 125 cd m-2 are of 1.17 +/- 0.61 msec for OP2, a value almost identical (P greater than 0.10; n = 36) to the one obtained at the geniculate level (1.11 +/- 0.88 msec). However, when a dimmer flash is used, the latency shifts are not so well correlated. The latter could be partly explained by the threshold nature of the resulting stimulus (i.e. high photopic background combined with a dim flash).

Action Potentials↗
PubMed1991

Responsiveness of reorganized primary somatosensory (SI) cortex after local inactivation of normal SI cortex in chronic spinal cats.

The cortical map of adult cats that sustained spinal cord transection at T12 when they were 2 weeks old is characterized by a clear duplication of the representation of the forelimb, rostral trunk, and neck. The novel representation is located in the cortical region that is, in nonoperated animals, normally devoted to the hindlimb representation. We have investigated the possibility that the reactivation of the deprived hindlimb cortex may be mediated by corticocortical projections from normal to reorganized cortex. The primary somatosensory (SI) cortex was initially mapped to determine the boundaries of the normal and reorganized cortical representations. Somatotopically corresponding regions in both normal and reorganized cortex representing the trunk, the web space, or the shoulder were more precisely mapped. Inactivation of normal cortex was achieved by the nanoinjection of a solution of lidocaine hydrochloride stained with Chicago sky blue. Two major findings are described. First, inactivation of a circumscribed region of normal cortex representing a given receptive field (RF) failed to reduce or inhibit the responsiveness of a somatotopically corresponding RF represented in reorganized cortex. Therefore, it is unlikely that intracortical connections between normal and reorganized cortex could account for the reorganizational processes observed in cats that sustained spinal cord transection at 2 weeks of age. Second, the chemical blockade of normal cortex provoked an increase of the responsiveness and of the size of the peripheral RFs represented in reorganized cortex. This finding suggests that there are corticocortical connections (possibly topographically organized) between normal and reorganized cortex, and that these connections are inhibitory.

Afferent Pathways↗