Visually-evoked potentials elicited by a light-emitting diode chronically implanted in the frontal sinus of the freely moving cat.

Visually-evoked potentials (VEPs) were obtained with a small yellow light-emitting diode (LED), chronically implanted in the frontal sinus of the cat. The VEPs obtained in scotopic conditions through the LED have a similar morphology to those obtained with a stroboscopic flash. A noticeable delay was observed in LED wave latencies versus flash values. Analysis of wave intervals indicated a long-lasting generation of LED VEPs primary components. Secondary responses have a similar pattern with both modalities of stimulation. Implanted LEDs can be used as a reliable method to obtain VEPs and especially in those behavioural or pathological conditions in which variations exist in the constancy of the ocular media.

Representation of edges of variable blur by neuronal responses in the lateral geniculate body and the visual cortex of cats: limits of linear prediction.

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6-1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to -50% in individual cells and -15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6-1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in "band pass neurons"). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.

Visual evoked cortical potentials in the cat after retinal modifications induced by intraocular kainic acid.

Intraocular injections of kainic acid (KA) induce changes in visual-evoked cortical potentials in the cat at the level of the primary and secondary wave complexes. These changes are correlated with histological findings that show degeneration of cells in the inner nuclear layer and ganglion cell layer. These results indicate that despite the cell death produced by KA, certain retinal circuits continue to function. The fact that the first modifications induced by KA are observed at the level of the primary complex agrees well with the proposed retinal origin of this complex. The modifications of the secondary complex may imply a different transmission and/or processing of visual information at the central level.

Effect of sleep stages on the parallel channeling of visual flicker information in cats.

An analysis of the temporal frequency properties at the lateral geniculate nucleus (LGN) and visual cortex (VC) levels during the phases of the sleep-wakefulness cycle (SWC) was carried out. The results indicated a poor selectivity in the filter properties of the LGN for the different stages of the SWC, showing no differences in the handling of visual flicker information at this level. However, at the cortical level, filter properties were clearly affected by the SWC; thus, clear differences were observed between the synchronized phase of sleep (SS) and the desynchronyzed stages (W and PS). For the first time different functional temporal frequency channels at the cortical level closely related with the states of the SWC in cats are reported.

Relationship between orientation tuning and spatial frequency in neurones of cat area 17.

The orientation bandwidth was measured at different spatial frequencies for simple and complex cells. With increasing spatial frequency, the orientation tuning of simple cells became progressively narrower. This tendency was much less marked in complex cells. The results are interpreted in support of geniculate cells with orthogonal orientation biases providing the excitatory and inhibitory inputs to a simple cell.

Fast response and temporal coherent oscillations in small-world networks.

We have investigated the role that different connectivity regimes play in the dynamics of a network of Hodgkin-Huxley neurons by computer simulations. The different connectivity topologies exhibit the following features: random topologies give rise to fast system response yet are unable to produce coherent oscillations in the average activity of the network; on the other hand, regular topologies give rise to coherent oscillations, but in a temporal scale that is not in accordance with fast signal processing. Finally, small-world topologies, which fall between random and regular ones, take advantage of the best features of both, giving rise to fast system response with coherent oscillations.

[Statistical analysis of the parameters defining the cortical visual evoked potentials in the phases of the sleep-wake cycle]. / Análisis estadístico de los parámetros que definen a los potenciales evocados visuales corticales durante las fases del ciclo vigilia-sueño.

Visual evoked potential parameters (latencies, intervals of latencies and amplitudes) obtained by photic stimulation using a light-emitting diode implanted in the frontal sinus of cats were studied by statistical methods (analysis of variance) during the stages of wakefulness, slow sleep and paradoxical sleep. The results show: a) greater intraindividual homogeneity in all cases with special emphasis on the latencies; b) the greatest homogeneity of responses was found during slow sleep and paradoxical sleep stages; c) in relation to the influences exerted by the sleep-wakefulness cycle on the visual evoked potentials, the parameters most affected were those closely related to the secondary complex. We conclude, that latency, due to its great homogeneity, is the most useful parameter in this kind of experiments and secondly, that it is the secondary complex of the visual evoked potentials that is affected by the endogenous conditions of the subject (in our case the sleep-wakefulness cycle stage).

Cortical visual evoked potentials during the sleep-wakefulness cycle in the freely moving cat. A dynamic study.

The dynamic features of visual evoked potentials, elicited by a light emitting diode chronically implanted in the frontal sinus of the freely moving cat, were studied during sleep stages by means of calculation of the power spectra of the EEG prior to the stimulus and the poststimulus EEG which contain the single evoked potential. The statistical analysis between mean frequencies from both EEG prior to the stimulus and single evoked potential, within each sleep stage, showed that the main changes occurred during slow sleep, with a significant increase in the percentage of the alpha band. These results are interpreted as a partial desynchronization evoked by the visual stimulation during the synchronized phase of sleep.

A central pattern generator to control a pyloric-based system.

A central pattern generator (CPG) is built to control a mechanical device (plant) inspired by the pyloric chamber of the lobster. Conductance-based models are used to construct the neurons of the CPG. The plant has an associated function that measures the amount of food flowing through it per unit of time. We search for the best set of solutions that give a high positive flow of food in the maximization function. The plant is symmetric and the model neurons are identical to avoid any bias in the space of solutions. We find that the solution is not unique and that three neurons are sufficient to produce positive flow. We propose an effective principle for CPGs (effective on-off connectivity) and a few predictions to be corroborated in the pyloric system of the lobster.

Cortical visual evoked potentials during the sleep wakefulness cycle of the freely moving cat. Characterization and statistical comparisons.

Visual evoked potentials (VEPs) were obtained during the stages of wakefulness (W), slow sleep (SS) and paradoxical sleep (PS) by means of a light-emitting diode chronically implanted in the frontal sinus of the freely moving cat. Statistical analysis of the variables: latencies, latency intervals and amplitudes, between each of the mentioned stages shows that, for the first components, variations occurred only in the first interval of latency during SS vs. W. Lengthening of VEP latencies and increase of VEP amplitudes were observed for all secondary components in the comparisons between both SS and W, and SS and PS. PS-VEPs vs. W-VEPs showed shortening of latencies and decrease of amplitudes of all secondary components of the former case. The results confirm that in the freely moving cat, the secondary VEP response is more intensely affected by sleep than the primary VEP response, but indicate that there are different mechanisms in the generation of the VEP during SS and PS.