A simple glass-coated, fire-polished tungsten electrode with conductance adjustment using hydrofluoridic acid

Topographical Aspects of Intracortical Excitation and Inhibition Contributing to Orientation Specificity in Area 17 of the Cat Visual Cortex

Intracortical mechanisms contributing to orientation and direction specificity were investigated with a method of local cortical inactivation. Single-unit activity was recorded in area 17 of the anaesthetized cat while a small volume of cortical tissue 400 - 2900 microm lateral to the recorded cell was inactivated by gamma-aminobutyric acid (GABA) microiontophoresis. Cells were stimulated with moving bars of variable orientation and changes of the response were monitored. Recording and inactivation sites were histologically verified. Statistically significant changes in orientation tuning during GABA-induced remote inactivation were observed in 80 of 145 cells (55%), and consisted in a reduced orientation specificity due to either increased (36%) or decreased (19%) responses. Increases of responses were more pronounced for the non-optimal orientations. This effect mainly occurred with GABA application at distances around 500 microm and is interpreted as loss of inhibition. Reduced orientation specificity as a result of decreasing response mainly to the optimal orientation was interpreted as loss of excitation. This effect most frequently occurred with inactivation at distances around 1000 microm. Loss of inhibition was also elicited from a distance of 1000 microm; such inhibition, however, affected only directionality, without inducing changes in orientation tuning. For several cells at distances >1000 microm from the inactivation site a temporal sequence consisting of a change in direction specificity followed by a reduction of orientation specificity, and finally by direct GABAergic inhibition of the cell under study, could be induced with gradually increasing ejecting currents. The results indicate that excitation and inhibition originating from populations of neurons at different horizontal distances differentially contribute to direction and orientation specificity of a given visual cortical cell.

On the significance of temporally structured activity in the dorsal lateral geniculate nucleus (LGN)

Higher organisms perceive information about external or internal physical or chemical stimuli with specialized sensors that encode characteristics of that stimulus by a train of action potentials. Usually, the location and modality of the stimulus is represented by the location and specificity of the receptor and the intensity of the stimulus and its temporal modulation is thought to be encoded by the instantaneous firing rate. Recent studies have shown that, primarily in cortical structures, special features of a stimulus also are represented in the temporal pattern of spike activity. Typical attributes of this time structure are oscillatory patterns of activity and synchronous discharges in spatially distributed neurons that respond to inputs evoked by a coherent object. The origin and functional significance of this kind of activity is less clear. Cortical, subcortical and even very peripheral sources seem to be involved. Most of the relevant studies were devoted to the mammalian visual system and cortical findings on temporally structured activity were reviewed recently (Eckhorn, 1994, Progr. Brain Res., Vol. 102, pp. 405-426; Singer and Gray, 1995, Annu. Rev. Neurosci., Vol. 18, pp. 555-586). Therefore, this article is designed to give an overview, especially of those studies concerned with the temporal structure of visual activity in subcortical centers of the primary visual pathway, which are the retina and the dorsal lateral geniculate nucleus (LGN). We discuss the mechanisms that possibly contribute to the generation and modulation of the subcortical activity time structure and we try to relate to each other the subcortical and cortical patterns of sensory activity.

Toward the ultimate metal microelectrode

The performance of metal microelectrodes for stimulating and recording neuronal action potentials depends on precise control of their geometrical, electrical and mechanical properties. We describe a combination of materials whose properties approach fundamental physical limitations on achievable performance and reproducible fabrication techniques that provide probes with very small dimensions. Pure iridium wire is electrolytically sharpened, vapor-coated with Parylene-C insulation and the tip exposed using an automatically steerable UV laser. Electrochemical activation of the iridium increases the capacitance of the metal-electrolyte interface so that the overall impedance in the relevant frequency band (100-10,000 Hz) is dominated by the access resistance of the surrounding tissues.

Fine structure analysis of temporal patterns in the light response of cells in the lateral geniculate nucleus of cat

This study focuses on the analysis of temporal patterns in the spike train of cells in the lateral geniculate nucleus (LGN) of cat. Two-hundred eighty-three units have been recorded extracellularly in anesthetized animals during visual stimulation with flashing spot stimuli of different size. We used a novel method of temporally local computed interval distributions (intervalogram; Funke & Wörgötter, 1995) to visualize the statistical distribution of interspike intervals during different phases of the visual response. Multimodal interval distributions were observed mainly in X- and Y-ON cells, reflecting the tendency of these cells to fire with preferred intervals during the sustained light response. The shortest preferred interval is called the fundamental interval and the longer ones (higher-order intervals) are, in general, multiples thereof. During increasing surround inhibition a redistribution of the intervals towards the higher orders was observed. We regarded the different in the interval distributions as different components of possible temporal spike sequences and performed a pattern search up to the level of five subsequent intervals. While it is obvious, that the dominant peak is most strongly represented in any interval sequence, we also show that a significant overrepresentation of short sequences of similar intervals exists. The repetition rate is rather small (4-5 intervals) and, therefore, no long-lasting oscillatory pattern was observed in the autocorrelograms. Power spectral analysis of the peristimulus-time histograms, however, revealed that the sequential firing pattern is strongly stimulus locked at least for the majority of sweeps in the records. The mean firing rate of an LGN cell decreases with increasing stimulus size as well as with decreasing contrast. Therefore, the mean rate cannot be used to distinguish between these situations. While in the whole network this tradeoff can be resolved by the combined activity of multiple cells, our findings additionally suggest that contrast and size can be distinguished already at the single-cell level using different temporal patterns.

A simplified method for manufacturing glass-insulated metal microelectrodes

A simplified method to manufacture durable, glass-insulated, tungsten microelectrodes with sufficient control of the final electrode impedance is described. This method requires only two instruments, an electrolytic etcher for wires and pipette puller, for manufacturing these electrodes. The manufacture of these electrodes involves 3 steps: (1) etching tungsten wire to sharpen the tip, (2) insulating the electrode by pulling a glass pipette over the sharpened tungsten wire and (3) assessing and adjusting the tip exposure and impedance of the electrode to meet recording requirements. Control over the electrode impedance is easily accomplished by varying the distance between the uppermost portion of the heating coil and the sharpened wire tip before a glass pipette is pulled over the wire tip. This distance determines the area of tip exposure and also the location where the glass insulation ends and the exposed electrode tip begins. A performance test of these electrodes in a chronically prepared monkey showed that they were strong enough to repeatedly penetrate thickened dura mater without significant changes in impedance and to isolate cortical neuronal activity after these multiple penetrations. Furthermore, the strength of these microelectrodes eliminated the need to remove reactive granular tissue from the dura overlying the recording site.

A tungsten-in-glass iontophoresis assembly for studying input-output relationships in central neurons

A method is described for the production of an electrode capable of monitoring and modulating the input-output relationship of thalamic neurons. Tungsten-in-glass electrodes were manufactured with the ability to simultaneously record lateral geniculate nucleus action potentials and associated, retinally evoked S-potentials. The recording electrodes were mounted onto multibarreled micropipettes with iontophoretic capability. The completed electrode assembly permitted micropharmacological modulation of the fraction of lateral geniculate nucleus output spikes to retinal input spikes (the transfer ratio). Iontophoretically applied gamma-aminobutyric acid (GABA) decreased the transfer ratio, an effect countered by the GABAA antagonist, bicuculline. Elevated transfer ratios produced by stimulation of an afferent pathway originating in the parabrachial region of the brainstem were decreased by concurrently applied GABA. The fabrication of this electrode assembly employs simple modifications of existing techniques and separate construction of recording and iontophoretic elements to provide high-quality single-unit recordings coupled with micropharmacological function.

Neuronal dysfunction at the border of focal lesions in cat visual cortex

Traditional concepts assume that traumatic or ischemic brain lesions are surrounded by regions with depressed neuronal function. More recently hyperactivity gained increasing attention as excitotoxic mechanisms become effective at certain stages of neuronal injury. Single cell recordings in the surrounding of small focal lesions in the cat visual cortex revealed both types of functional pathology 1-30 days after lesioning. A rim of suppressed neurons surrounded a completely silent core. Cells further away from the lesion showed bursts and long lasting hyperactivity with extremely high discharge rates. Consequently, the volume of disturbed tissue was considerably larger than the region of initial cell death. This halo of dysfunction may be important for neurological symptoms evoked by cortical lesions.

Axial responses in visual cortical cells: spatio-temporal mechanisms quantified by Fourier components of cortical tuning curves

The responses of 81 cells from area 17 in paralysed and anaesthetized cats were studied with moving spots and moving bars of different lengths. Tuning curves were measured and plotted as polar-plots. The strongest response of visual cortical cells to a moving bar occurs when the stimulus trajectory crosses the long axis of the receptive field (Hubel and Wiesel 1962). The optimal orientation for a moving and a flashing bar are identical, so that this response-type has been called the orientational component. For a moving spot, however, in most cases the strongest response occurs for motion along the receptive field long axis (axial component). Thus, the axial and orientational components are orthogonal (Wörgötter and Eysel 1989). It is shown that orientational and axial components can display direction selectivity and for short bar stimuli a superposition of the two orthogonal components is demonstrated. Such a superposition in general, resulted in a polar-plot with four peaks 90 degrees apart from each other (four-symmetrical polar-plot). Polar-plots with three or two response peaks were also found; the actual number of response peaks depending on the direction selectivity of the components. In many cells pure axial responses could be elicited with a light spot which stimulates only motion dependent mechanisms. Thus, it was concluded that temporal facilitation is strongly involved in the generation of axial responses. Fourier analysis of polar-plots (SDO-analysis, Wörgötter and Eysel 1987; Wörgötter et al. 1990) was applied to determine the tuning strengths of the different components.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantification and Comparison of Cell Properties in Cat's Striate Cortex Determined by Different Types of Stimuli

Direction and orientation tuning elicited by moving bars, flashing bars and a moving noise field were compared in cells in area 17 of the cat. Fourier analysis of tuning curves (SDO-analysis) was applied to quantify the general sensitivity (S) to visual stimulation, tuning strength to direction(D) and orientation (O), as well as the preferred direction (PD) and orientation (PO). Results from SDO-analysis were compared with the commonly used direction index and half-width-at-half-height orientational tuning parameter and it is demonstrated that the commonly used parameters can be replaced and are superseded by the results from SDO-analysis. The comparison of the responses elicited by the different types of stimuli showed that a linear correlation between D (or O) components was mainly found in simple cells, while in most cases no correlation was obtained for complex cells. Since several of the simple cells also showed no linear relationship, a direct mutual prediction of the S, D and O components can only be achieved for approximately 50% of the cortical cells applying commonly used stimulus types. The general responsiveness (S) shows that flashing bar stimuli are at least as effective as moving bars, whereas moving noise stimulates cortical cells more weakly. A moving bar tends to increase the orientation tuning (O) in most cells and with a moving noise stimulus predominantly the directional tuning (D) of complex cells is strongly enhanced. In conclusion, Fourier analysis of tuning curves (SDO-analysis) provides a valuable and simple tool for the quantification of direction and orientation specificity. Motion enhances the cortical response specificity which indicates the involvement of facilitation or inhibition exclusively induced by movement.