Stationary Spot Research Articles

Transitions in two-dimensional patterns in a ferrocyanide–iodate–sulfite reaction

Transitions in two-dimensional (2D) spatial patterns were investigated in a ferrocyanide–iodate–sulfite (FIS) reaction in a circular thin gel reactor. The state of the gel reactor was maintained by contact of one side of the gel with a continuously refreshed well-stirred reservoir. For long residence times of the chemicals in the reservoir, the gel reactor was in a spatially uniform state of low pH (about 4), while at short reservoir residence times the reactor was in a uniform state of high pH (about 7). At intermediate residence times the spatiotemporal 2D structures observed include a large low pH oscillating spot, small metastable high pH oscillating spots, shrinking rings, spirals that formed when the axisymmetry of shrinking rings was broken, self-replicating spots that either grew and divided or died from overcrowding, and highly irregular, stationary lamellae. Transitions among the different patterns were examined as a function of gel thickness (0.2–0.6 mm), reservoir residence time (0.6–4 min), and ferrocyanide concentration (12–80 mM). Iodate and sulfite concentrations were held fixed at 75.0 and 89.0 mM, respectively. Several transitions were examined in detail: from a stationary spot to an oscillating spot; from an oscillating spot to a shrinking ring or spirals; the onset of replicating spots; and the transition from a homogeneous state to lamellar patterns. The observed phenomena can all be described in terms of a parity-breaking front bifurcation (nonequilibrium Ising-Bloch bifurcation).

Ca2+ sparks involving multiple Ca2+ release sites along Z-lines in rat heart cells.

1. High spatial resolution confocal imaging was used to investigate the fundamental nature of "Ca2+ sparks' in rat cardiac myocytes loaded with the fluorescent calcium indicator, fluo-3. 2. The sites at which calcium sparks occurred (Ca2+ release sites) were packed closely and irregularly in transverse planes along Z-lines (mean spacing between sites of 0.76 microns). In contrast, sites were spaced more regularly in the longitudinal direction, at intervals of 1.8 microns (i.e. the sarcomere length). 3. Diffusion of released Ca2+ was slower transversely (apparent diffusion coefficient, D, 7.9 microns 2 s-1) than longitudinally (D, 17.1 microns 2 s-1). 4. Frequently, discrete sites several hundred nanometres apart transversely activated in near synchrony. The probability of transverse synchronous activity fell to low levels (< 20%) at sites separated by more than 1.0 micron. Synchronous activation was not observed between sites on different Z-lines (i.e. separated longitudinally by 1.8 microns). 5. High temporal resolution confocal microscopy (stationary spot) revealed Ca2+ sparks with "stepped' rises, consistent with multiple sites of origin. 6. We conclude that the Ca2+ spark as originally described is usually not an "elementary' event, in the sense of being indivisible, but is often comprised of yet smaller, triggered units of Ca2+ release.

Hot spot formation on a catalyst

Stationary hot spots with an amplitude of 50–100 K were observed by an IR imager during hydrogen oxidation on supported niclel-on-alumina catalytic rings placed in a mixed vessel. These states were found under reactor conditions for which the uniform system is bistable, i.e. when both uniformly ignited and extinguished states exist. The nonuniform states were generated either by short, local heating of an extinguished state or by slow cooling of a sufficiently large ignited ring. The hot spot could be created at any location on the ring. The width of the temperature fronts connecting the high- and low-temperature zones was rather insensitive to variations in the thickness and diameter of the ring. Stationary hot spots of slightly different shapes, with up to 8% different levels of conversion in the reactor effluent, were observed under the same operating conditions following different initial conditions. The region of reactor temperature vs feed concentration in which hot spots exist increases with ring diameter, and its boundaries are sensitive to the intensity of mixing in the vessel. The hot spots are stabilized bya global interaction between the catalytic surface and the oxygen concentration in the mixed vessel.

Breathing Spots in a Reaction-Diffusion System

A quasi-2-dimensional stationary spot in a disk-shaped chemical reactor is observed to bifurcate to an oscillating spot when a control parameter is increased beyond a critical value. Further increase of the control parameter leads to the collapse and disappearance of the spot. Analysis of a bistable activator-inhibitor model indicates that the observed behavior is a consequence of interaction of the front with the boundary near a parity breaking front bifurcation.

Scale-up of pulsed laser deposition (PLD) for 4″-wafer coating

PLD of uniform thin films on 4″-wafers has been realized by the integration of PLDLD-source into a commercial MBE-system. Thickness homogeneity over the total substrate area is obtained by precise spatial control of the plasma plumes excited simultaneously at two or more adjacent target locations. Computer controlled motion of a cylindrical target with respect to the stationary focal spots of the laser beams, has to provide (a) a suitable deflection of the plume axis and (b) a uniform target erosion. Process control was investigated by computer simulations. The efficiency of the two techniques for large-area coating described in this paper is illustrated by the preparation of DLC thin film specimens and their ellipsometric characterization. Special film thickness gradients were realized by a particular regime of target and substrate motion.

Brainstem modulation of signal transmission through the cat dorsal lateral geniculate nucleus.

We studied changes in retinogeniculate transmission that occur during variation of modulatory brainstem input and during variation of stimulus contrast. Responses of single cells in the dorsal lateral geniculate nucleus (dLGN) to a stationary flashing light spot of varying contrast were measured with and without electrical stimulation of the peribrachial region (PBR) of the brainstem. PBR stimulation increased the contrast gain (slope of response versus contrast curve) and the dynamic response range (range between spontaneous activity and maximal firing). Lagged and nonlagged X-cells reached the midpoint of the dynamic response range at lower contrasts during PBR stimulation than in the controls. No comparable change was seen for Y-cells. Only minor changes of threshold contrast were seen. The characteristics of the retinogeniculate transmission were directly studied by comparing the response of dLGN cells with their retinal input (slow potentials, S-potentials). With increasing contrast there was a marked increase in the transfer ratio (proportion of impulses in the input that generates action potentials in the dLGN cell). The transfer ratio seemed to be primarily determined by the firing rate of the retinal input. The transfer ratio increased with increasing input rates from low values near threshold to values that could approach 1 at high-input firing rates. PBR stimulation increased the transfer ratio, particularly at moderate input firing rates. The increased transfer ratio, caused by increasing input firing rates, enhanced the response versus contrast characteristics through an increase in contrast gain and dynamic response range. The modulatory input from the PBR further enhanced these characteristics.

Large-Area Pulsed Laser Deposition (PLD)

ABSTRACTPLD of uniform thin films on 4”-wafers has been realized by the integration of a PLD-source into a commercial MBE-system. Thickness homogeneity over the total substrate area is obtained by precise spatial control of the plasma plumes excited simultaneously at 4 adjacent target locations. Computer controlled motion of a cylindrical target with respect to the stationary focal spots of the laser beams, has to provide (a) a suitable deflection of the plume axis and (b) a uniform target erosion. Process control was investigated by computer simulations. The efficiency of the developed technique for large area coating is illustrated by the preparation of DLC thin film specimens and their ellipsometric characterization

Response variability of single cells in the dorsal lateral geniculate nucleus of the cat. Comparison with retinal input and effect of brain stem stimulation.

1. We studied the degree and source of response variability in different classes of cell in the dorsal lateral geniculate nucleus (dLGN). The response of single cells to a series of contrasts of a stationary flashing light spot was measured. The variability analyses were based on the mean and SD of the response to a number of repeated stimulus presentations. The relative variability was expressed by the coefficient of variation (Cv; SD/mean). 2. At a given contrast, the Cv for lagged cells was larger than for nonlagged cells. No difference was found between the Cv of X and Y cells. The magnitude of the Cv was about the same as previously found for cells in striate cortex. Accordingly, little variability is added at the cortical level. The Cv decreased with increasing contrast showing that the reliability of response and the signal-to-noise ratio was improved with increasing contrast. 3. For some cells, the retinal input was determined by recording S potentials in addition to action potentials. The Cv of the retinal input was smaller than the Cv of the dLGN cells at a given contrast. Thus in the paralyzed and anesthetized preparation, variability was added at the geniculate relay. 4. The additional variability was related to modulatory input from the brain stem. This was shown by comparing Cv versus contrast curves for the dLGN cells obtained during electrical stimulation of the peribrachial region of the brain stem (PBR) with corresponding curves obtained without PBR stimulation. During PBR stimulation, which presumably mimics the effects of arousal on the dLGN cell, the Cv at a given contrast was reduced toward the value for the retinal input to the cell. Furthermore PBR stimulation increased the signal-to-noise-ratio of the cell to the level of the retinal input. 5. When Cv was plotted against response rather than against contrast, approximately the same function was found for the various dLGN cell classes. This indicated that the variability basically depended on firing rate rather than on stimulus contrast. No difference of Cv was seen between lagged and nonlagged cells at a given level of response. The difference found at a given level of contrast reflected differences in firing rate of the two cell classes. During PBR stimulation, there was no clear difference between the Cvs of the dLGN cell and its retinal input at a given level of response.(ABSTRACT TRUNCATED AT 400 WORDS)

Orientation imaging of microstructures

The complexity of microstructures characteristic of polycrystalline materials presents the serious investigator with many challenges. The materials engineer hopes to associate the important technological properties of these materials with specific (quantifiable) attributes of the microstructure; however, microscopy presents an overwhelming myriad of details over a wide range of scales of inquiry. Thus, the persistent question becomes: What is important in the microstructure relative to a specific property or aspect of material performance? One particular viewpoint, which stems from the modern atomistic interpretation of the structure of solids, is that for polycrystalline materials it is the spatial placement of lattice orientation that is of essential interest.The past decade has seen some remarkable progress in microdiffraction technique in conjunction with the scanning electron microscope. This progress now makes it possible to vigorously pursue the aforementioned lattice-orientational viewpoint. Since 1987 modern SIT (silicon intensified target) vidicon and CCD (charge-coupled device) cameras have been used to capture the backscattered Kikuchi diffraction (BKD) formed in stationary spot mode in the scanning electron microscope.

Nonlinear heat structures and arc-discharge electrode spots.

Domainlike temperature distributions (solitary structures) are studied, which may arise in the half space with the surface heated by the external heat flux depending in a nonlinear way on the local surface temperature. Solutions describing stationary axisymmetric structures for some model dependencies of the external heat flux density on the surface temperature are found analytically or numerically. The stability of stationary axisymmetric structures is analyzed. When applied to an analysis of spots on electrodes in arc discharges, for given characteristics of the near-electrode plasma layer such an approach results in the complete description of the stationary spots on smooth surfaces, including the spot radius and the integral current

Brain-stem influence on visual response of lagged and nonlagged cells in the cat lateral geniculate nucleus.

This study examined the influence of the pontomesencephalic peribrachial region (PBR) on the visual response properties of cells in the dorsal lateral geniculate nucleus (LGN). The response of single cells to a stationary flashing light spot was recorded with accompanying electrical stimulation of the PBR. The major objectives were to compare the effects of PBR stimulation on lagged and nonlagged cells, to examine how the visual response pattern of lagged cells could be modified by PBR stimulation and to examine whether the physiological criteria used to classify lagged and nonlagged cells are applicable during increased PBR input to the LGN. During PBR stimulation, the visual response was enhanced to a similar degree for lagged and nonlagged cells and the latency to half-rise of the visual response was reduced, particularly for the lagged X cells. The latency to half-fall of the visual response of lagged cells was not changed by PBR stimulation. Accordingly, the division of LGN cells into lagged and nonlagged cells based on visual response latencies was maintained during PBR stimulation. The initial suppression that a visual stimulus evokes in lagged cells was resistant to the effects of PBR stimulation. For the lagged cells, the largest response increase occurred for the initial part of the visual response. For the nonlagged cells, the largest increase occurred for the tonic part of the response. The results support the hypothesis that the differences in temporal response properties between lagged and nonlagged cells belong to the basic distinctions between these cell classes.

Smooth eye movements elicited by microstimulation in the primate frontal eye field.

1. We electrically stimulated the macaque monkey's frontal eye field (FEF) region to localize and to analyze the smooth pursuit eye movement representation. Rhesus monkeys were trained to fixate stationary spots of light, and trains of stimulation (usually 250-500 ms at 10-100 microA) were applied while the fixation targets remained lit and stationary. This paradigm was used in a total of 485 electrode penetrations through the arcuate sulcus region of six hemispheres in three adult monkeys. Smooth eye movements (SEMs), clearly distinct from saccades, were elicited at 86 sites in 53 of these penetrations. These SEMs had an average peak velocity of 11 degrees/s and an average latency of 39 ms. 2. The initial acceleration and peak velocity of elicited SEMs increased with stimulation intensity at any given site. On the other hand, SEM direction was characteristic of a given stimulation site and did not vary with stimulation intensity. These findings indicate that SEM amplitude is coded by the intensity of neural activity, and SEM direction is coded by the location of this activity within the cortex ("rate" vs. "place" codes). 3. SEMs elicited in the presence of a stationary fixation target (closed-loop conditions) typically reached a plateau velocity early in the stimulation and maintained that velocity throughout most of the stimulation train. However, when retinal slip was eliminated by artificially stabilizing the fixation target on the fovea (open-loop conditions), the electrical stimulation caused the eye to accelerate for longer periods and to attain higher velocities than in the closed-loop condition. Eye velocities obtained at the same site in open- and closed-loop conditions diverged from one another approximately 100 ms after SEM onset, consistent with the visual latency of the pursuit system. These findings suggest that the FEF primarily conveys an eye acceleration signal, rather than an eye velocity goal, to the pursuit system, and that this signal can be affected by visual retinal errors before effecting the smooth eye movements. 4. SEMs were elicited from a small portion of the arcuate fundus and neighboring posterior bank lying directly posterior to the principal sulcus. Functionally, this SEM region was surrounded by the saccadic FEF and by somatic premotor cortex. 5. Even though ipsilateral, contralateral, and vertical SEMs were elicited, the distribution of SEM directions was skewed toward ipsilateral movements. This tendency was more pronounced for sites in the arcuate fundus, whereas SEMs elicited from the posterior arcuate bank were often directed contralaterally and vertically.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of contrast on the visual response of lagged and nonlagged cells in the cat lateral geniculate nucleus.

The response vs. contrast characteristics of different cell classes in the dorsal lateral geniculate nucleus (LGN) were compared. The luminance of a stationary flashing light spot was varied stepwise while the background luminance was constant. Lagged X cells had lower slope of the response vs. contrast curve (contrast gain), and they reached the midpoint of the response range over which the cells' response varied (dynamic response range) at higher contrast than nonlagged X cells. These results indicated that nonlagged cells are well suited for detection of small contrasts, whereas lagged cells may discriminate between contrasts over a larger range. The contrast gain and the contrast corresponding to the midpoint of the dynamic response range were similar for X and Y cells. The latency to onset and to half-rise of the visual response decreased with increasing contrast, most pronounced for lagged cells. Even at the highest contrasts, the latency of lagged cells remained longer than for nonlagged cells. For many lagged cells, the latency to half-fall decreased with increasing contrast. It is shown that the differences in the response vs. contrast characteristics between lagged and nonlagged X cells in the cat are similar to the differences between the parvocellular and magnocellular neurones in the monkey.

Simultaneous recording of lagged and nonlagged cells in the cat dorsal lateral geniculate nucleus.

It has been suggested that lagged and non-lagged cells in the cat dorsal lateral geniculate nucleus (LGN) represent state-dependent response modes of the same class of LGN cells. In two separate experiments with single-unit recording in the LGN of anaesthetized and paralysed adult cats, a lagged and a nonlagged X-cell were recorded simultaneously with the same microelectrode. For each pair of cells, the amplitude of the action potentials was sufficiently different to allow separate compilation of peri-stimulus-time-histograms. For all 4 cells, the visual response pattern to a stationary flashing light spot was typical of their respective cell class. These findings support the hypothesis that lagged and non-lagged cells are separate cell classes and indicate that the population of LGN cells do not appear as lagged during one state of modulatory input and as nonlagged during another.

Infant eye movements: Quantification of the vestibulo-ocular reflex and visual-vestibular interactions

The gain of the infant vestibulo-ocular reflex (VOR) was determined when infants were rotated either in total darkness or while they viewed visual targets consisting of a stationary spot or a full field of black and white stripes. The average VOR gain in the dark was 1.03 ± 0.014 for 1–4-month-old infants and 0.59 ± 0.03 for adult subjects tested with appropriate controls for psychological “set”. Longitudinal studies showed no significant change in gain over the first 4 months of life. Although the presence of the spot or full-field striped background increased adult compensatory gains from 0.59 to 1.0, the same visual targets had no effect on infant gains. Thus, an infant's VOR gain of nearly 1.0 apparently reduces reliance on the poorly developed smooth pursuit and optokinetic systems that, in adults, help the VOR provide perfect ocular stabilization.

Discharge patterns of neurons in the pretectal nucleus of the optic tract (NOT) in the behaving primate

1. To determine the possible role of the primate pretectal nucleus of the optic tract (NOT) in the generation of optokinetic and smooth-pursuit eye movements, we recorded the activity of 155 single units in four behaving rhesus macaques. The monkeys were trained to fixate a stationary target spot during visual testing and to track a small moving spot in a variety of visual environments. 2. The majority (82%) of NOT neurons responded only to visual stimuli. Most units responded vigorously for large-field (70 x 50 degrees) moving visual stimuli and responded less, if at all, during smooth-pursuit eye movements in the dark; many of these units had large receptive fields (greater than 10 x 10 degrees) that included the fovea. The remaining visual units responded more vigorously during smooth-pursuit eye movements in the dark than during movement of large-field visual stimuli; all but one had small receptive fields (less than 10 x 10 degrees) that included the fovea. For all visual units that responded during smooth pursuit, extinction of the small moving target so briefly that pursuit continued caused the firing rates to drop to resting levels, confirming that the discharge was due to visual stimulation of receptive fields with foveal and perifoveal movement sensitivity and not to smooth-pursuit eye movements per se. 3. Eighteen percent of all NOT units ceased their tonic discharge in association with all saccades including the quick phases accompanying optokinetic or vestibular nystagmus. The pause in firing began after saccade onset, was unrelated to saccade duration, and occurred even in complete darkness. 4. Most (90%) of the visual NOT units were direction selective. They exhibited an increase in firing above resting during horizontal (ipsilateral) background movement and/or during smooth pursuit of a moving spot and a decrease in firing during contralateral movement. 5. The firing rates of NOT units were highly dependent on stimulus velocity. All had velocity thresholds of less than 1 degree/s and exhibited a monotonic increase in firing rate with visual stimulus velocity over part (n = 90%) or all (n = 10%) of the tested range (i.e., 1-200 degrees/s). Most NOT units exhibited velocity tuning with an average preferred velocity of 64 degrees/s.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. I. Lagged cells

1. Synaptic mechanisms that might explain the functional properties of the recently discovered class of lagged cells in the dorsal lateral geniculate nucleus (LGN) were analyzed with electrophysiological and pharmacologic techniques. To study the type of excitatory amino acid (EAA) receptor that mediates visual responses of lagged cells, we recorded the response of single cells to a stationary flashing light spot before, during, and after microiontophoretic application of antagonists and agonists to EAA receptors. 2. The visual response of the lagged cells could be almost completely blocked by an antagonist to the N-methyl-D-aspartate (NMDA) receptor. The degree of suppression was dose dependent, and the average maximum degree of suppression for all the cells was 94%. NMDA enhanced the response, and this enhancement was antagonized by NMDA antagonists. A quisqualate/kainate receptor antagonist had no significant effect on the lagged cells. 3. These findings indicate that the visual response in lagged cells is dependent upon activation of NMDA receptors, which may directly result from activation of retinal inputs. 4. No pharmacologic difference was seen between lagged X- and Y-cells, or between lagged ON- and OFF-center cells. 5. gamma-Aminobutyric acid-A (GABA-A) receptor antagonists were used to study whether the characteristic lag of the visual response and the suppression of the initial transient response component of the lagged cells are dependent on geniculate inhibition. Beside enhancement of the visual response, the GABA antagonists strongly reduced the lag of the visual response, and an initial transient response component occurred instead of the initial suppression. The lag remained slightly longer than for nonlagged cells, and the peak firing rate of the transient was below values typical for nonlagged cells, indicating that the lagged cell properties are dependent on other factors beside GABA-A receptor-mediated inhibition. 6. The enhanced visual response during iontophoresis of GABA antagonists could be completely blocked by simultaneous iontophoresis of an NMDA-receptor antagonist. This gives further support to the hypothesis that the retinal input to these cells is mediated by NMDA receptors. 7. The NMDA-receptor/ionophore complex mediates excitatory postsynaptic potentials (EPSPs) characterized by slow rise and decay times and long duration. The ionophore is characterized by a voltage-dependent blockade that makes these receptors particularly sensitive to inhibitory input. The temporal interplay between the slow NMDA receptor-mediated EPSPs and the fast GABA receptor-mediated inhibitory postsynaptic potentials (IPSPs) may explain the characteristic response properties of the lagged cells.

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. II. Nonlagged cells

1. We studied the type of receptor for excitatory amino acids (EAA) that mediates visual responses of nonlagged cells in the dorsal lateral geniculate nucleus (LGN) by recording the visual response of single cells to a stationary flashing spot before, during, and after iontophoretical application of antagonists and agonists to EAA receptors. 2. The visual response of the nonlagged cells was strongly suppressed, in a dose-dependent manner, by the specific quisqualate/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). The average degree of suppression was 74%. Quisqualate enhanced the visual response. 3. Specific antagonists to the N-methyl-D-aspartate (NMDA) receptor had a weak suppressing effect on most nonlagged cells. The average degree of suppression was 22%. Measurement of such weak effects was complicated by the considerable spontaneous fluctuations of responsivity in the LGN cells. In the majority of cells where the visual response was suppressed by NMDA antagonists, the tonic response component was more strongly suppressed than the initial transient response component. The visual response was enhanced by NMDA, and this enhancement was antagonized by NMDA antagonists. 4. These findings suggest that the excitatory input from the retina to nonlagged LGN cells is mainly mediated by non-NMDA receptors. The non-NMDA receptors mediate fast EPSPs, and this can explain the fast onset and offset of the visual response of the nonlagged cells. 5. The generally small contribution from NMDA receptors to the visual response of the nonlagged cells might reflect a minor involvement of these receptors in the retinal input, or it could be related to the excitatory input to LGN from the visual cortex. 6. To study whether the expression of NMDA receptors was related to modulatory brain stem input to LGN, we examined the effects of the NMDA antagonists when the visual response was enhanced with gamma-aminobutyric acid (GABA) antagonists or acetylcholine (ACh). Neither of these pharmacologic manipulations consistently increased the relative contribution of NMDA receptors to the visual response. 7. No pharmacologic difference was found between nonlagged X- and Y-cells, or between ON- and OFF-center cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Three-dimensional transient model for arc welding process

A direct computer simulation technique, discrete element analysis (DEA), was utilized in the development of a transient multidimensional (2-D and 3-D) mathematical model for investi-gating coupled conduction and convection heat transfer problems associated with stationary and moving arc welding processes. The mathematical formulation considers buoyancy, electro-magnetic, and surface tension driving forces in the solution of the overall heat transfer conditions in the specimen. Furthermore, the formulation of the model allows realistic consideration of the geometrical variations in the workpiece. The model treats the -weld pool surface as a truly deformable free surface, allowing for the prediction of the weld surface deformations such as the “weld crown.≓ A marked element formulation was employed to monitor the transient de-velopment of the weld pool as determined by the latent heat considerations and the calculated velocities in the weld pool. The model was utilized to simulate the heat and fluid flows in the weld pool that occur during stationary (spot) and moving (linear) gas tungsten-arc welding. Also, the present analysis considers a simple rectangular specimen and a geometrically complex specimen to demonstrate the capability of the model to simulate realistic 3-D arc welding prob-lems. The results of the present investigation clearly demonstrate the significant influence of the heat and fluid flows and the specimen geometry on the development of the weld. Comparison of the predicted and the experimentally observed fusion zone and heat-affected zone (HAZ) geometries indicate good agreement.

Response properties of single units in the lateral terminal nucleus of the accessory optic system in the behaving primate.

1. To determine the potential role of the primate accessory optic system (AOS) in optokinetic and smooth-pursuit eye movements, we recorded the activity of 110 single units in a subdivision of the AOS, the lateral terminal nucleus (LTN), in five alert rhesus macaques. All monkeys were trained to fixate a stationary target spot during visual testing and to track a small spot moving in a variety of visual environments. 2. LTN units formed a continuum of types ranging from purely visual to purely oculomotor. Visual units (50%) responded best for large-field (70 x 50 degrees), moving visual stimuli and had no response associated with smooth-pursuit eye movement; some responded during smooth pursuit in the dark, but the response disappeared if the target was briefly extinguished, indicating that their smooth-pursuit-related response reflected activation of a parafoveal receptive field. Eye movement and visual units (36%) responded both for large, moving visual stimuli and during smooth-pursuit eye movements made in the dark. Eye movement units (14%) discharged during smooth-pursuit or other eye movements but showed no evidence of visual sensitivity. 3. Essentially all (98%) LTN units were direction selective, responding preferentially during vertical background and/or smooth-pursuit movement. The vast majority (88%) preferred upward background and/or eye movement. During periodic movement of the large-field visual background while the animal fixated, their firing rates were modulated above and below rather high resting rates. Although LTN units typically responded best to movement of large-field stimuli, some also responded well to small moving stimuli (0.25 degrees diam). 4. LTN units could be separated into two populations according to their dependence on visual stimulus velocity. For periodic triangle wave stimuli, both types had velocity thresholds less than 3 degrees/s. As stimulus velocity increased above threshold, the activity of one type reached peak firing rates over a very narrow velocity range and remained nearly at peak firing for velocities from approximately 4-80 degrees/s. The firing rates of the other type exhibited velocity tuning in which the firing rate peaked at an average preferred velocity of 13 degrees/s and decreased for higher velocities. 5. A close examination of firing rates to sinusoidal background stimuli revealed that both unit types exhibited unusual behaviors at the extremes of stimulus velocity.(ABSTRACT TRUNCATED AT 400 WORDS)