Methods

1. Timing criterion for LT bursts spike train ≥100 ms ≤4 ms S-potentials can arise from a single retinal ganglion cell.(Top) Average of 71 isolated S-potentials recorded with no other events occurring within 20 msec during spontaneous activity. Gray shading represents +/-1 standard deviation around the baseline noise. (Bottom) Average of 9,416 S-potentials during visually driven activity. The larger standard deviation before and after the S-potential reflects the occurrence of other spiking events. During the 2 msec prior to the S-potential, the standard deviation is identical to that seen in the upper trace, indicating an absolute refractory period. RGC spikes sweep LGN spikes sweep Growth of retinal ganglion cell S-potentials as microelectrode approaches an LGN neuron. During this 15 min trace, the electrode was advanced 80 mm from the point the unit was first recorded. The initial waveform (timepoint 1) was triphasic, characteristic of an LGN somatic spike recorded at a distance. By timepoint 5, S-potentials began to be observed in isolation and immediately preceding LGN spikes. After timepoint 8, the A-potential occasionally appears with S-potentials, but without the B-potential, indicating a failure of propagation into the somatodendritic tree. S-potentials precede and drive nearly all LGN spikes in a burst. (Top) A rastergram and spike histogram of 190 LT bursts, aligned on the first LGN spike of each burst. The preceding 100 msec spike-free epochs are thought to be definitive of IT activity. (Bottom) Rastergram and histogram of the S-potentials accompanying each LT burst shown above. The RGC is not silent during the pre-LGN burst interval, nor is its input ignored during secondary spikes in a burst. Before nearly all LGN bursts, one “priming” retinal input appears within the prior 20 ms, followed by the S-potential that generates the LGN burst. S-potentials driving LGN spikes can be unmasked using muscimol. (Top) During continuous stimulation with a light spot turning on and off at 3 Hz, a 0.5% solution of muscimol is injected in the LGN to hyperpolarize the neuron by opening GABAA receptor channels. After a few minutes, the LGN neuron begins to cease firing (left), while the RGC continues to fire at the same rate (right). Thus, muscimol does not block presynaptic GABA receptors in the macaque LGN. (Bottom) Expanded views of the recording episodes marked by red and green arrows above. During the control trace, S-potentials as well as S-A and S-A-B waveforms are present. After muscimol application, all but one of the A- and B-potentials have disappeared. This provides further evidence that S-potentials initiate most LGN spikes. Examples of LGN bursts during naturalistic visual stimulation. During episodes of burst firing (interspike intervals < 4 ms), S-potentials can be seen in isolation and clearly leading some of the LGN spikes, including those after the first spike. In other LGN spikes, the S-potentials are challenging to identify without a subtraction procedure. secondary burst spikes cardinal burst spikes 0.2 mV 1 sec S-potentials recorded during visual stimulation. (Top) Luminance variation of a spot of light centered on the receptive field of an parvocellular ON-center cell. (Middle) Raw electrode trace acquired during a 10 sec stimulus sample. (Bottom) Expanded segment of the recording shown above, illustrating the occurrence of S-potentials associated with LGN A-potentials and somatodendritic B-potentials. S-potentials which occur at the beginning of the B-potentials are often difficult to identify because the waveforms merge, but in this example a T-potential notch is still visible. Anatomy of an LGN spike and the S-potential. Averaged traces of LGN spikes (black) and isolated S-potentials (blue) in the macaque show stereotypic features in their waveforms. The S-potential is the excitatory postsynaptic potential (EPSP) originating from retinal input. It is monophasic and preceded by a smaller presynaptic potential designated “T” (Wang, Cleland & Burke, Brain Res.,1985). LGN spikes have these same features, although the S-potential partially merges with the rising phase of the action potential, itself exhibiting two components. The “A” potential is due to the action potential initiated at the axon hillock, and is followed by the “B” potential which is the backpropagation of the action potential into the somatodendritic tree. secondary burst spikes cardinal burst spikes Coordinated burst firing in the retina and lateral geniculate nucleus of the macaque Lawrence C. Sincich, Ken F. Park, Daniel L. Adams, John R. Economides and Jonathan C. Horton Beckman Vision Center, University of California San Francisco, San Francisco, CA 94143 Introduction Identifying retinal input to the LGN Bursting activity in the retina and LGN The spiking activity of neurons in the lateral geniculate nucleus (LGN) is often categorized into two modes: burst and tonic. The bursting mode has been shown in cats and guinea pigs to depend on activation of the low-threshold calcium current (IT). Characteristically, all spikes but the first one in a burst do not require additional synaptic input to occur because IT depolarizes the cell, generating several INa action potentials. Although bursting is commonly associated with sleep states, it is also found under awake conditions. Bursting has been proposed as mechanism for signaling novelty to neurons in V1. In the macaque monkey, it is not known if IT is active when stimuli are derived from natural scenes. It is also unclear how the firing pattern in the retina influences the mode of firing in the LGN. We have addressed several questions about firing activity in the macaque LGN: (1) What is the dominant firing mode during a stimulus paradigm that mimics the viewing of natural scenes? (2) Are LGN bursts simply due to bursting activity in the retina? (3) Does the LGN ignore retinal input during bursts because of IT activation? To answer these questions, we recorded the action potentials of LGN neurons along with their retinal input in the form of excitatory postsynaptic potentials (S-potentials) using a single extracellular electrode. This method offers the advantage of extended recording periods, wherein the LGN could switch between tonic and burst modes, while providing access to the firing patterns on both sides of the retinogeniculate synapse. LGN spikes and retinally driven postsynaptic potentials have distinct waveforms: Interspike intervals that are often used to identify when IT is active. From intracellular studies in cats, the combination of these interspike intervals has been correlated with IT current. Merged S-potentials are identifiable in both tonic and burst LGN spikes. A template LGN spike, stripped of its S-potential, was derived from a spike which had an long S-potential latency, but within the absolute refractory period of the cell (bottom left, in red). This template was aligned at the half-height point of the A-potential of other spikes, displayed in rank-order according to the S-potential latency (left column). After subtracting the template, the remaining waveforms reveal S-potentials (middle column). The distribution of S-potential latencies for 1,000 spikes show that most LGN spikes occur within 0.5 msec of an S-potential (right column). From this set, 42 spikes were categorized as secondary spikes in a burst, and their S-potential latencies had a similar distribution (black circles). The presence of S-potentials in burst spikes suggests that LGN bursting is inherited from retinal spike trains rather than the activation of IT. Retinal ganglion cells recorded directly show burst firing behavior to naturalistic stimuli. Two examples of retinal ON cells are shown bursting in response to a step increase in luminance (red arrow). The top trace was recorded in the LGN and contained S-potentials, while the bottom trace was recorded in the optic tract from a different animal. Both traces show LT burst firing in the retina, with strikingly similar latencies to stimulus onset. Methods Experiments were conducted in 6 adult male macaques using procedures approved by the UCSF Committee on Animal Research. Extracellular recordings of neurons in the LGN were made under standard conditions, with isoflurane as anesthetic. Paralysis with vecuronium bromide was used to eliminate eye movements. We searched for LGN neurons with associated S-potentials arising from retinal ganglion cells (RGCs). From a total of 240 cells recorded, 19 S-potential/LGN recordings with sufficient data quality were chosen for analysis. Each LGN cell’s receptive field center was stimulated with a light spot produced by an LED of the preferred color shining on the back of a translucent screen. The receptive field surround was masked with black flocking paper. The luminance of the LED was varied at 80 Hz according to a naturalistic temporal frequency distribution (van Hateren, Vision Res. 1997). The temporal power spectrum of this stimulus followed a 1/f power law, often called “pink noise”. LGN spikes and S-potentials were classified as either tonic or burst. A burst was any group of spikes with an interspike interval less than 4 msec. Bursts preceded by a silent period of at least 100 msec were subclassified as low-threshold (LT), reflecting the dynamics of the thalamic low-threshold calcium current IT (Lu et al., J. Neurophysiol., 1992). All remaining spikes were designated as tonic. Conclusions • During naturalistic stimulation, nearly all LGN spikes are driven by retinal spikes, and the tonic mode predominates. • When the LGN fires in burst mode, it is slavishly responding to burst firing in the retina. • LGN bursts require a “priming” input from the retina to be initiated. • Under our stimulus and recording conditions, the low-threshold calcium current IT is not required to drive LGN burst firing. During naturalistic stimuli, RGCs and LGN neurons fire predominantly in tonic mode. (Top) Scatterplot of pre- versus postsynaptic spike intervals for a parvocellular ON neuron reveals a wide range of interspike times (n=10,000). Less than 1% of these spikes could be classified as LT burst spikes, based on timing criteria (red dots). The “secondary” burst spikes are presumed to be caused by IT activation, theoretically requiring no input from the retina. (Bottom) Scatterplot of the S-potentials for the same LGN neuron. The proportion of bursting activity between RGCs and LGN neurons is similar.