Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2021 Sep 1.
Published in final edited form as: J Neurosci Methods. 2020 Jul 1;343:108839. doi: 10.1016/j.jneumeth.2020.108839

Long-term Labeling of Microelectrode Tracks with Fluorescent Latex Microspheres

Joshua B Simmons 1, Robert S Turner 2, Jonathan C Horton 1
PMCID: PMC7442242  NIHMSID: NIHMS1610929  PMID: 32621915

Abstract

Background:

After physiological recordings are performed in behaving animals, it is valuable to identify microelectrode tracks in histological sections so that neuronal responses can be correlated with brain anatomy. However, no good method currently exists for long-term labeling, so that microelectrode tracks can be recovered months or even years after recording sessions.

New Method:

Penetrations were made into the brains of mice with microelectrodes coated with fluorescent dyes packaged into 0.2 µm polystyrene microspheres, followed by survival periods of 3 days, 2, 4, or 6 months. Sections were examined by fluorescence microscopy before and after cytochrome oxidase histochemistry to identify microelectrode tracks.

Results:

After all 4 survival periods, 0.2 µm fluorescent microspheres clearly marked the tracks of microelectrode penetrations.

Comparison with Existing Methods:

Fluorescent microspheres label microelectrode penetrations for longer than do fluorescent lipophilic dyes, such as FM 1–43FX. The label appears punctate, and resistant to degradation, because it is protected by the barrier of the polystyrene micro-container.

Conclusions:

Coating of microelectrodes with fluorescent microspheres allows one to identify the penetration track in histological sections half a year later. This technique may be useful when electrophysiological recording sessions are being carried out in behaving animals, with plans to identify electrode tracks in histological sections many months later.

Keywords: microelectrode, latex bead, cytochrome oxidase, DiI, FM 1–43FX, neural recording

1. Introduction

To correlate single cell recordings with neuroanatomy it is exceedingly useful to mark the path followed by a microelectrode through the brain. It is not certain who should be credited with first accomplishing this feat, but Hess (1932) is a strong contender. He passed current through a metal electrode and then performed a Prussian blue reaction to reveal iron deposition at the tip. The tracks of microelectrodes made of glass were first illustrated by Renshaw and colleagues (1940). They described giving a twist to their stereotaxic micromanipulator to slice the microelectrode transversally through the brain, increasing the scar visible after Nissl staining. This procedure also created a shoulder along the track, corresponding to the intermediate point along the penetration where the electrode was moved laterally. Hubel (1959) devised a method to identify multiple intermediate points along an electrode track by passage of direct current to create small electrolytic lesions. Such lesions, and the electrode track connecting them, are easily visible in tissue sections processed for cytochrome oxidase (CO) histochemistry (Wong-Riley, 1989). They appear pale, silhouetted against the dark CO reaction product that accumulates diffusely within the neuropil (Fig. 1).

Fig. 1).

Fig. 1)

Open in a new tab

Tangential section through macaque striate cortex processed for CO showing patches in layers 2/3 and a Stoner-Mudge coated tungsten microelectrode track marked with 6 electrolytic lesions made by passing 5 µA for 5 s. The penetration terminated at the last lesion (arrow). The recording was made to determine if regions of low orientation specificity coincide with the patches. No relationship was found. (Horton and Hubel 1980, unpublished data).

Electrode tracks and lesions are relatively easy to recover when the tissue is processed soon after an experiment. It is more challenging when a long interval has elapsed between the recording session and histological examination of the brain. The tracks and lesions left by a microelectrode appear to fade eventually, although this healing process has not been well documented. In some cases, electrodes may be so fine that they never leave a trace. Fig. 2 shows a tissue section from a Rhesus macaque after 90 penetrations with a glass-insulated tungsten microelectrode (180 µm shaft diameter) made over 11 months into the globus pallidus (Zimnik et al., 2015). There is no evidence of any tracks in the target nucleus, but gliosis is present in the overlying cortex from insertion of the the 30-gauge guide tube. Perhaps because of difficulty in recovering electrode tracks after long-term recordings, some physiologists do not attempt to correlate their physiological findings with anatomy.

Fig. 2).

Fig. 2)

Open in a new tab

Coronal section stained for Nissl after 90 recording sessions from the Rhesus globus pallidus. There is gliosis (arrows) at the surface from the 30-gauge guide tube, but no electrode tracks are visible in the deeper target nucleus. Metal tubes were inserted post-mortem into the recording chamber, leaving parallel hollow tracks, to bracket the region of the basal ganglia reached by the microelectrodes.

Given the vital advances in neuroscience being made through daily electrophysiological recording sessions in alert, trained monkeys (Buffalo et al., 2019), it would be helpful to have some method to label microelectrode tracks so that they can be identified months or years later. Snodderly and Gur (1995) used a fluorescent dye called DiI (1,1′ - dioctadecyl – 3,3,3′,3′ - tetra- methylindo-carbocyanine perchlorate) to coat microelectrodes for recordings in awake monkeys (Honig and Hume, 1989). One month after electrode penetrations were made, tracks were easily visible in striate cortex. DiCarlo and colleagues (1996) also found that DiI-labeled tracks were present up to a month after electrode penetrations, although only fragments of tracks were recoverable in some cases. We have tested several compounds to find a method that labels electrode tracks for longer than just one month. Here we report that fluorescent microspheres can reliably mark microelectrode tracks for up to 6 months.

2. Materials and methods

2.1. Animals and Surgery

Thirteen female C57BL/6J black mice aged 3 months weighing 20–24 g were used in this study. All procedures were conducted in accordance with a protocol approved by the UCSF Institutional Animal Care and Use Committee.

Mice were anesthetized with a mixture of 2.0 mg ketamine HCl and 0.3 mg xylazine HCl delivered intraperitoneally. They were placed in a stereotaxic frame and supported on a heating pad. The scalp was infiltrated with 1% lidocaine HCl with epinephrine 1:100,000 (Hospira, Inc., Lake Forest, Il). A 4 mm sagittal incision was made to expose the skull. 1 mm diameter burr holes were drilled 1.25 mm on either side of the sagittal suture, centered 1.75 mm anterior to the interaural line, to expose the dura over the parietal cortex. After perpendicular microelectrode penetrations were made, the scalp was closed with two 7–0 polyglactin sutures. Mice survived for varying time intervals to evaluate the persistence of various compounds used to label the electrode tracks (Table 1).

Table 1.

Summary of Experiments.

Mouse Number Label Survival Time
1 * FM 1–43FX 20 minutes
2 FM 1–43FX 25 days
3 FM 1–43FX 25 days
4 FM 1–43FX 25 days
5 0.2 µm microspheres 3 days
6 0.2 µm microspheres 3 days
7 0.2 µm microspheres 2 months
8 0.2 µm microspheres 2 months
9 0.2 µm microspheres 4 months
10 0.2 µm microspheres 4 months
11 0.2 µm microspheres 6 months
12 0.02 µm microspheres 1 day
13 0.02 µm microspheres 1 day
Open in a new tab
*

Only two FM 1–43FX penetrations were made in this animal.

2.2. Fluorescent Dyes

A fluorescent dye related to DiI was tested: FM 1–43FX (Life Technologies, catalog # F35355). This lipophilic dye is a water-soluble, aldehyde-fixable (FX) analog of FM 1–43. It leaves vividly fluorescent tracks in mouse brain after tissue is processed with the CLARITY technique, whereas regular DiI is washed out by this lipid clearing procedure (Jensen and Berg, 2016). FM 1–43FX was dissolved in distilled water (0.1% w/v solution).

Fluorescent dyes contained in polystyrene microspheres were also tested as a method of marking electrode tracks. Three types of beads were tried: 0.2 µm diameter yellow-green sulfate FluoSpheres® (Invitrogen Molecular Probes™, catalog # F8848), 0.02 µm diameter yellow-green sulfate FluoSpheres® (catalog # F8845), and 0.02 µm diameter yellow-green aldehyde-sulfate FluoSpheres® (catalog # F8760). All are supplied as an aqueous suspension containing 2% polystyrene solids. These sulfate latex microspheres are relatively hydrophobic but adsorb well onto protein surfaces. The latter property allows them to stick to the walls of a track as the microelectrode is advanced through neural tissue. Two different microsphere diameters were chosen in order to determine whether the size of the microspheres affects the labeling quality.

2.3. Electrode Preparation

Penetrations were made with 125 µm diameter epoxy-coated tungsten microelectrodes with a rounded fine tip (Frederick Haer catalog #UEWLEHSM8NNE). The process of electrode coating was identical for FM 1–43FX and FluoSpheres® beads. The solutions were placed in a microcuvette and the microelectrode was lowered directly into the dye solutions to a depth of 5 mm and left in place for one minute. It was then raised in the air and allowed to dry for one minute. (Fig. 3). After each penetration the microelectrode was recoated by repeating this procedure. After the experiment, microelectrodes were cleaned with a 70% isopropyl alcohol gauze to remove the fluorescent dye.

Fig. 3).

Fig. 3)

Open in a new tab

Microelectrode coated with fluorescent microspheres. A) Epoxy-coated tungsten microelectrode after coating with 0.2 µm latex microspheres. The puncta presumably represent clumps of beads. B) Same coated electrode, after a single penetration in the mouse brain. Most of the fluorescent label is removed, underscoring the importance of re-coating between each penetration.

The dye-coated microelectrode was advanced through the intact dura to a depth of 4.00 mm in 30 s using a Narishige motorized hydraulic micromanipulator. The electrode was then left at that depth for one minute prior to withdrawal at the same speed. Two tandem sagittal penetrations 600 µm apart were made through each burr hole, for a total of 4 penetrations per mouse. We were concerned that dye might be scraped off the microelectrode by passage through the dura, but at least in mice this issue did not seem to arise. It might be a problem in animals with thicker dura, such as monkeys being used for long-term awake recordings.

2.4. Histology

After surviving differing time periods after electrode penetrations, mice were euthanized with 4 mg sodium pentobarbital intraperitoneally. Following transcardiac perfusion with 4% paraformaldehyde in 0.1 M phosphate buffer solution (PBS), the brain was extracted and submerged in 2% paraformaldehyde in 0.1 M PBS with 30% sucrose. After a minimum of 24 hours, 100 µm sagittal sections were cut using an American Optical 860 freezing microtome. The sections were mounted onto subbed slides and cover-slipped with 80% glycerin/20% 100 mM tris buffer while still slightly damp. Sections were examined in a Zeiss Axiophot microscope with a 5x Plan-Neofluar objective to identify microelectrode tracks. The filter set was ET-CY3 (ET545/25x, T565LPXR, ET605/70m, Chroma catalog #49004) for FM 1–43FX (peak absorption 510 nm/peak emission 626 nm). The filter set was FITC/Cy2 (ET470/40x, T495LP, ET525/50m, Chroma catalog #49002) for the FluoSpheres® (peak absorption 505 nm/peak emission 515 nm). Representative photographs are shown of microelectrode penetrations, which showed consistent labeling among animals tested under the same conditions.

After selected sections were photographed, the coverslips were removed and the slides were rinsed in 0.1 M PBS. The sections were then reacted for CO activity for about 6 hr, as described previously (Horton, 1984). Some CO sections were dehydrated in alcohol, cleared in xylene, and coverslipped with Permount. Other sections were re-coverslipped with glycerin/tris buffer. CO-processed sections were also photographed with the Zeiss Axiophot microscope.

3. Results

3.1. FM 1–43FX

In Mouse 1 two tandem penetrations were made in one hemisphere with a microelectrode coated with FM 1–43FX. The mouse was perfused 20 minutes after the second electrode penetration to simulate the timing of an acute physiology experiment. The brain was sectioned the next day.

As reported by Jensen and Berg (2016), FM 1–43FX produced a strong fluorescence signal. Fig. 4A shows a vertical electrode penetration labeled by FM 1–43FX through the parietal cortex and hippocampus, ending in the thalamus. The picture was taken the same day that the brain was sectioned. There is some lateral diffusion of the label but the track is sharply defined. Fig. 4B shows the same section 3 days later after reaction for CO activity. The electrode track is visible in the light microscope, but it would be difficult to identify without having the fluorescent label available as a guide. Fig. 4C shows a fluorescence image of the CO section, photographed the next day, after dehydration and 24 hr exposure to Permount®. The FM 1–43FX dye has diffused, but the track is still strongly labeled. After 15 months of dark storage, the FM1–43FX track was still vibrant, with hardly any reduction in fluorescence (data not shown).

Fig. 4).

Fig. 4)

Open in a new tab

FM 1–43FX labeling of microelectrode tracks. A) Parasagittal section from Mouse 1 showing 2 penetrations with an FM 1–43FX coated electrode. One penetration was recovered nearly completely in this single section (arrow). B) Same section photographed 3 days later after processing for CO. The electrode track is visible in CO, but the termination (arrow) is certain only in FM 1–43FX image. C) Same section re-photographed 24 hours after CO processing, dehydration, and coverslipping with Permount®. The electrode track is still well defined by FM 1–43FX. Note reduced label in the molecular layer of the dentate gyrus (*) compared to (A), but increased uptake by granule cells (arrow) and pyramidal cells of CA-1 (arrowhead).

In Fig. 4C FM 1–43FX label is visible in granule cells in the dentate gyrus, presumably from diffusion along their dendrites located in the molecular layer. There is also label in the pyramidal cells of the CA1. This labeling occurred post-mortem, because immediately after the section was cut, there was scant cell labeling (compare Fig. 4A and 4C).

3.2. Persistence of FM 1–43FX tracks

We next tested for how long FM 1–43FX tracks remain visible in the brain. Electrode penetrations were made in Mice 2 – 4, followed by a survival time of 25 days. Fig. 5A shows an example of an electrode track. The FM 1–43FX dye has diffused widely and the track no longer appears intensely labeled, as it did in the animal perfused immediately after the experiment (Fig. 4A). The point where the track ends is not certain. This is a critical deficiency, because it means that recording sites along the penetration cannot be scaled accurately. There are many labeled cells, especially in the hippocampus. After CO histochemistry, only a short fragment of the track is visible (Fig. 5B).

Fig. 5).

Fig. 5)

Open in a new tab

Fading of FM 1–43FX labeling. A) Parasagittal section from Mouse 4 processed after 25 days of survival showing an electrode penetration labeled with FM 1–43FX. There is substantial diffusion along the entire track and the termination is uncertain. Many cells are labeled, especially in CA1–4 (arrowhead) and the dentate gyrus (arrow). B) Same section after CO processing. After 25 days the electrode track is almost indiscernible.

Although all 12 penetrations were detectable in this group of 3 mice, FM 1–43FX had diffused considerably. As a result, after a survival time of less than a month, the electrode tracks were not well defined. We concluded that FM 1–43FX would not be a suitable method for long-term marking of electrode tracks in animals surviving for many months after recording sessions.

3.3. Polystyrene microspheres filled with fluorescent dyes

Electrodes coated with 0.2 µm diameter yellow-green sulfate FluoSpheres® were used to make penetrations in seven mice (Mice 5 – 11, Table 1). The animals survived for varying times after the experiment to test how well the label persists in the brain.

Mice 5 and 6 were perfused 3 days after electrode penetrations were made. All penetrations appeared intensely labeled. The fluorescence was contained in round, punctate structures, which varied in size, presumably because the beads tend to clump (Fig. 6).

Fig. 6).

Fig. 6)

Open in a new tab

Parasagittal section from Mouse 5 showing an electrode penetration (arrow marks end) labeled by 0.2 µm diameter yellow-green sulfate fluorescent microspheres after a 3 day survival period.

Mice 7 and 8 were perfused after a survival period of 2 months. All penetrations were easily visible (Fig. 7A). They were similar in appearance to the labeled penetrations in the mice that survived only 3 days, although some clumps of beads had migrated laterally up to 150 µm. After sections were processed for CO, microelectrode tracks could also be detected (Fig. 7B). However, without images of the section showing fluorescent label for reference, the electrode termination points would be uncertain. Mice 9 and 10 were perfused after a survival time of 4 months. The electrode penetrations appeared similar to those recovered after a 2 month survival period (data not shown).

Fig. 7).

Fig. 7)

Open in a new tab

Fluorescent microsphere labelling after 2 months. A) Parasagittal section from Mouse 8, showing a pair of electrode penetrations labeled by fluorescent microspheres. Both tracks are vividly outlined by fluorescent beads, with clear entry and endpoints (arrows). Lateral diffusion has occurred at some points along the track, but unlike FM 1–43FX, the microspheres do not appear to have labeled hippocampal cells, even after two months. B) Same section after CO reaction. Electrode tracks are difficult to identify.

Mouse 11 survived for 6 months after electrode penetrations. Compared with the electrode track after 2 months, the penetrations are more sparsely labeled by the fluorescent beads and a greater percentage of them has diffused laterally (Fig. 8A). Nonetheless, the center of the tracks and their termination points are clear. Although latex beads were originally used as retrograde tracers (Katz and Iarovici, 1990), there was little evidence of transport to distant neurons in these mice.

Fig. 8).

Fig. 8)

Open in a new tab

Fluorescent microspheres persist for 6 months. A) Parasagittal section showing a pair of bead-labeled tracks in Mouse 11, 6 months after the penetrations were made. A few scattered beads mark the ends of the tracks (arrows). B) Microelectrode tracks are hardly visible in the same section processed for CO activity. C) Same section re-photographed in fluorescence microscope after reaction for CO activity and coverslipping in glycerin/tris. There is loss of fluorescence compared to (A), but the penetrations remain identifiable.

After examination under fluorescence, the sections were then processed for CO activity. Microelectrode tracks were scarcely visible when the CO labelled sections were examined in the light microscope. For example, of the two tracks labeled by fluorescent microspheres in Fig. 8A, only a short proximal fragment of one track could be detected (Fig. 8B). To test if the fluorescent microspheres could survive CO histochemistry, the sections were re-coverslipped in glycerin/tris buffer. The intensity of bead labeling was reduced, but the tracks were still visible (Fig. 8C). After 3 months in dark storage, the fluorescence label diminished no further. A small number of bead-labeled sections were dehydrated in ethanol, cleared in xylene, and coverslipped in Permount. These procedures completely destroyed the bead labeling.

Mouse 12 was tested with 0.02 µm diameter sulfate FluoSpheres® and Mouse 13 was tested with 0.02 µm diameter aldehyde-sulfate FluoSpheres®. The survival time was one day. In both animals the labeling of the 4 microelectrode tracks was faint and incomplete. The 0.02 µm spheres appeared so inferior to the 0.2 µm spheres for marking electrode tracks that we did not test longer survival periods.

4. DISCUSSION

The use of extracellular recording microelectrodes has provided valuable insight into the workings of the mammalian brain. The information gained is amplified enormously if one can determine the precise anatomical site in the brain where each neuron was recorded. Techniques to accomplish this goal are well established for acute physiological experiments that are followed by immediate processing of brain tissue (Wässle and Hausen, 1981). CO histochemistry is especially useful (Fig. 1), because it reveals cortical columns and layers, and also shows in stark relief the tracks and electrolytic lesions made by microelectrodes (Adams and Horton, 2006; Horton, 1984).

Fluorescent lipophilic dyes have been introduced as a reliable method for marking the tracks of extracellular electrodes. DiI and DiI-C5 consistently mark the entire electrode penetration in macaque visual cortex when tissue is processed a month after the recording session (DiCarlo et al., 1996; Snodderly and Gur, 1995). Recently, various DiI analogues have been developed that are more strongly fluorescent than DiI and also have the advantage of becoming fixed to proteins by paraformaldehyde (Jensen and Berg, 2016). This property means that the fluorescent dyes remain in the tissue after lipids have been removed by passage through serial ethanol solutions and xylene. We found that FM1–43FX vividly marks electrode tracks even after tissue is reacted for CO and coverslipped with an aromatic hydrocarbon mounting medium (Fig. 4). This means that one does not need first to coverslip slides with an aqueous mounting medium, identify fluorescently-labeled tracks, photograph them, and then carry out CO histochemistry followed by dehydration and re-coverslipping. Instead, one can follow the more efficient procedure of reacting all sections for CO activity, dehydrating them, coverslipping permanently, and then searching for fluorescently-labeled tracks.

When tracks marked with FM1–43FX were examined in mice that survived for 25 days, the dye had faded and diffused considerably (Fig. 5). We expected the penetrations to be more strongly labeled, given that DiI intensely marks microelectrode tracks in monkey cortex after a one month survival period. No studies have compared the durability of DiI and FM1–43FX labeling. It is possible that DiI remains in tissue longer than FM1–43FX. It is known to persist in adult retinal ganglion cells for 9 months (Vidal-Sanz et al., 1988). Another possibility is that the young mouse brain is able to disperse lipophilic dyes more efficiently than the adult macaque brain. If so, FM1–43FX might be satisfactory for labeling microelectrode tracks in monkeys after one month.

In systems neuroscience, it has become common to make microelectrode recordings in monkeys on a daily basis, followed by a long period before anatomical studies can be undertaken. Microelectrode arrays that are implanted in the cortex leave a scar that makes their footprint easy to identify (Barrese et al., 2016; Economides et al., 2011). Chronically implanted probes induce proliferation of astrocytes (Potter et al., 2012). However, the track of a single microelectrode that has been advanced through the cortex and removed immediately is usually invisible when the tissue is examined months later (Fig. 2). To recover an electrode track in this setting, a long-lasting marker is needed to visualize the penetration. The fading of FM1–43FX detected in mice at one month motivated us to search for a more durable compound. Fluorescently labelled polystyrene microspheres have been used as microinjectable cell tracers (Wadsworth, 1987), in flow cytometry (Iannone, 2001), for blood flow measurements (Robertson and Hlastala, 2007), and studies of phagocytosis (Morris et al., 2003). Katz (1984; 1990) introduced fluorescent microspheres as a retrograde neuronal tracer. He found relatively little diffusion of the microspheres at the injection site and reported that rhodamine-labeled fluorescent microspheres persist at least 10 weeks upon being transported to neuronal somata. Divac and Mogensen (1990) reported neurons in rats labeled by rhodamine microspheres that showed undiminished fluorescence after a survival time of one year. Fluorescent microspheres are non-toxic, causing no damage to cells labeled in vivo (Katz and Iarovici, 1990). These properties motivated us to test fluorescent microspheres as a method for semi-permanent marking of microelectrode tracks.

Fluorescent microspheres showed excellent labeling 6 months after microelectrode penetrations were made into the brain. In fact, the labeling at 6 months (Fig. 8) was only slightly diminished compared to that seen at 2 months (Fig. 7). This finding implies that the beads are relatively inert and may label electrode tracks for years. Our observations were made using epoxy-coated metal electrodes. It is unclear if latex microspheres adhere as efficiently to glass-coated electrodes or pipettes.

The labeling by microspheres is quantal, in the sense that the fluorescence is contained inside discrete packages, whereas DiI compounds produce diffuse labeling. The advantage is that the fluorescent compound is protected from degradation by encapsulation in the latex bead. The disadvantage is that the signal is punctate, so there must be a sufficient quantity of microspheres along the track to identify it clearly. This can lead to some ambiguity in determining the track termination point, since it is defined only by the last visible microsphere. Another disadvantage of polystyrene microspheres is that they dissolve in ethanol, so sections must be coverslipped in an aqueous-based medium for recovery of fluorescent tracks. Even carrying out a CO reaction will cause some loss of fluorescence (compare Fig. 7A, C).

Fluorescent microspheres are available that absorb and emit at many different wavelengths. One could coat microelectrodes with at least five variously colored microspheres to distinguish between different penetrations.

These experiments in mice suggest that fluorescent microspheres may offer a solution to the problem of identifying microelectrode penetrations in monkeys, in situations where anatomical studies cannot be done until months after the physiological recordings were made.

Highlights.

  • No good method exists to label microelectrode tracks indefinitely in animals

  • Lipophilic dyes are popular for marking electrode tracks, but they fade over time

  • Fluorescent microspheres can be coated onto microelectrodes to label their tracks

  • Fluorescent microspheres can label electrode tracks for at least 6 months in mice

Acknowledgments:

This work was supported by grants EY029703 (J.C.H.) and EY02162 (Vision Core Grant) from the National Eye Institute and an unrestricted grant from Research to Prevent Blindness.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Declarations of Interest: None

REFERENCES

  1. Adams DL, Horton JC. Monocular cells without ocular dominance columns. J. Neurophysiol, 2006; 96: 2253–64. [DOI] [PubMed] [Google Scholar]
  2. Barrese JC, Aceros J, Donoghue JP. Scanning electron microscopy of chronically implanted intracortical microelectrode arrays in non-human primates. J Neural Eng, 2016; 13: 026003. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Buffalo EA, Movshon JA, Wurtz RH. From basic brain research to treating human brain disorders. Proc Natl Acad Sci U S A, 2019. [DOI] [PMC free article] [PubMed]
  4. DiCarlo JJ, Lane JW, Hsiao SS, Johnson KO. Marking microelectrode penetrations with fluorescent dyes. J. Neurosci. Methods, 1996; 64: 75–81. [DOI] [PubMed] [Google Scholar]
  5. Divac I, Mogensen J. Long-term retrograde labelling of neurons. Brain Res, 1990; 524: 339–41. [DOI] [PubMed] [Google Scholar]
  6. Economides JR, Sincich LC, Adams DL, Horton JC. Orientation tuning of cytochrome oxidase patches in macaque primary visual cortex. Nat. Neurosci, 2011; 14: 1574–80. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Hess WR. Beiträge zur Physiologie des Hirnstammes: Die Methodik der lokalisierten Reizung und Ausschaltung subkortikaler Hirnabschnitte. Georg Thieme, 1932.
  8. Honig MG, Hume RI. Dil and diO: versatile fluorescent dyes for neuronal labelling and pathway tracing. Trends Neurosci, 1989; 12: 333–5, 40–1. [PubMed] [Google Scholar]
  9. Horton JC. Cytochrome oxidase patches: a new cytoarchitectonic feature of monkey visual cortex. Philos. Trans. R. Soc. Lond. B. Biol. Sci, 1984; 304: 199–253. [DOI] [PubMed] [Google Scholar]
  10. Hubel DH. Single unit activity in striate cortex of unrestrained cats. The Journal of physiology, 1959; 147: 226–38. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Iannone MA. Microsphere-based molecular cytometry. Clin. Lab. Med, 2001; 21: 731–42. [PubMed] [Google Scholar]
  12. Jensen KH, Berg RW. CLARITY-compatible lipophilic dyes for electrode marking and neuronal tracing. Scientific reports, 2016; 6: 32674. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Katz LC, Burkhalter A, Dreyer WJ. Fluorescent latex microspheres as a retrograde neuronal marker for in vivo and in vitro studies of visual cortex. Nature, 1984; 310: 498–500. [DOI] [PubMed] [Google Scholar]
  14. Katz LC, Iarovici DM. Green fluorescent latex microspheres: a new retrograde tracer. Neuroscience, 1990; 34: 511–20. [DOI] [PubMed] [Google Scholar]
  15. Morris MR, Dewitt S, Laffafian I, Hallett MB. Phagocytosis by inflammatory phagocytes: experimental strategies for stimulation and quantification. Methods Mol. Biol, 2003; 225: 35–46. [DOI] [PubMed] [Google Scholar]
  16. Potter KA, Buck AC, Self WK, Capadona JR. Stab injury and device implantation within the brain results in inversely multiphasic neuroinflammatory and neurodegenerative responses. J Neural Eng, 2012; 9: 046020. [DOI] [PubMed] [Google Scholar]
  17. Renshaw B, Forbes A, Morison BR. Activity of isocortex and hippocampus: electrical studies with micro-electrodes. J. Neurophysiology, 1940; 3: 74–105. [Google Scholar]
  18. Robertson HT, Hlastala MP. Microsphere maps of regional blood flow and regional ventilation. J Appl Physiol (1985), 2007; 102: 1265–72. [DOI] [PubMed] [Google Scholar]
  19. Snodderly DM, Gur M. Organization of striate cortex of alert, trained monkeys (Macaca fascicularis): ongoing activity, stimulus selectivity, and widths of receptive field activating regions. J. Neurophysiol, 1995; 74: 2100–25. [DOI] [PubMed] [Google Scholar]
  20. Vidal-Sanz M, Villegas-Perez MP, Bray GM, Aguayo AJ. Persistent retrograde labeling of adult rat retinal ganglion cells with the carbocyanine dye diI. Exp. Neurol, 1988; 102: 92–101. [DOI] [PubMed] [Google Scholar]
  21. Wadsworth P Microinjected carboxylated beads move predominantly poleward in sea urchin eggs. Cell Motil. Cytoskeleton, 1987; 8: 293–301. [DOI] [PubMed] [Google Scholar]
  22. Wässle H, Hausen K. Extracellular Marking and Retrograde Labelling of Neurons In Heym C, Forssmann W-G, editors. Techniques in Neuroanatomical Research Springer Berlin Heidelberg: Berlin, Heidelberg, 1981: 317–38. [Google Scholar]
  23. Wong-Riley MT. Cytochrome oxidase: an endogenous metabolic marker for neuronal activity. Trends Neurosci, 1989; 12: 94–101. [DOI] [PubMed] [Google Scholar]
  24. Zimnik AJ, Nora GJ, Desmurget M, Turner RS. Movement-related discharge in the macaque globus pallidus during high-frequency stimulation of the subthalamic nucleus. J. Neurosci, 2015; 35: 3978–89. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES