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Cited by 32Neuroscience and Neural EngineeringTactile and Sensory InteractionsVisual perception and processing mechanisms

Eye movements and the perceived location of phosphenes generated by intracranial primary visual cortex stimulation in the blind

Open access
Caspi, Avi, Barry, Michael P., Patel, Uday K., Salas, Michelle Armenta, Dorn, Jessy D., Roy, Arup, Niketeghad, Soroush, Greenberg, Robert J. +1 more·2021·Brain Stimulation
DOI 10.1016/j.brs.2021.04.019
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Abstract

BACKGROUND: Restoring sight for the blind using electrical stimulation of the visual pathways is feasible but demands an understanding of the spatial mapping of the visual world at the site of targeted stimulation, whether in the retina, thalamus, or cortex. While a visual cortex stimulator can bypass the eye and create visual percepts, there is an inherent dissociation between this stimulation and eye movements. It is unknown whether and how robustly the brain maintains the oculomotor circuitry in patients with bare- or no-light perception. OBJECTIVE: To critically and quantitatively evaluate the effect of eye movements have on phosphene locations elicited by cortical stimulation that bypasses the eyes in order to restore sight in blind subjects. METHODS: The NeuroPace Responsive Neurostimulator (RNS) and the Orion visual cortical prosthesis devices were used to electrically stimulate the visual cortex of blind subjects with bare or no light perception. Eye positions were recorded synchronized with stimulation and the location of the percepts were measured using a handheld marker. RESULTS: The locations of cortical stimulation-evoked percepts are shifted based on the eye position at the time of stimulation. Measured responses can be remapped based on measured eye positions to determine the retinotopic locations associated with the implanted electrodes, with remapped responses having variance limited by pointing error. CONCLUSIONS: Eye movements dominate the perceived location of cortical stimulation-evoked phosphenes, even after years of blindness. By accounting for eye positions, we can mimic retinal mapping as in natural sight.

AI snapshot

Written from the paper’s full text.

TL;DR

In two blind volunteers with cortical implants, electrically evoked phosphene locations shifted with eye position and could be remapped to eye-position-independent retinotopic locations. The mapping depended on gaze at stimulation onset, not at delayed pointing, suggesting real-time eye-position compensation is needed for cortical visual prostheses.

Key findings

  • Perceived phosphene location correlated strongly with eye position at stimulation: for RNS electrode A1, Pearson r = 0.971–0.995 horizontal and 0.832–0.971 vertical; for B1, 0.979–0.966 and 0.776–0.961; all p < 1e-7.
  • Remapping by eye position reduced variance to pointing error; single-electrode precision after remapping was 2.2 (mean distance from centroid). In multi-electrode runs, remapped clusters separated: Table 1 centroid distances were 7.4, 13.9, and 7.0 for NeuroPace A1/B1, Orion A10/E09, and Orion B10/C07; spreads were 2.9/2.0, 2.8/3.0, and 2.2/2.1 (SEs in table).
  • Delayed-response experiment (12 trials, six left/right) showed significantly lower variance when correcting for eye position at stimulation than at pointing (Table S-4), indicating localization occurs at stimulation onset.
  • Multi-electrode stimulation in both subjects produced distinct remapped retinal locations per electrode, supporting retinotopic mapping of selected cortical areas.

Methods

  • Two blind volunteers: a 30-year-old female with 8-year bare-light perception from Vogt-Koyanagi-Harada syndrome implanted with NeuroPace RNS over right medial occipital lobe; and a 56-year-old male with optic neuropathy after burns/trauma implanted with Orion over left medial visual cortex. Direct biphasic monopolar stimulation; eye positions recorded with SMI ETG 2.0 or Pupil Labs Pupil Core synchronized to stimulation; subject pointed a handheld marker to phosphene location.
  • Three experiments: single electrode at different gaze directions (21 trials for A1); delayed response after about 5 s to dissociate gaze at stimulation vs response (12 trials); interleaved two-electrode stimulation in RNS and Orion subjects (18, 27, and 25 trials across runs).
  • Analysis: linear regression of head-centered marker location on eye-tracker output to estimate slopes/intercepts and remap to retina-centered coordinates; variance/precision and cluster separation (silhouette method) assessed.

Limitations

  • Stated: limited number of contacts tested; possible other stimulation-related factors may preclude generalization; more systematic studies needed, including higher-order visual areas (V2/V3). Eye tracking may not be feasible in all cortical prosthesis candidates (e.g., enucleation, corneal opacity, blurred pupil boundaries), though specialized contact lenses might help.
  • Stated: open-loop pointing in blind subjects combines percept variability, memory, motor control, and proprioceptive feedback; eye tracker could not be calibrated with visual stimuli in blind subjects.
  • Evident: only two subjects (one per device) and small, unequal trial counts (e.g., 21 single-electrode trials; 12 delayed-response trials; some trials removed for blinks/eye movements), limiting generalizability.

Model-written summary. Check it against the paper before relying on it.

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Registry

Crossref · 10.1016/j.brs.2021.04.019

Cites

35 of 35 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 36 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    Remapped responses have variance limited by pointing error.

    “with remapped responses having variance limited by pointing error.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.
  • Findingown data

    Measured responses can be remapped based on measured eye positions to determine the retinotopic locations associated with the implanted electrodes.

    “Measured responses can be remapped based on measured eye positions to determine the retinotopic locations associated with the implanted electrodes, with remapped responses having variance limited by pointing error.”
    • Also stated in: § Single-electrode stimulation · plain
      “Fig. 3d shows the remapped locations based on the linear relationship with eye position at the time of stimulation.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.
  • Findingown data

    Remapped spatial charts show that the variances of the corrected percepts are reduced significantly.

    “Remapped spatial charts show that the variances of the corrected percepts are reduced significantly and each electrode can be linked to a distinct retinal area.”
    Quote in the paperSubstantial claim
  • Findingown data

    The electrical stimulation in each trial resulted in phosphenes being perceived at different locations in head-centered coordinates.

    “The electrical stimulation in each trial was observed to result in the phosphenes being perceived at different locations in head-centered coordinates.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.
  • Findingown data

    The precision of responses, defined as the mean distance from the centroid over all trials, was 2.2 after remapping based on eye positions.

    “The precision of responses, defined as the mean distance from the centroid over all trials, was 2.2 after remapping based on eye positions.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.
  • May combine two claims. 12
  • Classified as probably not a substantial claim. 27
  • The quote only partly states this claim. 1

  • May combine two claims.

    Findingown data

    Electrical stimulation of implanted electrodes in the medial occipital lobe showed a consistent relationship between percept and electrode locations with common effects of eye position.

    “Electrical stimulation of implanted electrodes in the medial occipital lobe showed a consistent relationship between percept and electrode locations with common effects of eye position.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingown data

    Measurements of percepts and mapping of electrode B3 showed similar results.

    “Measurements of percepts and mapping of electrode B3 (green symbol in Fig. 1) with a comparison of the fitted coefficients relative to the results in Fig. 3, can be found in Table S-2 in the supplementary materials, showing similar results.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • May combine two claims.

    Findingown data

    There is still some correlation between eye position at the time of pointing and the time of stimulation, but there is a lack of meaningful correlation within rightward- or leftward-looking responses.

    “There is still some correlation between eye position at the time of pointing and the time of the stimulation, but note the lack of meaningful correlation within rightward-or leftward-looking responses.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    For electrode B1, the correlation coefficients are 0.979-0.966 and 0.776-0.961 for the horizontal and vertical axis, respectively.

    “For electrode B1 (presented in Figure S-2) the correlation coefficients are 0.979e0.966and 0.776e0.961for the horizontal and vertical axis, respectively.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • May combine two claims. The quote only partly states this claim.

    Findingreasoning

    Because the subjects directed their gazes in different directions for each trial, there was no clear separation between the phosphenes generated by each electrode in head-based coordinates.

    “Because the subjects directed their gazes in different directions for each trial, there was no clear separation between the phosphenes generated by each electrode.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    The mapping between retinotopic and spatiotopic coordinate systems takes into account instantaneous and intended eye positions.

    “The mapping between the coordinate systems takes into account instantaneous and intended eye positions [11].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [11] Duhamel, 1992, The updating of the representation of visual space in parietal cortex by intended eye movements, Science source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Brain activity in the occipital lobe at different eye positions, as measured by BOLD mapping, corresponds to retinal location.

    “This aforementioned work demonstrated that the brain activity in the occipital lobe at different eye positions, as measured by blood oxygen level dependent (BOLD) mapping, corresponded to the retinal location.”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [15] Benson, 2012, The retinotopic organization of striate cortex is well predicted by surface topology, Curr Biol source

      Not checked in the free run.

    • cites [16] Gardner, 2008, Maps of visual space in human occipital cortex are retinotopic, not spatiotopic, J Neurosci source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Retinotopic visual maps in the medial occipital lobe were observed in sighted subjects using fMRI.

    “Retinotopic visual maps in the medial occipital lobe were indeed observed in sighted subjects using fMRI [15,16].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [15] Benson, 2012, The retinotopic organization of striate cortex is well predicted by surface topology, Curr Biol source

      Not checked in the free run.

    • cites [16] Gardner, 2008, Maps of visual space in human occipital cortex are retinotopic, not spatiotopic, J Neurosci source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    The brain uses visual information in both retinotopic and spatiotopic coordinate systems.

    “Although action and perception use the transformed spatiotopic coordinates, there is experimental evidence that the brain uses visual information in both retinotopic [9] and spatiotopic [10] coordinate systems.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [9] Golomb, 2012, Retinotopic memory is more precise than spatiotopic memory, Proc Natl Acad Sci U S A source

      Not checked in the free run.

    • cites [10] Zimmermann, 2016, Spatiotopic adaptation in visual areas, J Neurosci source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The timing of visual cortex stimulation can be finely tuned to produce more complex visual perceptions.

    “A recent study also demonstrated that the timing of the stimulation could be finely tuned to produce more complex visual perceptions [4].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [4] Beauchamp, 2020, Dynamic stimulation of visual cortex produces form vision in sighted and blind humans, Cell source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The number of trials was balanced between directions to maintain the same average location.

    “The number of trials was balanced between directions to maintain the same average location.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductown data

    Post-hoc synchronization between the RNS neurostimulator and the eye tracking system was achieved using Camtasia Screen Recorder.

    “Post-hoc synchronization between the RNS neurostimulator and the eye tracking system was achieved using the Camtasia Screen Recorder (TechSmith, Okemos, MI), which captured and correlated the screen of the neurostimulator with the system timestamp from the eye tracker.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Previously, the ability to perform artificial vision scanning using eye movements could be restored in retinal-prosthesis users.

    “Previously, we demonstrated that the ability to perform artificial vision scanning using eye movements could be restored in retinal-prosthesis users.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [35] Caspi, 2018, Eye movement control in the Argus II retinal-prosthesis enables reduced head movement and better localization precision, Invest Ophthalmol Vis Sci source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductreasoning

    For the Orion stimulator, stimulation data were synchronized using the tracker scene camera.

    “For the Orion stimulator, the stimulation data were synchronized using the tracker scene camera.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • Classified as probably not a substantial claim.

    Study design and conductnone given

    Eye positions were recorded synchronized with stimulation.

    “Eye positions were recorded synchronized with stimulation and the location of the percepts were measured using a handheld marker.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • Classified as probably not a substantial claim.

    Premisecitation

    Some neurons in the lateral occipital lobe have nonretinotopic, e.g. head-based, receptive fields.

    “Some neurons in the lateral occipital lobe have nonretinotopic, e.g.head-based, receptive fields [34].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [34] McKyton, 2007, Beyond retinotopic mapping: the spatial representation of objects in the human lateral occipital complex, Cerebr Cortex source

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    Head movements shifted the entire field-of-view of the head-mounted camera and eye movements shifted the region of interest within the wide field-of-view of the camera.

    “Head movements shifted the entire field-of-view of the head-mounted camera and eye movements shifted the region of interest within the wide field-of-view of the camera.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [35] Caspi, 2018, Eye movement control in the Argus II retinal-prosthesis enables reduced head movement and better localization precision, Invest Ophthalmol Vis Sci source

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data

    The ability to remap the locations perceived by stimulating various electrodes to corrected locations based on eye position, with variability reduced to pointing error, validates the hypothesis that stimulation of the selected cortical areas is retinotopic.

    “The ability to remap the locations perceived by stimulating various electrodes to corrected locations based on eye position (Figs. 5 and 6), with variability reduced to pointing error, validates our hypothesis that stimulation of the selected cortical areas is indeed retinotopic.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Eye movements during electrical stimulation of the visual cortex affect the location of observed phosphenes.

    “While it has been previously reported [1e3] that eye movements during electrical stimulation of the visual cortex affect the location of the observed phosphenes, those observations were only based on the subjective description from the subjects and some incidental observations.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [1e3]

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Cortical prostheses must use dynamic stimulation for pattern vision, suggesting that the pattern is presented over a time window and not simultaneously.

    “The recent finding that cortical prostheses must use dynamic stimulation for pattern vision suggests that the pattern is presented over a time window and not simultaneously [4].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [4] Beauchamp, 2020, Dynamic stimulation of visual cortex produces form vision in sighted and blind humans, Cell source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The brain of a blind subject shifts the location of a percept elicited by electrical stimulation of a retinal location based on the instantaneous eye position at the time of the stimulation, as demonstrated with the Argus II retinal prosthesis system.

    “This finding is similar to results obtained using the Argus II retinal prosthesis system [12] which demonstrated that the brain of a blind subject shifts the location of a percept elicited by electrical stimulation of a retinal location based on the instantaneous eye position at the time of the stimulation.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [12] Caspi, 2017, Retinotopic to spatiotopic mapping in blind patients implanted with the Argus II retinal prosthesis, Invest Ophthalmol Vis Sci source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Study design and conductreasoning

    In blind subjects, calibration using visual stimuli is not feasible.

    “In blind subjects, this calibration is not feasible.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • May combine two claims.

    Interpretationown data

    The observation that mapping should be done based on eye position at the time of stimulation and not on the time of action demonstrates that spatial localization of the phosphene occurs at the time of stimulation.

    “The observation that the mapping should be done based on the eye position at the time of the stimulation and not on the time of action (Fig. 4), demonstrates that spatial localization of the phosphene occurs at the time of stimulation.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The subjects were blinded to which electrode was stimulated in each trial.

    “The subjects were blinded to which electrode was stimulated in each trial.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The research protocols were approved by the Institutional Review Board of the University of California in Los Angeles.

    “The research protocols were approved by the Institutional Review Board of the University of California in Los Angeles.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Study design and conductnone given

    The experimenter verified that the patient moved the eye to the instructed direction prior to the stimulation.

    “The experimenter verified that the patient moved the eye to the instructed direction prior to the stimulation.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    Attention can shift visual maps in the brain, both in BOLD signals and in percepts elicited through direct stimulation.

    “Nonetheless, there is evidence that attention can shift visual maps in the brain, both in BOLD [17] signals and in the percepts elicited through direct stimulation [18].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [17] Crespi, 2011, Spatiotopic coding of BOLD signal in human visual cortex depends on spatial attention, PloS One source

      Not checked in the free run.

    • cites [18] Bestmann, 2007, Spatial attention changes excitability of human visual cortex to direct stimulation, Curr Biol source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    The study was registered on ClinicalTrials.gov with identifiers NCT02747589 and NCT03344848.

    “The ClinicalTrials.govidentifier is NCT02747589 for the RNS NeuroPace neurostimulator and NCT03344848 for the Orion device.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Recent evidence suggests a link between phosphenes derived from brain stimulation and areas identified through retinal visual stimulus.

    “Recent evidence suggests that there is a link between the phosphenes derived from brain stimulation and areas identified through retinal visual stimulus [7], but more studies are needed to determine how reliable and reproducible these percept overlaps would be.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [7] Winawer, 2016, Linking electrical stimulation of human primary visual cortex, size of affected cortical area, neuronal responses, and subjective experience, Neuron source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Electrical stimulation of the lateral occipital lobes of sighted patients with intractable epilepsy who were implanted with subdural electrodes showed a retinotopic mapping of phosphenes.

    “Electrical stimulation of the lateral occipital lobes of sighted patients with intractable epilepsy who were implanted with subdural electrodes showed a retinotopic mapping of phosphenes [33].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [33] Kaido, 2004, Retinotopy with coordinates of lateral occipital cortex in humans, J Neurosurg

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    There are differences between human and non-human primate brain neurophysiology that might be crucial to the function of a cortical visual prosthesis.

    “Although much information can be acquired from non-human primates, there are differences between human and non-human primate brain neurophysiology that might be crucial to the function of a cortical visual prosthesis [19,20].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [19] Passingham, 2009, How good is the macaque monkey model of the human brain?, Curr Opin Neurobiol source

      Not checked in the free run.

    • cites [20] Orban, 2004, Comparative mapping of higher visual areas in monkeys and humans, Trends Cognit Sci source

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data

    Perceived locations in coordinates of the head-mounted camera can be mapped to defined retinal locations, demonstrating that eye scanning is feasible with a cortical prosthesis that completely bypasses the eyes.

    “Our present results, that perceived locations in coordinates of the headmounted camera can be mapped to defined retinal locations, demonstrate that eye scanning is likewise feasible with a cortical prosthesis that completely bypasses the eyes.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Subjects were able to use a combined eye-head scanning mode.

    “Subjects were able to use a combined eye-head scanning mode [35].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [35] Caspi, 2018, Eye movement control in the Argus II retinal-prosthesis enables reduced head movement and better localization precision, Invest Ophthalmol Vis Sci source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    If the percept was leftward, the subject looked leftward for pointing, which can contribute to some correlation between eye position and pointing/perceived location.

    “If the percept was leftward the subject looked leftward for pointing, which can contribute to some correlation between eye position and pointing/perceived location.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    Trials with blinks or eye movements close to the stimulation were removed in the offline analysis.

    “The number of samples at each direction of gaze is unequal because trials with blinks or eye movements close to the stimulation were removed in the offline analysis.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

  • Classified as probably not a substantial claim.

    Study design and conductreasoning

    Experiments were conducted with more than one electrode in each subject.

    “To ensure the results were not a result on a specific electrode test, experiments were conducted with more than one electrode in each subject.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Localization by extra-retinal signals in sighted subjects has limited accuracy with significant variability among subjects and between the horizontal and vertical axes.

    “In addition, it was demonstrated with sighted subjects that localization by extra-retinal signals, as in our case, has limited accuracy with significant variability among subjects and between the horizontal and vertical axes [26,27].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [26] Bock, 1986, Contribution of retinal versus extraretinal signals towards visual localization in goal-directed movements, Exp Brain Res source

      Not checked in the free run.

    • cites [27] Henriques, 1998, Gaze-centered remapping of remembered visual space in an open-loop pointing task, J Neurosci source

      Not checked in the free run.

37 of 37

  • The paper relies on this without support.

    The location where the subject places the handheld marker accurately indicates the perceived location of the phosphene.

    “the subject was instructed to place a red racquetball (57-mm diameter) at the location where they perceived the “light.””
    “The subject placed the marker in space at arm's length to indicate the location of the percept in egocentric world coordinates, as shown in Fig. 2.”

  • The paper relies on this without support.

    Self-calibrated eye tracker data provide a valid estimate of gaze direction in blind subjects.

    “In blind subjects, this calibration is not feasible.Therefore, in the current study the head-centered angular percept positions were correlated with the self-calibrated eye data provided in units of pixels of the scene-camera.”

  • The paper relies on this without support.

    In humans, visual information is acquired in retinotopic coordinates and then re-mapped by the brain into spatiotopic coordinates.

    “In humans, visual information is acquired in retina-centered, i.e., retinotopic, coordinates and then re-mapped by the brain into world-centered, i.e., spatiotopic, coordinates.”

  • The paper relies on this without support.

    The brain performs the retinotopic-to-spatiotopic transformation based on eye and head positions.

    “The brain performs this transformation based on eye and head positions.”

  • The authors state this as an assumption.

    Eye positions at response time determine percept localization.

    “Assuming that eye positions at response time determine percept localization, the difference in gaze-shift instructions would have no impact on a correlation between such eye positions and marker locations.”

  • The paper relies on this without support.

    In blind subjects, the placement of a marker in an open-loop pointing task can only be guided by proprioceptive signals about head and hand position.

    “In blind subjects, the placement of the marker can only be guided by proprioceptive signals about head and hand position.”
6 of 6

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Gaze orientation plays an important role in both phosphenes location and interpretability for mobility and visual tasks.

    “Gaze orientation also plays an important role in both phosphenes location59 and interpretability for mobility and visual tasks.60”

    From Visual Prostheses in the Era of Artificial Intelligence Technology (2025) · cites this paper as [59]

  • Intracortical prostheses are more invasive but take advantage of the close contact between the electrodes and the neurons to provide a more selective stimulation at much lower current amplitudes.

    “Intracortical prostheses are more invasive but take advantage of the close contact between the electrodes and the neurons to provide a more selective stimulation at much lower current amplitudes [100, 101].”

    From Advances in visual prostheses: engineering and biological challenges (2022) · cites this paper as [101]

  • Voluntary and involuntary eye movements affect the spatial localisation of the phosphene elicited by retinal and cortical stimulation, even after years of blindness.

    “They affect the spatial localisation of the phosphene elicited by retinal [200] and cortical [101] stimulation, even after years of blindness.”

    From Advances in visual prostheses: engineering and biological challenges (2022) · cites this paper as [101]

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • B1 confidence-interval bounds are reversed

    Minor

    The stated horizontal interval for electrode B1 runs from 0.979 to 0.966, but the paper says these values are the lower and upper bounds, respectively. A lower bound cannot exceed its upper bound.

1 of 1

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Authors
Caspi, A., Barry, M. P., Patel, U. K., Salas, M. A., Dorn, J. D., Roy, A., Niketeghad, S., Greenberg, R. J., & Pouratian, N.
Affiliations
  • Johns Hopkins University (US)
  • Second Sight Medical Products (US)
  • University of California, Los Angeles (US)
  • Alfred Mann Foundation (US)
  • The University of Texas Southwestern Medical Center (US)
Corresponding author
Caspi, Avi
ORCID
Venue
Brain Stimulation
Publisher
Elsevier BV
Published
2021-07
Volume
14
Issue
4
Pages
851-860
ISSN
  • 1876-4754 (ISSN-L)
  • 1935-861X
DOI
10.1016/j.brs.2021.04.019
OpenAlex
W3162346430
Semantic Scholar
d8ac0a82dd35e5852a08103e211f5d8b02ce7408
Cited by
32
References
35
Citations by year
2026: 6 · 2025: 7 · 2024: 6 · 2023: 5 · 2022: 7 · 2021: 1
FWCI
2.19
Type
journal article
Language
en
Open access
Licence
Locations
  • Brain stimulation (publishedVersion, cc-by-nc-nd)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
National Institutes of Health (UH3NS103442)
Topics
  • Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neuroscience and Neural Engineering
  • Life Sciences › Neuroscience › Cognitive Neuroscience › Tactile and Sensory Interactions
  • Life Sciences › Neuroscience › Cognitive Neuroscience › Visual perception and processing mechanisms
Keywords
eye movements, phosphenes, visual cortex stimulation, blindness, visual prosthesis, retinotopic mapping, no light perception, neurostimulation
MeSH terms
Blindness, Electric Stimulation, Eye Movements, Humans, Phosphenes, Visual Cortex, Visual Perception, Visual Prosthesis

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-08.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-08.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

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