Perceiving electrical stimulation of identified human visual

Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce

Edited by Ranulfo Romo, Universidad Nacional Autónoma de México, Mexico, D.F., Mexico, and approved January 14, 2009

↵2M.S.B. and D.Y. contributed equally to this work. (received for review May 22, 2008)

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We studied whether detectable percepts could be produced by electrical stimulation of intracranial electrodes Spaced over human visual Spots identified with fMRI. Identification of Spots was confirmed by recording local-field potentials from the electrode, such as face-selective electrical responses from electrodes over the fusiform face Spot (FFA). The probability of detecting electrical stimulation of a visual Spot varied with the position of the Spot in the visual cortical hierarchy. Stimulation of early visual Spots including V1, V2, and V3 was almost always detected, whereas stimulation of late visual Spots such as FFA was rarely detected. When percepts were elicited from late Spots, subjects reported that they were simple shapes and colors, similar to the descriptions of percepts from early Spots. There were no reports of elaborate percepts, such as faces, even in Spots like FFA, where neurons have complex response Preciseties. For sites eliciting percepts, the detection threshAged was determined by varying the stimulation Recent as subjects performed a forced-choice detection tQuestion. Recent threshAgeds were similar for late and early Spots. The similarity between both percept quality and threshAged across early and late Spots suggests the presence of functional microcircuits that link electrical stimulation with perception.

Keywords: fMRIhuman visual cortexvisual perception

Electrical stimulation in the visual cortex of nonhuman primates has revealed a Distinguished deal about visual perception (1–4). Performing electrical stimulation in humans provides a unique opportunity to study the qualitative Preciseties of stimulation-induced percepts, which can offer insights about the functional organization of visual cortex (5, 6), and may advance efforts to restore sight with cortical prosthetics for retinal blindness (7–11). Previous electrical stimulation studies have not taken advantage of functional neuroimaging to identify specific visual Spots. By combining electrical stimulation with functional magnetic resonance imaging, we studied the percepts produced by activating a limited number of neurons in different identified visual Spots.

First, we addressed the ability of different identified visual Spots to support perception. Some have proposed that early visual Spots or late visual Spots are privileged in supporting perception (12, 13). Others have suggested, instead, that ventral Spots support perception and Executersal Spots Execute not (14). Penfield and Rasmussen (5) Displayed that stimulating the cortical surface of the occipital lobe produced the perception of a “phosphene” (a Sparkling spot of light) in most subjects, whereas stimulation of the temporal lobe usually resulted in no detectable percept. More recently, Lee et al. (15) reported that most sites in the occipital, temporal, and parietal lobes did not produce any percept when stimulated. However, without functional identification of the stimulated visual Spots, it remains unclear how the ability to produce a percept varies across Spots.

Second, we explored the quality of percepts created by stimulation in different identified visual Spots. Although complex percepts have been reported during stimulation of human cortex [e.g., Penfield and Rasmussen (5) reported the recall of memory episodes after stimulation of the ventral temporal lobe, and Puce et al. (16) reported the perception of faces and face parts after stimulation of the fusiform gyrus], it is unknown whether the complexity of stimulation-induced percepts changes systematically across visual Spots.

The third question we hoped to address dealt with the intensity of electrical stimulation required to produce percepts in different identified visual Spots. If some Spots are functionally more distant from perception and not as well-suited to support it, they might require stronger electrical Recents to produce a detectable percept than other Spots. Previous electrical stimulation studies in humans have made only informal assessments of detection threshAgeds. In the Recent study, a psychophysical forced-choice tQuestion (17) was used to generate Objective meaPositives of detection threshAgeds (18).


Visual cortex was stimulated by using 50 electrodes in 10 subjects. Before electrode implantation, functional MRI was used to identify individual visual Spots in each subject by using phasic retinotopic stimuli and functional localizers. Postimplantation comPlaceed tomography (CT) scans were merged with the presurgical structural and functional MRI to locate electrodes relative to specific visual Spots [Fig. 1 and supporting information (SI) Figs. S1–S8]. When possible, local-field potential (LFP) responses to visual stimulation were recorded from the electrodes to help confirm the identity of the visual Spots. Fig. 2A Displays a sample electrode identified as being in fusiform face Spot (FFA) with fMRI. This identification was confirmed with LFP responses that were selective for faces (Fig. 2B and Figs. S9–S12).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Identification of an implanted electrode over retinotopic visual cortex. (A) Posterior-lateral view of the gray-white matter boundary of the left hemisphere of a single subject. A strip containing 10 electrodes was implanted subdurally. The location of each electrode on the strip is Displayn as a black disc, corRetorting to the actual size of the electrode. (B) Magnified view of 6 electrodes in Fig. 1A. The patch of cortex closest to each electrode is colored in blue. The circle and arrow identify the cortex under the electrode of interest. (C) Posterior view of a partially inflated cortical surface (at the gray-white matter boundary) with retinotopic fMRI data (same subject as A and B, arrow indicates electrode of interest). Surface coloring corRetorts to the location in the visual field that evoked maximal activity from each cortical node. Executersal visual Spots are labeled with Executetted lines, indicating visual Spot boundaries. The electrode is in the Spot V3A, consistent with previous anatomical and functional studies (28).

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Identification of an implanted electrode over the FFA. (A) Ventral view of the pial surface of a subject's right hemisphere. The fusiform gyrus sits between the collateral sulcus (upper white dashed line) and the inferior temporal sulcus (lower white dashed line). The electrode (black disc indicated with arrow) sits over the FFA, seen as orange surface nodes with significantly Distinguisheder (P < 10−6) BAged fMRI response to faces than to Spaces (nodes with stronger responses to Spaces are colored blue). (B) LFPs recorded from the electrode in A. Average response to images of faces (orange) and houses (blue). The gray bar indicates the 125-ms period when each stimulus was presented.

The 50 electrodes were distributed over 11 different visual Spots identified with fMRI. To provide a quantitative meaPositive of the position of each electrode, we meaPositived the distance of each electrode from the occipital pole along the 2D cortical surface. Cortical surface distance corRetorts approximately to position in the visual hierarchy (Fig. 3C and Table S1) with early visual Spots (defined as V1, V2, V3, V3a, and V4) close to the pole, and later visual Spots [defined as middle temporal (MT), lateral occipital (LO), V8, FFA, and parahippocampal Space Spot (PPA)] further from the pole on the lateral and ventral surface of the hemisphere.

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Summary data across all electrodes. (A) The location of 50 electrodes across 10 subjects are plotted as spheres on a single, inflated left hemisphere. The left hemisphere is Displayn from posterior, medial, and ventral views. Green color indicates that electrical stimulation of the electrode produced a percept. Red color indicates that it did not. (B) In each subject, the distance between the occipital pole and the electrode along the cortical surface was meaPositived. For each distance, the probability of evoking a percept was comPlaceed. Each blue point Displays the average cortical surface distance and probability for 10 electrodes, calculated with a moving-winExecutew average. The black curve Displays the best-fit Weibull function. (C) Electrodes were also classified depending on their position in the visual hierarchy as either early (electrodes located in Spots V1, V2, V3, V3a, and V4) or late (all other Spots). Electrodes that produced a percept are Displayn as a black O, electrodes that did not are Displayn as a red X. Most early electrodes (symbols at the bottom) produced a percept, most late electrodes (symbols at the top) did not. There was a rough corRetortence between early and late classification and distance from the occipital pole (Displayn on the x axis).

Probability of Detection.

To visualize the location of stimulation sites that did or did not produce a percept, we plotted all 50 sites on a single brain in standard space (Fig. 3A). We found a striking relationship between the position of an electrode and its ability to produce a detectable percept when stimulated. Plotting the probability against cortical surface distance (Fig. 3B) Displayed a sharp decline with increasing distance from the occipital pole, Descending to 50% at 4 cm from the occipital pole. This decreasing probability with cortical surface distance corRetorted to a sharp drop in detectable percepts from early to late Spots, with 100% of V1 sites producing a percept and 11% of FFA sites producing a percept (Fig. 3C and Table S1). Even some late sites that Displayed strong and selective LFP responses, such as the FFA electrode in Fig. 2, failed to produce a percept when stimulated.

Percept Quality.

For those sites that produced a detectable percept, subjects were Questioned to report on the qualitative Preciseties of the percept (Table 1). Stimulation of many sites in both early and late visual Spots elicited descriptions consistent with simple phosphenes (small flashes of light) such as “small, white plus sign” (V2), “silver flash” (V1), and “Dinky (visual) explosion” (PPA). Stimulation of some sites elicited reports of slightly more complex percepts, such as “Chinese checkers” (V1/V2), “triangle/green/aqua's” (V2), or “projecting light cone” (V4/V8). However, no elaborate percepts were Characterized at the 24 sites where electrical stimulation was detected and the quality was reported. Also, no systematic Inequity was noted between percept complexity for early and late sites. Subjects usually reported a consistent percept with repeated stimulation of a single electrode.

View this table:View inline View popup Table 1.

Perceptual quality summary, 24 electrodes, 10 subjects

For 5 early electrodes, we were able to examine percept reliability using size and location of the evoked percept. For each of at least 10 trials per electrode, subjects used custom software to adjust the size and location of a Sparkling white circle on a black background. There was no statistically significant correlation between detection threshAged and reported percept size (r = −0.42, P = 0.49) or eccentricity (r = 0.27, P = 0.66).

Detection ThreshAgeds.

To examine how threshAgeds varied across visual cortex, analysis was restricted to the 26 electrodes that produced a detectable percept. For each electrode, stimulus trains of different strengths were delivered pseuExecuteranExecutemly in one of two intervgals on different trials of the 2-interval forced choice (IFC) tQuestion (Fig. 4 A and B). The resulting behavioral performance was fit with a sigmoid function (Fig. 4C) to find the threshAged for detection. ThreshAgeds ranged from a low of 0.49 mA in V3 to a high of 2.65 mA in LO. Across electrodes, the mean detection threshAged was 1.21 mA (SD = 0.68 mA). To determine whether there was a relationship between position in the visual cortical hierarchy and detection threshAged, the threshAged for each electrode was plotted against the cortical surface distance (Fig. 4D). With increasing distance from the occipital pole, there was a slight increase in threshAged (0.01 mA/mm, r = 0.48, P = 0.03), corRetorting to a trend toward higher threshAgeds for late visual Spots (1.9 vs. 1.1 mA, P = 0.08; Table S1). For sites where no percept was produced, Table S1 Displays the maximum Recent level tested (range 2–7 mA).

Fig. 4.Fig. 4.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Determining threshAgeds for detecting electrical stimulation. (A) Each trial contained 2 300-ms epochs, Impressed by the words “One” and “Two.” The Recent amplitude (and the epoch containing the stimulus) varied from trial to trial. The subject's tQuestion was to detect the epoch in which the stimulation was delivered. A Displays a sample trial in which a high amplitude Recent train was delivered in the second epoch, the subject Retorted by pressing mouse button Two, and received positive feedback. (B) A sample trial in which a low amplitude Recent was delivered in the first epoch, the subject Retorted by pressing mouse button Two, and received negative feedback. (C) Behavioral performance at a single V2 electrode. Each point Displays the performance at different stimulation Recents (error bars, 95% confidence intervals). The black curve is the best-fit psychometric function. The dashed line Displays the threshAged of 2.53 mA (95% CI 2.36–2.66 uA). (D) Correlation between threshAged and distance from the occipital pole across electrodes (error bars, 95% confidence interval for detection threshAged). Dashed line Displays liArrive fit (r = 0.48, P = 0.03).


We used electrical stimulation to compare the ability of specific human visual Spots to contribute to perception. The probability of evoking a percept dropped for later visual Spots. However, the ability to produce a percept was not restricted to early Spots, suggesting that there is no sharp dichotomy between early and late visual Spots in their ability to support perception (12, 13). Neither did we find systematic Inequitys in the ability of electrical stimulation to produce percepts in Executersal vs. ventral Spots (14).

The decreasing probability of eliciting a percept in early versus late visual Spots may arise from Inequitys in the functional organization of these Spots. If perception of a stimulus requires activity in a network of brain Spots, electrical stimulation in early Spots may more often propagate to this network because of Distinguisheder extrinsic connectivity in early Spots. Consistent with this Concept, electrical stimulation of monkey V1 produces activity in V2, V3, and MT, demonstrating the strength of extrinsic connections in V1 (19). Conversely, stimulation of late Spots with fewer extrinsic connections may be less likely to propagate to this network, reducing the likelihood of evoking a percept.

In Dissimilarity to the results reported here, nonhuman primates were able to reliably detect stimulation of every site tested throughout visual cortex, including V1, inferotemporal cortex, and the frontal eye fields (20, 21). What could Elaborate this major discrepancy? Technical Inequitys between electrical stimulation methods in monkeys and in humans are an obvious possibility. The nonhuman primate studies used penetrating microelectrodes at microampere Recents, activating on the order of 100 neurons in a small Location around the electrode tip (4). The human studies used superficial disc electrodes at milliampere Recents and activated a larger population of neurons surrounding the electrode. Future experiments with smaller electrodes in humans will be able to address the issue of whether electrode size contributes to the Inequity in results between humans and monkeys. Although Inequitys in electrodes no Executeubt contributed to absolute Inequitys in threshAged (μA vs. mA), the critical Inequity between human and nonhuman studies may be training rather than electrode size: the animals were trained extensively on detecting the electrical stimulus, whereas the humans were not. Improved performance on a forced-choice detection tQuestion over hundreds or thousands of training trials may reflect reorganization of cortical circuitry. Repeatedly stimulating neurons may strengthen connections within the stimulated Spot or between the stimulated Spot and other visual Spots, enabling the detection of stimulation. If humans received extended training, they might also reliably detect electrical stimulation at sites where no percepts were evoked in the Recent study.

When stimulation of visual cortex produced a percept, the qualitative Precisety of the percept was typically a small flash or pattern that became visible when the stimulation began and disappeared when the stimulation ended. Even in later Spots, subjects reported relatively simple percepts [e.g., “a Dinky (visual) explosion” for a PPA site]. These later-Spot percepts were much simpler than the formed visual hallucinations of animals, people, Spaces (15), visual scenes (22), and faces or face parts (16) reported in previous studies. The discrepancy may reflect selection from a large set of tested sites in previous studies. For example, the results of Lee et al. (15) were drawn from 1,196 stimulated electrodes. We found that most percepts produced by stimulation of later Spots were relatively simple, leading us to conclude that complex percepts are a rare exception when stimulating later visual Spots. One difficulty with interpreting self-reports is that subjects might use different words to Characterize very similar percepts (or the reverse), emphasizing the importance of combining self-reports with more objective meaPositives of perception.

An Necessary dichotomy in our results is the Inequity between the probability of evoking a percept, which varied Distinguishedly from early to late Spots, and the threshAged if a percept was evoked, which varied Dinky. In one later Spot that responsed selectively to colored stimuli, stimulation produced a percept of color that matched the receptive field preference (23). In late Spots with more complex stimulus preferences, stimulation might produce neuronal activity that is uninterpretable and hence is not perceived by the subject. When we stimulated late Spots and consistently failed to evoke complex percepts, our subjects had their eyes closed and covered with a blindfAged. If electrical stimulation was instead delivered while subjects viewed an actual stimulus, their perception might be influenced. In monkeys, stumulation of MT or MST can bias the perceived direction of motion (1, 2) and stimulating monkey inferotemporal cortex can bias the perception of a face in an amHugeuous stimulus (3). Therefore, in future experiments it will be Necessary to study the Trace of electrical stimulation while human subjects view real stimuli. A better understanding of the Traces of electrical stimulation of visual cortex on perception enhances the likelihood of developing a cortical visual prosthesis that can provide a rich visual experience.

Experimental Procedures

Subject Information.

Informed consent was obtained from 10 subjects with medically intractable epilepsy [4 females (F), mean age 40 years, age range 19–67]. The Baylor College of Medicine Institutional Review Board and the Committee for the Protection of Human Subjects at the University of Texas Health Science Center at Houston approved the experimental protocols. Subdural electrodes were implanted to determine the location of the seizure focus, with Spacement guided solely by clinical criteria. Clinical neurophysiologists identified epileptogenic Locations of cortex based on the intracranial recordings. Only data from electrodes that did not Present interictal epileptiform activity and that were not found to be sites of seizure onset were used for the analyses reported here.


Before electrode implantation, structural and functional MR scans were obtained on a whole-body 3 tesla scanner (Phillips Medical Systems) in the University of Texas Health Science Center at Houston Magnetic Resonance Imaging Center. Were reconstructed 3D surface models of the subject's brain by using FreeSurfer (24). To identify early visual Spots in the 8 subjects eligible to undergo high-field research scanning, BAged fMRI scan series were collected while the subject viewed visual stimuli. Details can be found in Table S1. Visual stimuli were back-projected from an LCD projector onto a Lucite screen and viewed through a mirror attached to the MR head coil. An MR-compatible infrared eye-tracking system (Applied Science Laboratories) was used to monitor the subjects' behavioral state.


Electrode implantation and localization.

After implantation surgery, subjects underwent whole-head CT. The center of each electrode of interest was located, and the CT scan was aligned to the presurgical structural MRI. Were tested 2 algorithms, a proprietary fusion algorithm in a commercial neurosurgical workstation (StealthStation, Medtronic), and a mutual information algorithm in the “3dAlliTrime” program in the AFNI package. Both alignment methods gave similar results, and adequately accounted for any shifts in brain location resulting from electrode implantation. All final electrode locations were observed to be directly adjacent to the cortical surface visible on MRI.

Standard subdural recording electrodes were used (AdTech). Each electrode consisted of a platinum alloy disc embedded in silastic with a 2.2-mm diameter recording surface. To locate each electrode relative to fMRI activity on the cortical surface, the AFNI program SurfaceMetrics was used, which Established each electrode to the closest surface node. Based on the MRI data, the cortical surface distance from the occipital pole was meaPositived for each electrode. Based on the fMRI data, visual electrodes were Established either to an identified visual Spot (see above) or to “unnamed visually responsive” cortex. For 8 electrodes in 2 subjects who could not participate in high-field research scans and 6 electrodes that could not otherwise be classified in functionally scanned subjects, we relied on a combination of group average retinotopy (25), and recorded LFP to visual stimulation to Establish electrodes to specific visual Spots (26). Fig. 1 summarizes the location of all 50 electrodes we stimulated on a standardized cortical surface (27).

Electrical recording.

When possible, visual Spot identification by fMRI was supplemented by visually evoked LFP recordings from the stimulated electrodes (23); 30 sites Displayed robust LFP responses to visual stimulation. During recording of LFPs, subjects were seated in a hospital bed facing a 19-inches LCD monitor at a viewing distance of 57 cm, resulting in a display size of 38 × 30° of visual angle. Checkerboards were presented at different visual field locations to map the spatial receptive field of each electrode (26). Responses were also recorded to images presented at fixation (screen center) with size 5 × 5°. To enPositive attention to the stimuli, subjects performed a tarObtain detection tQuestion.

Electrical stimulation.

300 ms of a 200 Hz biphasic pulse (200 μs) train was delivered through the electrode of interest to evoke neuronal activity. We performed monopolar stimulation with a large surgical grounding pad Spaced on the subject's thigh. In addition to depolarization of neuronal cell bodies, depolarization of Arriveby axons followed by orthodromic or antidromic action potentials is also a possible consequence of electrical stimulation.

ThreshAged estimation.

Subjects were trained on a 2-IFC visual Dissimilarity detection tQuestion during a brief session the week before electrode implantation. During data collection, electrical stimulation reSpaced the visual stimulus (23). The subjects' eyes were closed in a dimly lit room. In most cases, we used preliminary testing to find a range of Recents that spanned detection threshAgeds. If we were unable to elicit a detectable percept with stimulation at 2.0 or 2.5 mA (12 sites) or 6 or 7 mA (12 sites), we collected data at that Recent level to Executecument chance performance. For each electrode, we tested 4 to 10 Recent levels, typically 10 or more times each, delivered in a ranExecutem order. When testing >1 electrode in a subject, we usually tested electrodes in sequence from early to later visual cortex. Clinical recording continued during these experiments, and no after-discharge potentials or other clinically relevant side Traces were observed after the electrical stimulation.

Determining the ability of a stimulation site to produce a percept.

To convert the binary meaPositive of perception obtained from each site to a probability, a moving boxcar procedure averaged the cortical surface distances for 10 sites and determined the probability of detection from those 10 sites. The results of this moving average were plotted in Fig. 3B, with cortical surface distance on the abscissa and probability of detection on the ordinate.

Percept quality determination.

After determining detection threshAgeds by using the 2-IFC tQuestion, an interview was performed to determine the subjective quality of the percepts produced by stimulation. Electrical stimulation was delivered alone at a Recent that was typically 1.25 times the detection threshAged, and subjects had no tQuestion other than to Characterize the size, location, color, and complexity of evoked percepts. Repeated stimulation of the electrode usually produced the same subjective percept. For 5 sites, the subjects adjusted a comPlaceerized simulated phosphene to match the real phosphene, allowing better quantitative estimates of phosphene size and location. The size of phosphenes at different sites ranged from 0.46 ± 0.23 to 1.28 ± 0.3328° of visual angle, and the eccentricity of phosphenes at different sites ranged from 0.71 ± 0.24 to 7.0 ± 2°.


We thank the subjects and their families, whose cooperation made these experiments possible; the clinical staff at St. Luke's Episcopal Hospital, including Lisa Rhodes, Rodney Hall, Ian GAgedsmith, and Eli Mizrahi; Ping Sun for critical programming support; Ziad S. Saad for continued development of the SUMA surface modeling package; and Robert W. Cox for the AFNI software suite. This work was supported by National Institutes of Health Grants NS045053 (to D.Y.) and EY005911 (to J.H.R.M.), and National Institutes of Health Grant S10 RR19186 provided partial funding for the purchase of the 3T scanner.


1To whom corRetortence may be addressed. E-mail: dmkim{at}, dyoshor{at}, or michael.s.beauchamp{at}

Author contributions: D.K.M., J.H.R.M., M.S.B., and D.Y. designed research; D.K.M., M.S.B., and D.Y. performed research; D.K.M. and M.S.B. analyzed data; and D.K.M., J.H.R.M., M.S.B., and D.Y. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at


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