Visual Acuity of Simulated Thalamic Visual Prostheses in Normally Sighted Humans

Simulation in normally sighted individuals is a crucial tool to evaluate the performance of potential visual prosthesis designs prior to human implantation of a device. Here, we investigated the effects of electrode count on visual acuity, learning rate and response time in 16 normally sighted subjects using a simulated thalamic visual prosthesis, providing the first performance reports for thalamic designs. A new letter recognition paradigm using a multiple-optotype two-alternative forced choice task was adapted from the Snellen eye chart, and specifically devised to be readily communicated to both human and non-human primate subjects. Validation of the method against a standard Snellen acuity test in 21 human subjects showed no significant differences between the two tests. The novel task was then used to address three questions about simulations of the center-weighted phosphene patterns typical of thalamic designs: What are the expected Snellen acuities for devices with varying numbers of contacts, do subjects display rapid adaptation to the new visual modality, and can response time in the task provide clues to the mechanisms of perception in low-resolution artificial vision? Population performance (hit rate) was significantly above chance when viewing Snellen 20/200 optotypes (Log MAR 1.0) with 370 phosphenes in the central 10 degrees of vision, ranging to Snellen 20/800 (Log MAR 1.6) with 25 central phosphenes. Furthermore, subjects demonstrated learning within the 1–2 hours of task experience indicating the potential for an effective rehabilitation and possibly better visual performance after a longer period of training. Response time differences suggest that direct letter perception occurred when hit rate was above 75%, whereas a slower strategy like feature-based pattern matching was used in conditions of lower relative resolution. As pattern matching can substantially boost effective acuity, these results suggest post-implant therapy should specifically address feature detection skills

Mapping the primate lateral geniculate nucleus: A review of experiments and methods

Mapping neuronal responses in the lateral geniculate nucleus (LGN) is key to understanding how visual information is processed in the brain. This paper focuses on our current knowledge of the dynamics the receptive field (RF) as broken down into the classical receptive field (CRF) and the extra-classical receptive field (ECRF) in primate LGN. CRFs in the LGN are known to be similar to those in the retinal ganglion cell layer in terms of both spatial and temporal characteristics, leading to the standard interpretation of the LGN as a relay center from retina to primary visual cortex. ECRFs have generally been found to be large and inhibitory, with some differences in magnitude between the magno-, parvo-, and koniocellular pathways. The specific contributions of the retina, thalamus, and visual cortex to LGN ECRF properties are presently unknown. Some reports suggest a retinal origin for extra-classical suppression based on latency arguments and other reports have suggested a thalamic origin for extra-classical suppression. This issue is complicated by the use of anesthetized animals, where cortical activity is likely to be altered. Thus further study of LGN ECRFs is warranted to reconcile these discrepancies. Producing descriptions of RF properties of LGN neurons could be enhanced by employing preferred naturalistic stimuli. Although there has been significant work in cats with natural scene stimuli and noise that statistically imitates natural scenes, we highlight a need for similar data from primates. Obtaining these data may be aided by recent advancements in experimental and analytical techniques that permit the efficient study of nonlinear RF characteristics in addition to traditional linear factors. In light of the reviewed topics, we conclude by suggesting experiments to more clearly elucidate the spatial and temporal structure of ECRFs of primate LGN neurons

Primary experiment optotype sizes.

<p>Snellen and equivalent LogMAR visual acuity values of the optotype sizes used in the Primary experiment.</p

Validation results.

<p>Hit rate as function of font sizes (translated to the equivalent acuity) for the standard Snellen chart test (red unfilled squares) and the multiple-optotype two alternative forced choice (MO2AFC) letter recognition task (blue filled circles). Error bars are two-sigma confidence levels. Smooth traces are sigmoid fits to each data set. Uneven spacing across the horizontal axis reflects the step sizes between lines in the standard Snellen chart. Results from the letter recognition task have been normalized to span [0–1], an equivalent range as the Snellen chart test (0%–100%), for ease of comparison. The two curves are in high agreement, although there appears to be a trend to a shallower transition with the letter recognition task, possibly due to uncorrected false negatives at the upper end of the range.</p

Visual acuity values for each phosphene pattern.

<p>For each phosphene pattern under test, P<i>_i</i>, the number of phosphenes in the central part of vision is shown, followed by the population mean acuity using the two criteria of first significant deviation from chance, and interpolated 75% hit rate (see main text). The FSD acuity corresponds to the stimuli where the population response first differs significantly from chance (<i>p</i><0.001 for all except P₃ for which <i>p</i><0.02), whereas the i75 acuity corresponds to the 75% performance level of a sigmoid fitted to the population data. Uncertainties shown for i75 values represent 95% confidence ranges.</p

Phosphene patterns.

<p>Numeric details of the experimental parameter of phosphene count. Electrode spacing refers to the distance between electrode tips implanted in LGN tissue in a 3D regular grid pattern that will produce a center-weighted phosphene pattern in visual space <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0073592#pone.0073592-Rizzo1" target="_blank">[27]</a>. Total electrode count includes electrodes that will generate phosphenes anywhere in the entire visual field, most of which would not be active in the Primary experiment, whereas the count within 10 degrees is for those electrodes generating phosphenes that lie within the central part of visual space corresponding to the approximate location of the letterform stimuli in this report.</p

Photographs of the apparatus.

<p>A subject is shown performing the experiment (left) along with a close-up of the goggles with eye tracking camera (right). In use, the head frame is adjusted to the seated height of each subject, and the monitor position adjusted to maintain a consistent distance to each subject's eyes. The base of the head frame is mechanically secured to the desk, but the upper part can be raised or lowered. The camera is mounted on a flexible arm that holds position once adjusted for a close-up view of the subject's eye. Experiment workstations and experimentor controls are not shown.</p

Cues and distractors.

<p>The nine letters used from the Snellen set are shown paired with their three distractors. Each letter appears in the table once as a cue, and three times as a distractor. Each trial uses one of the cues and one of the three distractors associated with that cue. Distractors were selected to match cues in identification difficulty, and to form a closed, balanced set. The letter recognition paradigm involves a total of twenty seven randomly presented alternative choice combinations.</p

Comparison between paradigms.

<p>Testing using the Snellen chart (left) proceeds top to bottom, and left to right, with the subject calling out each letter on successive rows, or declaring their inability to do so. Testing started with the 20/50 line as shown here, which is the fourth line of standard charts. Testing using our letter recognition task (right) is performed in a balanced, randomly interleaved pattern; the figure, designed to show the equivalence between tests at given acuities, uses shuffle arrows to imply the interleaving. Trial conditions are represented as a cue letter followed by the two alternatives. Red and Green highlights illustrate single letter scoring (correct/incorrect) during an example data collection session where the subject was assessed with 20/18 on the Snellen task and 20/20 on the letter recognition task.</p

Learning effects.

<p>Scatter plots of population hit rate and response time in the first versus last segment of trials. (LEFT) Population hit rates split by condition (stars) and overall mean (bulls eye). Data are above the line of equality as subjects perform more accurately in the last 200 trials (10 per condition) than in the first 200 trials. (CENTER) Population response time for each condition (stars) and overall mean (bulls eye). Nearly every datum is below the line of equality as subjects have faster responses in the last 200 trials than in the first 200. (RIGHT) Combining population response times and hit rates from the two previous graphs with linkages between first (open circles) and last (filled stars) segments reveals two distinct spans of behavior, one at low hit rates that is more disorganized, and one at higher hit rates that displays strong structure. The lower hit rates correspond to the Chance regime where subjects are more likely to be guessing; the higher hit rates correspond to the Mid-Range and High-Performance regimes. The largest improvements, as shown by the longest linkages, cluster around the threshold between Chance and Mid-Range regimes.</p