- Artificial Vision: A Clinical Guide - Google книги
- Artificial vision: what people with bionic eyes see
- Artificial Vision for the Blind
This shortcoming complicates the creation of percepts in world-centered coordinates, which highly depends on conveying information to the brain that is associated with the correct location in the visual scene. Inspired by the need to overcome this hurdle, this review aims to address gaze contingency as one of the major challenges for artificial vision by presenting both its neural substrate and the means current prosthesis projects include gaze-contingent information in their studies.
Artificial Vision: A Clinical Guide - Google книги
Finding new ways to mimic the normal oculomotor and visual function is important in order to develop prostheses that could benefit blind individuals in their mobility and navigational performance, while also increasing their independence when performing activities of daily living ADL. Motion is an inherent aspect of human life. We move our arms, legs and body in order to navigate in our environment.
Most importantly, even in the absence of whole-body movements, we move our eyes to capture objects of interest and scan the world around us. These eye movements cause the visual representation of objects in our world to move across our retinas Burr, ; Klier and Angelaki, ; Inaba and Kawano, With each new eye movement, a given location in the world is projected to a new location on the retina Burr, ; Heiser and Colby, ; Klier and Angelaki, And yet, despite these frequent displacements, the visual brain is able to create a stable and continuous mental image of the external world by compensating each retinal snapshot with the gaze direction used to make it Klier and Angelaki, ; Pezaris and Eskandar, ; Rao et al.
There must be a mechanism supporting the perceptual stability of a visual scene, raising the question of how the percepts resulting from each retinal displacement are updated in the appropriate coordinates and integrated into a whole. For example, to correctly fixate two subsequently presented visual stimuli at different screen locations, the information about the intervening eye movement caused by the gaze shift toward the first stimulus needs to be taken into account in order to correctly localize the second stimulus see Figure 1.
This observation suggests that the brain must combine different kinds of information: the retinal signal caused by the position of the stimulus on the retinal surface i. Widely known as spatial updating Duhamel et al. This ability enables us to reach objects accurately and interact with them effectively, which makes spatial updating highly relevant for visual prosthesis development.
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The double-step saccade task used to illustrate spatial updating. Subjects fixate a centrally presented stimulus FP and subsequently they are asked to fixate two successively and briefly presented stimuli at different screen locations T1, T2. The eye movements required to correctly foveate each target are in turn called the motor errors.
Artificial vision: what people with bionic eyes see
To make a saccade to the first target T1, the motor error ME1 can be deduced directly from the retinal error RE1 for T1 and therefore correctly executed. However, the first saccade to T1 displaces T2 from the location where T2 initially appeared on the retina.
Thus, executing a second saccade based purely on the originally observed retinal error RE2 would lead to a failed attempt to foveate T2 orange dashed line. Instead, the motor plan ME2 for T2 needs to compensate for the intervening saccade to T1. This is accomplished by subtracting RE1 from RE2. Recall that both T1 and T2 are only briefly presented, and are extinguished prior to the execution of eye movements. Adapted from Mays and Sparks, and Klier and Angelaki, Over the last decades, there has been a growing interest in investigating the mechanisms that mediate spatial updating.
To this end, most studies have used the double-step saccade task, as first introduced by Hallett and Lightstone a , b ; see Figure 1. During the generalized version of this task, subjects are instructed to fixate a target FP at the center of the screen while two peripheral visual stimuli are successively flashed e.
However, for the second saccade to be accurate i. At the neural level, spatial updating seems to be mediated by RF locations that shift as our gaze moves to different locations Klier and Angelaki, The first evidence showing that retinal signals are combined with information about the gaze position at a given instance an example of extra-retinal information came from the seminal double-step saccade study of Sparks and Mays ; see Figure 2. In a typical trial, after fixating a central target, the monkey had to generate an eye movement to the location of a peripheral target that was briefly flashed.
This methodology allowed them to test whether the electrically evoked saccade would affect the characteristics of the subsequent naturally made saccade toward the target. This finding indicates that neurons in SC are responsible for recomputing the motor error resulting from intervening eye movements, thereby providing strong evidence supporting the idea that retinal signals are combined with information about instantaneous eye position. The double-step saccade task used by Sparks and Mays This study showed that the electrically induced saccade S2 was followed by a saccade S3 toward the location of the target stimulus TP , which allowed the animal to correctly localize the target TP.
Saccades that did not take into account the electrically induced perturbation dashed line were not observed, demonstrating that the perception of TP occurred in spatial coordinates that are deduced from a combination of retinal activity and eye position. Adapted from Sparks and Mays, Similar findings have been reported in other areas as well such as frontal eye field FEF and the lateral intraparietal area LIP for a review see Rao et al.
It has been shown that neurons in FEF responded to stimuli presented as targets for second saccades in double-saccade tasks, although these stimuli were not shown in their unadapted RF Bruce and Goldberg, ; Bruce et al. Single neurons in LIP have been found to respond in a predictive manner to anticipate what the visual scene would look like after a saccade Heiser and Colby, These neurons seem to play a crucial role in spatial updating, with most studies reporting LIP activity when a saccade shifted the RF onto a previously stimulated location Andersen and Mountcastle, ; Andersen et al.
Inspired by these findings, Duhamel et al. As the animal shifted its gaze to the locus of the second target, the RF shifted as well, and the cell began to fire.
Surprisingly, the discharge of the cell preceded the saccade, indicating that the location of the RF shifted before the onset of the eye movement. But, what is the source of this information? The predictive-remapping task from Duhamel et al. Then, a target point TP to which the animal was required to make a saccade was presented simulatenously with a peripheral visual stimulus Stimulus in the future, post-saccade, RF location of the neuron dashed circle.
B During initial fixation, there is no neural response blue histogram.
Artificial Vision for the Blind
However, slightly preceding the initiation of the saccade solid red line , the cell begins to fire gray background. Saccades typically completed in 30 ms dashed red line , placing the classical RF over the Stimulus.
enter With normal latency, the response would be expected to start 75 ms later dashed green line , but the cell has continued to respond in the meanwhile gray hatched background. We expect that, as the animal shifs its gaze to the locus of the target, the RF shifts as well, and the cell would begin to fire after the normal response latency following the saccade completion.
However, portions of the discharge of the cell not only preceded that expected latency gray hatched background , but also preceded the saccade gray area , suggesting that the location of the RF shifted to accurately anticipate the position after the eye movement. Data extracted from Duhamel et al. This information has been suggested to come from a motor efference copy or corollary discharge, i. For example, in order to change our gaze, a neural command must be generated and then sent to the motor neurons of the brainstem that are responsible for controlling the eye muscles.
A copy of this motor command could then be sent to visual mapping areas and can be, subsequently, used by the brain for several tasks and processes, one of them being spatial updating Klier and Angelaki, ; Caspi et al. While direct evidence for an efference copy has not yet been identified, the mounting indirect evidence is substantial, with most studies pointing to sub-cortical and extra-striate areas Heiser and Colby, ; Sommer and Wurtz, ; Inaba and Kawano, ; Rao et al.
Specifically, SC and FEF have been implicated as candidate areas responsible for generating and supplying a copy of the eye movement command to LIP Heiser and Colby, , while additional evidence suggests that the corollary discharge mechanism may result from the operations of the pathway from SC to mediodorsal thalamus MD to FEF Sommer and Wurtz, Converging evidence has attributed the RF shifts to a predictive remapping mechanism that contributes in maintaining visual stability.
According to the predictive remapping account, cells in several brain areas exhibit a transient change of their RF location immediately before the initiation of a saccadic eye movement Duhamel et al. Supporting data comes from studies showing that presaccadic remapping occurs in both cortical and subcortical areas e. This presaccadic remapped response differs from the normal visual response in two important aspects. First, its spatial location depends on the initial RF, as well as on the vector of the subsequent saccade.
Second, the visual latency of a given neuron does not affect the timing of the remapped response Rao et al. The temporal alignment between RF shifts and saccade onset further supports the hypothesis implicating a corollary discharge signal as the underlying cause for the RF shifting. The second candidate mechanism to explain this remapping phenomenon is known as spatiotopic representation or the gain field account Andersen and Mountcastle, ; Andersen et al.
The gain field framework holds that spatiotopic representation is mediated through neurons whose visual responses are multiplicatively modulated by eye position Andersen and Mountcastle, ; Andersen et al. In other words, the firing frequency of gain field neurons increases or decreases as if it were being multiplied by gaze angle, scaled by some gain factor, while the shape and the location of their RF remains unaffected by gaze position Andersen and Mountcastle, ; for a review see Blohm and Crawford, Contrary to the eye-centered representations of retinal neurons, the gain-modulated responses observed in parietal regions indicate that visual images in higher-order brain areas are represented in spatiotopic, rather than retinotopic, coordinates Andersen and Mountcastle, ; Andersen et al.
This finding has led researchers to propose this mechanism as being responsible for combining retinal information with eye position signals Klier and Angelaki, This combining, in turn, allows the forming of head-centered target representations, which are necessary for object localization, motor execution, and visuomotor coordination Salinas and Abbott, Neurons whose visual responses are modulated by gaze position i.
In an important study, Andersen et al. They employed a memory saccade task, as first introduced by Hikosaka and Wurtz and later developed by Andersen and colleagues e. During the initial baseline period , the animals fixated at one of 9 fixation points. Subsequently, an eccentric saccade target was presented for ms the light-sensitive period. Finally, the animal had to initiate a saccade toward the remembered location the saccade period. Neural direction tuning was determined by measuring responses during each period for targets around a circle.
The effect of eye position on all three responses for this direction was then tested so as to examine gain fields, i. They found that both LIP and 7a neurons yielded significant responses for all three types of activity light-sensitive, memory, and saccade , with the majority of the cells exhibiting a tonic background activity closely linked to eye position. Although direction tuning remained unaffected by eye position, the magnitude of the response was influenced, revealing a modulatory role of eye position in determining these three response types in both cortical areas.
Taken together, these findings indicate that LIP and 7a neurons display gaze-dependent activity, which, operating simultaneously at different processing stages, could possibly generate a large final effect Salinas and Abbott, Although motor efference copies and gain fields have been proposed as the mechanisms underlying visual mapping in sighted individuals, it remains unknown whether these processes continue to operate in the same fashion with artificial vision in blind individuals Caspi et al.
As current prosthetic devices do not provide foveal vision, the question of whether implanted individuals maintain the ability to employ these mechanisms to achieve visual stability has not been addressed Caspi et al.