Recent Findings

Prof. Ehud Zohary (Hebrew University)

The children that have been included in our research have all suffered from bilateral cataracts since birth or a few months afterwards. The cataract removal surgeries have been carried out in Hawassa referral hospital since 2007, and have been part of the project since May 2013. The children who are traceable are being followed up till this day. Surgery was done on 53 children from 4 to 19 years of age. The figure below depicts their distribution. Each arrow denotes one patient, and its length indicates the time since the operation.

Generally, we can see clear signs of improvement of visual capabilities with time, though there is considerable variance in the children's performance. Below is a summary of their performance in some basic tests.

The Contrast Sensitivity Function

Optometrists and ophthalmologists have their technique for assessing visual acuity. When testing people with very poor vision, the standard Snellen chart is irrelevant. Typically, patients are categorized to ones having only light perception (LP), being able to sense hand motion (HM), or counting fingers from X meters (FC, x). These tests are generally only qualitative and may vary considerably upon the optometrist's specific technique, the lighting conditions in the room and other factors.

 

We use the contrast sensitivity test:  a more controlled technique to assess spatial acuity. We present sinusoidal gratings at different spatial frequencies and contrasts and find the threshold contrast per spatial frequency in which the participant can still discriminate between a vertical and horizontal grating (in 75% of the cases).  The results are presented as the contrast sensitivity function (CSF). The measure of the CSF is commonly used in vision science and allows us to follow the children's acuity as they improve following surgery.

 

The figure below depicts the CSF of 3 patients, and their development of visual acuity following surgery.Typically, the patients are unable to see high spatial frequencies. This results in a blurry image perception.  The image of the lion below schematically illustrates the image which is seen depending on the cutoff frequency of the CSF. There is also great variability in the extent to which the patients improve, following surgical treatment. One of the major goals of project eye-opener is to understand the factors contributing to this variability, to allow better treatment for the newly sighted. 

Mid-level Visual Functions

Visual analysis is typically divided into three processing stages. Crudely, low-level vision is engaged in contour extraction and boundary assessment; mid-level vision in the assessment of surfaces, and high-level vision is involved in making sense of the visual image: identifying objects, actors, social interactions, etc. We wanted to assess at what level of processing do the newly-sighted face serious problems. To that end, we asked our subjects to point out the odd element in an array. The odd element could be different in its color, size or shape (low level visual cues, above) or in its pictorial depth cues (shading, box; mid-level conditions). The patients had no trouble recognizing the odd object based on color or shape, but they were unable to infer the 3D shape of an object based on pictorial cues. For further details, see our publication "The Limits of Shape Recognition following Late Emergence from Blindness" in the publications section.

Size Constancy

One key feature of visual perception is object constancy: The ability to perceive an object as it is, despite radical changes in its physical appearance due to variation in the level of illumination, shading, viewpoint, physical distance, etc. For example: we do not perceive an object as getting smaller when we move away from it, even though its projection on our retina (e.g. its retinal size) is becoming smaller. Apparently, our brain makes use of the pictorial information (such as perspective and other 3D cues) to derive the true size of the object. This capability is termed Size Constancy. But what if you've had very poor vision from birth. Are you able to acquire such an ability later in life?

To study this size constancy in the newly-sighted we placed two balls of various sizes in two distances (close and far) from the subjects and asked them to estimate which ball was bigger. If the visual system of the newly-sighted cannot take the distance of the object into account one would expect subjects to estimate the further ball as being smaller than the closer one, even if it’s actually physically bigger. This was clearly not the case in our newly-sighted group. We therefore suggest that the brain mechanisms supporting size constancy can develop even in patients with highly blurred vision and very poor shape recognition. In hindsight, this may not be that surprising since a highly blurred retinal image is still inversely scaled with distance, and object size can be confirmed, at least for objects within hand range. A paper summarizing this project will be published in Current Biology on July 24th, 2017. For further information, See "publications" in this website.

Gaze following requires early visual experience

Joint attention – realizing the object of another individual's gaze and switching attention to that object – is a key component of behavior with clear evolutionary advantages. But can this strategy develop despite years of extremely poor vision from infancy? We studied 14 congenital bilateral-cataract patients, surgically treated only in late childhood, to test the effect of viewed gaze direction on target localization. On average, the late-treated participants were faster to point to a target matching a gaze-cue indicated by head-direction than when the target appeared at the opposite side. No such cue-compatibility effect was seen when gaze was indicated by eye-direction. This matches a case study of a late-treated patient who identified gaze direction by head-orientation, suggesting that gaze understanding can be acquired late, following surgery. However, we recently tested performance in three participants prior to surgery. All showed a head-direction cue-effect, implying that their limited pre-surgical vision (available to variable extent in all cataract cases), was sufficient for the development of head-directed joint attention. We therefore suggest that joint attention acquisition requires visual experience at an early age. Our study stresses the importance of thorough investigation of pre-surgical vision for better characterization of visual function acquisition following prolonged visual deprivation.

Imitation Behavior

Much of our behavior is mediated by a functioning visual system. One example of a visually guided action is our tendency to automatically imitate the motor actions of someone else (as in the “Simon says” game). In this study, we wanted to test whether viewing human gestures elicits automatic mimicry behavior in the newly-sighted children as in controls. To that end, we only tested children who could understand human gestures and presented them with short videos of hands tapping on a table. They were asked to tap with their right or left hand on the basis of the viewed hands’ color (red: left tap; blue: right tap). Crucially, the viewed hands were seen in a similar tapping action using either a spatially-corresponding (i.e. compatible) hand or the other (incompatible) hand.  Sighted subjects typically respond faster when the seen action is compatible with required response and slower in incompatible trials. But this does not occur if they watch balls moving in the same path rather than hand action. This hand-specific automatic imitation effect suggests a direct mapping between action-viewing and motor-performance, a key notion of the mirror neuron system hypothesis. Crucially, the newly-sighted were almost unaffected by the viewed hands’ action, showing a much smaller automatic imitation effect. These results suggest that newly-sighted may have a dysfunctional mirror neuron system. This deficit could influence the newly sighted abilities to learn new motor skills or to understand the intentions of others.

Much of our behavior is mediated by a functioning visual system. One example of a visually guided action is our tendency to automatically imitate the motor actions of someone else (as in the “Simon says” game). In this study, we wanted to test whether viewing human gestures elicits automatic mimicry behavior in the newly-sighted children as in controls. To that end, we only tested children who could understand human gestures and presented them with short videos of hands tapping on a table. They were asked to tap with their right or left hand on the basis of the viewed hands’ color (red: left tap; blue: right tap). Crucially, the viewed hands were seen in a similar tapping action using either a spatially-corresponding (i.e. compatible) hand or the other (incompatible) hand.  Sighted subjects typically respond faster when the seen action is compatible with required response and slower in incompatible trials. But this does not occur if they watch balls moving in the same path rather than hand action. This hand-specific automatic imitation effect suggests a direct mapping between action-viewing and motor-performance, a key notion of the mirror neuron system hypothesis. Crucially, the newly-sighted were almost unaffected by the viewed hands’ action, showing a much smaller automatic imitation effect. These results suggest that newly-sighted may have a dysfunctional mirror neuron system. This deficit could influence the newly sighted abilities to learn new motor skills or to understand the intentions of others.

Much of our behavior is mediated by a functioning visual system. One example of a visually guided action is our tendency to automatically imitate the motor actions of someone else (as in the “Simon says” game). In this study, we wanted to test whether viewing human gestures elicits automatic mimicry behavior in the newly-sighted children as in controls. To that end, we only tested children who could understand human gestures and presented them with short videos of hands tapping on a table. They were asked to tap with their right or left hand on the basis of the viewed hands’ color (red: left tap; blue: right tap). Crucially, the viewed hands were seen in a similar tapping action using either a spatially-corresponding (i.e. compatible) hand or the other (incompatible) hand.  Sighted subjects typically respond faster when the seen action is compatible with required response and slower in incompatible trials. But this does not occur if they watch balls moving in the same path rather than hand action. This hand-specific automatic imitation effect suggests a direct mapping between action-viewing and motor-performance, a key notion of the mirror neuron system hypothesis. Crucially, the newly-sighted were almost unaffected by the viewed hands’ action, showing a much smaller automatic imitation effect. These results suggest that newly-sighted may have a dysfunctional mirror neuron system. This deficit could influence the newly sighted abilities to learn new motor skills or to understand the intentions of others.

Visual inference capacity

We typically recognize visual objects, by utilizing the spatial layout of their parts, simultaneously present on our retina. However, a retinotopic image of an object is not a necessary condition for perception: When an object moves behind a
narrow slit, only a tiny fraction of the object is visible at any instant, activating the same place on the retina. Yet, we can recognize the object in such conditions. Here, shape can only be reconstructed by an integration of successive shape-views over time, using a non-retinotopic reference frame. This impressive visual inference capacity normally develops by early childhood. But what if you have had very limited vision during the early years of life?

We studied whether the newly-sighted are able to acquire this capacity. On each trial, a shape moved behind a horizontal or vertical slit (orthogonal to the slit orientation). In such conditions, one must assess the velocity of the moving object to correctly piece together its fragments into a coherent whole. Therefore, we first tested whether the newly-sighted are able to recognize the direction of shape motion. In other task they were asked to recognize the slit-viewed shape. The newly-sighted children judged shape motion as good as their normally sight-developing peers. However, in general, they failed to judge shape configuration. Image reconstruction in such conditions requires a complex interaction between motion- and shape-related visual processes and sensory memory.  Our finding suggests that this capability cannot develop when visual experience is extremely limited at early age.

© 2019 Ehud Zohary`s Lab