New publication: Centre-surround visual processing of contrast: ageing affects mechanisms that operate within eyes but not between eyes

As described in our previous lab blog posts, we have a long standing interest in trying to understand how healthy aging affects vision. Specifically, many of our research projects involve the design of experiments that can tell us more about which visual neural processes are altered by the aging process. A number of our experiments have investigated a phenomenon called centre surround contrast suppression.  This phenomenon describes the visual experience in which a high contrast grey and white pattern will appear to be of lower contrast when surrounded by another higher contrast pattern. By manipulating different aspects of the central or surround pattern, we can test separate neural pathways that are responsible for visual experience.

In this particular experiment, we take advantage of the anatomical fact that neurons that carry information from the two eyes do not directly communicate until they reach the areas of the brain responsible for visual processing.

Centre surround contrast suppression can be elicited when by showing the centre and surround pattern to the same eye (‘within eye’ suppression). This is how most studies are conducted. Centre surround contrast suppression can also be elicited by showing the centre pattern to one eye and the surround pattern to the other eye (‘between eyes’ suppression). ‘Between eyes’ suppression is particularly informative because we know that it can only occur once information from the two eyes is combined in the brain. In this study, we measured both ‘within eye’ and ‘between eyes’ suppression. The figure below shows the visual patterns used in the study and summarises the results.

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When the centre and surround patterns were shown to both eyes, the amount of suppression (i.e. the amount by which the contrast of the centre pattern appeared lower than its actual contrast) was higher in older individuals than in younger individuals. However, when the centre and surround patterns were shown to different eyes, both younger and older individuals showed similar amounts of suppression. This indicates that the difference between older and younger adults is not general, but quite specific to the “within eye” condition.

The complete findings of this study have been recently published in Journal of Vision and can be accessed here.

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New paper: ageing effects on peripheral vision

We have recently published the first paper from Menaka’s PhD, investigating how healthy ageing affects the perception of visual stimuli in the presence of surrounding features in peripheral vision. In particular, we are interested in the area of our field of view that is just outside of the centre (the fovea), which we refer to as the parafoveal area. Studying parafoveal vision is important in older adults, especially as foveal damage occurs in age-related conditions like macular degeneration, which may lead to increased reliance on parafoveal vision.

One aspect of vision that occurs in the parafovea is crowding, which is typically described as reduced object recognition in a cluttered scene. Another visual phenomenon that occurs in the parafovea is surround suppression. An example of suppression occurs when a person’s ability to detect that a visual target is present is worse when there are surrounding features. We measured crowding and surround suppression effects in 20 older and 21 younger adults. Observers focused on the centre of a computer screen and visual stimuli were presented in their peripheral vision (8 degrees from where they were fixating). Examples of what were presented are shown in the figure below.

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Top: To measure the effect of crowding, we presented a target (right, striped pattern) next to an extra piece of clutter known as a flanker (left, cross hatched pattern) Bottom: To measure the effect of surround suppression, the target (right, centre striped pattern) is presented with or without a surrounding half annulus. The white dot is the fixation spot for both tasks.

We found that older adults have reduced ability to detect a visual target when it was surrounded (more surround suppression) compared to the younger observers, but crowding was unaffected by the type of stimuli used in our study. This suggests that crowding and surround suppression may be distinct visual phenomena, despite both occurring in parafoveal vision.

An extra question we asked was: given previous reports of decreased visual attention with healthy ageing, could altered performance on the crowding and surround suppression tasks be explained by changes to visual attention? Our specific measure of attention was a visual search task. Indeed, our group of older adults showed decreased visual attention but neither crowding nor surround suppression showed a relationship to the attentional aspect.

Advancing our knowledge on the ageing effects on peripheral visual function is useful for future remediation strategies using peripheral vision in older individuals where foveal vision is hindered. The article can be viewed online here.

2016: It’s a wrap!

2016 has again been a busy year for our lab. Key highlights from our research areas of interest:

  1. Glaucoma: A highlight of our program this year was a visiting PhD student, Clement Beugnet, from the University of Lille, France. Clement’s research is interested in how glaucoma impacts on complex visual processing of objects and scenes. 2016 also provided an opportunity for PhD candidate, Nikki Rubinstein, to present her work at an international conference (22nd International Perimetry and Imaging Symposium) in Udine, Italy. We also welcomed two additional postdoctoral researchers to the lab: Astrid Zeman and Phil Bedggood.
  2. Migraine: In 2016 we continued a large, 3 year study that will determine whether migraine events can be predicted by daily testing of vision on an portable tablet device (iPad), which has primarily been conducted by postdoctoral research fellow Janet Chan. We have also been involved in a collaborative project with the Royal Melbourne Hospital investigating a rare condition known as visual snow. Part of this work was presented this year at the international Asia Pacific Conference on Vision in Fremantle, Western Australia.
  3. Healthy ageing: In 2016, we welcomed two Honours students (Chongyue He and Huda Waraich) to the lab who conducted novel experiments looking at how healthy ageing impacts on the processing of moving objects. These types of experiments may have relevance to driving or other tasks where detecting moving objects is critical to performance. Both students have successfully completed their Honours year, so congratulations!

A recent highlight was Allison winning the ‘No-Bell Prize’ at the Melbourne Neuroscience Institute’s final event for the year in December. The competition requires researchers to communicate what they do to a wider audience without the use of jargon or technical words. Each time a scientific word is used that is not comprehensible, one of the judges can ring a bell. The least number of bells wins!

Thank you to all of our marvelous volunteers for assisting in our research program during 2016, and we wish you all the best for the festive season.

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Above: Chongyue He (Honours student) graduated in December 2016

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Above: Our research leader Allison brings home the ‘No-Bell Prize’ trophy for the Department of Optometry & Vision Sciences for the second year running (last year’s winner was Dr Christine Nguyen).

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Above: Our end-of-year event was held at the Brunswick Bowling Club in mid December with barefoot bowls and a bbq dinner

Imaging of the eye shows individual differences in the shape of the macula

One of our major research areas is in the field of glaucoma, an eye disease that affects the retinal ganglion cells (nerve cells). These cells are most densely located within the central 10° of the retina from the very centre, the region which is called the macula. Recently there has been an increased interest in the study of the morphology (shape) and functionality of the macula in people suffering from glaucoma, because we expect that this is where the initial pathological changes would appear.

Many researchers have tried to connect functional changes with morphological changes within the central 10° of the retina in people with glaucoma by creating a ‘structure-function map’. In particular, some researchers have included individual anatomical information to improve these maps. For example, at the very centre (foveola), one photoreceptor (the light-sensitive cell of the retina) connects to approximately one retinal ganglion cell, but the connection is slightly displaced spatially. Correcting for this factor would be unnecessary, however, if the maculae of all eyes were equal.

In this paper we explored whether some macular parameters such as the maximum thickness, the minimum thickness and the horizontal distance from the two previous values (we call it “radius”) of high-resolution optical coherence tomography (OCT) scans centered at the foveola are different between individuals. The figure below shows where the scans are centred.

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Retinal photo showing where the scans are taken (at the very centre of the retina, the fovea)

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The three parameters compared: central thickness, maximum thickness (height) and radius.

We found that the measured foveal shape parameters are different between different people. We also found that the foveal shape is not symmetrical between superior and inferior parts of the retina, which is important because the damage is often asymmetric between the superior and inferior hemiretinae in glaucoma.

This work formed part of Juan Sepulveda’s Masters project. You can read the full published article in Investigative Ophthalmology and Visual Science here.

New paper: Automatic identification of the temporal raphe from standard retinal scans

Glaucoma is a disease of the optic nerve. The nerve fibres that travel along the optic nerve to the brain are spread out across the retina, and can be visualised using an imaging technique called  optical coherence tomography (OCT). Individual nerve fibre bundles cannot be seen with standard clinical tests. It is typically assumed that if a nerve fibre bundle is lost in one part of the retina, then a person will have corresponding visual loss in that part of their field of view (i.e. visual field). Recently it has been recognised that different people can have quite different distributions of retinal nerve fibres, which means that the relationship between the location of loss of visual field and the location of nerve fibre damage differs between individuals and can be hard to predict.

Our lab has recently developed sophisticated models that overcome this problem to predict just how a particular nerve fibre pattern should map onto an individual patient’s visual field. These models depend critically upon a single measured parameter from the patient’s eye, namely the axis of symmetry between fibres that travel along the upper (superior) retina and those that travel along the lower (inferior) retina. This axis is known as the temporal nerve fibre raphe or temporal raphe for short (shown in the figure below as a red line).

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Figure 1. The temporal raphe (red), where the nerve fibres travelling from the upper and lower retina meet, can be identified using retinal scans

The location of the temporal raphe is typically difficult to appreciate with standard imaging techniques. However, it has been shown recently that by collecting OCT data with scans of very high spatial density, the raphe can be visualised in exquisite detail and its orientation hence measured very precisely. Unfortunately, collection of such data requires a lot of time, good image quality, and particularly steady fixation, which are luxuries that we do not have in a clinical setting.

In this paper we set out to determine just how well the temporal raphe can be determined from standard scans acquired routinely in the clinic (‘macular cube’ scans). By acquiring additional high density scan data on the same eyes, we were able to measure how accurately several automated algorithms can estimate the orientation of the temporal raphe. Our results show that the best performing algorithm gave a mean error of 1.5°. The ability to quantify the orientation of the raphe with this level of precision paves the way for future work that uses these measurements to accurately map retinal structure (by OCT) to retinal function (by visual field), in order to more accurately assess the health of the retina and optic nerve in glaucoma.

This paper has been published in Biomedical Optics Express and is available to read here.

A trip to Italy: Imaging and Perimetry Society Symposium

Members of our lab have just returned from a trip to sunny Italy to attend the 22nd International Visual Field Imaging Symposium. The meeting was held in the small town of Udine, in north east Italy, with the conference itself being held in the Castle of Udine.

Nikki presented a talk “Incorporating probabilistic graphical models into perimetric test procedures”, which describes work directly from her PhD studies.

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Allison was busy with her role as Secretary of the Imaging and Perimetry Society  which is an international organisation of researchers that aims to: 1) to promote the study of normal and abnormal visual function and of ocular imaging, and  2) to ensure and facilitate the cooperation and friendship of scientists of different countries working and interested in these disciplines. The next IPS meeting (23rd International Visual Field and Imaging Symposium) will be held in Kanazawa, Japan, May 9-12, 2018.

Lab adventures down the Great Ocean Road

On a winter’s weekend in August, our lab went on a roadtrip down the Great Ocean Road. Our main activity was to walk along part of the Great Ocean Walk from Blanket Bay campground to the Cape Otway lighthouse. Highlights from the trip were:

  • Koala spotting in the Cape Otway National Park
  • Friendly alpacas (and ‘cowpacas’) on the drive out from the lighthouse
  • Hearty bowls of pasta arabbiata and board games
  • Barefoot leaps across the chilly water to reach the hidden cove of Parker’s Inlet
  • Marine mammal spotting from Apollo Bay and the Cape Otway lighthouse
  • Free French lessons from our visiting PhD student from Lille, Clement
  • Quick detour on the drive home to the Erskine Falls outside of Lorne
  • Our very own resident paparazzo (Honours student, Chongyue) taking photos of our every move

 

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Our enthusiastic bunch before we embarked on the 11.5 km walk

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Halfway stop at Parker’s Inlet between Blanket Bay and Cape Otway lighthouse

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We made it to the Cape Otway lighthouse!