Neurotransmitters are chemical messengers that are abundant in our brains and are responsible for communicating information throughout the nervous system. One key neurotransmitter, called gamma aminobutyric acid (or GABA), is critical for normal function of our visual system. GABA is an inhibitory neurotransmitter, which reduces rather than augments nerve impulses.
In the past there have been reports of healthy ageing being associated with changes in visual perception that are attributed to impaired GABAergic inhibition in the visual part of the brain. There are also suggestions from non-human primate studies of the visual cortex that ageing is associated with reduced GABA levels. However, no studies have directly measured GABA levels in the visual cortex of older humans.
In collaboration with colleagues at the Monash Biomedical Imaging facility of Monash University, we used a technique called magnetic resonance spectroscopy to measure neurotransmitter levels in the brains of older and younger adults. Magnetic resonance spectroscopy allows us to non-invasively compare the chemical composition of different parts of the brain. We found increased levels of GABA in the visual cortex in the older adults (see left hand side of Figure), but no changes in a non-visual part of the brain (dorsolateral prefrontal cortex, see right hand side of Figure). Furthermore, performance on two visual tasks that are considered measures of perceptual suppression (or inhibition), was correlated with GABA levels in the visual brain. These results challenge prior assumptions about how GABA is altered with healthy ageing.
Above: GABA levels are increased in the visual cortex of older adults relative to younger adults, but no difference was observed in a non-visual part of the brain (dorsolateral prefrontal cortex, or DLPFC).
You can read the full article at Scientific Reports here.
On a weekend in August our lab went on a winter retreat down at Wilson’s Promontory, the southernmost tip of mainland Australia. Fortunately the bucketing rain in Melbourne did not follow us south, and we enjoyed spectacular winter weather at ‘The Prom’.
Highlights of our trip include:
- Spotting wombats in the national park
- Dipping our feet into the (freezing) water at Squeaky Beach
- Gas heated accommodation at the Tidal River huts
- Short walks near Tidal River (Squeaky Beach track, Pillar Point, Loo Errn track)
- Dinner, drinks and games and a big breakfast
Part of the coastline at Wilson’s Promontory (Photo credit: Juan Sepulveda)
On the Tidal River footbridge before heading on the Squeaky Beach walking track (Photo credit: Chongyue He)
At Squeaky Beach (Photo credit: Chongyue He)
As we wrote previously in our lab blog, Nikki successfully submitted her PhD thesis within minutes of her 4-year deadline. It had been promised to Nikki that if she submitted her thesis before the 4 year due date, Allison and Andrew would throw a party in her honour. So this is how our lab ended up having a lab celebration party in July. It was also a party for Cassie and Kabilan who submitted their Masters and PhD theses, respectively.
The party was a chance to have some food and drink together, and to demonstrate our competitive spirits in a group scavenger hunt around the University of Melbourne. In order to gain points on the hunt, we circled the University campus, high-fived random strangers, took endless selfies at commemorative statues, tracked down obscure signs and patted dogs (real or not) along the way, while Allison tracked our progress in real time from the comfort of her desk. Thanks to Allison and Andrew for organising a fun way to get to know our work environment better!
Above: Kabilan holding the traditional PhD submission balloon at the 1888 building, University of Melbourne. Below: Evidence of our scavenger hunt tasks
Later that week, it was time to farewell Astrid and Kabilan, who have both left our lab to start new projects in Europe. Astrid has relocated to Belgium to begin the next stage of her career, and Kabilan will soon start his first postdoctoral position working in Hamburg, Germany. We wish both of you all the best in your future projects!
Astrid and Kabilan at Tsubu farewell drinks
Our environment often contains visual and auditory information originating from separate events. Our brains need to be able to combine related sensory information coming from the same event and separate unrelated information from different events. One of the few common approaches to investigate such behaviour in human observers is known as audiovisual synchrony judgement: visual and auditory stimuli are presented at the same time or at some time offset from each other. Observers judge whether each stimulus pair is perceived as occurring at the same time (synchronous) or at different times (asynchronous). Previous work in our lab has shown that older adults are more likely to perceive asynchronous pairs as synchronous – i.e. older adults are more likely to incorrectly combine unrelated sensory information. How then does this reflect in the underlying brain processes involved in multisensory combination?
In this newly published study, we evaluated the impact of healthy ageing on the brain processes underlying audiovisual synchrony judgement using electroencephalography (EEG). EEG is used to measure brain activity in response to a stimulus (in this case, visual and auditory information). In the particular task used in this study, older and younger adults performed the same perceptually, but older adults recruited more widespread brain areas (specifically fronto-polar and frontal regions) to maintain the similar level of performance (see Figure 1 below).
Figure 1: EEG activity indicating more widespread brain areas recruited by the older adults (bottom panel) to perform the audiovisual synchrony judgement task for synchronous pairs as compared to younger adults (top panel). A similar finding was also found for asynchronous audiovisual pairs (see published paper for details). Figure adapted from Figures 3 and 4 of Chan, Pianta, Bode & McKendrick (2017).
This work has been published in the journal Neurobiology of Aging (click here to view the abstract). A full copy of the paper can be requested by contacting Allison at email@example.com
Visual snow is a rare neurological condition where people see static-like ‘snow’ (continuous tiny dots similar to the noise of an analogue TV) in their vision all of the time. Other common complaints are seeing afterimages and excessive floaters and experiencing tinnitus. The precise cause of visual snow is not understood, however, symptoms are thought to be due to excessive neural firing in the visual areas of the brain (cortical hyperexcitability).
In this study, we tested patients with visual snow on four visual perceptual tasks that are believed to indirectly measure visual cortical hyperexcitability. Two of the tasks, luminance increment detection in spatial noise and centre surround contrast matching, were chosen as they test early stages of the visual processing. The other two tasks, global form perception and global motion discrimination, assess relatively later stages of the visual processing pathway.
We found that people with visual snow process luminance and contrast differently from controls, consistent with elevated excitation in the early stages of the visual processing pathway (higher luminance increment detection threshold and higher perceived contrast in the presence of a high contrast surround grating). This work reveals promise for the future development of visual tests that may help differentiate visual snow from other disorders and quantify the effectiveness of treatments.
The paper has been published in Neurology and was conducted in collaboration with Assoc Prof Owen White from Melbourne Health (Royal Melbourne Hospital); and Assoc Prof Joanne Fielding from Monash University. To access a full copy of the paper, please contact Allison directly at firstname.lastname@example.org
The tradition at the University of Melbourne is to receive a well-earned balloon once you have submitted your thesis. Our lab celebrated two theses submissions recently:
1) Cassie Brooks submitted her MPhil thesis on 28th April 2017. One paper has been published describing work from her thesis in Journal of Vision, which can be accessed in full here.
2) On 13th May 2017, Nikki Rubinstein submitted her PhD thesis “Incorporating spatial information into visual field testing algorithms”. To date, Nikki has successfully published one paper from her PhD work in Translational Vision Science and Technology, which you can read at this link.
Congratulations to our balloon-holding students!
This week we said farewell to Dr Fumi Tanabe, who is an ophthalmologist from Osaka, Japan. Fumi visited our lab for approximately 1.5 years with a special interest in glaucoma, and worked on a number of research projects using the high resolution optical imaging device in our laboratory (optical coherence tomography). She will be missed!
Fumi (middle, bottom) and her farewell cake with ‘fluorescein green’ topping, pictured with members of our laboratory and other staff and students at the Department of Optometry and Vision Sciences, University of Melbourne