Members of our laboratory (Allison, Bao, Kabilan and Cassie) have recently returned from the Australasian Experimental Psychology Conference at the University of Sydney, April 8-11. Vision science is very multidisciplinary area and is of interest to a range of scientific conferences and research groups.
Our postdoctoral research fellow, Bao, presented a talk about the effects of normal ageing on vision.
Ageing is known to reduce visual contrast sensitivity, which is typically clinically measured on plain uniform backgrounds. However, in the natural world, objects of interest are typically embedded in non-uniform backgrounds. In this study, we were interested in how older (62-78 years) and younger people (24-35 years) perceive the contrast of a striped pattern when the pattern is surrounded by another pattern, i.e. a non-uniform background. Previous research from our lab shows that, on average, older people see the central pattern as lower contrast (grayer) than younger observers (Karas and McKendrick 2009 J Vis, Karas and McKendrick 2011 Optom Vis Sci, Karas and McKendrick 2015 Vision Res). In this study we investigated performance on these tasks in peripheral vision. The aim is to understand the neural changes that underpin these effects, in order to better understand healthy brain ageing. Furthermore, understanding performance changes in older adults may assist in predicting changes in performance on more complex, real-world tasks involving vision.
Cassie, who is currently finishing her MPhil, presented her work, which is related to audiovisual integration. The sensory input we receive from the world around us changes continually as time passes, sometimes with a regular rhythm like the flashing lights and simultaneous noise of a police or ambulance siren rising and falling over time. When fluctuating auditory and visual stimuli, such as a sequence of beeps and flashes, are presented together, each stimulus can influence the other’s perceived timing because the brain combines information across the senses. In fact, auditory and visual stimuli may seem to be “beeping” or “flashing” at a different rate when presented together than when presented on their own. Cassie has been studying how normal ageing impacts the brain’s ability to combine this fluctuating information across vision and audition. In her study, we matched how well older and younger participants could discriminate changes in the perceived beeping and flashing rates when each signal was presented on its own. This controlled for the subtle changes to vision and audition that occur as part of normal ageing, which alter stimulus perceptibility. We then measured the perceived speed of oscillation for the matched auditory and visual stimuli when presented in combination. Older and younger adults performed similarly under these controlled conditions, suggesting that healthy ageing doesn’t greatly influence the brain processes that combine information about the timing of auditory and visual stimuli.
Finally, Kabilan,one of our PhD students presented a poster titled “In vivo Gamma Aminobutyric Acid (GABA) concentration assessed using 1H Magnetic Resonance Spectroscopy and its relationship with contrast suppression, motion suppression and binocular rivalry in young adults”. An object’s contrast is influenced by the contrast of surrounding objects. In the image on the left side, the contrast of the central target appears dimmer when it is surrounded by a high contrast annulus.
This phenomenon is known as centre surround contrast suppression and it is believed to be essential for perceiving edges and contours of objects. A key brain neurochemical thought to be relevant to this phenomena is Gamma Aminobutyric Acid. The amount of brain GABA levels cannot be quantified using conventional brain imaging procedures. In this study, we used a cutting-edge technique called MEGA PRESS to quantify the brain GABA levels from 15 younger participants (aged 24 to 34) and related it to visual performance on these tasks. This forms part of a larger body of work in the lab investigating the mechanisms underpinning these visual phenomena.