Perceptual suppression mechanisms in healthy ageing

© 2017 Dr. Kabilan PitchaimuthuHealthy ageing alters contextual spatial interactions such as centre surround contrast suppression and spatial suppression of motion discrimination. However, the exact neural mechanisms that underlie age related changes to contextual spatial interactions are still elusive. The body of work reported in this thesis explored the perceptual mechanisms behind altered contextual spatial interactions in younger and older adults using psychophysical and neuroimaging methods. Experiment 1 investigated the strength of foveal centre surround contrast suppression under intraocular and interocular viewing for two stimulus durations (40 ms and 200 ms) in younger and older adults. Foveal intraocular center surround contrast suppression decreased with longer stimulus duration whereas interocular surround suppression did not, suggesting contributions from separate mechanisms to these forms of suppression. In addition, intraocular center surround contrast suppression was increased in older adults compared to younger adults; however, interocular suppression was similar in both groups. Experiment 2 studied the effect of orientation of the surround grating in relation to the orientation of the centre grating (surround orientation effect) on centre surround contrast suppression under intraocular and interocular viewing in younger and older adults. Interocular and intraocular centre surround contrast suppression showed different surround orientation effect, and older adults demonstrated unaltered levels of surround orientation effect compared to younger adults under both intraocular and interocular viewing. Experiment 3 measured Gamma Aminobutyric Acid (GABA, the principal inhibitory neurotransmitter of the adult brain) levels in visual cortex of younger and older adults using magnetic resonance spectroscopy. Visual cortical GABA levels were increased in older adults compared to younger adults. In addition, visual cortical Glx (combined estimate of glutamate - the principal excitatory neurotransmitter, and glutamine) levels were reduced in older adults compared to younger adults. Neither GABA levels nor Glx levels in visual cortex correlated with foveal centre surround contrast suppression. The final experiment of this thesis (Experiment 4) investigated the relationship between visual cortical GABA levels and performance on two visual tasks that are hypothesised to be mediated, at least in part, by GABAergic inhibition: spatial suppression of motion discrimination and binocular rivalry. Both younger and adults participated in this experiment as well. Increased visual cortical GABA levels were associated with prolonged binocular rivalry percept durations and reduced spatial suppression of motion discrimination. The experimental frameworks used in the thesis were based on our modern understanding of cortical circuits that are implicated in mediating centre surround contrast suppression, and a neuroimaging technique that could potentially link physiology with behaviour. The novel findings reported in thesis answered some of the important questions related to perceptual surround suppression in younger and older adults. The current findings suggested that healthy ageing differentially affects distinct forms of suppression arising at various levels of the visual pathway, and challenged prior assumptions regarding age related changes to GABA levels within the visual cortex and its association with centre surround contrast suppression and spatial suppression of motion discrimination

Sound suppresses earliest visual cortical processing after sight recovery in congenitally blind humans

Abstract Neuroscientific research has consistently shown more extensive non-visual activity in the visual cortex of congenitally blind humans compared to sighted controls; a phenomenon known as crossmodal plasticity. Whether or not crossmodal activation of the visual cortex retracts if sight can be restored is still unknown. The present study, involving a rare group of sight-recovery individuals who were born pattern vision blind, employed visual event-related potentials to investigate persisting crossmodal modulation of the initial visual cortical processing stages. Here we report that the earliest, stimulus-driven retinotopic visual cortical activity (<100 ms) was suppressed in a spatially specific manner in sight-recovery individuals when concomitant sounds accompanied visual stimulation. In contrast, sounds did not modulate the earliest visual cortical response in two groups of typically sighted controls, nor in a third control group of sight-recovery individuals who had suffered a transient phase of later (rather than congenital) visual impairment. These results provide strong evidence for persisting crossmodal activity in the visual cortex after sight recovery following a period of congenital visual deprivation. Based on the time course of this modulation, we speculate on a role of exuberant crossmodal thalamic input which may arise during a sensitive phase of brain development

Dataset and Code: Sound Suppresses Earliest Visual Cortical Processing After Sight Recovery in Congenitally Blind Humans

Dataset and code for Sound Suppresses Earliest Visual Cortical Processing After Sight Recovery in Congenitally Blind Humans. The zip file ERP_RT_sLORETA contains three folders: ERP: ERP analysis scripts and data RT: RT analysis scripts and data sLORETA: Source analysis (sLORETA) protocol for brainstor

Audiovisual spatial recalibration but not integration is shaped by early sensory experience

To clarify the role of sensory experience during early development for adult multisensory learning capabilities, we probed audiovisual spatial processing in human individuals who had been born blind because of dense congenital cataracts (CCs) and who subsequently had received cataract removal surgery, some not before adolescence or adulthood. Their ability to integrate audio-visual input and to recalibrate multisensory spatial representations was compared to normally sighted control participants and individuals with a history of developmental (later onset) cataracts. Results in CC individuals revealed both normal multisensory integration in audiovisual trials (ventriloquism effect) and normal recalibration of unimodal auditory localization following audiovisual discrepant exposure (ventriloquism aftereffect) as observed in the control groups. In addition, only the CC group recalibrated unimodal visual localization after audiovisual exposure. Thus, in parallel to typical multisensory integration and learning, atypical crossmodal mechanisms coexisted in CC individuals, suggesting that multisensory recalibration capabilities are defined during a sensitive period in development

Biological Action Identification Does Not Require Early Visual Input for Development

Visual input during the first years of life is vital for the development of numerous visual functions. While normal development of global motion perception seems to require visual input during an early sensitive period, the detection of biological motion (BM) does not seem to do so. A more complex form of BM processing is the identification of human actions. Here, we tested whether identification rather than detection of BM is experience dependent. A group of human participants who had been treated for congenital cataracts (CC; of up to 18 years in duration, CC group) had to identify ten actions performed by human line figures. In addition, they performed a coherent motion (CM) detection task, which required identifying the direction of CM amid the movement of random dots. As controls, developmental cataract (DC) reversal individuals (DC group) who had undergone the same surgical treatment as CC group were included. Moreover, normally sighted controls were tested both with vision blurred to match the visual acuity (VA) of CC individuals [vision matched (VM) group] and with full sight [sighted control (SC) group]. The CC group identified biological actions with an extraordinary high accuracy (on average ∼85% correct) and was indistinguishable from the VM control group. By contrast, CM processing impairments of the CC group persisted even after controlling for VA. These results in the same individuals demonstrate an impressive resilience of BM processing to aberrant early visual experience and at the same time a sensitive period for the development of CM processing

Occipital GABA levels in older adults and their relationship to visual perceptual suppression

Abstract Several studies have attributed certain visual perceptual alterations in older adults to a likely decrease in GABA (Gamma Aminobutyric Acid) concentration in visual cortex, an assumption based on findings in aged non-human primates. However, to our knowledge, there is no direct evidence for an age-related decrease in GABA concentration in human visual cortex. Here, we estimated visual cortical GABA levels and Glx (combined estimate of glutamate and glutamine) levels using magnetic resonance spectroscopy. We also measured performance for two visual tasks that are hypothesised to be mediated, at least in part, by GABAergic inhibition: spatial suppression of motion and binocular rivalry. Our results show increased visual cortical GABA levels, and reduced Glx levels, in older adults. Perceptual performance differed between younger and older groups for both tasks. When subjects of all ages were combined, visual cortical GABA levels but not Glx levels correlated with perceptual performance. No relationship was found between perception and GABA levels in dorsolateral prefrontal cortex. Perceptual measures and GABA were not correlated when either age group was considered separately. Our results challenge current assumptions regarding neurobiological changes that occur within the aging human visual cortex and their association with certain age-related changes in visual perception

Relating excitatory and inhibitory neurochemicals to visual perception:A magnetic resonance study of occipital cortex between migraine events

Certain perceptual measures have been proposed as indirect assays of brain neurochemical status in people with migraine. One such measure is binocular rivalry, however, previous studies have not measured rivalry characteristics and brain neurochemistry together in people with migraine. This study compared spectroscopy-measured levels of GABA and Glx (glutamine and glutamate complex) in visual cortex between 16 people with migraine and 16 non-headache controls, and assessed whether the concentration of these neurochemicals explains, at least partially, inter-individual variability in binocular rivalry perceptual measures. Mean Glx level was significantly reduced in migraineurs relative to controls, whereas mean occipital GABA levels were similar between groups. Neither GABA levels, nor Glx levels correlated with rivalry percept duration. Our results thus suggest that the previously suggested relationship between rivalry percept duration and GABAergic inhibitory neurotransmitter concentration in visual cortex is not strong enough to enable rivalry percept duration to be reliably assumed to be a surrogate for GABA concentration, at least in the context of healthy individuals and those that experience migraine