Academic literature on the topic 'Magnocellular Processing'

Research has shown an association between functioning of the visual magnocellular system and dyslexia. While many studies have provided evidence supporting the magnocellular deficit, some studies have failed to replicate these findings. The main aim of the current study was to examine different theoretical explanations of reduced motion sensitivity in dyslexia. These included: (1) a sensory deficit caused by a structural abnormality in the magnocellular system affecting the processing of sparse motion signals (Talcott et al., 1998), (2) a deficit in temporal integration (Raymond & Sorensen, 1998), (3) a deficit at extrastriate visual areas only (e.g., Hill & Raymond, 2002), and (4) a deficit in noise exclusion (Sperling, Lu, Manis, & Seidenberg, 2005, 2006b). Three global motion experiments were conducted to investigate motion extraction, motion integration, and simultaneous motion processing. A local motion control task was also administered. Participants were two groups of high functioning adults with and without dyslexia. The dyslexia group were significantly less sensitive than the skilled reader group on each of the global motion processing tasks, but not on the local motion processing task. Manipulations of dot density, the number of animation frames presented in the random dot kinematogram (RDK), and signal dot lifetime affected motion sensitivity in the dyslexia and skilled reader groups similarly. A combination of high dot density and presentation of an increased number of animation frames in the global motion stimulus increased sensitivity for both reader groups. These results suggest that the global motion deficit found in dyslexia can partially be explained by sensory and perceptual motion processing deficits mediated by visual area V5.<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy in Clinical Psychology (PhD ClinPsych)<br>School of Psychology<br>Griffith Health<br>Full Text

To perceive and interact with the environment functionally, our brain must simultaneously encode and decode various temporal cues and efficiently combine them across different senses. Nonetheless, not all sensory modalities exhibit equally valuable temporal resolutions, being auditory the primary sensory modality through which time is precisely experienced. Consequently, such a combination is not a linear process, and temporal information perceived across individual sensory streams shows several grades of relevance when performing duration estimations. To date, however, it is still unclear how temporal processing occurs within the brain, along with how auditory impairments during development might determine plastic changes that negatively affect time perception. Powerful insights can be obtained by investigating temporal processing in individuals whose auditory perception was impaired during critical developmental stages, when the brain experienced the most plastic properties. Interestingly, both the absence of hearing experience during development, such as in congenital and early deafness, and its deterioration, such as in Developmental Dyslexia, seem to determine a generalized reduction of temporal sensitivity across the lifespan. Even though there is solid evidence suggesting that abnormal auditory experience might affect temporal processing, there is a surprising lack of behavioral evidence aiming at bridging temporal impairment in deafness and Developmental Dyslexia. I thereby first investigated temporal processing in these populations using classing psychophysical techniques. I thus developed two different tasks directly designed to evaluate visual temporal abilities, with a specific focus on the magnocellular properties of temporal perception. Being particularly prone to be shaped by abnormal sensory experience during development, magnocellular cells should in fact reflect different temporal encoding according to the nature of the auditory impairment. Then, due to the COVID-19 pandemic and the consequent, inevitable difficulties emerged in data collection, I decided to foster a remote and safe approach to psychophysical testing. I therefore conceptualized, designed, and validated PsySuite, an Android App aimed at providing a reliable and easy-to-use tool to assess perceptual measurements while respecting social distancing restrictions. PsySuite’s feasibility in performing remote data collection was evaluated through extensive hardware and behavioral testing, allowing to determine the reliability of the produced stimuli and the generalizability of the results obtained with the App. After establishing PsySuite’s effectiveness and stability, I developed a portable version of a psychophysical task specifically designed to gauge and enhance temporal perceptual skills in typical adults. With this newfound, novel approach, I then investigated temporal processing in deaf and dyslexic participants.

While spatial determinants of emmetropization have been examined extensively in animal models and spatial processing of human myopes has also been studied, there have been few studies investigating temporal aspects of emmetropization and temporal processing in human myopia. The influence of temporal light modulation on eye growth and refractive compensation has been observed in animal models and there is evidence of temporal visual processing deficits in individuals with high myopia or other pathologies. Given this, the aims of this work were to examine the relationships between myopia (i.e. degree of myopia and progression status) and temporal visual performance and to consider any temporal processing deficits in terms of the parallel retinocortical pathways. Three psychophysical studies investigating temporal processing performance were conducted in young adult myopes and non-myopes: (1) backward visual masking, (2) dot motion perception and (3) phantom contour. For each experiment there were approximately 30 young emmetropes, 30 low myopes (myopia less than 5 D) and 30 high myopes (5 to 12 D). In the backward visual masking experiment, myopes were also classified according to their progression status (30 stable myopes and 30 progressing myopes). The first study was based on the observation that the visibility of a target is reduced by a second target, termed the mask, presented quickly after the first target. Myopes were more affected by the mask when the task was biased towards the magnocellular pathway; myopes had a 25% mean reduction in performance compared with emmetropes. However, there was no difference in the effect of the mask when the task was biased towards the parvocellular system. For all test conditions, there was no significant correlation between backward visual masking task performance and either the degree of myopia or myopia progression status. The dot motion perception study measured detection thresholds for the minimum displacement of moving dots, the maximum displacement of moving dots and degree of motion coherence required to correctly determine the direction of motion. The visual processing of these tasks is dominated by the magnocellular pathway. Compared with emmetropes, high myopes had reduced ability to detect the minimum displacement of moving dots for stimuli presented at the fovea (20% higher mean threshold) and possibly at the inferior nasal retina. The minimum displacement threshold was significantly and positively correlated to myopia magnitude and axial length, and significantly and negatively correlated with retinal thickness for the inferior nasal retina. The performance of emmetropes and myopes for all the other dot motion perception tasks were similar. In the phantom contour study, the highest temporal frequency of the flickering phantom pattern at which the contour was visible was determined. Myopes had significantly lower flicker detection limits (21.8 ± 7.1 Hz) than emmetropes (25.6 ± 8.8 Hz) for tasks biased towards the magnocellular pathway for both high (99%) and low (5%) contrast stimuli. There was no difference in flicker limits for a phantom contour task biased towards the parvocellular pathway. For all phantom contour tasks, there was no significant correlation between flicker detection thresholds and magnitude of myopia. Of the psychophysical temporal tasks studied here those primarily involving processing by the magnocellular pathway revealed differences in performance of the refractive error groups. While there are a number of interpretations for this data, this suggests that there may be a temporal processing deficit in some myopes that is selective for the magnocellular system. The minimum displacement dot motion perception task appears the most sensitive test, of those studied, for investigating changes in visual temporal processing in myopia. Data from the visual masking and phantom contour tasks suggest that the alterations to temporal processing occur at an early stage of myopia development. In addition, the link between increased minimum displacement threshold and decreasing retinal thickness suggests that there is a retinal component to the observed modifications in temporal processing.

Visual information is processed by two separate visual pathways. One is the magnocellular visual pathway (M-pathway), which carries high temporal frequency information but low spatial frequency information. The other is the parvocellular visual pathway (P-pathway), which carries low temporal information but high spatial information. Kveraga and colleagues (2007) presented participants with high and low spatial frequency images and found that participants made faster and more accurate categorization responses to the low spatial frequency images. They hypothesized this was due to low spatial frequency “gist” information being rapidly carried by the M-pathway. Using diffuse light and hand posture manipulations, we replicated the advantage for low spatial frequency (LSF) images in both experiments, and also found a larger advantage for LSF information when biasing the M-pathway (using hand posture). We were unable to inhibit the M-pathway using red diffuse light. Thus, it does appear “gist” processing is uniquely carried by the M-pathway.