Deborah Giaschi

The typical development of motion perception is commonly assessed with tests of global motion integration using random dot kinematograms (RDKs). There are discrepancies, however, with respect to when typically-developing children reach adult-like performance on this task, ranging from as early as 3 years to as late as 12 years. While much research characterizes performance in terms of dot speed, there is evidence that different spatial and temporal components can impact performance on this task in adults and in children. Other studies suggest that the distance that dots are displaced each animation frame (∆x), rather than frame duration (∆t) or dot speed (∆x/∆t) per se, determines performance in developing macaques. No studies have directly investigated whether psychophysical performance follows this pattern in children. The current studies measured motion coherence thresholds in adults and children in two experiments. Experiment 1 examined differences in adult performance in two studies with similar RDK parameters except for the ∆x and ∆t used to make up similar speeds. This experiment tested four ∆x/∆t pairs yielding a speed of 1 deg/s, and held the number of presented frames constant, or the duration of the stimulus constant. These factors had no effect on the thresholds of adults. Experiment 2 was designed to replicate the results of macaque studies in human children, and compare their thresholds to adults. Two ∆t values were used in combination with seven ∆x values, for a range of speeds (0.3-38 deg/s). Adult thresholds followed a u-shape as a function of ∆x, with lower coherence thresholds for larger ∆t when ∆x was small. Child thresholds followed a rough u-shape as a function of ∆x, regardless of ∆t. Developmental comparisons showed children performed as well as adults for larger ∆x, and were immature for smaller ∆x. When parameters were expressed as speed, there was a range of intermediate speeds (4-12 deg/s) for which maturity was dependent on the values of ∆x and ∆t tested. These results resolve previous discrepancies by showing that motion sensitivity to a given speed may be mature, or not, depending on the underlying spatial and temporal properties of the motion stimulus.

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Amblyopia is a visual developmental disorder defined by reduced visual acuity in one (amblyopic) eye, while the other (fellow) eye has normal visual acuity and is otherwise healthy. Deficits in motion perception – such as multiple object tracking - affect both the amblyopic eye and the fellow eye. This thesis examined the neural correlates of the multiple-object tracking deficit to further understand the cortical deficit in amblyopia. Functional data were collected as participants with and without a history of amblyopia performed the multiple-object tracking task monocularly inside a 3T MRI scanner. Participants were asked to use their attention to track 0, 1, 2 or 4 of 9 moving balls (6 deg/s) for 12 seconds. MR signal change relative to fixation, as a function of target numerosity (track 0, track 1, track 2, track 4), group (control, amblyopia), and eye were examined in six regions of interest: putative V1, MT, superior parietal lobule, frontal eye fields, anterior intraparietal sulcus, and posterior intraparietal sulcus. For all four tracking conditions, area MT was found to be less active in participants with amblyopia with both fellow and amblyopic eye viewing. When tracking 4 balls, the anterior intraparietal sulcus was found to be less active in participants with amblyopia, only with amblyopic eye viewing. This finding suggests the functional differences in this region may be subtle. Future investigations targeting the network involved in sustained attention can determine the extent to which posterior parietal function may be impaired in amblyopia. Overall, this thesis provided neuroimaging evidence that the MT region is affected in human amblyopia, and that both eyes are affected by underlying cortical changes in dorsal extra-striate areas.

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Children with developmental dyslexia have difficulty learning to read. These children may also have deficits in temporal processing, which is the perception and integration of rapidly presented stimuli. Behavioural research indicates a link between reading and temporal processing ability; however, the cortical relationship between these two skills has not been established. This thesis examined whether tasks of reading and temporal processing activate similar cortical regions in children with average reading ability and in children with dyslexia. Using functional magnetic resonance imaging (fMRI), activity for two reading tasks (phonological and orthographic) and two temporal processing tasks (dichotic pitch and global motion perception) was assessed. Three regions of interest were established in each participant: the lateral occipital cortex (LOC) and areas engaged by dichotic pitch and global motion tasks. Results demonstrated that both groups had increased activity in bilateral LOC during reading. In average readers, left LOC was more active than right regions during the phonological task, while dyslexic readers showed equivalent activity between left and right LOC for both reading tasks. The dichotic pitch regions did not show any evidence of activation during reading in either group. However, children with dyslexia exhibited significant activity in right global motion regions during the phonological task, but only on the difficult word condition. Average readers did not illustrate activation in global motion areas during reading. The current results suggest that LOC is involved with the reading process and children with dyslexia may have a deficit in left LOC. It was hypothesized that dyslexic readers may have increased attentional processing and recruitment from additional cortical regions during difficult tasks, which may explain the similar activity between global motion and phonological reading. Since there were no similar regions between dichotic pitch and reading, this suggests that these may not be directly related through cortical activity. The current results provide novel evidence that reading and visual temporal processing may involve some of the same cortical areas, at least in children with dyslexia. Future research will investigate links between reading and temporal processing in younger children and will examine differences in white matter connectivity.

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