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Visual Neuroscience Lab

fMRI RESEARCH

Select Abstracts from Completed Projects

Secen, J, Cullham, J, Ho C. & Giaschi, D. (2011). Neural correlates of the multiple-object tracking deficit in amblyopia.Vision Research, 51, 2517-2527.

Deficits in multiple-object tracking have previously been reported in both the amblyopic and the clinically unaffected fellow eye of patients with amblyopia. We examined the neural correlates of this deficit using functional MRI. Attentive tracking of 1, 2 or 4 moving targets was compared to passive viewing and to baseline fixation in an amblyopic group and an age-matched control group in six regions of interest: V1, middle temporal complex (MT+), superior parietal lobule (SPL), frontal eye fields (FEF), anterior intra- parietal sulcus (IPS), and posterior IPS. Activation in all regions of interest, except V1, increased with attentional load in both groups. MT+ was less active in both eyes of the amblyopic group relative to controls for passive viewing and each of the tracking conditions. Anterior IPS and FEF were less active with amblyopic eye viewing when tracking four targets. These results implicate both the low-level passive and high-level active motion systems in the multiple-object tracking deficit in amblyopia.

Ho C, Giaschi D (2009) Low- and high-level motion perception deficits in anisometropic and strabismic amblyopia: evidence from fMRI. Vision Research, 49, 2891-901

Maximum motion displacement (Dmax) is the largest dot displacement in a random-dot kinematogram (RDK) at which direction of motion can be correctly discriminated [Braddick, O. (1974). A short-range process in apparent motion. Vision Research, 14, 519­527]. For first-order RDKs, Dmax gets larger as dot size increases and/or dot density decreases. It has been suggested that this increase in Dmax reflects greater involvement of high-level feature-matching motion mechanisms and less dependence on low-level motion detectors [Sato, T. (1998). Dmax: Relations to low- and high-level motion processes. In T. Watanabe (Ed.), High-level motion processing, computational, neurobiological, and psychophysical perspectives (pp. 115­151). Boston: MIT Press]. Recent psychophysical findings [Ho, C. S., & Giaschi, D. E. (2006). Deficient maximum motion displacement in amblyopia. Vision Research, 46, 4595­4603; Ho, C. S., & Giaschi, D. E. (2007). Stereopsis-dependent deficits in maximum motion displacement. Vision Research, 47, 2778­2785] suggest that this “switch” from low-level to high-level motion processing is also observed in children with anisometropic and strabismic amblyopia as RDK dot size is increased and/or dot density is decreased. However, both high- and low-level Dmax were reduced relative to controls. In this study, we used functional MRI to determine the motion-sensitive areas that may account for the reduced Dmax in amblyopia. In the control group, low-level RDKs elicited stronger responses in low-level (posterior occipital) areas and high-level RDKs elicited a greater response in high-level (extra-striate occipital­parietal) areas when activation for high-level RDKs was compared to that for low-level RDKs. Participants with anisometropic amblyopia showed the same pattern of cortical activation although extent of activation differences was less than in controls. For those with strabismic amblyopia, there was almost no difference in the cortical activity for low-level and high-level RDKs, and activation was reduced relative to the other groups. Differences in the extent of cortical activation may be related to amblyogenic subtype.

Ho C, Giaschi D (2009) Low- and high-level first-order random-dot kinematograms: evidence from fMRI. Vision Research, 49, 1814-24

Maximum motion displacement (Dmax) represents the largest dot displacement in a random-dot kinematogram (RDK) at which direction of motion can be discriminated. Direction discrimination thresholds for maximum motion displacement (Dmax) are not fixed but are stimulus dependent. For first-order RDKs, Dmax is larger as dot size increases and/or dot density decreases. Dmax may be limited by the receptive field size of low-level motion detectors when the dots comprising the RDK are small and densely spaced. With RDKs of increased dot size/decreased dot density, however, Dmax exceeds the spatial limits of these detectors and is likely determined by high-level feature-matching mechanisms. Using functional MRI, we obtained greater activation in posterior occipital areas for low-level RDKs and greater activation in extra-striate occipital and parietal areas for high-level RDKs. This is the first reported neuroimaging evidence supporting proposed low-level and high-level models of motion processing for first-order random-dot stimuli.

Giaschi D, Zwicker A, Young SA, Bjornson B. (2007) The role of cortical area V5/MT+ in speed-tuned directional anisotropies in global motion perception. Vision Research, 47(7), 887-898.

Several different directional anisotropies have been found in global motion perception. The purpose of this study was to examine the role of the motion sensitive cortical area V5/MT+ in directional anisotropies for translational flow fields. Experiments 1 and 2 tested direction discrimination and detection of moving random dot patterns. When the speed of motion was 8 deg/s, lower coherence thresholds were found for centripetal relative to centrifugal hemifield motion. When the speed of motion was 1 deg/s, coherence thresholds were similar in all directions. Experiment 3 used fMRI to measure the BOLD response to different directions of motion at speeds of 1 and 8 deg/s. Greater activity was found in V5/MT+ for centripetal motion than for centrifugal motion at both speeds. These results suggest that V5/MT+ does play a role in directional motion anisotropies. This role is discussed with respect to visually-guided reaching and locomotion.

Giaschi D, Edwards V, Au Young S, Bjornson B (2005) Asymmetrical cortical activation by global motion in children with dyslexia. Journal of Vision, 5(8):848a.

Several groups have reported elevated motion coherence thresholds on global motion tasks in children and adults with dyslexia (eg. Edwards et al., 2004; Raymond & Sorensen, 1998; Talcott et al., 1998). The nature of the relationship between motion perception and reading deficits, however, has not been established. We used functional MRI to study the neural basis of the global motion deficit in 12 right-handed children with dyslexia and 12 age-matched controls. Area V5/MT+ was identified with a localizer task in which blocks of dots in expanding/contracting radial motion alternated with blocks of stationary dots. Activation in the V5/MT+ region was observed in 23 of 24 hemispheres in the control group and in all 24 hemispheres in the dyslexic group. This result is contrary to previous reports of reduced or no activation in response to moving stimuli in V5/MT+ in adults with dyslexia (Demb et al., 1998; Eden et al., 1996). Global motion direction discrimination was assessed using blocks of discrete trials of horizontally moving dots alternating with blocks of stationary dots. The coherence level was 85% or 30% on alternate motion blocks. Both groups showed more widespread activation when the coherence level was 30% than when the coherence level was 85%. At 30% coherence, activation was bilateral and symmetric in the controls. In contrast, the dyslexic group showed asymmetric activation with significantly reduced left hemisphere activation in V5/MT+, posterior occipital (putative V3A, V1, V2) and posterior parietal cortex. This finding, on motion tasks, is notable because, on reading tasks: 1) normal young readers show increasing left hemispheric lateralization as their reading fluency increases (Turkeltaub et al., 2003), and 2) children with dyslexia show reduced activation in left posterior regions compared to control children (Shaywitz et al., 2002). These results implicate left posterior cortex in both reading and global motion deficits in children with dyslexia.

Giaschi D, Jan J, Bjornson B, Au Young S, Tata M, Good W, Lyons C, Wong P (2003) Conscious visual abilities in a patient with early bilateral occipital damage. Developmental Medicine & Child Neurology, 45, 772-781.

A 21 year old male is presented whose occipital lobes were extensively damaged by bilateral infarcts present at birth. The absence of striate cortex was confirmed with anatomic and functional MRI and high-resolution EEG. His cortical visual impairment is severe, but he retains a remarkable ability to see fast moving stimuli. Horizontal optokinetic nystagmus could be elicited from either eye. Resolution acuity was close to normal providing the patient was allowed to move his head and eyes. The direction of motion in random-dot patterns could be discriminated with perfect accuracy at speeds above 2 deg/s, and the patient reported that he could “see" the motion at fast but not at slow speeds. This conscious residual vision for motion is known as Riddoch's phenomenon, but it has never been reported in the complete absence of striate cortex. Functional neuroimaging revealed activation that was outside the motion-responsive regions of the extrastriate cortex. This case demonstrates remarkable plasticity in the human visual system and may have implications for understanding the functional organization of the motion pathways.

Chen C-C, Giaschi D, Bjornson B, Au Young S (2003) Bold activation for detection and identification of motion-defined form in human brain. Society for Neuroscience Abstracts, 29, 591.18.

Figure-ground segregation can be achieved in a random dot kinematogram when the dots of the figure and the ground move in different directions. Focal cortical lesions in the region of the temporoparietal occipital junction disrupt the perception of such motion-defined (MD) forms, but some patients show dissociation between the detection and identification of MD form. We used functional MRI to investigate the neural basis of this dissociation by examining brain activation during passive viewing and during active identification of MD form.



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