Monday, September 21, 2015  12:00 – 1:30 pm, in 489 Minor Hall

Graduate Student Seminar

presented by

 Christy Sheehy, PhD Candidate (Austin Roorda’s Lab)

and

Wesley Chaney, PhD Candidate (David Whitney’s Lab)

 

SESSION 1
 
Speaker: Christy Sheehy, PhD Candidate (Roorda Lab)
 
Title: Usage of the Tracking Scanning Laser Ophthalmoscope (TSLO)
 
The human eye is always moving, even when it’s fixating. With the eye as an ever moving object, recording high-resolution images of the retina can be difficult. Additionally, targeted light delivery to the retina remains uncontrolled with constant eye motion. To address these issues, we have built a tracking scanning laser ophthalmoscope (TSLO) that images the retina while simultaneously providing a high-fidelity eye motion trace. The TSLO tracks the retina at a rate of 960 Hz, with a tracking accuracy of 0.66 arcminutes (~3 µm) – down to the size of an individual cone photoreceptor (Sheehy et al. 2012). The TSLO system itself is robust, easy to use, cheap to build compared with many other eye trackers, and flexible in its usage. It can be operated as a stand-alone monocular eye tracker or can be converted into a binocular eye tracker (Stevenson et al. 2015). Furthermore, the system can be combined with other high-resolution imaging systems such as optical coherence tomography (OCT) (Vienola et al. 2012, Braaf et al. 2013) and adaptive optics scanning laser ophthalmoscopes (AOSLO) (Sheehy et al. 2015) to actively steer an imaging beam to stay on target. We will summarize how the TSLO improved the image quality and residual motion in these imaging modalities and report on future goals and usages of the system.

 
SESSION 2
 
Speaker: Wesley Chaney, PhD Candidate (Whitney Lab)
 
Title: Spatial Attention Reduces Correlated Noise in the fMRI Response
 
Spatial attention modulates sensory neural activity, enhancing the representation of relevant stimuli and leading to enhanced performance in a variety of tasks. However, the exact mechanisms by which attention modulates early sensory activity and how this impacts the fMRI BOLD response are not yet fully understood. Previous studies recording from single units in area V4 have demonstrated that attention reduces correlations in the noise of simultaneously observed neurons (Cohen and Maunsell, 2009; Mitchell et.al., 2009). We tested whether analogous reductions of correlated noise also occur at the population level using the fMRI BOLD signal. We examined the effects of attention on the representation of four Gabor stimuli presented simultaneously in each quadrant of the visual field at jittered eccentricities. Subjects attended for contrast decrements in the Gabors in one visual field (either upper or lower in alternating runs) while ignoring the Gabors in the other visual field. After regressing out stimulus driven activity in V1 and V2 in a General Linear Model analysis, we analyzed pairwise correlations of the residual timecourses in voxels representing either the attended or unattended visual fields. We found that attention to either the upper or lower visual field reduces correlation of these timecourses. This reduction is opposite to what would be predicted from reduced noise within individual voxels or an increase in stimulus driven activity due to attention and it cannot be explained by vasculature differences or local scanner artifacts as each region is attended or ignored for an equal number of runs. This reduction in correlated noise within attended locations allows for more accurate signal estimation at the population level, and may facilitate readout by higher level processes that pool over information in early visual cortex. We also show that decorrelation is associated with improved position discrimination in early visual cortex.

 

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