Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2009.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
"February 2009."
Includes bibliographical references.
The visually evoked potential (VEP) is a local field potential (LFP) evoked in visual cortex in response to visual stimuli. Unlike extracellular single unit recordings, which allow us to probe the function of single spiking cells acutely, the chronic VEP technique gives us insight into ensemble synaptic activity. However, while action potentials are easily interpreted as the output of the recorded neuron, LFPs are difficult to interpret because they may reflect the sum of activity occurring at or beyond the site of recording. The goal of this study was to use the current source density (CSD) method to derive information about synaptic activity occurring at the site of recording and to determine how this activity relates to the concurrent LFP. The mouse has recently become a widely-used experimental model for studying the mechanisms of plasticity and there has been an increase in the use of VEP recordings to study experience-dependent changes in mouse primary visual cortex (V1). These studies typically focus on changes occurring in the layer 4 VEP after a variable period of visual deprivation. Layer 4 of mouse V1 receives heavy direct input from the lateral geniculate nucleus. This initial input is followed by strict hierarchical connectivity from cortical layer 4 to superficial layers 2/3 and from 2/3 to deep layers 5/6. Using a method for silencing cortical activity without affecting geniculate input activity in conjunction with CSD analyses, we found that the laminar flow of activity in mouse V1 in response to various grating stimuli was consistent with the anatomical connectivity going from layer 4 ?? 2/3 ?? 5/6. To determine if the layer 4 VEP is indeed reflecting synaptic activity occurring in layer 4, we applied the CSD method to field potentials recorded from mouse V1. Our results indicate that changes in the layer 4 VEP strongly and significantly covaries with changes in layer 4 current sink activity suggesting that the layer 4 VEP is indeed reflecting local layer 4 synaptic activity.
(cont.) This layer 4 activity is likely due to direct geniculate input since it persisted after intracortical activity was blocked. If the layer 4 VEP reflects synaptic activity due to direct geniculo-cortical input and if this input is carrying information about the visual world then we would expect the VEP to change as the parameters of the stimuli vary. Indeed the binocular-driven VEP broadened in shape as we increased the spatial frequency (SF) of grating stimuli. Using CSD analyses, we were able to trace the transformations of the layer 4 VEP waveform to changes happening in layer 4 current sinks and layer 4 current sinks were in turn affected by events in deep layers. Specifically, increasing SF of the grating stimuli led to a reduction of current sink activity in deep layers and this unmasked prolonged current sink activity in layer 4. This prolonged layer 4 current sink activity persisted after cortical silencing suggesting that it is likely due to late-onset direct geniculate input. We suggest that late-onset activity from the ipsilateral-eye may be unmasked with increasing SF. VEPs have been used extensively in the clinical and laboratory setting to determine visual acuity in humans as well as anaesthetized animals. If the layer 4 VEP is to be a useful measure of visual function in awake head-fixed mice, VEP-assessed visual acuity and contrast sensitivity should be consistent with behaviorally-assessed measures. We found that VEP-assessed visual acuity agreed with previous behaviorally-assessed acuity; however, VEP-assessed contrast-sensitivity values were slightly higher. One of the reasons why inbred laboratory mice are becoming increasingly useful in Neuroscience is because individual mice are genetically identical and any behavioral variability should be experience-driven. While this is true for mice within a given strain, it is not true between strains since strains are genetically different. Therefore, it is crucial to understand how strain differences in genes affects neural activity before comparing results from different strains.
(cont.) To this end, we compared the VEP response of two commonly used laboratory mouse strains: C57BL/6 and 129/Sv and found important differences in the VEP waveform which may translate into differences in visual function. Specifically, our data suggest that 129/Sv mice may have better acuity than C57BL/6 mice. The advent of molecular engineering tools is another reason why the mouse has become the preferred model system for studying the cellular and molecular mechanisms underlying behavioral and physiological phenomena. Genetically modified mice are routinely screened for behavioral deficits using tasks such as the Morris watermaze -- test for spatial navigation which assumes that the mice have functional vision. In order to remove the experimental confound of vision, the layer 4 VEP can be used to assay the visual function of mice prior to behavioral experimentation. Using the VEP technique, we determined the visual function of Shank1-/- mice to be normal in response to low SF gratings but impaired in response to high SF gratings. Shank1-/- mice were not impaired in the eight-arm radial maze task - another test of spatial navigation suggesting that low SF vision may be sufficient for performing this task. Taken together, this study demonstrates that the VEP is an interpretable and useful recording technique which can be combined with CSD analysis to determine the laminar activity patterns which underlie visual function in the awake mouse.
by Rahmat Muhammad.
Ph.D.