Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Neurobiology and Anatomy, 2011.
Self-movement creates a radial pattern of optic #ow that tells us where we are
going. Recent studies have shown that the processing of visual motion is actively
controlled by frontal-parietal cortical networks to satisfy the demands of ongoing
behavior. Navigation-related visual motion perceptual de!cits in aging and
Alzheimer’s disease (AD) suggest a systems level disruption of optic flow processing in
these conditions. We have now examined this system using human evoked potentials
and probed the underlying neuronal mechanisms using monkey single neuron
physiology.
In our studies of aging and AD, we combine a continuous visual discrimination
task with simultaneous visual motion and word stimulus streams to assess task effects
on stimulus evoked cortical activity. We !nd that the cognitive control of motion and
word processing are fundamentally different in aging and AD. Impairments in verbal
#uency among our AD patients and impairments in optic #ow perceptual thresholds
among our older adult subjects are associated with selective reductions in cortical
responsiveness to word and optic #ow stimuli, respectively. Thus, diagnostic criteria
for AD may not appropriately consider impairments in visuospatial processing.
Monkeys trained in a memory guided steering task show single neurons with
task-dependent optic #ow processing in cortical areas MST and LIP. One third of MST
neurons show task effects on basic neuronal response properties. In contrast, LIP
neurons show minimal optic #ow stimulus selectivity but pronounced task effects on
their responses. Both MST and LIP show evidence of cognitive signals related to motor
planning just prior to steering. These results suggest that posterior parietal cortical
areas form a distributed network which is dynamically shaped by task demands.
Despite species and methodological differences, the dynamics of cortical
activity in humans and monkeys show sensory signals that are followed shortly by
cognitive signals to actively modulate self-movement perception. In young adult
human subjects and monkey single neurons, we see obligate optic #ow processing in
all task conditions. In older adult humans, we see a loss of obligate optic #ow
responsiveness which may suggest a selective vulnerability of visuospatial processing
networks in cognitive aging.
