The early visual system processes sensory input in a linear, feed-forward manner: afferent inputs are combined according to area-specific pooling rules to form a new representation of the sensory input; the output of one area is then fed forward to others, where it becomes the input for a subsequent stage of processing. This model accurately describes sensory processing in early visual areas, ranging from the retina up to primary visual cortex, but cannot adequately describe the responses of cortical visual neurons, particularly in extrastriate visual areas. This thesis begins by reviewing the evidence for feed-forward processing in early visual areas. Next, we present data from three situations that suggest the involvement of locally-recurrent connections or feedback from extra-retinal visual areas.
In Chapter 2, we examine the representations of orientation and spatial frequency in area V4, a mid-level visual involved in form processing, attention, and occulomotor planning, and compare these representations to those found in primary visual cortex. While selectivity for these features is superficially similar in both visual areas, we observed considerably more complex temporal dynamics in V4, including changes in tuning shape and preference as well as long-lasting response suppression. This suggests that the V4 representation, unlike the V1 version, is generated or refined by local recurrency or inter-areal feedback. On the other hand, we found relatively little interaction between the V4 representations of orientation and spatial frequency: they remained as separable in V4 as they are in V1. These results are consistent with V4s known involvement in feature-based attention, but are somewhat surprising given its role in form processing.
In Chapter 3, we describe a relationship between the V4 local field potential and occulomotor activity. Specifically, we found that saccade onset is phase-locked to specific frequency components of the V4 LFP. Depending on the site, we observed phase-locking in bands ranging from delta (<4 Hz) to beta (13-40 Hz) frequencies. Some sites showed phase-locking across multiple frequency bands, but phase-locking in one band was not predictive of phase-locking in others. This phenomenon is reminiscent of the association between spatial navigation and theta-band EEG and LFP oscillations. It may serve to synchronize the flow of visual information across multiple brain areas or may facilitate the decoding of time-varying neuronal coding schemes, such as the one described in Chapter 2.
In Chapter 4, we examine how stimulation of the non-classical receptive field (nCRF) affects responses to stimuli inside a cells classical receptive field (CRF). While nCRF stimulation alone cannot drive a cell, we found that simultaneous stimulation of the CRF and nCRF with naturalistic movies caused the cell to respond more selectively, more reliably, and more precisely than when the same stimulus was placed only in the CRF. We used intracellular recordings from electrophysiologically and histologically identified cell types to identify the underlying biophysical mechanism, an increase in intracortical inhibition. We then examined the effect of different types of visual stimulation and compared these results to previously published data.
Taken together, these results indicate that local and inter-areal recurrency play an important role in generating reliable, accurate, and flexible visual representations of the external world.
|Advisor:||Mazer, James A.|
|Commitee:||Chun, Marvin, Lee, Daeyeol, McCormick, David|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 74/05(E), Dissertation Abstracts International|
|Keywords:||Cortical circuits, Eye movements, Surround suppression, Vision, Visual perception, nCRF|
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