| Description |
Primate primary visual cortex (V1) consists of six anatomical layers. There are both heterogeneous and homogeneous functional properties found across layers. Surround modulation (SM) occurs when neuronal responses to stimulation of a neuron's receptive field (RF) is modulated by simultaneous simulation outside of the RF. There are three potential candidates for SM: feedforward (FF) and intra-V1 horizontal (HZ) connections underpin the region nearby RF (near surround), while the modulatory signal arising from distant regions (far surround) are conveyed by feedback (FB) connections from higher visual areas. Also, V1 layers show distinct patterns of FF, HZ, and FB terminations. The goal of my dissertation research was to study 1) the properties of SM across V1 layers, 2) how simple visual stimuli in the RF and surround of a V1 column activate V1 layers, and (3) what specific afferent circuits to and within the V1 column of these stimuli recruit. Using single electrode recordings sampling from all the layers of V1, I found that near SM is more sharply orientation-tuned in the superficial layers (L3B, 4B and 4C?), where there are prominent horizontal connections. However, far SM is more orientation-tuned in L4B, possibly reflecting the orientation organization of feedback connections to this layer. Using laminar recordings, I investigated the temporal dynamics of inputs (local field potentials, LFPs) to each layer when stimulating surround elements. Near surround stimulation simultaneously localized the first inputs in superficial and deep layers with a significant delay in L4C, suggesting both HZ and FB contribute to near SM. Feedback recipient layers (L1/2A and L5/6) received the earliest inputs with far surround stimulation. Measuring the latency of spiking activity while co-stimulating RF and surround, the untuned near SM first emerged in L4C, but, tuned near SM and far SM, emerged outside thalamic recipient layers, suggesting a cortical origin. Finally, I found that brain oscillations in response to stimuli in the surround, mirror the structure of the underlying horizontal and feedback connections. Grating patches positioned on the collinear axis to a cell's preferred orientation, evoke a greater power in different frequency bands of LFP, including alpha, beta, and gamma, compared to orthogonal position in both near and far surround. We propose that horizontal and feedback connections, substrates of near and far surround, are aligned collinearly in the visual field and help generate brain oscillations. |
