An important issue in visual neuroscience is to understand how neural networks (or circuits of neurons, i.e. brain cells) in the visual cerebral cortex determine neuronal responses to visual objects and visual perception.
The goal of this project is to identify the circuitry and mechanisms for surround modulation in visual cortex. Surround modulation is the ability of visual cortical neurons to adjust their responses to an object (e.g. a vertically-oriented bar) depending on the visual context within which it is presented (e.g. a background of many vertical or horizontal bars). This phenomenon is thought to underlie ability to perceive objects in cluttered scenes. For example, a background of vertical bars suppresses the response to the single vertical bar, making it perceptually invisible. Instead, a background of horizontal bars increases the response to the vertical bar, making it perceptually to stand out from the background.
A combination of computational modeling and experimental studies will be used to identify the circuitry and mechanisms for surround modulation. Specifically, to gain insights into the underlying circuitry, the spatial, temporal and tuning properties of surround modulation will be studied using visual stimuli designed to probe the underlying pathways. These data will be used to generate computational models of visual cortical circuitry and function. Modeling will be used to understand the mechanisms generating surround modulation and its role in visual perception.
This research will uncover the neural substrates for higher visual cortical processing and perception. The interdisciplinary nature of the proposed work will create novel training opportunities for graduate students at the University of Utah. This project will support the training of several graduate students, and expose to the field of neuroscience undergraduate students from the field of bioengineering.
The response of neurons in the visual cerebral cortex to a target visual stimulus is influenced by the visual context within which the stimulus is embedded, i.e. other stimuli present in its surround. Similarly, in human vision the perception of a target visual object is influenced by its perceptual context, the objects present in the surround. This phenomenon is called surround modulation and is thought to paly a role in the perception of object contours and salient visual targets. Our laboratory recently discovered that there are two regions in the surround that have distinct effects on an object perception and the response it evokes in visual cortex neurons. These are termed the "near" surround, which is the surround region in the immediate vicinity of the target object, and a far surround region, which is farther out. Our hypothesis is that these different surround regions are generated by different neuronal circuits, and have different roles in visual perception. The research funded by this NSF award characterized the property of signals arising from the near and far surround, and the organization of the circuits that may generate them. For this investigation we used simple stimuli consisisting of gratings whose orientation could be varied. We found that the relative orientation of the target and surround gratings has a profound influence on both the neuronal responses to the target grating and the grating's perception. In particular, the neurons' responses to a target grating are strongest (and teh grating's perception is best) when the target and surround gratings are orthogonally oriented, while they are weakest (and perception worse) when they are of similar orientation. This sensitivity to the stimulus orientation was much more pronounced for the near surround than for the far surround. Thus, large orientation differences between target and surround stimuli caused significant suppression of neuronal responses (and impaired perception) when the stimuli were in the far surround, but had no effect when the stimuli were in the near surround. We also found that the neruonal circuits that may generate near and far surround effects have different orientation organizations, with circuits for the near surround linking preferentially neurons which respond best to the same edge orientation. In contrast, the circuits for the far surround show a more loose orientation organization, linking neurons with more disparate orientation preferences. Our results suggest that near and far surround modulation have different perceptual roles. In particular, the different tuning of near and far surround modulation seem to reflect the statistical relation observed in natural images between edge orientation and distance between edges. Specifically, in natural images nearby edges have a higher probability than distant edges of being similarly-oriented and of belonging to the same physical contour. If near and far surround modulation reflect this statistical bias, then surround modulation should be more sensitive to orientation differences for nearby edges, and less sensitive for distant edges, which is consistent with our results. We conlcude that near surround modulation may serve to detect small orientation differences in nearby edges (which useful for the perception of contours), while far surround modulation may serve to direct saccades (i.e. fast eye movements) and/or attention to visually salient distant locations in the visual field, i.e. l where objects have a markedly different orientation from that of a target stimulus
