IBN: 9634367 PI: Finkel Gain control is a powerful process that is used in many biological and engineering applications. In a videocamera, the photodetectors can only operate over a limited range of light levels, so in order to be able to record in bright daylight and dim internal light, the signal must be re-scaled to stay within a limited range of intensities. A slightly smarter system, used in more expensive devices, is to change the gain (how much the signal is amplified) separately in each local region. Changing the amount of gain in an adaptive manner allows signals to be extracted from ambient noise. The same principles of gain control act in our visual system-- in a simple manner on the pupil to control light entering the eye, but in increasingly more sophisticated ways as one goes to higher levels in the visual cortex. Our goal is to investigate these mechanisms of cortical gain control to better understand how the visual sytem works, and as a means of uncovering new image processing applications. The cortex has the ability to control gain not only based on spatial location, but also in a context-dependent manner. The response to a particular feature, say a short line segment, depends upon what other features are present in the image. If the line segment forms part of a circle or an extended line, the response is increased. Psychophysical studies have identified the conditions under which certain features "pop-out" and become the focus of visual attention. Recent physiological studies have shown that cortical cells are exquisitely sensitive to small changes in the context of the scene. It is believed that response changes are a result of a change in gain of individual cortical cells, but the mechanisms controlling cell gain are poorly understood. We are particularly interested in differential changes of gain among different cells in a neural population. Most features are represented, in the cortex, by a set of graded responses ove r a population of cells. The orientation of a line, for example, is coded by a distribution of responses over cells that prefer various orientations (vertical, horizontal, oblique). We hypothesize that inputs from stimuli in the visual image act to differentially change the gain of cells. A differential change in gain across a population would lead to increased sensitivity--a better estimate of line orientation, or whatever feature is being analyzed. This type of context-dependent gain control could lead to enhanced discrimination of the salient features in an image. We will carry out a series of physiological experiments in cat visual cortex to investigate this hypothesis. We also will conduct a set of computer simulations of various cellular mechanisms to test our model. Finally, we will build a prototype image processing device, based on these findings, and apply it to real images.
