The question of whether the visual system uses a short-term memory store and the nature of that storage is a topic of ongoing research. Electrophysiological studies have found that neurons in visual cortex modify their activity within a fraction of a second in response to stimulus changes and do not seem to show evidence of memory. However, the long phases of stable perception of reversible figures suggest a memory that persists for seconds.
Often the ambiguity in reversible figures is in the foreground-background structure, whether they be Necker cubes, rotating spheres of dots, or images like Rubin's vase-face figure that have multiple coherent interpretations which vary based on the assignment of foreground and background in the image elements. One of the ways in which the brain encodes foreground-background structure is by assigning contours to figures. Neurons in area V2 of macaques are selective to which side of a contour is figure and which side is ground. This selectivity has been termed border ownership tuning, and it very likely plays a role in the perception of figure and ground. Therefore, it seems possible that the perceptual persistence found in these ambiguous displays could come from persistence in the border ownership signal.
This thesis tested this hypothesis by examining how the border ownership signal in single neurons decayed when the figure ground cues changed in different ways. Primarily, the goal was to see if the signal showed persistence when the figure ground cues that assigned border ownership were removed and the image became ambiguous regarding figure-ground.
I found that the border ownership signal did persist, slowly decaying over more than one second after the stimulus became ambiguous for figure-ground. However, when the figure-ground cues reversed, reversing the border ownership assignment, the signal rapidly reversed in response to this change. The signal did not accumulate over multiple figure presentations, but instead each new figure reset the encoding mechanism. Thus the system is both able to preserve the information that has been removed and reset in response to new information that comes in.
Further experiments were performed to study the characteristics of the persisting signal. I found that the persistence was robust, existing for border ownership signals generated by a variety of figure-ground cues. It also could survive the interruption of activity in the border ownership encoding neurons, ruling out an activity dependent loop in these neurons as a mechanism.
I also show in preliminary results from one monkey that this signal may contribute to stable perception across saccades and object movements. The results of a third monkey who did not seem to show the same persistence as the other two are also discussed.
This thesis shows that the visual system does have a memory store for figure-ground information. This should contribute to our understanding of how the border ownership mechanism works and how the visual system performs the daunting task of perception in an actively changing visual world.
|Advisor:||Heydt, Rudiger von der|
|School:||The Johns Hopkins University|
|School Location:||United States -- Maryland|
|Source:||DAI-B 71/01, Dissertation Abstracts International|
|Keywords:||Electrophysiology, Macaque, Single-unit recording, Visual cortex|
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