Overall Research Aims

The investigation of the processes intervening between the presentation of an object in the world and the emergence of a visual percept can be separated into three non-interacting stages:

Formation of the optical image on the retina;

Signal transduction and neural transformation in the retina; and

Central processing in the brain.

This division of the overall task is possible because retinal image formation takes place entirely in the realm of optics and because in the mammal there is no descending pathway to the retina, which carries out its function in isolation. It follows that, if adequate knowledge is available about the first two stages, and if precautions are taken to retain stability and stationarity between the visual world and the eye, then the pre-conditions are met for a scientific study of cortical processing of perception.

Two radically different modes of approach are available. The "hardware" of the brain can be studied by such means as direct recording the activity of brain cells and their connections as well as by the less selective procedures of electro-encephalography and imaging. Or analytical experiments can be performed to gauge neural processing from changes in the intact organism -- externally observed behavior in animals or the reports of the personal experience of a human observer (psychophysics).

The enormous amount of interaction continuously taking place within the functioning brain demands specificity in the conditions of the experiment. To home in on the operation of the first neural stages of brain processing of visual stimuli, it is necessary to keep the more global operations of memory, attention and learning invariant, or, reciprocally, if the specifics of either memory formation or the plastic changes in learning or attention are the subject of study, one should use simple and repeatable stimulus sets.

Current Experimental Program

The focus is on the perception of spatial and spatio-temporal relationships in the visual world. Within the sensory modality of vision, one can distinguish between such attributes of a seen feature as location, contrast, color, depth, and motion. That there is universal agreement on the separability of such properties of a visual sensation -- in subjective personal impressions, in language, and in animal behavior -- implies that they each have their own stream of brain processing. In other words, they are processed in parallel. Anatomical and neurophysiological studies have born this out. Yet these studies have only very partially elucidated the channeling of information within the streams, in part because the neural circuits involved are intricate and difficult to disentangle by current neurobiological techniques, and in part because in the absence of an astronomical set of random probing stimuli, insight from the perceptual experience still remains the most powerful guide. Models based on mathematical concept also come to mind as possible tools, and the laboratory takes their study seriously, specifically as regards the validity of their fundamental assumptions and the possibility of their yielding testable propositions.

An experimental program is actively being pursued in which the sensitivity of human observers to variations in spatial and temporal parameters of simple visual configurations is examined. The aim is to accurately describe the ultimate differentiating capacity of the organism and hence to delineate the operating characteristics of the neural circuits that sort incoming visual signals in the cortex. Results reveal quite surprising abilities to discriminate between stimuli, and serve as benchmarks that future descriptions of neural circuitry must eventually reach. In the process, interesting new findings emerge on the interacting influences of learning and attention on even these early stages of cortical processing.