Correspondence Noise and Signal Pooling in the Detection of Coherent Visual Motion

Abstract

In the random dot kinematograms used to analyze the detection of coherent motion in the middle temporal visual area (MT) and in psychophysical experiments the exact way that dots are paired between successive presentations is not known by the observer. We show how to calculate the limit to coherence threshold caused by this uncertainty, which we call "correspondence noise." We compare ideal thresholds limited only by this noise with those of human observers when dot density, ratio of dot numbers in two fields, area of stimulus, number of fields, and method of generation of the coherent dots are varied. The observed thresholds vary in the same way as the ideal thresholds over wide ranges, but they are much higher. We think this difference is because the ideal detector takes advantage of the high precision with which dots are placed in the kinematograms, whereas the neural motion system can only operate with low precision. When kinematograms are generated with decreased precision of dot placement, the ideal detector no longer has this advantage, and the gap between ideal and actual performance is greatly reduced. Because the signals that result from objects moving in the real world are scattered over broad ranges of direction and velocity, high precision is not needed, and it is advantageous for the motion system to pool information over broad ranges. Other mismatches between kinematograms and the neural motion system, and internal noise, may also elevate human thresholds relative to the ideal detector. The importance of external noise suggests that the neurons of MT form a vast array of optimal filters, each matched to a different combination of parameters in the multidimensional space required to define motion in patches of the visual field. Key words: correspondence noise; coherent motion; statistical efficiency; integration; matched filters; MT or V5; global motionThe motivation for the work to be described here was to find the natural difficulties and limiting factors for detecting motion in the random dot kinematograms that have been used so successfully to analyze the neuronal basis for the detection of coherent motion by monkeys (Newsome et al., 1989(Newsome et al., , 1990Britten et al., 1992Britten et al., , 1995Celebrini and Newsome, 1994). In this paradigm some of the dots are moved coherently in the same direction from field to field, whereas the remainder are replaced at random positions; the behavioral responses of the monkey, and the discharges of its cortical neurons, are tested for their ability to detect motions with varying percentages of coherence, and a fraction as low as 5% is often reliably detected both by the whole monkey and by single neurons in the middle temporal visual area (MT or V5). We thought that the value of the comparison between neurophysiology and behavior would be much increased if the limiting factors were better understood. Figure 1, top, illustrates the correspondence problem, which arises whenever motion has to be detected and is specially important in random dot kinemato...

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