Hcd

Figure 3 (A) A space-time plot of a dark rectangle translating rightward. It starts at x position = 0, and its x position (horizontal position) increases with time, whereas its y position (vertical position) remains constant. (B) An x-t plot of the rectangle's motion created by taking a slice through the space-time plot at a constant y value. Note that the x axis is horizontal and the t axis is vertical. The origin is at the midpoint of the x axis. Rightward motion starts at x = 0 and lies along a line that is tilted down and to right in this plot. (C) Leftward motion starts at x = 0 and lies along a line that is tilted down and to the left. (D) A static rectangle whose position remains fixed at x = 0. Because its position does not change with time, its graph is a vertical straight line.

estimates that show that primate MT neurons are arranged into direction columns according to their preferred directions of motion. There is a smooth gradation of preferred direction across neurons in neighboring columns. For similar moving dot stimuli, physiological estimates of the direction bandwidth of MT neurons range from 40 to 52°.

C. Speed Coding

Suzanne McKee and coworkers showed that humans are very sensitive to changes in speed and can discriminate a 5% speed difference despite small variations in the contrast, duration, and temporal frequency content of the stimulus. Currently, the way in which the brain processes and codes speed information is less well understood than the neural processing and coding of the direction of motion. Although the space-time-oriented receptive fields of motion energy detectors and complex cells in striate cortex of the cat do respond optimally to a unique velocity, the output of a motion energy unit also depends strongly on both the contrast and size of a moving stimulus. To extract velocity information, which is independent of stimulus contrast and size, Ted Adelson and Jim Bergen proposed a velocity computation stage that calculates the ratio of the outputs of pairs of motion-sensitive and static neurons. If the response of both the static and motion-sensitive cells depends equally on contrast and size, then this divisive normalization largely eliminates the effect of contrast and size on the output of the velocity computation stage. Nonetheless, large changes in contrast affect estimates of perceived speed, with higher contrasts associated with higher perceived speeds.

For a drifting sine wave grating, speed is the ratio of its temporal frequency to spatial frequency. Although earlier models proposed that the visual system codes speed by directly measuring temporal frequency, there is currently robust evidence that the visual system is sensitive to speed per se and not to the combinations of spatial and temporal frequency that characterize the motion. Several studies working with moving bars and dots have provided evidence for speed selectivity in areas V1 and MT. However, these studies do not distinguish between the possibility that the neurons are responding to changes in temporal frequency rather than speed. Bill Newsome and Tony Movshon have found evidence for MT neurons that are tuned to the speed of a moving grating rather than to its temporal or spatial frequency. Further experiments are needed to establish that MT is indeed the site that encodes speed.

III. INTEGRATING LOCAL MOTION SIGNALS A. The Aperture Problem

As mentioned earlier, there is evidence for space-time-oriented units in visual area 1 (V1), the earliest cortical processing area for visual information. This area has units with small receptive fields that respond best to bars or edges that move in the cell's preferred direction within the receptive fields. Thus, these small units do not directly signal the motion of complex real-world objects but respond only to the motion of image fragments that fall within the aperture that is their receptive field. The problem that a local neuron faces is evident if one views the motion of an extended edge through an aperture. The direction of motion is ambiguous. This is the aperture problem, and it is

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