Knowing the direction and speed of moving objects is often critical for survival. However, it is poorly understood how cortical neurons process the speed of image movement. Here we tested MT neurons using moving sine-wave gratings of different spatial and temporal frequencies, and mapped out the neurons' spatiotemporal frequency response profiles. The maps typically had oriented ridges of peak sensitivity as expected for speed-tuned neurons. The preferred speed estimate, derived from the orientation of the maps, corresponded well to the preferred speed when moving bars were presented. Thus, our data demonstrate that MT neurons are truly sensitive to the object speed. These findings indicate that MT is not only a key structure in the analysis of direction of motion and depth perception, but also in the analysis of object speed.

No abstract

I present a method by which direction- and speed-tuned cells, such as those commonly found in the middle temporal area of the primate brain, can be used to analyze the patterns of retinal image motion that are generated during observer movement through the environment. For pure translation, the retinal image motion is radial in nature and expands out from a point that corresponds to the direction of heading. This heading direction can be found by the use of translation detectors that act as templates for the radial image motion. Each translation detector sums the outputs of direction- and speed-tuned motion sensors arranged such that their preferred direction of motion lies along the radial direction out from the detector center. The most active detector signifies the heading direction. Rotation detectors can be constructed in a similar fashion to detect areas of uniform image speed and direction in the motion field produced by observer rotation. A model consisting of both detector types can determine the heading direction independently of any rotational motion of the observer. The model can achieve this from the outputs of the two-dimensional motion sensors directly and does not assume the existence of accurate estimates of image speed and direction. It is robust to the aperture problem and is biologically realistic. The basic elements of the model have been shown to exist in the primate visual cortex.