The Biology

The V1 cells (layer 4B) that project to MT have the following properties [26]:

• They are direction selective, but are relatively poorly tuned for orientation and spatial frequency;
• They are insensitive to color;
• They have large receptive field for V1 cells.

These properties generally resemble the MT cells they project to. Similar to these V1 cells, almost all neurons in the MT area are known to be selectively responsive to orientation and velocity of visual motion (both speed and direction). However, there are major response differences between the neurons in V1 and MT, as listed below [27], [28], [29], [35], [39]. All these findings show that V1 is an upstream visual area of MT, and it provides important visual input to MT.

• The receptive fields of MT neurons are generally an order of magnitude larger than that of the V1 neurons at equivalent eccentricities. The MT neurons are directionally selective for higher speeds ( $10^{\circ}/sec.$ to $150^{\circ}/sec.$) than the V1 neurons ( $4^{\circ}/sec.$ to $20^{\circ}/sec.$), The direction tuning of MT neurons (bandwidth $95^{\circ}$) is wider than that of the V1 neurons (bandwidth $68^{\circ}$).
• There are two types MT neurons. The first type, called component MT neurons, responds to the velocity of the local motion. The component MT cells are subject to the aperture problem, just like the velocity selective V1 cells. The second type, called pattern MT neurons, responds to the true 2D global motion of a visual pattern (the pattern motion).
• The V1 and MT cells respond differently to transparent motions which have more than one velocity vector at each location in the visual field (usually two local velocities of different directions). V1 cells can respond to their preferred directions of movement under transparent conditions, whereas MT cells are suppressed when two motions are in opposite directions ([40]).
• Some non-motion segmentation cues also influence visual motion detection at both the neuronal and perceptual levels. For example, when two grating patterns moving in different directions across each other, a subject will perceive either two grating motions if the luminance of the intersection areas of the gratings is such that the gratings are perceptually transparent, or a coherent plaid motion when the luminance of the intersection areas is either too high or too low for the perceptual transparency [44]. Consistent result was also found later at the neuronal level in the MT area [45].