Animal Motion Perception

Visual Neurons

Early research on frogs and insects showed that some visual neurons were specialized to respond only to relatively small moving visual stimuli, and often these preferred a particular direction of motion. These directional specific neurons were sequence detectors. If two adjacent areas in the visual field (say A and B) were stimulated by a moving stimulus that moved over A then B, the neuron would fire to signal this event. However, if the direction were reversed so that B then A was stimulated, the neuron would not fire, but would be inhibited. Early experiments showed that two flashed stimuli one at A and then the next at B after a short delay, also stimulated these movement neurons, but again, they did not fire when B then A were flashed without movement. Thus, these neurons respond to apparent motion as well as real motion. These motion-detecting neurons that respond to small moving things are precisely the stimuli that capture an animal's attention and typically produce a turning of the head and eyes (“orienting response”) to place the object's image onto the fovea or retinal area of high resolution. In frogs, these were nicknamed “Bug Detectors” because frogs turn and snap at bug-like moving patterns. Typically, larger moving stimuli produce escape or avoidance behavior in animals.

Movement and direction-specific neurons have been found in the visual system of all invertebrate and vertebrate animals where moving stimuli have been used to interrogate the visual system, and so it seems safe to assume these neural specializations are universal. Moreover, wherever motion-detecting neurons occur, it seems that the underlying mechanism in all species is produced by delayed asymmetric lateral inhibition. Object motion sensitive neurons have been found in flies, bees, locusts, crickets, praying mantises, and dragonflies, and also in fish, salamanders, turtles, frogs, toads, chickens, pigeons, owls, mice, rats, cats, ferrets, wallabies, and several species of monkey. Perhaps the most extensive studies have been conducted on the middle temporal cortical area of the macaque monkey cortex.

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