(Lecture 20 - Motion Detection)

Required Reading : Chapter 5 - Simmons and Young

Recommended Reading :

Rind, F.C. & Simmons, P.J. (1998) Local circuit or the computation of object approach by an identified visual neuron in the locust. J.comp.Neurol. 395:405-415

 Kern, R., Egelhaaf, M., & Srinivasan, M.V. (1997) Edge detection by landing honeybees: behavioural analysis and model simulations of the underlying mechanism. Vision Res. 37:2103-2117

Optomotor neurons in flies

• large, fan-shaped neurons in the lobula plate
• respond to movement over a large receptive field (flow field) - probably control motion in flight
• exhibit directional sensitivity (preferred direction versus null direction)
• 3 HS neurons - sensitive in yaw axis
  (right lobula plate - clockwise movement : left - anti-clockwise)
• 11 VS neurons - sensitive in pitch and roll axes
  (each VS probably has different sensitivity)
• neurons such as H1 have processes in both lobula plates and carry information between pairs of optomotor neurons - allows comparison of flow fields (moving forwards, or spinning in place...)
• each optomotor neuron receives input from an array of elementary motion detectors over a large receptive field
• elementary motion detectors occur in groups of at least 4 - one pair responds to light objects (+ve and -ve response) and one pair responds to dark objects (+ve and -ve response)

The response of this elementary motion detector (shown on the left as two individual photoreceptors within an ommatidium, and on the right in a circuit diagram) is dependent on the posterior receptor p being stimulated before the anterior receptor a. If p and a are stimulated simultaneously, or if a is stimulated before p, the motion detector will show no response or will be inhibited.

Figure-ground neurons

• 4 neurons - also found in the lobula plate
• have large receptive fields
• respond to small stimuli moving independently of background
• max response at about 5 degrees of visual field
• larger stimuli produce smaller response
• used to track movement of small objects relative to the background - such as other insects, or potential landing sites

Collision avoidance in locusts

• information about rapidly approaching objects conveyed to motor system by descending contralateral movement detector (DCMD) - triggers escape jump
• DCMD receives input from one neuron in the lobula - the lobula giant movement detector (LGMD)
• LGMD responds briefly (rapid adaptation) to movements in the visual field, but responds vigorously to an approaching object
• tightly tuned response: a deviation of 2-3 degrees from direct approach halves response of LGMD
• LGMD also receives widefield inhibition, which inhibits firing if the entire visual field moves at once (such as when the animal moves its head)
• response is the result of balanced excitation and lateral inhibition (both at the LMCs and the direct inputs to the LGMD)
• if the object is approaching rapidly the spread of excitation occurs faster than the corresponding lateral inhibition