Reading 5: How do visual neurons detect direction of motion? Flashcards
Why is it not easy for neurons to integrate direction of stimuli movement?
The problem is that neuron C doesn’t have your larger vantage point. All it can do is integrate synaptic inputs and fire action potentials. Individual neurons have a narrow point of view.
Development of direction selectivity model
A highly influential model to explain direction selectivity was developed more than 60 years ago at the Max Planck Institute of Biology in Tubingen, Germany by Bernard Hassenstein and Werner Reichardt. They used a beetle, Chlorophanus (Fig. 4), as their model system. When these bugs are presented with a moving visual grating, they move to follow the motion. By studying this sensitivity to visual motion, Hassenstein and Reichardt developed a model which has come to be called a Reichardt detector. A central feature of this model is a filter or delay that alters the timing of inputs. A simple way to introduce this delay in one of the two inputs in our example is to make its axon longer.
Tweak to the development of direction selectivity model
One more tweak! We’ll make each EPSP too small to trigger an action potential in neuron C; however, if two the EPSPs arrive at the same time at neuron C, their summed input will trigger an action potential. Let’s see how this changes things.
Imagine a visual stimulus moving from left-to-right. It will activate neuron A first, triggering an action potential. But because of its longer axon, it will take a bit longer for the action potential to reach the synapse. The stimulus reaches the receptive field of neuron B a bit later; however, neuron B has a shorter axon, so the action potential reaches its synapse more quickly. If we adjust the lengths of the axons just right, we can make it so that the two EPSPs arrive at neuron C at the same time, triggering an action potential. For a stimulus moving in the other direction, the timing is off and there is no action potential. Neuron C is now a direction-selective neuron!
What is the basic idea of direction selective neurons is also used in what other system?
This basic idea, using axon length as a timing mechanism, is also used in the auditory system to determine where sounds are coming from.
Consider a sound source that is directly in front of you. The sound from this source will arrive at your two ears at the same time. Now imagine the sound source moving farther and farther to the right. In this case, the sound will reach your right ear slightly before your left ear. As the sound source moves progressively farther to the right, this interaural time delay will
become progressively greater. The interaural time delay is detected by an array of neurons in the medial superior olivary nucleus. An individual neuron in this array fires only when it receives coincident input from both ears. The sound hits the right ear first, but the long axons connecting the right cochlea to the contralateral olivary nucleus introduces a delay.
Neurons in the olivary array only fire when they get coincident input from both ears. In this way, the array of neurons in medial superior olivary nucleus form a map of contralateral auditory space, with sounds directly in front represented rostrally and sounds at progressively more lateral locations represented progressively more caudally.
