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A. Sedlackova et al.
of directionally-selective neurons [3]. The LPTCs depolarize in response to background
motion in their preferred direction and hyperpolarize in response to motion in their null
direction. The LPTCs, however, do not compute motion locally but rather integrate over
a large part of the visual field. The lobula plate tangential cells inhibit each other such
that right inhibits left and vice versa, and up inhibits down and vice versa [3]. These
cells are commonly thought to detect optical flow that arises from self-motion.
In our model, the LPTCs sum the outputs of the T4 cells in the medulla in a
directionally-selective way. We only model the right- and left-sensitive LPTCs. Figure 1E
shows how the medulla neurons all feed into one of two LPTCs. These neurons have
the same properties as the rest of the network. The synapses from the medulla to the
LPTCs have the same properties as those from the lamina to the medulla. None of these
synapses possess delays.
Motor Neurons. The motor centers receive input from the LPTCs via descending
interneurons [5]. We modeled such connections by synapsing the LPTC neurons onto
nonspiking neurons that represent the combined motor neuron (MN) and muscle membrane. These MNs function as leaky integrators, integrating incoming spikes over time
but “leaking” to 0 activity when no spikes are incoming. These neurons have the same
parameter values as the other neurons in the network, except that the spiking mechanism has been disabled. As described above, the MN voltages specify rotation and speed
commands for the neck servomotor.
3 Results
LPTC Voltage Encodes Wide-Field Visual Motion. Figure 2 displays raster plots of
retina, lamina, and medulla spiking activity in response to background motion. Each point
represents one action potential. The retina encodes the stripe pattern of the background
as it moves right for five seconds, and then left for five seconds. The lamina increases the
contrast by amplifying range of firing frequencies seen in the retina (Fig. 3). The medulla
neurons encode the direction of the background’s velocity, with separate subpopulations
encoding each direction. Each one of these layers encodes visual information as observed
in animal systems.
LPTC Activity Reflects Wide-Field Pattern Velocity. Figure 4 shows how the LPTC
neurons encode the velocity of the background. The LPTC spikes have been removed
from the membrane voltage curves to more clearly show how the LPTC depolarization
(solid lines) encodes the velocity of the background (dotted lines). However, when
the background’s speed becomes too high, the correlation operation performed by the
medulla breaks down, and the LPTC voltage no longer reflects the background speed.
This same phenomenon is observed in Drosophila [3].
A. Sedlackova et al.
of directionally-selective neurons [3]. The LPTCs depolarize in response to background
motion in their preferred direction and hyperpolarize in response to motion in their null
direction. The LPTCs, however, do not compute motion locally but rather integrate over
a large part of the visual field. The lobula plate tangential cells inhibit each other such
that right inhibits left and vice versa, and up inhibits down and vice versa [3]. These
cells are commonly thought to detect optical flow that arises from self-motion.
In our model, the LPTCs sum the outputs of the T4 cells in the medulla in a
directionally-selective way. We only model the right- and left-sensitive LPTCs. Figure 1E
shows how the medulla neurons all feed into one of two LPTCs. These neurons have
the same properties as the rest of the network. The synapses from the medulla to the
LPTCs have the same properties as those from the lamina to the medulla. None of these
synapses possess delays.
Motor Neurons. The motor centers receive input from the LPTCs via descending
interneurons [5]. We modeled such connections by synapsing the LPTC neurons onto
nonspiking neurons that represent the combined motor neuron (MN) and muscle membrane. These MNs function as leaky integrators, integrating incoming spikes over time
but “leaking” to 0 activity when no spikes are incoming. These neurons have the same
parameter values as the other neurons in the network, except that the spiking mechanism has been disabled. As described above, the MN voltages specify rotation and speed
commands for the neck servomotor.
3 Results
LPTC Voltage Encodes Wide-Field Visual Motion. Figure 2 displays raster plots of
retina, lamina, and medulla spiking activity in response to background motion. Each point
represents one action potential. The retina encodes the stripe pattern of the background
as it moves right for five seconds, and then left for five seconds. The lamina increases the
contrast by amplifying range of firing frequencies seen in the retina (Fig. 3). The medulla
neurons encode the direction of the background’s velocity, with separate subpopulations
encoding each direction. Each one of these layers encodes visual information as observed
in animal systems.
LPTC Activity Reflects Wide-Field Pattern Velocity. Figure 4 shows how the LPTC
neurons encode the velocity of the background. The LPTC spikes have been removed
from the membrane voltage curves to more clearly show how the LPTC depolarization
(solid lines) encodes the velocity of the background (dotted lines). However, when
the background’s speed becomes too high, the correlation operation performed by the
medulla breaks down, and the LPTC voltage no longer reflects the background speed.
This same phenomenon is observed in Drosophila [3].