Lecture for Neural Networks study group held on January 11, 2020.
Reference book: http://hagan.okstate.edu/nnd.html
Video: https://youtu.be/H4NKgliTFUw
Initiated by Taiwan AI Group (https://www.facebook.com/groups/Taiwan.AI.Group/permalink/2017771298545301/)
2. *Copyright Notice:
Most materials from this presentation are taken
from the book “Neural Network Design 2nd edition”
authored by Martin T. Hagan, Howard B. Demuth,
Mark Hudson Beale and Orlando De Jesús. The other
quoted sources are mentioned in the respective
slides. This presentation itself adopts Creative
Commons license.
3. A Quote from this Chapter
“Although the original inspiration for the field of
artificial neural networks came from biology, at times
we forget to look back to biology for new ideas. It will be
the blending of biology, mathematics, psychology and
other disciplines that will provide the maximum growth
in our understanding of neural networks.”
「雖然類神經網路這一領域的最初靈感來自生物學,
但有時我們會忘了回顧生物學以啟迪新的想法。融合
生物學、數學、心理學和其他學科將使我們對神經網
路的理解得以最大的發展。」
8. Test for the Blind Spot
Look at the blue circle on the left side of above figure with
your right eye while covering your left eye and move your
head back and forth perpendicularly to the circle to find
the point at which the circle on the right will disappear
from your field of vision.
18. Leaky Integrate-and-Fire
*Figure adopted from Wulfram Gerstner, Werner M. Kistler, Richard Naud and Liam Paninski “Neuronal Dynamics:
From Single Neurons to Networks and Models of Cognition” Cambridge University Press. 2014. Page 11.
26. Characteristics of Layer 1
• The network is sensitive to relative intensities of the
input pattern, rather than absolute intensities.
• The output of Layer 1 is a normalized version of the
input pattern.
• The on-center/off-surround connection pattern and the
nonlinear gain control of the shunting model produce
the normalization effect.
• The operation of Layer 1 explains the brightness
constancy and brightness contrast characteristics of the
human visual system.
31. Characteristics of Layer 2
• As in the Hamming and Kohonen networks, the inputs to
Layer 2 are the inner products between the prototype
patterns (rows of the weight matrix W2) and the output
of Layer 1 (normalized input pattern).
• The nonlinear feedback enables the network to store the
output pattern (pattern remains after input is removed).
• The on-center/off-surround connection pattern causes
contrast enhancement (large inputs are maintained,
while small inputs are attenuated).
35. Sigmoid Transfer Function
• A sigmoid function is faster-than-linear for small
signals, approximately linear for intermediate
signals and slower-than-linear for large signals.
• When a sigmoid transfer function is used in Layer 2,
the pattern is contrast enhanced; larger values are
amplified, and smaller values are attenuated.
• All initial neuron outputs that are less than a certain
level decay to 0.
• This merges the noise suppression of the faster-
than-linear transfer functions with the perfect
storage produced by linear transfer functions.
41. Three major differences:
• The Grossberg network is a continuous-time network.
• Layer 1 of the Grossberg network automatically
normalizes the input vectors.
• Layer 2 of the Grossberg can perform a “soft”
competition, rather than the winner-take-all
competition of the Kohonen network. This soft
competition allows more than one neuron in Layer 2 to
learn. This causes the Grossberg network to operate as
a feature map.
Grossberg vs. Kohonen Networks
42. Potential Issues
• One key problem of the Grossberg network is the
stability of learning. As more inputs are applied to
the network, there is no guarantee that the weight
matrix will eventually converge (form stable clusters
/ categories). [Refer to Chapter 19: Adaptive
Resonance Theory, ART]
• Another problem is the stability of the differential
equations that implement Grossberg’s continuous-
time competitive recurrent network. The output of a
recurrent network could converge, oscillate, or even
diverge. [Refer to Chapter 20: Stability]
43. Basic ART Architecture (Ch.19)
Add-ons:
Layer 2 to Layer 1 expectations
The orienting subsystem (reset)
Modified gain control
Let’s move on…