CMU 11-785 L09 Cascade-Correlation and Deep Learning

Cascade-Correlation Algorithm

• Start with direct I/O connections only. No hidden units.
• Train output-layer weights using BP or Quickprop.
• If error is now acceptable, quit.
• Else, Create one new hidden unit offline.
  • Create a pool of candidate units. Each gets all available inputs. Outputs are not yet connected to anything.
  • Train the incoming weights to maximize the match (covariance) between each unit’s output and the residual error:
  • When all are quiescent, tenure the winner and add it to active net. Kill all the other candidates.
• Re-train output layer weights and repeat the cycle until done.

Why Is Backprop So Slow?

• Moving Targets
  • All hidden units are being trained at once, changing the environment seen by the other units as they train.
• Herd Effect
  • Each unit must find a distinct job – some component of the error to correct.
  • All units scramble for the most important jobs. No central authority or communication.
  • Once a job is taken, it disappears and units head for the next-best job, including the unit that took the best job.
  • This is a very inefficient way to assign a distinct useful job to each unit.

Advantages of Cascade Correlation

• No need to guess size and topology of net in advance.
• Can build deep nets with higher-order features.
• Much faster than Backprop or Quickprop.
• Trains just one layer of weights at a time (fast).
• Works on smaller training sets (in some cases, at least).
• Old feature detectors are frozen, not cannibalized, so good for incremental “curriculum” training.
• Good for parallel implementation.

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