Schould be able to set a callback the rate

dist/synaptic.js

@@ -1988,6 +1988,10 @@
 	        var currentBucket = Math.floor(iterations / bucketSize);
 	        currentRate = this.rate[currentBucket] || currentRate;
 	      }
+	      
+	      if(typeof this.rate === 'function') {
+	        currentRate = this.rate(iterations, error);
+	      }
 
 	      if (crossValidate) {
 	        this._trainSet(trainSet, currentRate, cost);

src/trainer.js

@@ -86,6 +86,10 @@ Trainer.prototype = {
         var currentBucket = Math.floor(iterations / bucketSize);
         currentRate = this.rate[currentBucket] || currentRate;
       }
+      
+      if(typeof this.rate === 'function') {
+        currentRate = this.rate(iterations, error);
+      }
 
       if (crossValidate) {
         this._trainSet(trainSet, currentRate, cost);