diff --git a/js/NumberControl.js b/js/NumberControl.js
index 71ec9eb45..647827b43 100644
--- a/js/NumberControl.js
+++ b/js/NumberControl.js
@@ -58,12 +58,12 @@ define( function( require ) {
       endCallback: null, // called when interaction ends, default value set in validateCallbacksAndSetDefault()
 
       // Specific callbacks for each component
-      sliderStartCallback: null, // called when specific interaction begins
-      sliderEndCallback: null, // called when specific interaction ends
-      leftArrowStartCallback: null, // called when specific interaction begins
-      leftArrowEndCallback: null, // called when specific interaction ends
-      rightArrowStartCallback: null, // called when specific interaction begins
-      rightArrowEndCallback: null, // called when specific interaction ends
+      sliderStartCallback: null, // called when dragging starts on the slider
+      sliderEndCallback: null, // called when dragging ends on the slider
+      leftArrowStartCallback: null, // called when left arrow is pressed
+      leftArrowEndCallback: null, // called when left arrow is released
+      rightArrowStartCallback: null, // called when right arrow is pressed
+      rightArrowEndCallback: null, // called when right arrow is released
 
       enabledProperty: new Property( true ), // {Property.<boolean>} is this control enabled?
       disabledOpacity: 0.5, // {number} opacity used to make the control look disabled
@@ -303,15 +303,16 @@ define( function( require ) {
   /**
    * Validate all of the callback related options. There are two types of callbacks. The "start/endCallback" pair
    * are passed into all components in the NumberControl. The second set are start/end callbacks for each individual
-   * component. This was added to support multitouch in Rutherford Scattering as part of https://github.com/phetsims/rutherford-scattering/issues/128.
+   * component. This was added to support multitouch in Rutherford Scattering as part of
+   * https://github.com/phetsims/rutherford-scattering/issues/128.
    *
-   *
-   * This function mutates the options by initializing general callbacks from null (in the extend call) to a no-op function.
+   * This function mutates the options by initializing general callbacks from null (in the extend call) to a no-op
+   * function.
    * @param {Object} options
    */
   function validateCallbacksAndSetDefault( options ) {
 
-    var normalCallbacksPresent = options.startCallback || options.endCallback;
+    var normalCallbacksPresent = !!( options.startCallback || options.endCallback );
 
     var specificCallbacksPresent = false;
     SPECIFIC_COMPONENT_CALLBACK_OPTIONS.forEach( function( callbackOption ) {
@@ -321,7 +322,8 @@ define( function( require ) {
       }
     } );
 
-    assert && assert( normalCallbacksPresent !== specificCallbacksPresent,
+    // only general or component specific callbacks are supported
+    assert && assert( ( !normalCallbacksPresent && !specificCallbacksPresent ) || ( normalCallbacksPresent !== specificCallbacksPresent ),
       'Use general callbacks like "startCallback" or specific callbacks like "sliderStartCallback" but not both.' );
 
     // Set here so that we can validate above based on falsey.