diff --git a/+neurostim/plugin.m b/+neurostim/plugin.m
index 18327de8..ca8db06e 100644
--- a/+neurostim/plugin.m
+++ b/+neurostim/plugin.m
@@ -564,7 +564,7 @@ function setChangesInTrial(o,prm)
         end
     end
     
-    methods (Access={?neuorstim.plugin,?neurostim.parameter})
+    methods (Access={?neuorstim.plugin,?neurostim.parameter,?neurostim.stimulus})
         
         %Accessing neurostim.parameters is much slower than accessing a raw
         %member variable. Because we define many such parms (with addProperty) in the base
diff --git a/+neurostim/stimulus.m b/+neurostim/stimulus.m
index 7d6188ed..fdc74756 100644
--- a/+neurostim/stimulus.m
+++ b/+neurostim/stimulus.m
@@ -237,17 +237,19 @@ function addDiodeFlasher(s,varargin)
     
     methods (Access=private)
         
-        function s = updateRSVP(s)
+        function s = updateRSVP(s,sOnFrame,cFrame)
             % Called from baseBeforeFrame only when the stimulus is on.
-            %How many frames for item + blank (ISI)?
-            nFramesPerItem = s.cic.ms2frames(s.rsvp.duration+s.rsvp.isi);
-            %How many frames since the RSVP stream started?
-            rsvpFrame = s.cic.frame-s.onFrame;
+            % How many frames for item + blank (ISI)?
+            durationInFrames = s.cic.ms2frames(s.rsvp.duration,true);
+            isiInFrames = s.cic.ms2frames(s.rsvp.isi,true);
+            nrFramesPerItem = durationInFrames+isiInFrames;
+            % What is the frame we are preparing (base -0 relative to the first frame of the stimulus)
+            rsvpFrame = cFrame-sOnFrame;
             %Which item frame are we in?
-            itemFrame = mod(rsvpFrame, nFramesPerItem);
+            itemFrame = mod(rsvpFrame, nrFramesPerItem);
             %If at the start of a new element, move the design to the
             % next "trial"
-            if itemFrame==0
+            if itemFrame==0  % First of an item
                 ok = beforeTrial(s.rsvp.design);
                 if ~ok
                     % Ran out of "trials"
@@ -258,11 +260,11 @@ function addDiodeFlasher(s,varargin)
                 for sp=1:size(specs,1)
                     s.(specs{sp,2}) = specs{sp,3};
                 end
+                localizeParms(s,true);  % Update those loc parameters that change within a trial
             end
             
             %Blank now if it's time to do so.
-            startIsiFrame = s.cic.ms2frames(s.rsvp.duration);
-            s.flags.on = itemFrame < startIsiFrame;  % Blank during rsvp isi
+            s.flags.on = itemFrame < durationInFrames;  % Blank during rsvp isi  (< because itemFrame is base-0)
             if s.rsvp.log
                 if itemFrame == 0
                     s.rsvpIsi = false;
@@ -349,8 +351,13 @@ function baseBeforeTrial(s)
             beforeTrial(s);
         end
         
-        function baseBeforeFrame(s)
-            
+        function baseBeforeFrame(s)         
+            % Because this function is called for every stimulus, every
+            % frame, try to optimize as much as possible by avoiding
+            % duplicate access to member properties and by using the localized
+            % member variables for dynprops (see loc_X definition and the
+            % plugin.localizeParms function).
+             
             if s.loc_disabled
                 % Diode flasher should match the stimulus state always,
                 % eevn if disabled
@@ -358,13 +365,10 @@ function baseBeforeFrame(s)
                 return;
             end
             
-            % Because this function is called for every stimulus, every
-            % frame, try to optimize as much as possible by avoiding
-            % duplicate access to member properties and by using the localized
-            % member variables for dynprops (see loc_X definition and the
-            % plugin.localizeParms function).
-           
-            %Should the stimulus be drawn on this frame?
+            
+          
+            %% Determine the s.flags.on flag
+            % Should the stimulus be drawn on this frame?
             % This partially duplicates get.onFrame get.offFrame
             % code to minimize computations (and especially dynprop
             % evaluations which can be '@' functions and slow)
@@ -373,32 +377,34 @@ function baseBeforeFrame(s)
             if isinf(s.loc_on)
                 s.flags.on =false; %Dont bother checking the rest
             else
-                sOnFrame = round(s.loc_on.*s.cic.screen.frameRate/1000)+1;
-                %sOnFrame = s.cic.ms2frames(sOn,true)+1; % rounded==true
+                % Time is base-0 but frames are base-1 (frame 1 is the
+                % first that can be visible on the screen).
+                sOnFrame = round(s.loc_on.*s.cic.screen.frameRate/1000)+1;               
                 if cFrame < sOnFrame % Not on yet.
                     s.flags.on = false;
-                else % Is on already or turning on. Checck that we have not
-                    % reached full duration yet.
-                    sOffFrame = round((s.loc_on+s.loc_duration)*s.cic.screen.frameRate/1000);
-                    %sOffFrame = s.cic.ms2frames(sOn+s.duration,true);
+                else % Is on already or turning on. 
+                    % Checck that we have not  reached full duration yet.
+                    % No +1 here.
+                    sOffFrame = sOnFrame + round(s.loc_duration*s.cic.screen.frameRate/1000);                    
                     s.flags.on = cFrame <sOffFrame;
+                    
+                    % This is the only path where s.flags.on can be true
+                    % Update RSVP parameter values if necesssary
+                    if s.rsvp.active && s.flags.on
+                        s=updateRSVP(s,sOnFrame,cFrame);
+                    end            
                 end
             end
             
-            %% RSVP mode
-            %   Update parameter values if necesssary
-            if s.rsvp.active && s.flags.on
-                s=updateRSVP(s);
-            end
-            
-            %%
-            
-            %If this is the first frame on which the stimulus will NOT be drawn, schedule logging after the pending flip
+                        
+            %% Setup offset logging
+            % If this is the first frame on which the stimulus will NOT be drawn, schedule logging after the pending flip
             if cFrame==sOffFrame
                 s.cic.addFlipCallback(s);
                 s.logOffset = true;
             end
             
+            %% Draw to the backbuffer
             %If the stimulus should be drawn on this frame:
             locWindow =s.window;      
             if s.flags.on                      
@@ -420,10 +426,11 @@ function baseBeforeFrame(s)
                     s.logOnset = true;
                 end
                 
-                %Pass control to the child class
+                %Pass control to the child class to do its drawing
                 beforeFrame(s);                                
             end
-                        
+
+            %% Flash
             if s.diodeFlasher.enabled
                 Screen('glLoadIdentity',locWindow);% Don't rotate or scale with the stimulus
                 if s.flags.on