When attempting to evoke SSVEPs, it is important to record the real flicker rate directly from the screen using a light detector. The FFT of this signal will let you know the actual power at the particular frequencies of stimuli, which could be different or degraded than what you are attempting to display. For instance, if many other processes are running on your computer, your program will not actually be running at those frequencies. This is most often the problem in my experience.

However SSVEPs could not be evoked due to low contrast stimuli, small stimuli, the goal of the subject during the task, subjects' attention, etc. A few preliminary experiments with subjects are usually required to find appropriate stimuli and frequencies for the particular task.

Are you using Psychtoolbox 3? Companies like Cedrus sell light detectors:

http://cedrus.com/stimtracker/

Attached is a link to an example MATLAB stimulus code that may be helpful to read. However it is written for Psychtoolbox 2 and will not run without some significant work.

https://github.com/mdnunez/mcntoolbox/tree/master/nunez_etal2016_mathpsych/visual_stimuli

Thank you for your reply. I am using cogent graphics toolbox for generating the stimuli. But the software seems pretty heavy for the system to run. I installed Psychtoolbox and ran the code you mentioned in the above link. But I am not getting any required output. The first m file gave a plot and the second m file is giving some error like 

Undefined function or variable 'PTB2'.

Error in pdmexp3_public (line 57)

PTB2;

I am totally new to this Psychtoolbox so can you help me out with this? 

Psychtoolbox 3 on a machines running with OS Linux Ubuntu work well for us. Like I said, my script is not intended to be run, because it is not going to run without significant work. One of the reasons is that it was written for Psychtoolbox 2, not 3 (the main difference being the use of "Screen('Flip')" instead of "Screen('CopyWindow')").

Unfortunately I don't yet have an example written in Psychtoolbox 3. However there is most likely an example available, probably from the BCI community.

I found one here:

https://github.com/jadref/buffer_bci/blob/master/ssep/ssepFeedbackStimulusPTB.m

However it is difficult to read, but perhaps this repository has a working example.

I created a quick script from an existing stimulus to give an example of PTB3 code for SSVEP experiments. The squares on the left of the screen are "photocells" that we use to track the true frequency of stimuli with light detectors.

https://github.com/mdnunez/mcntoolbox/blob/master/PTB3_examples/photocells.m