How to filter out the noise and then get the frequency characteristics?

I want to extract frequency information between 6Hz and 15Hz from an EEG signal (sampling rate: 128Hz). I first used a bandpass filter on the signal, and then did a FFT on the filtered signal. The strange thing is that after FFT, the frequency peak is always around 10Hz. It is supposed that the frequency peak is different for different signals. The code is as follows:

Hd =filterdesign;

NFFT = 2^nextpow2(L); % Next power of 2 from length of y

z = filter(Hd,y); % y is signal

Z = fft(z,NFFT);

**************************************************

function Hd = filterdesign % Equiripple Bandpass filter designed using the FIRPM function.

% All frequency values are in Hz. Fs = 128; % Sampling Frequency

N = 10; % Order

Fstop1 = 6; % First Stopband Frequency

Fpass1 = 6.5; % First Passband Frequency

Fpass2 = 15; % Second Passband Frequency

Fstop2 = 15.5; % Second Stopband Frequency

Wstop1 = 1; % First Stopband Weight

Wpass = 1; % Passband Weight

Wstop2 = 1; % Second Stopband Weight

dens = 2000; % Density Factor

% Calculate the coefficients using the FIRPM function.

b = firpm(N, [0 Fstop1 Fpass1 Fpass2 Fstop2 Fs/2]/(Fs/2), [0 0 1 1 0 ... 0], [Wstop1 Wpass Wstop2], {dens});

Hd = dfilt.dffir(b);

*************************************

What is the standard way to do such signal processing? What I need is to filter out some band of noise and then find out the frequency peaks.

Thanks a lot.

Jun

[Image Analyst]

Are the different signals from different people? Don't brain waves look similar for different people? So why would you expect them to be different? Like for heart beats, I wouldn't be surprised if the dominant frequency is always around 75 +/- 10 beats per minute.

[jun]

The signal is SSVEP signal. My bad, I should say that the frequency peaks should be different for different stimuli. Thanks.

[Rick Rosson]

What is the value of NFFT for your signals? Perhaps the Frequency spacing is not small enough to distinguish between locations of different peaks from one spectrum to another...?

[jun]

NFFT is 1024, so the resolution is 1/8 Hz. Is the procedure correct, i.e, filtering and then doing FFT?

[mausam]

i am in similar situation, if you have made through it, please help me out.

    % code
W=fastica([EEGdata(2,:);EEGdata(3,:);EEGdata(4,:);EEGdata(5,:);]);
% amplitudeestimation
Fs = 256;
x=W(1,186:6330);
freqres = Fs/length(x);
xdft = fft(x);
xdft = xdft(1:length(x)/2+1);
xdft = 1/length(x).*xdft;
xdft(2:end-1) = 2*xdft(2:end-1);
freq = 0:Fs/length(x):Fs/2;
plot(freq,abs(xdft));
xlabel('Hz'); ylabel('Amplitude');
h = line(freq,ones(length(x)/2+1,1));
set(h,'color',[1 0 0],'linewidth',2);

end

%%%%%%%%%%%%%%%%

i should be getting at least one peak at 8 Hz but its all flat. thanks