Hi Anders, I agree with the points Jon made. 8 Hz flicker is also what I use. I haven't tried lower frequencies, but I think you would definitely have reduced SNR if you did. I also use rather thin wedges and rings and smoothly vary their position.
I would add that a longer cycle duration (e.g. 64 seconds) seems to provide better phase estimates. I had been using a shorter duration (32 seconds) for awhile, and while you can get reasonable results (and better SNR because of more cycles/scan), I was also noticing irregularities in the maps (e.g. spots of upper field preference where it should be lower field). When I did a direct comparison between the two cycle durations, the maps looked much cleaner with the longer cycle. Because the data can be rather noisy (regardless of field strength), multiple scans are important. I use 4 scans for polar angle (2 CW, 2 CCW) and 2 for eccentricity (1 expanding, 1 contracting), although I would do more in a session if I thought the typical subject could stand it. My current settings are 5 cycles/scan, 64 second cycle duration, 128 TRs, TR = 2.5 sec, so ~5.5 minutes / scan. A 3 sec TR would work fine too, and allow for 6 cycles/scan, which would improve SNR (and increase total scan duration). Don > Date: Thu, 7 Jul 2011 11:30:13 -0400 > From: j...@nmr.mgh.harvard.edu > To: ahouga...@dadlnet.dk > CC: freesurfer@nmr.mgh.harvard.edu > Subject: Re: [Freesurfer] Retinotopy stimuli > > > hi anders, > > the optimal set of stimulus parameters will depend somewhat on what > cortical area(s) you are trying to map and on the details of your > acquisition. in general, stimulating with both clockwise- and > counterclockwise-rotating wedges for polar angle mapping and with both > expanding and contracting rings for eccentricity mapping improves the > accuracy of the maps, and the proper analysis of this data is implemented > fsfast. the width of the wedge or the ring will depend on the cortical > area you are trying to stimulate, but you may want to consider a thinner > wedge. also, 8 Hz flickering has been shown to activate area V1 more > strongly than other frequencies, and smoothly moving stimuli can also > help. then, depending on your voxel size, field strength, coil array, > etc., you could include more cycles and may need to average together > multiple runs. > > marty, roger et al. have a few nice papers that i'd recommend that discuss > some of these details relating to phase-encoded retinotopic stimuli. > > Sereno & Tootell. Curr Opin Neurobiol. 2005;15(2):135-44. > Tootell et al. J Neurosci. 1997;17(18):7060-78. > Sereno et al. Science. 1995;268(5212):889-93. > > (the '97 paper discusses the ring/wedge thickness, the '95 paper discusses > the advantages of using both expanding + contracting stimuli, and the '05 > paper discusses the use of smoothly varying stimuli.) > > hope this helps. were you able to get reasonable looking maps with the > stimuli you described below? > > > -jon > > > > > On Thu, 7 Jul 2011, Anders Hougaard wrote: > > > Dear freesurfers, > > > > > > Which stimuli do you think provide the best retinotopic maps? > > > > I have done a retinotopy analysis using the following stimuli: > > > > Polar angle: > > > > - 45 deg counter-clockwise rotating wedge > > - 8 unique positions > > - flickering at 2 Hz > > - stimulation period: 36 sec > > - no. of cycles: 6 > > - TR = 3 > > > > Eccentricity: > > > > - expanding ring > > - 8 unique positions > > - flickering at 2 Hz > > - stimulation period: 36 sec > > - no. of cycles: 6 > > - TR = 3 > > > > Any suggestions on how to optimize this stimulation? > > E.g. different period length, more runs, bi-directional, different TR > > > > Thank you! > > > > Best regards, > > > > Anders > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > The information in this e-mail is intended only for the person to whom it is > addressed. If you believe this e-mail was sent to you in error and the e-mail > contains patient information, please contact the Partners Compliance HelpLine > at > http://www.partners.org/complianceline . If the e-mail was sent to you in > error > but does not contain patient information, please contact the sender and > properly > dispose of the e-mail. >
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