Alex
Retinotopy/Attenotopy
This project is about mapping field maps in high level visual areas. Over the last years it has been shown that with a few modifications to the classic retinotopy paradigm and better processing techniques, more maps can be consistently found in more areas in addition to occipital cortex. One version of these paradigm modifications, is making the subject covertly attend to the rotating wedge by having a target detection task, while the subject maintains fixation. This is method is appropriately named attenotopy. Furthermore, evidence shows that there are attention processes occurring in parietal cortex. By having an embedded attention task in the classic retinotopy task, we will be able to map visual fields in parietal cortex in addition to all the other occipital cortex maps.
Background
Many cortical areas have a one to one mapping between visual field and cortical location (Bressler and Silver 2010). A natural way to explore this is to have a stimulus that traverses the visual field in some fashion, and then do some type of correlation analysis between visual field location and brain activity. Engel et al. 1994, were one of the first experimenters to develop a continuos stimuli that covers all of the visual space for an fMRI experiment. They showed that by using a traveling wave analysis for a rotating wedge and moving concentric circles stimuli, the visual field maps could be produced in occipital cortex. Figure 1, taken from Wandell et al. 2007 illustrates this method.
Several other cortical areas have been mapped with fMRI including: V1,V2,V3,V4, V3A, V3B, V6, V7 or IPS0, IPS1, IPS2, IPS3 , IPS4, V01, V02, PHC1,PHC2 among many others (Silver and Kastner 2009), Figure 2 and Figure 3. Not that some of these areas extend beyond what was classically thought to be visual cortex. Visual maps in IPS(0-4) lie on the posterior part of parietal cortex. This particular region has been associated with cognitive processes such as attention and memory (Silver and Kastner 2009, see also Bressler and Silver 2010). Specifically, it is now widely accepted that allocation of attention while doing a retinotopic experiment increases the ability to produce field maps (Wandell and Winawer 2011, Bressler and Silver 2010). To this effect, Bressler and Silver made a quantitative study in order to analyze the gain in terms of SNR of this effect. This project uses the same stimuli as the Bressler study and produces similar maps.
Methods
Stimuli
As mentioned before the stimuli used was as in Bressler and Silver 2010, Figure 2. A flickering checkerboard rotating wedge traverses the visual field in clockwise fashion. The wedge is 45 degrees wide, and extends 0.5 degrees to 10.9 degrees from a central fixation. The wedge rotates 22.5 deg locked to the scanner's TR at 2.13s. It takes the wedge 34.13 seconds to complete a cycle and there are 8 cycles that go into the analysis. A target appears with 50% probability inside the wedge. The subject is instructed to respond when a target appears, while maintaining fixation. Refer to Bressler and Silver 2010 for more details.
Preprocessing
For this project only one subject was analyzed, and only the right hemisphere is presented here. mrVista was used for data preprocessing and analysis. Traditional retinotopy analyzes were used, i.e. the coherence between the frequency produced by a stimulus cycle of 34.13s and the average fMRI time series for each voxel was calculated (Engel et al. 1994). The resulting maps are transformed into an inflated 3D cortical view. In this view the regions of interest were defined and stored.
Analysis and Results
Visual field maps were defined following Wandell et al. 2007 definitions. Briefly, a visual field area should: 1) represent a substantial portion of the visual field, 2) it must do so in an orderly fashion, 3) the basic features should be consistant within subjects. Starting from V1 (easily identified by the Calcarine Sulcus) contrast reversals are followed dorsally and ventrally to define the rest of the maps. The results are shown in the following figures.
Figure 5 shows the maps produced using this procedure in ventral visual cortex. Starting in V1, moving dorsally a blue contrast reversal is seen, indicating the end of that hemifield and the marks the start of V2d. The distinction between V2 and V3 is not as marked, but it is there. From V3d there is a clear blue and then red reversal, which marks the start of V4. After V4; VO1, VO2, PHC1 and PHC2 can be easily identified. Also from VO1 all the ROI's follow the colateral sulcus.
V2d is defined by continuing dorsally from V1 in the Calcarine sulcus. V2d and V3d were a bit tricky to identify since the reversals were not as marked as in other areas. V3dAB follows V3d by going red to blue, then IPS0 by going blue to red.
In this subject the parietal field maps are not as clear as the ventral ones. But some of the reversals can still be seen in these areas. Continuing dorsally from IPS0 a red reversal is observed to start IPS1. The start of IPS2 is a bit tricky since there wasn't too much signal. However, maps continue dorsally and this subject showed a very clear SPL1.
Conclusions
The goal of this project was to receive training in how to do retinotopy/ attenotopy using mrVista. I defined a total of 15 maps in ventral visual cortex and in parietal cortex. There is definitely an element of judgement when defining these maps as must of the boundaries are defined by approximating where one thinks the reversal takes place. Moreover, these maps are highly variable across subjects and a good amount of experience is needed in order to be consistant in defining these maps. Nonetheless, I must admit that it is very cool seeing and defining these maps, and also to see how the brain organizes what we observe. Special thanks to Kevin Weiner who helped at every step, from obtaining the data getting everything running, to checking that the ROI's made were not in frontal cortex.
References
Bressler DW, Silver MA (2010) Spatial attention improves reliability of fMRI retinotopic mapping signals in occipital and parietal cortex. NeuroImage
Brian A. Wandell, Serge O. Dumoulin, Alyssa A. Brewer, Visual Field Maps in Human Cortex, Neuron, Volume 56, Issue 2, 25 October 2007
Brian A. Wandell, Jonathan Winawer, Imaging retinotopic maps in the human brain, Vision Research, Volume 51, Issue 7, 13 April 2011
Engel SA, Rumelhart DE, Wandell BA, Lee AT, Glover GH, Chichilnisky EJ, Shadlen MN(1994) fmri of human visual cortex. Nature 369:525
Silver MA, Kastner S (2009) Topographic maps in human frontal and parietal cortex. Trends in Cognitive Sciences