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Using an achiasmic human visual system to quantify the relationship between the fMRI BOLD signal and neural response.

Bao P, Purington CJ, Tjan BS - Elife (2015)

Bottom Line: Achiasma in humans causes gross mis-wiring of the retinal-fugal projection, resulting in overlapped cortical representations of left and right visual hemifields.Specifically, we can non-invasively double local neural responses by stimulating both neuronal populations with identical stimuli presented symmetrically across the vertical meridian to both visual hemifields, versus one population by stimulating in one hemifield.Reanalyzing published data shows that this inferred relationship is general.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Graduate Program, University of Southern California, Los Angeles, United States.

ABSTRACT
Achiasma in humans causes gross mis-wiring of the retinal-fugal projection, resulting in overlapped cortical representations of left and right visual hemifields. We show that in areas V1-V3 this overlap is due to two co-located but non-interacting populations of neurons, each with a receptive field serving only one hemifield. Importantly, the two populations share the same local vascular control, resulting in a unique organization useful for quantifying the relationship between neural and fMRI BOLD responses without direct measurement of neural activity. Specifically, we can non-invasively double local neural responses by stimulating both neuronal populations with identical stimuli presented symmetrically across the vertical meridian to both visual hemifields, versus one population by stimulating in one hemifield. Measurements from a series of such doubling experiments show that the amplitude of BOLD response is proportional to approximately 0.5 power of the underlying neural response. Reanalyzing published data shows that this inferred relationship is general.

No MeSH data available.


Robustness of BOLD summation results.(A) BOLD amplitudes of every voxel in each ROI (V1-V3) as evolved by the two versions of the single-sided stimulus (Stim A and B of Figure 3) in the 6-s experiment. The voxels responded approximately equally to both versions of the stimulus, indicating that any imperfect homotopy and/or gaze control did not significantly affect the stimuli's ability to equally co-activate the voxels. Red dots represent voxels in the ROIs that were strongly responsive (uncorrected p<0.001) to both versions of the stimulus. Reanalyzing the data using only these strongly responsive voxels yielded essentially the same results as in Figure 4 when all the voxels with the ROIs were used (right vs. left column, respectively, of B–D.DOI:http://dx.doi.org/10.7554/eLife.09600.013
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fig4s2: Robustness of BOLD summation results.(A) BOLD amplitudes of every voxel in each ROI (V1-V3) as evolved by the two versions of the single-sided stimulus (Stim A and B of Figure 3) in the 6-s experiment. The voxels responded approximately equally to both versions of the stimulus, indicating that any imperfect homotopy and/or gaze control did not significantly affect the stimuli's ability to equally co-activate the voxels. Red dots represent voxels in the ROIs that were strongly responsive (uncorrected p<0.001) to both versions of the stimulus. Reanalyzing the data using only these strongly responsive voxels yielded essentially the same results as in Figure 4 when all the voxels with the ROIs were used (right vs. left column, respectively, of B–D.DOI:http://dx.doi.org/10.7554/eLife.09600.013

Mentions: We defined the amplitude of a BOLD response to be the peak of the BOLD time course. We sought to express BOLD amplitude (B) as a function of neural response (Z), where Z refers to the local aggregate of neural response with unspecified constituents. At each cortical region of interest, the BOLD summation experiment provided five levels of Z, corresponding to the five levels of luminance contrast for the single-sided stimuli (A, B). (The single-sided stimuli A and B resulted in nearly identical time courses (Figure 3B, see also Figure 3—figure supplements 2 and Figure 4—figure supplement 2 on robustness), indicating that the inputs from the left and right visual fields were balanced. We therefore use their averages in the analysis.) For each level of Z evoked by a single-sided stimulus, the level of neural response evoked by the corresponding double-sided stimulus (A+B) is doubled under the assumptions of co-localization and independence, which we have empirically validated. We therefore have five pairs of BOLD measurements that correspond to Zi and 2Zi (Figure 4A, left column). We do not know the values of Zi, but we can reasonably assume that these levels of neural response were ordered by stimulus contrast: .


Using an achiasmic human visual system to quantify the relationship between the fMRI BOLD signal and neural response.

Bao P, Purington CJ, Tjan BS - Elife (2015)

Robustness of BOLD summation results.(A) BOLD amplitudes of every voxel in each ROI (V1-V3) as evolved by the two versions of the single-sided stimulus (Stim A and B of Figure 3) in the 6-s experiment. The voxels responded approximately equally to both versions of the stimulus, indicating that any imperfect homotopy and/or gaze control did not significantly affect the stimuli's ability to equally co-activate the voxels. Red dots represent voxels in the ROIs that were strongly responsive (uncorrected p<0.001) to both versions of the stimulus. Reanalyzing the data using only these strongly responsive voxels yielded essentially the same results as in Figure 4 when all the voxels with the ROIs were used (right vs. left column, respectively, of B–D.DOI:http://dx.doi.org/10.7554/eLife.09600.013
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Related In: Results  -  Collection

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fig4s2: Robustness of BOLD summation results.(A) BOLD amplitudes of every voxel in each ROI (V1-V3) as evolved by the two versions of the single-sided stimulus (Stim A and B of Figure 3) in the 6-s experiment. The voxels responded approximately equally to both versions of the stimulus, indicating that any imperfect homotopy and/or gaze control did not significantly affect the stimuli's ability to equally co-activate the voxels. Red dots represent voxels in the ROIs that were strongly responsive (uncorrected p<0.001) to both versions of the stimulus. Reanalyzing the data using only these strongly responsive voxels yielded essentially the same results as in Figure 4 when all the voxels with the ROIs were used (right vs. left column, respectively, of B–D.DOI:http://dx.doi.org/10.7554/eLife.09600.013
Mentions: We defined the amplitude of a BOLD response to be the peak of the BOLD time course. We sought to express BOLD amplitude (B) as a function of neural response (Z), where Z refers to the local aggregate of neural response with unspecified constituents. At each cortical region of interest, the BOLD summation experiment provided five levels of Z, corresponding to the five levels of luminance contrast for the single-sided stimuli (A, B). (The single-sided stimuli A and B resulted in nearly identical time courses (Figure 3B, see also Figure 3—figure supplements 2 and Figure 4—figure supplement 2 on robustness), indicating that the inputs from the left and right visual fields were balanced. We therefore use their averages in the analysis.) For each level of Z evoked by a single-sided stimulus, the level of neural response evoked by the corresponding double-sided stimulus (A+B) is doubled under the assumptions of co-localization and independence, which we have empirically validated. We therefore have five pairs of BOLD measurements that correspond to Zi and 2Zi (Figure 4A, left column). We do not know the values of Zi, but we can reasonably assume that these levels of neural response were ordered by stimulus contrast: .

Bottom Line: Achiasma in humans causes gross mis-wiring of the retinal-fugal projection, resulting in overlapped cortical representations of left and right visual hemifields.Specifically, we can non-invasively double local neural responses by stimulating both neuronal populations with identical stimuli presented symmetrically across the vertical meridian to both visual hemifields, versus one population by stimulating in one hemifield.Reanalyzing published data shows that this inferred relationship is general.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Graduate Program, University of Southern California, Los Angeles, United States.

ABSTRACT
Achiasma in humans causes gross mis-wiring of the retinal-fugal projection, resulting in overlapped cortical representations of left and right visual hemifields. We show that in areas V1-V3 this overlap is due to two co-located but non-interacting populations of neurons, each with a receptive field serving only one hemifield. Importantly, the two populations share the same local vascular control, resulting in a unique organization useful for quantifying the relationship between neural and fMRI BOLD responses without direct measurement of neural activity. Specifically, we can non-invasively double local neural responses by stimulating both neuronal populations with identical stimuli presented symmetrically across the vertical meridian to both visual hemifields, versus one population by stimulating in one hemifield. Measurements from a series of such doubling experiments show that the amplitude of BOLD response is proportional to approximately 0.5 power of the underlying neural response. Reanalyzing published data shows that this inferred relationship is general.

No MeSH data available.