Given the complexity of our visual environment, the ability to selectively attend to certain locations, while ignoring others, is crucial for reducing the amount of visual information to manageable levels and for optimizing behavioral performance. Sustained allocation of spatial attention causes persistent increases in functional magnetic resonance imaging (fMRI) signals in portions of early visual cortex that retinotopically represent the attended location, even in the absence of a visual stimulus. Here we test the hypothesis that topographically organized posterior parietal cortical areas IPS1 and IPS2 transmit top–down spatial attention signals to early visual cortex. We employed fMRI and coherency analysis to measure functional connectivity among cortical areas V1, V2, V3, V3A, V3B, V7, IPS1, and IPS2 during sustained visual spatial attention. Attention increased the magnitude of coherency for many pairs of areas in occipital and parietal cortex. Additionally, attention-related activity in IPS1 and IPS2 led activity in several visual cortical areas by a few hundred milliseconds. These results are consistent with transmission of top–down spatial attention signals from IPS1 and IPS2 to early visual cortex.

*f*

_{ x}(

*ν*)〉 and 〈

*f*

_{ y}(

*ν*)〉 are the means of the power spectra of segments of time series

*x*and

*y*at frequency

*ν*(see Coherency calculations section below), and 〈

*f*

_{ xy}(

*ν*)〉 is the mean of the cross spectrum of segments of

*x*and

*y*(Sun et al., 2004).

*R*

_{xy}(

*ν*) is a complex quantity (

*a*+

*ib*). In polar coordinates, the absolute value (modulus) is the length of the vector

*R*

_{xy}(

*ν*),

*x*and

*y*(taking a value between 0 and 1). The complex argument, or polar angle, of

*R*

_{xy}(

*ν*) defines the phase spectrum. Dividing this phase spectrum by a given temporal frequency yields the delay between

*x*and

*y*in units of time (taking positive or negative values, depending on the direction of the delay between the time series). Thus, coherency analysis allows the measurement of both the strength of functional coupling between two time series (magnitude) as well as the direction of signal transmission (phase). A more detailed description of coherency analysis and its application to neural signals can be found in Rosenberg, Amjad, Breeze, Brillinger, and Halliday (1989).

*y*-intercept, possibly leading to false positive errors in estimates of attention-specific inter-areal temporal latencies.

*p*< 0.05).

*π*and

*π*radians. We therefore employed a bootstrap procedure (Efron & Tibshirani, 1993) to test whether coherency magnitude and phase differences between attention and fixation were significantly different from zero. For each pairwise combination of cortical areas in each subject, one thousand bootstrap data samples of the coherency magnitude difference values were sampled with replacement over the nine frequency bands. This bootstrap procedure was repeated for phase difference values. Note that this analysis assumes statistical independence of the coherency magnitude and phase values across frequency bands, an assumption that may not be strictly correct. A non-parametric rank test was employed to determine statistical significance of the mean coherency magnitude and phase differences for each pair of cortical areas. For analysis of the average of the four subjects, samples were drawn from data pooled across all subjects and frequency bands. We corrected for multiple comparisons using the false discovery rate (FDR) method (Genovese, Lazar, & Nichols, 2002) with 28 values in each coherency difference matrix, comprising all possible pairwise comparisons between the eight cortical areas. All analysis code was written in MATLAB (The MathWorks, Inc., Natick, MA).

IPS2 | ||||||||
---|---|---|---|---|---|---|---|---|

IPS1 | −47 ± 44 | |||||||

V7 | − 150 ± 56 | − 147 ± 62 | ||||||

V3B | −88 ± 60 | − 112 ± 49 | − 190 ± 86 | |||||

V3A | −30 ± 73 | −127 ± 84 | − 335 ± 66 | − 255 ± 100 | ||||

V3 | − 165 ± 141 | − 187 ± 62 | − 203 ± 69 | −238 ± 135 | − 482 ± 139 | |||

V2 | 101 ± 47 | 26 ± 153 | 23 ± 78 | − 233 ± 74 | 192 ± 153 | −142 ± 118 | ||

V1 | − 87 ± 35 | −4 ± 62 | −83 ± 125 | 18 ± 74 | − 197 ± 48 | −45 ± 86 | − 406 ± 119 | |

V1 | V2 | V3 | V3A | V3B | V7 | IPS1 | IPS2 |

*p*< 0.01, corrected for multiple comparisons.