\documentclass[10pt,letterpaper]{article}
\usepackage{amsmath,amssymb}

\begin{document} 

  \begin{table}[ht!]
    \centering
    \begin{tabular}{rcccccc}
      \multicolumn{7}{c}{single sweeps (experiment I)} \\
      \hline
      $\bar{f} = $& \multicolumn{2}{c}{ 900\,Hz} & 
                    \multicolumn{2}{c}{1200\,Hz} & 
                    \multicolumn{2}{c}{1500\,Hz} 
                    \\
      probe $\leftrightarrows$ sweep & $\leftarrow$ & $\rightarrow$ & 
                    $\leftarrow$ & $\rightarrow$ & 
                    $\leftarrow$ & $\rightarrow$ \\
      \hline
      slope  ($\pm 0.03$)     &   0.36  &   0.30  &   0.38   &   0.42   &   0.41   &   0.40 \\
      $r_p$  ($p < 10^{-22}$) &   0.88  &   0.68  &   0.86   &   0.89   &   0.90   &   0.88 \\
      $r_s$  ($p < 10^{-33}$) &   0.91  &   0.78  &   0.92   &   0.96   &   0.94   &   0.96 \\    
      \hline
      & & & & & & \\
      \multicolumn{7}{c}{sweep trains (experiment II)} \\
      \hline
      $\bar{f} = $& \multicolumn{2}{c}{ 900\,Hz} & 
                    \multicolumn{2}{c}{1200\,Hz} & 
                    \multicolumn{2}{c}{1500\,Hz} 
                    \\
      probe $\leftrightarrows$ sweep & $\leftarrow$ & $\rightarrow$ & 
                    $\leftarrow$ & $\rightarrow$ & 
                    $\leftarrow$ & $\rightarrow$ \\
      \hline
      slope ($\pm 0.04$)     & $0.19$ & $0.08$ & $0.17$ & $0.18$ & $0.22$ & $0.21$ \\
      $r_p$ $(p < 10^{-7})$  & $0.54$ & n.s.   & $0.60$ & $0.61$ & $0.66$ & $0.59$ \\
      $r_s$ $(p < 10^{-3})$  & $0.55$ & $0.26$ & $0.58$ & $0.60$ & $0.66$ & $0.56$ \\
    \end{tabular}
  \end{table}

\end{document}