The neural mechanism for the negative BOLD response

is also still unknown (Pasley et al., 2007; Shmuel et al., 2002, 2006; Wade and Rowland, 2010). Inhibition via horizontal connections may play a role, although the spatial extent of the negative BOLD response suggests longer range interactions. Other possible neural mechanisms that could account for the decreases in the fMRI responses are a reduced input from the lateral geniculate nucleus, reduced or inhibitory feedback from higher cortical areas like V2 or MT (Angelucci and Bressloff, 2006; Angelucci and Bullier, 2003), or a reduction in inhibitory as well as excitatory activity, which can be explained by an inhibition-stabilized network (Ozeki et al., 2009; Tsodyks et al., 1997). selleck products Further study is needed to resolve the neural mechanism, for instance, by using different stimuli, elimination of feedback by pharmacological inactivation, or by combining high-resolution fMRI with multisite electrophysiological recording.

Our results indicate that CBV-based Olaparib research buy fMRI measures different properties than BOLD-based fMRI (Smirnakis et al., 2007). This indicates that CBV-based fMRI signals cannot be assumed to always reflect the same underlying processes as BOLD-based fMRI. On one hand, this can complicate the interpretation of comparative fMRI studies or of VASO- and BOLD-based responses. On the other hand, these differences can potentially be exploited to better understand fMRI signals or to disentangle different neural processes. The results presented

here have implications for comparative fMRI studies between macaques and humans. In the majority of the macaque fMRI studies, iron-based contrast agents are used to boost the contrast-to-noise ratio (CNR) of the functional signal (Vanduffel et al., 2001). Although comparative studies allow direct comparison between monkeys and humans under the same stimulus or task (Nasr et al., 2011; Tsao et al., 2003, 2008; Vanduffel et al., 2002), when iron-based contrast agents are used, the results may not always be exactly comparable. Our results indicate that BOLD and functional Ketanserin CBV responses are not fully equivalent, and CBV-based methods may be unable to unambiguously discriminate between processes that result in positive and negative BOLD signals, for instance, excitation versus inhibition. The similarity of the results obtained with VASO- and MION-based CBV suggests that the VASO- and MION-based CBV methods measure similar properties (Jin and Kim, 2008). The VASO- and MION-based CBV signals both suffer from the drawback that they cannot unambiguously distinguish processes leading to positive and negative BOLD responses. However, this may also be advantageous, because if the VASO and BOLD responses always reflect the same processes, VASO would just be a low-SNR version of BOLD.