User:Antonio Pereira/Proposed/Vibrissal coding in hippocampus

• Prof. Miguel A. L. Nicolelis, Duke University, NC, USA

• Prof. Antonio Pereira, Universidade Federal do Rio Grande do Norte/Edmond and Lily Safra International Institute of Neuroscience of Natal, Brazil

Anatomy of the tactile input to the hippocampus

The gateway to the hippocampal formation is the entorhinal cortex, which receives both unimodal and polymodal sensory inputs via the perirhinal and postrhinal cortices in rodents (Burwell and Amaral, 1998). In that study, retrograde tracing from the entorhinal cortex has shown very little staining in the primary somatosensory cortex, in contrast to cortical areas dedicated to olfaction and vision. Tactile information could reach the hippocampal formation through either polysynaptic pathways from the barrel cortex, or directly from subcortical nuclei of the somatosensory system.

<review> Please add to the section above an overview of the anatomical pathways between the vibrissal cortex and the hippocampus, suggesting how the hippocampus may receive the information from the vibrissae. Perhaps the following review can be of help (for example, figure 5 in that review): Whitlock JR, Sutherland RJ, Witter MP, Moser M-B, Moser EI (2008) Navigating from hippocampus to parietal cortex. PROC NAT ACAD SCI 105:14755-14762. </review>

Tactile processing in the hippocampus

Despite the wealth of anatomical connections capable of bringing sensory inputs to the hippocampus, little is known about the processing of sensory signals in the hippocampus other than vision <review> Reference missing </review>.

Recently, an analysis of CA1 hippocampal ensembles in a tactile discrimination task was performed using multielectrode arrays in rats (Pereira et al., 2007) <review> Citation missing from list below </review>. Chemical inactivation of key synaptic stations in the trigeminal somatosensory pathway showed that inputs from the whiskers reach the CA1 region through thalamic and cortical relays associated with discriminative touch <review> What relays? </review>. Further, ensembles of hippocampal neurons also carry precise information about stimulus identity when recorded during performance in an aperture-discrimination task (described originally in Krupa et al., 2001) using the whiskers. There are also broad similarities between tactile responses in trigeminal stations and in the hippocampus during different vigilance states (wake and sleep). These results show that tactile information associated with fine whisker discrimination is readily available in CA1 for retrieval.

<review> It is important to discuss the uses of such information in the hippocampus. Consider the following points: 1. Place cells function in the dark 2. The rat can navigate in the dark. 3. Although some of the information about navigation in the dark can come from the path-integration system, it may very well be that the whiskers are actually the main source used for discerning landmarks in the dark (more than olfaction, for example). The following very early seminal publication demonstrates the importance of whiskers in navigation tasks: Vincent, S.B. (1912) The Functions of the vibrissae in the behavior of the white rat. Animal behavior monographs 1(5) [1]

I also see that some of this discussion, and additional related discussion, appears in Pereira et al. (2007), and thus can be used here. </review>

<review> (Reviewer B) Data on vibrissal coding in the hippocampus is extremely scarce, so that it is quite surprising to dedicate a Scholarpedia page to the subject. Other than the one study from the authors, and the anatomical data which has to be expanded above, the authors should discuss the article by Berg, Whitmer and Kleinfeld, J Neurosci 2006 on the (lack of) synchronization between theta and whisking. That study does not rule out that theta and whisking might be locked during learning involving the vibrissae, as occurs in the olfactory domain (see the discussion in the Berg paper). Likewise, it is possible that vibrissal responses recorded by the authors appear in hippocampus only when the vibrissal input carries a behavioral significance. This should be discussed in more depth, as suggested by Reviewer A. </review>

References

<review> Please update the reference list according to the changes above </review>

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