Multisensory convergence in calcarine visual areas in macaque monkey
Introduction
The importance of multisensory perception and integration is well-established. The particular neural substrates, however, have remained unclear, especially at the cortical level. According to one hypothesis, multisensory integration may be predominantly a ‘bottom–up’ process, where different modalities converge in higher order multimodal areas in the cerebral cortex. An alternative view emphasizes instead the importance of feedback (‘top–down’) projections from multimodal to unimodal areas (Stein and Meredith, 1993, Driver and Spence, 1998, Driver and Spence, 2000, Pascual-Leone and Hamilton, 2001, Meredith, 2002, Molholm et al., 2002).
In this article, we present evidence for direct anatomic connections to areas V1 and V2 in the macaque monkey from both auditory and parietal association cortices. These may be considered a type of top–down connection (but see Section 4), and may contribute to the efficacy of bimodal interactions, especially at short latencies (Fort et al., 2002, Molholm et al., 2002). Some of these results have been presented in abstract form (Rockland and Ojima, 2001, Rockland and Ojima, 2002).
Section snippets
Methods
Results are based on extracellular injections of anterograde tracers in parietal (n=8) or auditory (n=3) association cortex of macaque monkeys (Macaca mulatta or Macaca fuscata). Animals were tranquilized with ketamine (11 mg/kg) and maintained under deep anesthesia with Nembutal (25 mg/kg sodium pentobarbital, delivered i.v.). Two monkeys, in which physiological mapping of auditory areas was carried out, were intubated and maintained on isoflurane anesthesia. All surgery was performed on
Parietal injection sites (Fig. 1)
Large injections were placed in seven monkeys so as to distribute over the inferior parietal lobule. How best to subdivide this territory is still under investigation (Lewis and van Essen, 2000), and, as a neutral alternative, we will continue to use the traditional nomenclature, which distinguished broadly between a posterior field, PG, and an anterior field, PF (Bonin and Bailey, 1947). These are closely equivalent to areas 7a and 7b, respectively, in the nomenclature proposed by Vogt and
Discussion
In the primate, areas V1 and V2 are generally believed to be visually dedicated areas and to receive cortical connections only from visually related areas (but in rodent, see Sanderson et al., 1991). The present report shows that areas V1 and V2 receive projections from several parietal and auditory association areas (Fig. 4). This seems to be a dramatic endorsement of the contribution of ‘top–down’ processes in multisensory integration, although, as discussed below, this is probably only one
Acknowledgements
We thank Adrian Knight for assistance with figure preparation, Michiko Fujisawa for assistance with manuscript preparation, and other lab members for excellent help with histological preparation. This work was supported by research funds from RIKEN Brain Science Institute.
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