To investigate brain function in context of the finer anatomy of the brain, CT imaging of dolphin anatomy must be replaced by an imaging modality sensitive to soft tissue.
Houser et al.( Houser et al., 2004) expanded the use of medical imaging modalities on live cetaceans to include functional scanning (SPECT and PET) and coupled the images obtained with these scans to structural imagery obtained via CT. Prior to our recent studies( Houser et al., 2004), live cetacean scans were limited to one computed tomography CT) study of a pygmy sperm whale with a sinus abscess ( Tristan et al., 2001). However, non-invasive means of investigating this large and highly organized brain in the living animal have been quite limited and there is little understanding of the neurotransmitter and neuromodulator distribution in the dolphin brain as a whole. Modern morphomolecular studies of fixed material have begun to reveal information relative to the neurochemistry of some regions of the dolphin brain (cf. It can be said that bottlenose dolphins and their close relatives in the cetacean family, Delphinidae, have large brains and have reached the zenith of cetacean brain development ( Marino,1998 Ridgway,1999 Marino et al.,2004). What triggers one hemisphere to go into SWS while the other hemisphere often displays an EEG indistinguishable from that of an awake animal remains to be determined.
These observations include independent eye movement and closure ( McCormick, 1969 Dawson et al., 1981 Lyamin et al., 2001 Lyamin et al., 2004),observations of behavior in nocturnal rest periods( Flanigan, Jr, 1974 Goley, 1999), a small corpus callosum ( Tarpley and Ridgway,1994), complete crossing of the nerves at the optic chiasm( Tarpley et al., 1994), and absence of an arterial Circle of Willis( McFarland et al., 1979). Several physiological and anatomical observations suggest a degree of dolphin brain hemispheric independence.
Left and right hemispheres alternate SWS by some unknown mechanism. The findings also demonstrate the potential value of functional scans to explore other aspects of dolphin brain physiology as well as pathology.ĭolphins and related small whales in the delphinoid cetacean family have shown slow wave sleep (SWS) electroencephalograms (EEG) in one brain hemisphere while producing waking EEG in the other( Serafetinides et al., 1970 Mukhametov et al., 1977 Mukhametov, 1984 Mukhametov, 1987 Ridgway, 2002 Lyamin et al., 2001 Lyamin et al., 2004). The findings suggest that unihemispheric reduction in blood flow and glucose metabolism in the hemisphere showing USW are important features of unihemispheric sleep.įunctional scans may also help to elucidate the degree of hemispheric laterality of sensory and motor systems as well as in neurotransmitter or molecular mechanisms of unihemispheric sleep in delphinoid cetaceans. Scans using PET revealed hemispheric differences in brain glucose consumption when scans with and without diazepam were compared.
Scans using SPECT revealed unihemispheric blood flow reduction following diazepam doses greater than 0.55 mg kg -1 for these 180-200 kg animals. MRIs were used to register functional brain scans with single photon emission computed tomography (SPECT)and positron emission tomography (PET) in trained dolphins. Diazepam has been shown to induce unihemispheric slow waves (USW), therefore we used functional imaging of dolphins with and without diazepam to observe hemispheric differences in brain metabolism and blood flow. This report documents the first use of magnetic resonance images (MRIs) of living dolphins to register functional brain scans, allowing for the exploration of potential mechanisms of unihemispheric sleep.