Synchrony of dolphin eye movements and their power density spectra

Peripheral and central sensation: multisensory orienting and recognition across species

Attentional bottlenecks force animals to deeply process only a selected fraction of sensory inputs. This motivates a unifying central-peripheral dichotomy (CPD), which separates multisensory processing into functionally defined central and peripheral senses. Peripheral senses (e.g., human audition and peripheral vision) select a fraction of the sensory inputs by orienting animals' attention; central senses (e.g., human foveal vision) allow animals to recognize the selected inputs. Originally used to understand human vision, CPD can be applied to multisensory processes across species. I first describe key characteristics of central and peripheral senses, such as the degree of top-down feedback and density of sensory receptors, and then show CPD as a framework to link ecological, behavioral, neurophysiological, and anatomical data and produce falsifiable predictions.

The primary visual cortex of Cetartiodactyls: organization, cytoarchitectonics and comparison with perissodactyls and primates

Cetartiodactyls include terrestrial and marine species, all generally endowed with a comparatively lateral position of their eyes and a relatively limited binocular field of vision. To this day, our understanding of the visual system in mammals beyond the few studied animal models remains limited. In the present study, we examined the primary visual cortex of Cetartiodactyls that live on land (sheep, Père David deer, giraffe); in the sea (bottlenose dolphin, Risso’s dolphin, long-finned pilot whale, Cuvier’s beaked whale, sperm whale and fin whale); or in an amphibious environment (hippopotamus). We also sampled and studied the visual cortex of the horse (a closely related perissodactyl) and two primates (chimpanzee and pig-tailed macaque) for comparison. Our histochemical and immunohistochemical results indicate that the visual cortex of Cetartiodactyls is characterized by a peculiar organization, structure, and complexity of the cortical column. We noted a general lesser lamination compared to simians, with diminished density, and an apparent simplification of the intra- and extra-columnar connections. The presence and distribution of calcium-binding proteins indicated a notable absence of parvalbumin in water species and a strong reduction of layer 4, usually enlarged in the striated cortex, seemingly replaced by a more diffuse distribution in neighboring layers. Consequently, thalamo-cortical inputs are apparently directed to the higher layers of the column. Computer analyses and statistical evaluation of the data confirmed the results and indicated a substantial correlation between eye placement and cortical structure, with a markedly segregated pattern in cetaceans compared to other mammals. Furthermore, cetacean species showed several types of cortical lamination which may reflect differences in function, possibly related to depth of foraging and consequent progressive disappearance of light, and increased importance of echolocation.

Aerial and aquatic visual acuity of the grey bichir Polypterus senegalus, as estimated by optokinetic response

The present study assessed the aerial and aquatic visual abilities of juvenile grey bichir Polypterus senegalus, fish capable of terrestrial locomotion, by measuring the optokinetic response to stimuli of varying speed and spatial frequency. In water, fish tracked slow-moving (2° s^-1 ) stimuli moderately well and fast-moving stimuli very poorly. Spatial acuity was very low compared with many other species, with maximum response observed at 0.05-0.075 stimulus cycles per degree of visual arc; however, it should be noted that adult fish, with their larger eyes, are likely to have somewhat improved spatial acuity. Low spatial acuity and limited stimulus tracking ability might be expected in a nocturnal ambush predator such as P. senegalus, where gaze stabilization may be less crucial and other sensory inputs may have greater importance in perception of the environment. In air, spatial and temporal acuity were both poorer by every measure, but some visual ability persisted. As the eye shows no anatomical specialization for aerial vision, poor vision was expected; however, the large decrease in saccade velocity observed in air trials was unexpected. Stimulus parameters typically have little effect on the characteristics of the saccade, so this finding may suggest that the function of the reflex system itself could be compromised in the aerial vision of some fishes capable of terrestrial locomotion.

Dolphins maintain cognitive performance during 72 to 120 hours of continuous auditory vigilance

The present study reports the first use of a choice visual–vocal response time cognitive task, during 72 or 120 h of continuous auditory vigilance. Two adult bottlenose dolphins (Tursiops truncatus), NAY(male) and SAY (female), maintained a very high detection rate(91.1–98.7%) of random 1.5 s goal tones infrequently substituted in a background of frequent 0.5 s equal-amplitude tones over continuous 72 or 120 h sessions. In addition, a choice visual–vocal response time task (CVVRT)tested cognitive performance during night time sessions, when the dolphins would have ordinarily been resting or asleep as we had observed in previous studies. NAY and SAY detected a single-bar, posterior, vertical, green (S1g)or 3-bar, anterior, horizontal, red (S2r) LED light stimulus presented randomly to each eye. They responded with a different vocalization (whistle or pulse burst) to each stimulus (S1g or S2r) presented randomly to left and right eyes. The animals maintained high levels of goal tone detection without signs of sleep deprivation as indicated by behavior, blood indices or marked sleep rebound during 24 h of continuous post-experiment observation. Acoustic goal tone response time (AGTRT) overall did not change during the 72 h(F=0.528, P=0.655) or 120 h (F=0.384, P=0.816) sessions. Nor did CVVRT slow or degrade over the 72 h(F=4.188, P=0.104) or 120 h (F=2.298, P=0.119) AGTRT sessions.

Dolphin continuous auditory vigilance for five days

The present report describes the first study of continuous vigilance in dolphins. Two adult bottlenose dolphins (Tursiops truncatus), WEN (male) and SAY (female), maintained a very high detection rate of randomly presented, infrequent, 1.5-s target tones in a background of frequent 0.5-s equal-amplitude tones over five continuous 120-h sessions. The animals were able to maintain high levels (WEN 97, 87, 99%; SAY 93, 96%) of target detection without signs of sleep deprivation as indicated by behavior, blood indices or marked sleep rebound during 24 h of continuous post-experiment observation. Target response time overall (F = 0.384; P = 0.816) did not change between day 1 and day 5. However, response time was significantly slower (F = 21.566, P = 0.019) during the night (21.00-04.00 h) when the dolphins would have ordinarily been resting or asleep.

Functional imaging of dolphin 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. 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. MRIs were used to register functional brain scans with single photon emission computed tomography (SPECT) and positron emission tomography (PET) in trained dolphins. Scans using SPECT revealed unihemispheric blood flow reduction following diazepam doses greater than 0.55 mg kg(-1) for these 180-200 kg animals. Scans using PET revealed hemispheric differences in brain glucose consumption when scans with and without diazepam were compared. The findings suggest that unihemispheric reduction in blood flow and glucose metabolism in the hemisphere showing USW are important features of unihemispheric sleep. Functional 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. The findings also demonstrate the potential value of functional scans to explore other aspects of dolphin brain physiology as well as pathology.

Dolphin pointing is linked to the attentional behavior of a receiver

In 2001, Xitco et al. (Anim Cogn 4:115-123) described spontaneous behaviors in two bottlenose dolphins (Tursiops truncatus) that resembled pointing and gaze alternation. The dolphins' spontaneous behavior was influenced by the presence of a potential receiver, and the distance between the dolphin and the receiver. The present study adapted the technique of Call and Tomasello [(1994) J Comp Psychol 108:307-317], used with orangutans to test the effect of the receiver's orientation on pointing in these same dolphins. The dolphins directed more points and monitoring behavior at receivers whose orientation was consistent with attending to the dolphins. The results demonstrated that the dolphins' pointing and monitoring behavior, like that of apes and infants, was linked to the attentional behavior of the receiver.