Late EEG after-effects in humans following hyperventilation. II. A comparison between subjects with different level of adaptation to hypoxia

The late EEG after-effects following application of a short-lasting ventilatory interoceptive influence (3 min hyperventilation-HV) were studied in humans with three degrees of adaptation: students (ST) with a lower degree of training, professional alpine climbers with a high level of training (AL1) and the same subjects (AL2) in a middle position of adaptation i.e. 6 months after an expedition. ST developed late EEG after-effects, consisting mainly in an increase of the beta-2 EEG activity; AL1 showed very slight changes, while in AL2 the EEG after-effects were intermediate. It is suggested, that a lower level of adaptation facilitates the triggering through HV of processes in the cortical EEG which accompany an improvement of the brain tone.     

Influences of attention on auditory aftereffects following purely visual adaptation

Recently, Kitagawa and Ichihara (2002) demonstrated that visual adaptation to an expanding or contracting disk produces a cross-modal visually-induced auditory loudness aftereffect (VALAE), which they attributed to cross-correlations of motion in three-dimensional space. Our experiments extend their results by providing evidence that attending selectively to one of two competing visual stimuli of the same saliency produces a cross-modal VALAE that favors the attended stimulus. These cross-modal attentional effects suggest the existence of integrative spatial mechanisms between vision and audition that are affected by attention.     

The dynamics of visual adaptation to faces

Several recent demonstrations using visual adaptation have revealed high-level aftereffects for complex patterns including faces. While traditional aftereffects involve perceptual distortion of simple attributes such as orientation or colour that are processed early in the visual cortical hierarchy, face adaptation affects perceived identity and expression, which are thought to be products of higher-order processing. And, unlike most simple aftereffects, those involving faces are robust to changes in scale, position and orientation between the adapting and test stimuli. These differences raise the question of how closely related face aftereffects are to traditional ones. Little is known about the build-up and decay of the face aftereffect, and the similarity of these dynamic processes to traditional aftereffects might provide insight into this relationship. We examined the effect of varying the duration of both the adapting and test stimuli on the magnitude of perceived distortions in face identity. We found that, just as with traditional aftereffects, the identity aftereffect grew logarithmically stronger as a function of adaptation time and exponentially weaker as a function of test duration. Even the subtle aspects of these dynamics, such as the power-law relationship between the adapting and test durations, closely resembled that of other aftereffects. These results were obtained with two different sets of face stimuli that differed greatly in their low-level properties. We postulate that the mechanisms governing these shared dynamics may be dissociable from the responses of feature-selective neurons in the early visual cortex.     

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.     

Increased renal dopamine and acute renal adaptation to a high-phosphate diet

The current experiments explore the role of dopamine in facilitating the acute increase in renal phosphate excretion in response to a high-phosphate diet. Compared with a low-phosphate (0.1%) diet for 24 h, mice fed a high-phosphate (1.2%) diet had significantly higher rates of phosphate excretion in the urine associated with a two- to threefold increase in the dopamine content of the kidney and in the urinary excretion of dopamine. Animals fed a high-phosphate diet had a significant increase in the abundance and activity of renal DOPA (l-dihydroxyphenylalanine) decarboxylase and significant reductions in renalase, monoamine oxidase A, and monoamine oxidase B. The activity of protein kinase A and protein kinase C, markers of activation of renal dopamine receptors, were significantly higher in animals fed a high-phosphate vs. a low-phosphate diet. Treatment of rats with carbidopa, an inhibitor of DOPA decarboxylase, impaired adaptation to a high-phosphate diet. These experiments indicate that the rapid adaptation to a high-phosphate diet involves alterations in key enzymes involved in dopamine synthesis and degradation, resulting in increased renal dopamine content and activation of the signaling cascade used by dopamine to inhibit the renal tubular reabsorption of phosphate.     

Profound contrast adaptation early in the visual pathway

Prior exposure to a moving grating of high contrast led to a substantial and persistent reduction in the contrast sensitivity of neurons in the lateral geniculate nucleus (LGN) of macaque. This slow contrast adaptation was potent in all magnocellular (M) cells but essentially absent in parvocellular (P) cells and neurons that received input from S cones. Simultaneous recordings of M cells and the potentials of ganglion cells driving them showed that adaptation originated in ganglion cells. As expected from the spatiotemporal tuning of M cells, adaptation was broadly tuned for spatial frequency and lacked orientation selectivity. Adaptation could be induced by high temporal frequencies to which cortical neurons do not respond, but not by low temporal frequencies that can strongly adapt cortical neurons. Our observations confirm that contrast adaptation occurs at multiple levels in the visual system, and they provide a new way to reveal the function and perceptual significance of the M pathway.     

Retinal mechanisms of visual adaptation in the skate

Electrical potentials were recorded from different levels within the skate retina. Comparing the adaptive properties of the various responses revealed that the isolated receptor potential and the S-potential always exhibited similar changes in sensitivity, and that the b-wave and ganglion-cell thresholds acted in concert. However, the two sets of responses behaved differently under certain conditions. For example, a dimly iluminated background that had no measurable effect on the senitivities of either of the distal responses, raised significantly the thresholds of both the b-wave and the ganglion cell responses. In addition, the rate of recovery during the early, "neural" phase of dark adaptation was significantly faster for the receptor and S-potentials than for the b-wave or ganglion cell discharge. These results indicate that there is an adaptive ("network") mechanism in the retina which can influence significantly b-wave and gaglion cell activity and which behaves independently of the receptors and horizontal cells. We conclude that visual adaptation in the skate retina is regulated by a combination of receptoral and network mechanisms.     

Response to visual threat following damage to the pulvinar

We present a unique case demonstrating contributions of the pulvinar in response to visual threat. Substantial evidence demonstrates that the amygdala contributes to the emotion of fear and the response to threat. Traditionally, two routes to amygdala activation have been distinguished: a "slow cortical" route through visual and association cortex and a "fast subcortical" route through the thalamus. The pulvinar nucleus of the thalamus is well connected to the amygdala, suggesting that pulvinar damage might interfere with amygdala activation and response to threat. We tested this possibility in patient SM, who suffered complete loss of the left pulvinar. We measured interference from threatening images on goal-directed behavior. In SM's ipsilesional field, threatening images slowed responses more than pleasant images did. This interference decreased rapidly over time. In contrast, in SM's contralesional field, interference from threatening images was initially absent and then increased rather than decreased over time. Processing through the pulvinar therefore plays a significant role in generating response to visual threat. We suggest that, with disruption of the subcortical route to the amygdala, briefly presented images were not fully processed for threat. The reemergence of interference over time may reflect contributions of a slower route.     

Fast visual categorization and speed of processing in migraine]

It has been suggested that the visual cortex of migraine sufferers is hypersensitive. To test this hypothesis, we compared the speed of visual processing in eight migraine patients (including three with aura) and nine controls performing two categorisation tasks of different complexity. In the complex task developed by Thorpe et al. (1996), subjects had to detect the presence of animals in briefly presented photographs (20 ms). The speed of visual processing was measured by the latency of the differential cerebral activity that appears around 150 ms between target and non-target trials. In the simple task, subjects performed a square/circle categorisation. All subjects were faster (445 ms/497 ms) with a higher rate of success (97%/91%) in the simple task. But in both tasks, the behavioural performance and the associated evoked potentials did not show any difference suggesting a cortical hypersensitivity in migraine sufferers.     

Increased prevalence of mitral valve prolapse in patients with migraine.

Patients with classic migraine (69 women and 31 men) selected randomly from a practice list of over 1000 were matched for age, sex and neighbourhood with 100 people who did not have headache problems, and both groups underwent M-mode and two-dimensional echocardiography and clinical examination by cardiologists blinded to the subjects'' clinical status. The mean ages were 34.9 +/- 11.3 years for the migraine group and 33.1 +/- 9.9 years for the control group. Definite and possible mitral valve prolapse (MVP), diagnosed according to predefined echocardiographic criteria, were found about twice as often in the migraine group as in the control group (in 15 v. 7 and 16 v. 8 patients respectively); the echocardiograms were definitely normal in 69 migraine patients and 85 controls (chi 2 = 8.39, p less than 0.025). Altogether 25% of the migraine group and 11% of the control group had evidence of MVP from a combination of the echocardiographic and auscultatory findings (chi 2 = 5.72, p less than 0.025). The odds ratio was 2.7, with 95% confidence limits of 1.17 and 6.29. The association between migraine and MVP has implications for the understanding of platelet abnormalities and episodes of cerebral ischemia occurring in both these conditions.     

Molecular ecology and adaptation of visual photopigments in craniates

In craniates, opsin‐based photopigments expressed in the eye encode molecular ‘light sensors’ that constitute the initial protein in photoreception and the activation of the phototransduction cascade. Since the cloning and sequencing of the first vertebrate opsin gene (bovine rod opsin) nearly 30 years ago (Ovchinnikov Yu 1982, FEBS Letters, 148, 179–191; Hargrave et al. 1983, Biophysics of Structure & Mechanism, 9, 235–244; Nathans & Hogness 1983, Cell, 34, 807–814), it is now well established that variation in the subtypes and spectral properties of the visual pigments that mediate colour and dim‐light vision is a prevalent mechanism for the molecular adaptation to diverse light environments. In this review, we discuss the origins and spectral tuning of photopigments that first arose in the agnathans to sample light within the ancient aquatic landscape of the Early Cambrian, detailing the molecular changes that subsequently occurred in each of the opsin classes independently within the main branches of extant jawed gnathostomes. Specifically, we discuss the adaptive changes that have occurred in the photoreceptors of craniates as they met the ecological challenges to survive in quite differing photic niches, including brightly lit aquatic surroundings; the deep sea; the transition to and from land; diurnal, crepuscular and nocturnal environments; and light‐restricted fossorial settings. The review ends with a discussion of the limitations inherent to the ‘nocturnal‐bottleneck’ hypothesis relevant to the evolution of the mammalian visual system and a proposition that transition through a ‘mesopic‐bottleneck’ may be a more appropriate model.