Remembering visual motion: neural correlates of associative plasticity and motion recall in cortical area MT

The pictorial content of visual memories recalled by association is embodied by neuronal activity at the highest processing stages of primate visual cortex. This activity is elicited by top-down signals from the frontal lobe and recapitulates the bottom-up pattern normally obtained by the recalled stimulus. To explore the generality and mechanisms of this phenomenon, we recorded motion-sensitive neurons at an early stage of cortical processing. After monkeys learned to associate directions of motion with static shapes, these neurons exhibited unprecedented selectivity for the shapes. This emergent shape selectivity reflects activation of neurons representing the motion stimuli recalled by association, and it suggests that recall-related activity may be a general feature of neurons in visual cortex.     

Manipulation of pre-target activity on the right frontal eye field enhances conscious visual perception in humans

The right Frontal Eye Field (FEF) is a region of the human brain, which has been consistently involved in visuo-spatial attention and access to consciousness. Nonetheless, the extent of this cortical site's ability to influence specific aspects of visual performance remains debated. We hereby manipulated pre-target activity on the right FEF and explored its influence on the detection and categorization of low-contrast near-threshold visual stimuli. Our data show that pre-target frontal neurostimulation has the potential when used alone to induce enhancements of conscious visual detection. More interestingly, when FEF stimulation was combined with visuo-spatial cues, improvements remained present only for trials in which the cue correctly predicted the location of the subsequent target. Our data provide evidence for the causal role of the right FEF pre-target activity in the modulation of human conscious vision and reveal the dependence of such neurostimulatory effects on the state of activity set up by cue validity in the dorsal attentional orienting network.     

Passive avoidance deficits following lesions of the posteroventral hippocampo-subiculo-entorhinal area in the developing rat

Young rats, 13, 16, and 20 days of age, underwent discrete bilateral electrolytic lesions of the posteroventral hippocampo-subiculo-entorhinal area, and were trained on a cool-draft-stimulus passive avoidance task 20 min later. Significant deficits in passive avoidance learning were observed at all ages studied following either small or more extended damage as compared to performance of sham-lesioned animals. The impairment was dependent upon the size of the lesion. Extended bilateral lesions of the parietal cortex overlying hippocampus induced no deficit. These results confirm that this part of the hippocampal complex plays a role in passive avoidance learning in the rat. They also show that this control of behavior is already established by the second week of life, thus supporting our previous experiments that demonstrate a cholinergic nicotinic involvement of this region in acquisition of passive avoidance as early as the 11th day of age.     

The parvocellular LGN provides a robust disynaptic input to the visual motion area MT

Dorsal visual cortical areas are thought to be dominated by input from the magnocellular (M) visual pathway, with little or no parvocellular (P) contribution. These relationships are supported by a close correlation between the functional properties of these areas and the M pathway and by a lack of anatomical evidence for P input. Here we use rabies virus as a retrograde transynaptic tracer to show that the dorsal area MT receives strong input, via a single relay, from both M and P cells of the lateral geniculate nucleus. This surprising P input, likely relayed via layer 6 Meynert cells in primary visual cortex, can provide MT with sensitivity to a more complete range of spatial, temporal, and chromatic cues than the M pathway alone. These observations provide definitive evidence for P pathway input to MT and show that convergence of parallel visual pathways occurs in the dorsal stream.     

Selective reflex activation of the genioglossus in humans

In anesthetized or decerebrate animals, negative pressure applied to the upper airway selectively activates the hypoglossal nerve compared with the phrenic nerve. Conversely, positive pressure reduces hypoglossal nerve activity out of proportion to any change in the phrenic neurogram. We have tested the hypothesis that analogous pressure changes applied to awake humans would selectively inhibit or activate genioglossal electromyographic (EMGge) activity relative to diaphragmatic electromyographic activity (EMGdi). We studied seven normal subjects in a head-out body plethysmograph. Pressure at the mouth was either atmospheric, +10 cmH2O, or -10 cmH2O, and lung volume was held constant by applying an identical pressure to the body surface. Thus the transmural pressure distorting the respiratory system was applied only to the upper airway. Subjects breathed CO2-enriched (2-3%) room air to stimulate phasic respiratory EMGge activity. We found that -10 cmH2O pressure applied selectively to the upper airway resulted in a 49% enhancement of peak-integrated EMGge activity, but EMGdi activity remained at control levels. Positive pressure did not result in any changes in EMGge or EMGdi activity. Neither pressure resulted in significant changes in the magnitude or pattern of ventilation. We conclude that reflex mechanisms maintaining upper airway patency are demonstrable in awake humans and probably have an important role in moment-to-moment modulation of upper airway muscle activity in normal awake humans.     

Sensitive period for a multimodal response in human visual motion area MT/MST

The middle temporal complex (MT/MST) is a brain region specialized for the perception of motion in the visual modality. However, this specialization is modified by visual experience: after long-standing blindness, MT/MST responds to sound. Recent evidence also suggests that the auditory response of MT/MST is selective for motion. The developmental time course of this plasticity is not known. To test for a sensitive period in MT/MST development, we used fMRI to compare MT/MST function in congenitally blind, late-blind, and sighted adults. MT/MST responded to sound in congenitally blind adults, but not in late-blind or sighted adults, and not in an individual who lost his vision between ages of 2 and 3 years. All blind adults had reduced functional connectivity between MT/MST and other visual regions. Functional connectivity was increased between MT/MST and lateral prefrontal areas in congenitally blind relative to sighted and late-blind adults. These data suggest that early blindness affects the function of feedback projections from prefrontal cortex to MT/MST. We conclude that there is a sensitive period for visual specialization in MT/MST. During typical development, early visual experience either maintains or creates a vision-dominated response. Once established, this response profile is not altered by long-standing blindness.     

Attenuation of radiation- and drug-induced conditioned taste aversions following area postrema lesions in the rat

The effects of lesions of the area postrema on the acquisition of radiation- and drug-induced (histamine and lithium chloride) conditioned taste aversions were investigated. The results indicated that area postrema lesions caused a significant attenuation of the aversion produced by pairing a novel sucrose solution with radiation (100 rad) or drug injection. Further, the area postrema lesions produced a similar level of attenuation of the taste aversion in all three treatment conditions. The results are discussed in terms of the implications of this finding for defining the mechanisms by which exposure to ionizing radiation can lead to the acquisition of a conditioned taste aversion.     

Head-centred motion perception in the ageing visual system

Stationary objects appear to move in the opposite direction to a pursuit eye movement (Filehne illusion) and moving objects appear slower when pursued (Aubert-Fleischl phenomenon). Both illusions imply that extra-retinal, eye-velocity signals lead to lower estimates of speed than corresponding retinal motion signals. Intriguingly, the velocity (i.e. speed and direction) of the Filehne illusion depends on the age of the observer, especially for brief display durations (Wertheim and Bekkering, 1992). This suggests relative signal size changes as the visual system matures. To test the signal-size hypothesis, we compared the Filehne illusion and Aubert-Fleischl phenomenon in young and old observers using short and long display durations. The trends in the Filehne data were similar to those reported by Wertheim and Bekkering. However, we found no evidence for an effect of age or duration in the Aubert-Fleischl phenomenon. The differences between the two illusions could not be reconciled on the basis of actual eye movements made. The findings suggest a more complicated explanation of the combined influence of age and duration on head-centred motion perception than that described by the signal-size hypothesis.     

Identification of visual paternity cues in humans

Understanding how individuals identify their relatives has implications for the evolution of social behaviour. Kinship cues might be based on familiarity, but in the face of paternity uncertainty and costly paternal investment, other mechanisms such as phenotypic matching may have evolved. In humans, paternal recognition of offspring and subsequent discriminative paternal investment have been linked to father–offspring facial phenotypic similarities. However, the extent to which paternity detection is impaired by environmentally induced facial information is unclear. We used 27 portraits of fathers and their adult sons to quantify the level of paternity detection according to experimental treatments that manipulate the location, type and quantity of visible facial information. We found that (i) the lower part of the face, that changes most with development, does not contain paternity cues, (ii) paternity can be detected even if relational information within the face is disrupted and (iii) the signal depends on the presence of specific information rather than their number. Taken together, the results support the view that environmental effects have little influence on the detection of paternity using facial similarities. This suggests that the cognitive dispositions enabling the facial detection of kinship relationships ignore genetic irrelevant facial information.     

Segregation of object and background motion in visual area MT: effects of microstimulation on eye movements

To track a moving object, its motion must first be distinguished from that of the background. The center-surround properties of neurons in the middle temporal visual area (MT) may be important for signaling the relative motion between object and background. To test this, we microstimulated within MT and measured the effects on monkeys' eye movements to moving targets. We found that stimulation at "local motion" sites, where receptive fields possessed antagonistic surrounds, shifted pursuit in the preferred direction of the neurons, whereas stimulation at "wide-field motion" sites shifted pursuit in the opposite, or null, direction. We propose that activating wide-field sites simulated background motion, thus inducing a target motion signal in the opposite direction. Our results support the hypothesis that neuronal center-surround mechanisms contribute to the behavioral segregation of objects from the background.     

Neural correlates of the contents of visual awareness in humans

The immediacy and directness of our subjective visual experience belies the complexity of the neural mechanisms involved, which remain incompletely understood. This review focuses on how the subjective contents of human visual awareness are encoded in neural activity. Empirical evidence to date suggests that no single brain area is both necessary and sufficient for consciousness. Instead, necessary and sufficient conditions appear to involve both activation of a distributed representation of the visual scene in primary visual cortex and ventral visual areas, plus parietal and frontal activity. The key empirical focus is now on characterizing qualitative differences in the type of neural activity in these areas underlying conscious and unconscious processing. To this end, recent progress in developing novel approaches to accurately decoding the contents of consciousness from brief samples of neural activity show great promise.     

Changes in electrical thresholds for evoking movements from the cat cerebral cortex following lesions of the sensori-motor area

We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4gamma, 4delta, 6aalpha, 6agamma, 3a. The lesions were mainly in area 4gamma. In the lesioned hemisphere there were many points around the lesion site (in areas 4gamma and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6agamma) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4gamma, 6agamma and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.     

Changes in electrical thresholds for evoking movements from the cat cerebral cortex following lesions of the sensori-motor area

We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4γ, 4δ, 6aα, 6aγ, 3a. The lesions were mainly in area 4γ. In the lesioned hemisphere there were many points around the lesion site (in areas 4γ and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6aγ) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4γ, 6aγ and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.