Bottom-up dependent gating of frontal signals in early visual cortex

The frontal eye field (FEF) is one of several cortical regions thought to modulate sensory inputs. Moreover, several hypotheses suggest that the FEF can only modulate early visual areas in the presence of a visual stimulus. To test for bottom-up gating of frontal signals, we microstimulated subregions in the FEF of two monkeys and measured the effects throughout the brain with functional magnetic resonance imaging. The activity of higher-order visual areas was strongly modulated by FEF stimulation, independent of visual stimulation. In contrast, FEF stimulation induced a topographically specific pattern of enhancement and suppression in early visual areas, but only in the presence of a visual stimulus. Modulation strength depended on stimulus contrast and on the presence of distractors. We conclude that bottom-up activation is needed to enable top-down modulation of early visual cortex and that stimulus saliency determines the strength of this modulation.     

A cortical area selective for visual processing of the human body

Despite extensive evidence for regions of human visual cortex that respond selectively to faces, few studies have considered the cortical representation of the appearance of the rest of the human body. We present a series of functional magnetic resonance imaging (fMRI) studies revealing substantial evidence for a distinct cortical region in humans that responds selectively to images of the human body, as compared with a wide range of control stimuli. This region was found in the lateral occipitotemporal cortex in all subjects tested and apparently reflects a specialized neural system for the visual perception of the human body.     

Neuroscience. More to seeing than meets the eye

A single event may elicit several different sensory stimuli such as vision, sound, and touch. But how does the brain know which of the many different stimuli arriving in the sensory cortex of the brain are connected? In her Perspective, de Gelder discusses new findings showing that when a touch is applied on the same side of the body as a visual cue, vision is enhanced (Macaluso et al.). She explains that this effect is due to neurons projecting from the somatosensory (touch) area of the sensory cortex back to the visual cortex. These neurons keep the visual cortex informed about tactile stimuli elicited at the same time as the visual stimuli.     

Aversive learning enhances perceptual and cortical discrimination of indiscriminable odor cues

Learning to associate sensory cues with threats is critical for minimizing aversive experience. The ecological benefit of associative learning relies on accurate perception of predictive cues, but how aversive learning enhances perceptual acuity of sensory signals, particularly in humans, is unclear. We combined multivariate functional magnetic resonance imaging with olfactory psychophysics to show that initially indistinguishable odor enantiomers (mirror-image molecules) become discriminable after aversive conditioning, paralleling the spatial divergence of ensemble activity patterns in primary olfactory (piriform) cortex. Our findings indicate that aversive learning induces piriform plasticity with corresponding gains in odor enantiomer discrimination, underscoring the capacity of fear conditioning to update perceptual representation of predictive cues, over and above its well-recognized role in the acquisition of conditioned responses. That completely indiscriminable sensations can be transformed into discriminable percepts further accentuates the potency of associative learning to enhance sensory cue perception and support adaptive behavior.     

Selective attention from voluntary control of neurons in prefrontal cortex

Animals can learn to voluntarily control neuronal activity within various brain areas through operant conditioning, but the relevance of that control to cognitive functions is unknown. We found that rhesus monkeys can control the activity of neurons within the frontal eye field (FEF), an oculomotor area of the prefrontal cortex. However, operantly driven FEF activity was primarily associated with selective visual attention, and not oculomotor preparation. Attentional effects were untrained and were observed both behaviorally and neurophysiologically. Furthermore, selective attention correlated with voluntary, but not spontaneous, fluctuations in FEF activity. Our results reveal a specific association of voluntarily driven neuronal activity with "top-down" attention and suggest a basis for the use of neurofeedback training to treat disorders of attention.     

Is face processing species-specific during the first year of life?

Between 6 and 10 months of age, the infant's ability to discriminate among native speech sounds improves, whereas the same ability to discriminate among foreign speech sounds decreases. Our study aimed to determine whether this perceptual narrowing is unique to language or might also apply to face processing. We tested discrimination of human and monkey faces by 6-month-olds, 9-month-olds, and adults, using the visual paired-comparison procedure. Only the youngest group showed discrimination between individuals of both species; older infants and adults only showed evidence of discrimination of their own species. These results suggest that the "perceptual narrowing" phenomenon may represent a more general change in neural networks involved in early cognition.     

Gamma oscillations and object processing in the infant brain

An enduring controversy in neuroscience concerns how the brain "binds" together separately coded stimulus features to form unitary representations of objects. Recent evidence has indicated a close link between this binding process and 40-hertz (gamma-band) oscillations generated by localized neural circuits. In a separate line of research, the ability of young infants to perceive objects as unitary and bounded has become a central focus for debates about the mechanisms of perceptual development. Here we demonstrate that binding-related 40-hertz oscillations are evident in the infant brain around 8 months of age, which is the same age at which behavioral and event-related potential evidence indicates the onset of perceptual binding of spatially separated static visual features.     

Prefrontal regions orchestrate suppression of emotional memories via a two-phase process

Whether memories can be suppressed has been a controversial issue in psychology and cognitive neuroscience for decades. We found evidence that emotional memories are suppressed via two time-differentiated neural mechanisms: (i) an initial suppression by the right inferior frontal gyrus over regions supporting sensory components of the memory representation (visual cortex, thalamus), followed by (ii) right medial frontal gyrus control over regions supporting multimodal and emotional components of the memory representation (hippocampus, amygdala), both of which are influenced by fronto-polar regions. These results indicate that memory suppression does occur and, at least in nonpsychiatric populations, is under the control of prefrontal regions.     

Cortical afferents to the entorhinal cortex of the Rhesus monkey

Although the entorhinal cortex is a major contributor of afferents to the hippocampus and dentate gyrus, knowledge of its own afferents has been vague. Regions of both the frontal and temporal lobes were found to contribute afferents to this region of the brain. These afferents form probable multisynaptic links in pathways connecting the classical sensory areas of the cortex and the limbic system.     

How learning to read changes the cortical networks for vision and language

Does literacy improve brain function? Does it also entail losses? Using functional magnetic resonance imaging, we measured brain responses to spoken and written language, visual faces, houses, tools, and checkers in adults of variable literacy (10 were illiterate, 22 became literate as adults, and 31 were literate in childhood). As literacy enhanced the left fusiform activation evoked by writing, it induced a small competition with faces at this location, but also broadly enhanced visual responses in fusiform and occipital cortex, extending to area V1. Literacy also enhanced phonological activation to speech in the planum temporale and afforded a top-down activation of orthography from spoken inputs. Most changes occurred even when literacy was acquired in adulthood, emphasizing that both childhood and adult education can profoundly refine cortical organization.     

Prolonged immobilization of the body: changes in performance and in the electroencephalogram

Subjects who were immobilized for a week but otherwise were exposed to a normal and varied sensory environment showed intellectual and perceptual deficits similar in many respects to those occurring after prolonged visual and auditory deprivation. A significant change in the electroencephalogram was also observed.     

Physiological limits for "subliminal" perception

Cortical, subcortical, and peripheral sensory nerve potentials were studied in cats to determine comparative thresholds of response. Stimuli capable of eliciting responses in peripheral nerve also invariably elicited responses from thalamus and cortex. Together with relevant data from human studies, the results indicate that stimuli subthreshold for perception do not affect the nervous system. They also have implications for studies of "subliminal" perception.     

采用双任务范式使被试将工作记忆信息保存在工作记忆当中并完成选择性注意任务,结果发现:视觉空间工作记忆负荷不能使熟悉面孔的干扰效应消失,语义工作记忆负荷使熟悉面孔的干扰效应在高知觉负荷条件下消失.因此,熟悉面孔在高知觉负荷条件下可能是以语义的方式存储在工作记忆当中,在低知觉负荷条件下可能是同时存储在视觉空间工作记忆和语义工作记忆中.

采用双任务范式使被试将工作记忆信息保存在工作记忆当中并完成选择性注意任务,结果发现:视觉空间工作记忆负荷不能使熟悉面孔的干扰效应消失,语义工作记忆负荷使熟悉面孔的干扰效应在高知觉负荷条件下消失.因此,熟悉面孔在高知觉负荷条件下可能是以语义的方式存储在工作记忆当中,在低知觉负荷条件下可能是同时存储在视觉空间工作记忆和语义工作记忆中.     

How visual stimuli activate dopaminergic neurons at short latency

Unexpected, biologically salient stimuli elicit a short-latency, phasic response in midbrain dopaminergic (DA) neurons. Although this signal is important for reinforcement learning, the information it conveys to forebrain target structures remains uncertain. One way to decode the phasic DA signal would be to determine the perceptual properties of sensory inputs to DA neurons. After local disinhibition of the superior colliculus in anesthetized rats, DA neurons became visually responsive, whereas disinhibition of the visual cortex was ineffective. As the primary source of visual afferents, the limited processing capacities of the colliculus may constrain the visual information content of phasic DA responses.     

Functional neuroanatomical differences between adults and school-age children in the processing of single words

A critical issue in developmental cognitive neuroscience is the extent to which the functional neuroanatomy underlying task performance differs in adults and children. Direct comparisons of brain activation in the left frontal and extrastriate cortex were made in adults and children (aged 7 to 10 years) performing single-word processing tasks with visual presentation; differences were found in circumscribed frontal and extrastriate regions. Conceivably, these differences could be attributable exclusively to performance discrepancies; alternatively, maturational differences in functional neuroanatomy could exist despite similar performance. Some of the brain regions examined showed differences attributable to age independent of performance, suggesting that maturation of the pattern of regional activations for these tasks is incomplete at age 10.     

A map of visual space induced in primary auditory cortex

Maps of sensory surfaces are a fundamental feature of sensory cortical areas of the brain. The relative roles of afferents and targets in forming neocortical maps in higher mammals can be examined in ferrets in which retinal inputs are directed into the auditory pathway. In these animals, the primary auditory cortex contains a systematic representation of the retina (and of visual space) rather than a representation of the cochlea (and of sound frequency). A representation of a two-dimensional sensory epithelium, the retina, in cortex that normally represents a one-dimensional epithelium, the cochlea, suggests that the same cortical area can support different types of maps. Topography in the visual map arises both from thalamocortical projections that are characteristic of the auditory pathway and from patterns of retinal activity that provide the input to the map.     

Complementary neural mechanisms for tracking items in human working memory

Recognition of a specific visual target among equally familiar distracters requires neural mechanisms for tracking items in working memory. Event-related functional magnetic resonance imaging revealed evidence for two such mechanisms: (i) Enhanced neural responses, primarily in the frontal cortex, were associated with the target and were maintained across repetitions of the target. (ii) Reduced responses, primarily in the extrastriate visual cortex, were associated with stimulus repetition, regardless of whether the stimulus was a target or a distracter. These complementary neural mechanisms track the status of familiar items in working memory, allowing for the efficient recognition of a currently relevant object and rejection of irrelevant distracters.     

Auditory pathways to the frontal cortex of the mustache bat, Pteronotus parnellii

In primates, certain areas of the frontal cortex play a role in guiding movements toward visual or auditory objects in space. The projections from auditory centers to the frontal cortex of the bat Pteronotus parnellii were examined because echolocating bats utilize auditory cues to guide their movements in space. An area in the frontal cortex receives a direct projection from a division of the auditory thalamus, the suprageniculate nucleus, which in turn receives input from the anterolateral peri-olivary nucleus, an auditory center in the medulla. This pathway to the frontal cortex bypasses the main auditory centers in the midbrain and cortex and could involve as few as four neurons between the cochlea and the frontal cortex. The auditory cortex is also a major source of input to the frontal cortex. This area of the frontal cortex may link the auditory and motor systems by its projections to the superior colliculus.     

Reorganization of retinotopic cortical maps in adult mammals after lesions of the retina

The organization of the visual cortex has been considered to be highly stable in adult mammals. However, 5 degrees to 10 degrees lesions of the retina in the contralateral eye markedly altered the systematic representations of the retina in primary and secondary visual cortex when matched inputs from the ipsilateral eye were also removed. Cortical neurons that normally have receptive fields in the lesioned region of the retina acquired new receptive fields in portions of the retina surrounding the lesions. The capacity for such changes may be important for normal adjustments of sensory systems to environmental contingencies and for recoveries from brain damage.     

Neuroscience. Where the brain monitors the body

Researchers haven't known exactly where in the brain all the signals that allow an animal to keep track of its limbs are located, but now a team may have located some of the neurons that first make these multisensory connections. On page 1782, researchers report evidence that a small region of the parietal cortex of the monkey brain known as area 5 may enable the monkey to integrate many sources of information about its body and thereby update its mental model of what the body is doing. The researchers based this conclusion on their finding that some area 5 neurons fire at their fastest rates when the visual feedback from a monkey's arm matches the sensory feedback, an indication that the neurons are sensitive to both streams of information.