Tuned responses of astrocytes and their influence on hemodynamic signals in the visual cortex

Astrocytes have long been thought to act as a support network for neurons, with little role in information representation or processing. We used two-photon imaging of calcium signals in the ferret visual cortex in vivo to discover that astrocytes, like neurons, respond to visual stimuli, with distinct spatial receptive fields and sharp tuning to visual stimulus features including orientation and spatial frequency. The stimulus-feature preferences of astrocytes were exquisitely mapped across the cortical surface, in close register with neuronal maps. The spatially restricted stimulus-specific component of the intrinsic hemodynamic mapping signal was highly sensitive to astrocyte activation, indicating that astrocytes have a key role in coupling neuronal organization to mapping signals critical for noninvasive brain imaging. Furthermore, blocking astrocyte glutamate transporters influenced the magnitude and duration of adjacent visually driven neuronal responses.     

Selective attention gates visual processing in the extrastriate cortex

Single cells were recorded in the visual cortex of monkeys trained to attend to stimuli at one location in the visual field and ignore stimuli at another. When both locations were within the receptive field of a cell in prestriate area V4 or the inferior temporal cortex, the response to the unattended stimulus was dramatically reduced. Cells in the striate cortex were unaffected by attention. The filtering of irrelevant information from the receptive fields of extrastriate neurons may underlie the ability to identify and remember the properties of a particular object out of the many that may be represented on the retina.     

Motor cortical encoding of serial order in a context-recall task

The neural encoding of serial order was studied in the motor cortex of monkeys performing a context-recall memory scanning task. Up to five visual stimuli were presented successively on a circle (list presentation phase), and then one of them (test stimulus) changed color; the monkeys had to make a single motor response toward the stimulus that immediately followed the test stimulus in the list. Correct performance in this task depends on memorization of the serial order of the stimuli during their presentation. It was found that changes in neural activity during the list presentation phase reflected the serial order of the stimuli; the effect on cell activity of the serial order of stimuli during their presentation was at least as strong as the effect of motor direction on cell activity during the execution of the motor response. This establishes the serial order of stimuli in a motor task as an important determinant of motor cortical activity during stimulus presentation and in the absence of changes in peripheral motor events, in contrast to the commonly held view of the motor cortex as just an "upper motor neuron."     

Unsupervised natural experience rapidly alters invariant object representation in visual cortex

Object recognition is challenging because each object produces myriad retinal images. Responses of neurons from the inferior temporal cortex (IT) are selective to different objects, yet tolerant ("invariant") to changes in object position, scale, and pose. How does the brain construct this neuronal tolerance? We report a form of neuronal learning that suggests the underlying solution. Targeted alteration of the natural temporal contiguity of visual experience caused specific changes in IT position tolerance. This unsupervised temporal slowness learning (UTL) was substantial, increased with experience, and was significant in single IT neurons after just 1 hour. Together with previous theoretical work and human object perception experiments, we speculate that UTL may reflect the mechanism by which the visual stream builds and maintains tolerant object representations.     

Categorical representation of visual stimuli in the primate prefrontal cortex

The ability to group stimuli into meaningful categories is a fundamental cognitive process. To explore its neural basis, we trained monkeys to categorize computer-generated stimuli as "cats" and "dogs." A morphing system was used to systematically vary stimulus shape and precisely define the category boundary. Neural activity in the lateral prefrontal cortex reflected the category of visual stimuli, even when a monkey was retrained with the stimuli assigned to new categories.     

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.     

Visual receptive fields sensitive to absolute and relative motion during tracking

Some neurons in the visual cortex of awake monkeys visually tracking a moving target showed receptive fields that were excited only by stimulus motion relative to a background, while other neurons responded to any kind of stimulus motion. This result was found with two methods, one in which tracking eye movements were identical in both relative-motion and absolute-motion conditions, and another in which stimulus motions on the retina were identical in both conditions. This response pattern can differentiate translation of the retinal image during eye movement from motion of objects in the world.     

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.     

Reward timing in the primary visual cortex

We discovered that when adult rats experience an association between visual stimuli and subsequent rewards, the responses of a substantial fraction of neurons in the primary visual cortex evolve from those that relate solely to the physical attributes of the stimuli to those that accurately predict the timing of reward. In addition to revealing a remarkable type of response plasticity in adult V1, these data demonstrate that reward-timing activity-a "higher" brain function-can occur very early in sensory-processing paths. These findings challenge the traditional interpretation of activity in the primary visual cortex.     

Contrast tuning in auditory cortex

The acoustic features useful for converting auditory information into perceived objects are poorly understood. Although auditory cortex neurons have been described as being narrowly tuned and preferentially responsive to narrowband signals, naturally occurring sounds are generally wideband with unique spectral energy profiles. Through the use of parametric wideband acoustic stimuli, we found that such neurons in awake marmoset monkeys respond vigorously to wideband sounds having complex spectral shapes, preferring stimuli of either high or low spectral contrast. Low contrast-preferring neurons cannot be studied thoroughly with narrowband stimuli and have not been previously described. These findings indicate that spectral contrast reflects an important stimulus decomposition in auditory cortex and may contribute to the recognition of acoustic objects.     

Integrating what and when across the primate medial temporal lobe

Episodic memory or memory for the detailed events in our lives is critically dependent on structures of the medial temporal lobe (MTL). A fundamental component of episodic memory is memory for the temporal order of items within an episode. To understand the contribution of individual MTL structures to temporal-order memory, we recorded single-unit activity and local field potential from three MTL areas (hippocampus and entorhinal and perirhinal cortex) and visual area TE as monkeys performed a temporal-order memory task. Hippocampus provided incremental timing signals from one item presentation to the next, whereas perirhinal cortex signaled the conjunction of items and their relative temporal order. Thus, perirhinal cortex appeared to integrate timing information from hippocampus with item information from visual sensory area TE.     

Modulation of oscillatory neuronal synchronization by selective visual attention

In crowded visual scenes, attention is needed to select relevant stimuli. To study the underlying mechanisms, we recorded neurons in cortical area V4 while macaque monkeys attended to behaviorally relevant stimuli and ignored distracters. Neurons activated by the attended stimulus showed increased gamma-frequency (35 to 90 hertz) synchronization but reduced low-frequency (<17 hertz) synchronization compared with neurons at nearby V4 sites activated by distracters. Because postsynaptic integration times are short, these localized changes in synchronization may serve to amplify behaviorally relevant signals in the cortex.     

Kitten visual cortex: short-term, stimulus-induced changes in connectivity

Single neurons in the kitten visual cortex can be induced to increase their responsiveness to a repeated stimulus applied while the neurons are under observation. These short-term changes are in the same direction as the permanent modifications produced in whole populations of neurons following environmental manipulations during the "critical period" of cortical development, but are less pronounced and probably transient.     

V1 neurons signal acquisition of an internal representation of stimulus location

A fundamental aspect of visuomotor behavior is deciding where to look or move next. Under certain conditions, the brain constructs an internal representation of stimulus location on the basis of previous knowledge and uses it to move the eyes or to make other movements. Neuronal responses in primary visual cortex were modulated when such an internal representation was acquired: Responses to a stimulus were affected progressively by sequential presentation of the stimulus at one location but not when the location was varied randomly. Responses of individual neurons were spatially tuned for gaze direction and tracked the Bayesian probability of stimulus appearance. We propose that the representation arises in a distributed cortical network and is associated with systematic changes in response selectivity and dynamics at the earliest stages of cortical visual processing.     

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.     

The role of the primate extrastriate area V4 in vision

Area V4 is a part of the primate visual cortex. Its role in vision has been extensively debated. Inferences about the functions of this area have now been made by examination of a broad range of visual capacities after ablation of V4 in rhesus monkeys. The results obtained suggest that this area is involved in more complex aspects of visual information processing than had previously been suggested. Monkeys had particularly severe deficits in situations where the task was to select target stimuli that had a lower contrast, smaller size, or slower rate of motion than the array of comparison stimuli from which they had to be discriminated. Extensive training on each specific task resulted in improved performance. However, after V4 ablation, the monkeys could not generalize the specific task to new stimulus configurations and to new spatial locations.     

Increased attention enhances both behavioral and neuronal performance

Single cells were recorded from cortical area V4 of two rhesus monkeys (Macaca mulatta) trained on a visual discrimination task with two levels of difficulty. Behavioral evidence indicated that the monkeys' discriminative abilities improved when the task was made more difficult. Correspondingly, neuronal responses to stimuli became larger and more selective in the difficult task. A control experiment demonstrated that changes in general arousal could not account for the effects of task difficulty on neuronal responses. It is concluded that increasing the amount of attention directed toward a stimulus can enhance the responsiveness and selectivity of the neurons that process it.     

Receptive fields in the cat retina: a new type

A new type of receptive field of cat retinal ganglion cells is described and termed the "suppressed-by-contrast" type. The firing rate of these cells is suppressed by a variety of visual stimuli. However, it has not been possible to find a stimulus that increases the firing rate above the maintained level.     

Neuroscience. Noise makes sense in neuronal computing

The united efforts of assemblies of neurons in the brain's primary visual cortex translate incoming visual signals into action potentials. These action potentials encode, for example, the contrast and orientation of different parts of the image. Some neurons are sensitive to one particular orientation, other are sensitive to other orientations, but all neurons respond equally well to the image contrast. In a Perspective, Volgushev and Eysel explain the finding (Anderson et al.) that neurons are able to maintain this sensitivity to the orientation of a stimulus regardless of the contrast by adding noise to the membrane potential, such that action potentials can also be generated in response to weak signals at low contrast.     

Acetylcholine facilitation, atropine block of synaptic excitation of cortical neurons

Sinigle neurons in cat visual cortex were driven by afferent stimuli to determine the effects of drugs liberated from multibarreled micropipettes on synaptic transmission. Acetylcholine increased, and atropine decreased, the number of neuron discharges fired in response to stimulation of the mesencephalic reticular formation without necessarily affecting responses to other stimuli or affecting spontaneous firing.