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.     

Attention units in the auditory cortex

In the course of examining single unit responses from the cortex of unrestrained and unanesthetized cats, we have come upon a population of cells that appears to be sensitive to auditory stimuli only if the cat "pays attention" to the sound source. We have described these responses, since they have not been previously reported and since they illustrate an important difference between the information which can be gleaned from experiments of this type and that obtained in the usual "acute" microelectrode experiment.     

Amygdalectomy impairs crossmodal association in monkeys

Monkeys trained on both visual and tactual versions of an object memory task (delayed nonmatching-to-sample) received bilateral ablations of either the amygdaloid complex or the hippocampal formation of the brain. Although both groups performed well on the two intramodal versions (visual-to-visual and tactual-to-tactual), the amygdalectomized monkeys were severely impaired relative to the hippocampectomized monkeys on a crossmodal version (tactual-to-visual). The findings suggest that the amygdala is critical for certain forms of crossmodal association and that the loss of such associations underlies many of the bizarre behaviors that make up the Klüver-Bucy syndrome.     

Evoked potentials and auditory reaction time in monkeys

Monkeys with bipolar stimulating and recording electrodes in primary auditory cortex were trained to release a key to the onset of a pure tone. Substitution of direct cortical stimulation for the pure tone resulted in a reduction of 15 milliseconds In the latency of the behavioral response. This changed latency agreed with the latency of the primary evoked potential recorded from the animals. Systematic related changes in the amplitude of the central response and in the latency of the behavioral response followed changes in the intensity and frequency of the acoustic stimulus.     

Decay of information in short-term memory

Self-paced recall in Broadbent's simultaneous listening task shows particular temporal patterns associated with report strategies. Accuracy is a decreasing function of the interval between presentation and report of an item, irrespective of report strategy. Results are related to an interaction between strategies of response selection and decay of information from memory.     

Functional specialization in rhesus monkey auditory cortex

Neurons in the lateral belt areas of rhesus monkey auditory cortex prefer complex sounds to pure tones, but functional specializations of these multiple maps in the superior temporal region have not been determined. We tested the specificity of neurons in the lateral belt with species-specific communication calls presented at different azimuth positions. We found that neurons in the anterior belt are more selective for the type of call, whereas neurons in the caudal belt consistently show the greatest spatial selectivity. These results suggest that cortical processing of auditory spatial and pattern information is performed in specialized streams rather than one homogeneously distributed system.     

Entorhinal cortex layer III input to the hippocampus is crucial for temporal association memory

Associating temporally discontinuous elements is crucial for the formation of episodic and working memories that depend on the hippocampal-entorhinal network. However, the neural circuits subserving these associations have remained unknown. The layer III inputs of the entorhinal cortex to the hippocampus may contribute to this process. To test this hypothesis, we generated a transgenic mouse in which these inputs are specifically inhibited. The mutant mice displayed significant impairments in spatial working-memory tasks and in the encoding phase of trace fear-conditioning. These results indicate a critical role of the entorhinal cortex layer III inputs to the hippocampus in temporal association memory.     

Localization of a short-term memory in Drosophila

Memories are thought to be due to lasting synaptic modifications in the brain. The search for memory traces has relied predominantly on determining regions that are necessary for the process. However, a more informative approach is to define the smallest sufficient set of brain structures. The rutabaga adenylyl cyclase, an enzyme that is ubiquitously expressed in the Drosophila brain and that mediates synaptic plasticity, is needed exclusively in the Kenyon cells of the mushroom bodies for a component of olfactory short-term memory. This demonstrates that synaptic plasticity in a small brain region can be sufficient for memory formation.     

Attention-like processes in Drosophila require short-term memory genes

Although there is much behavioral evidence for complex brain functions in insects, it is not known whether insects have selective attention. In humans, selective attention is a dynamic process restricting perception to a succession of salient stimuli, while less relevant competing stimuli are suppressed. Local field potential recordings in the brains of flies responding to visual novelty revealed attention-like processes with stereotypical temporal properties. These processes were modulated by genes involved in short-term memory formation, namely dunce and rutabaga. Attention defects in these mutants were associated with distinct optomotor effects in behavioral assays.     

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.     

Spatiotemporal pattern of neural processing in the human auditory cortex

The principles that the auditory cortex uses to decipher a stream of acoustic information have remained elusive. Neural responses in the animal auditory cortex can be broadly classified into transient and sustained activity. We examined the existence of similar principles in the human brain. Sound-evoked, blood oxygen level-dependent signal response was decomposed temporally into independent transient and sustained constituents, which predominated in different portions-core and belt-of the auditory cortex. Converging with unit recordings, our data suggest that this spatiotemporal pattern in the auditory cortex may represent a fundamental principle of analyzing sound information.     

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.     

Number coding in association cortex of the cat

In electrophysiological investigations of single neurons in cortical association response areas of the cat, cells have been encountered that appear to code the property of number. In a sequence of stimulus presentations, these cells characteristically discharge to a particular numbered stimulus in the series. This effect is independent of stimulus modality, intensity, and interstimulus interval; thus, the cells seem to be responding to the number of stimulus presentations.     

Development of polysensory responses in association cortex of kitten

Sensory responsiveness of single neurons in posterior association cortex of kittens that were 7 to 50 days old was investigated. The percentage of trimodal cells (that is, cells that respond to visual, auditory, and somesthetic stimulation) increased gradually until day 50, when percentages of trimodally responsive cells approached the adult level. In the youngest kittens, cells were predominantly responsive to only visual stimulation. With maturation, responsiveness to auditory and then to somesthetic stimulation was observed in increasing percentages of cells.     

Autoimmune encephalomyelitis and ocular lesions in monkeys sensitized during the neonatal period

The neonatal rhesus monkey is susceptible to the induction of autoimmune encephalomyelitis. The disease has been produced regularly by injection of neonatal animals with guinea pig spinal cord antigen in complete Freund's adjuvant. The onset of the disease, as compared with onset in adults, is delayed and is most often heralded by intrinsic eye lesions, notably widespread retinal hemorrhages.     

Unilateral ablation of the auditory cortex in the cat impairs complex sound localization

Unilateral ablation of the auditory cortex in the cat results in a profound deficit in attending to stimuli on the side contralateral to the lesion. The deficit is also manifested in an abnormal perception of left-right pulse pairs when the pulse which leads by a few milliseconds is contralateral to the damaged hemisphere.     

Task-relevant late positive component of the auditory event-related potential in monkeys resembles P300 in humans

A long-latency (300-millisecond), vertex-positive component of the event-related potential recorded from monkeys was present only when the eliciting stimulus was relevant to the task. The amplitude of this component varied inversely with stimulus probability and was dissociable from motor responses.     

Fast-forward playback of recent memory sequences in prefrontal cortex during sleep

As previously shown in the hippocampus and other brain areas, patterns of firing-rate correlations between neurons in the rat medial prefrontal cortex during a repetitive sequence task were preserved during subsequent sleep, suggesting that waking patterns are reactivated. We found that, during sleep, reactivation of spatiotemporal patterns was coherent across the network and compressed in time by a factor of 6 to 7. Thus, when behavioral constraints are removed, the brain's intrinsic processing speed may be much faster than it is in real time. Given recent evidence implicating the medial prefrontal cortex in retrieval of long-term memories, the observed replay may play a role in the process of memory consolidation.     

D1 dopamine receptors in prefrontal cortex: involvement in working memory

The prefrontal cortex is involved in the cognitive process of working memory. Local injections of SCH23390 and SCH39166, selective antagonists of the D1 dopamine receptor, into the prefrontal cortex of rhesus monkeys induced errors and increased latency in performance on an oculomotor task that required memory-guided saccades. The deficit was dose-dependent and sensitive to the duration of the delay period. These D1 antagonists had no effect on performance in a control task requiring visually guided saccades, indicating that sensory and motor functions were unaltered. Thus, D1 dopamine receptors play a selective role in the mnemonic, predictive functions of the primate prefrontal cortex.     

Coupling between neuronal firing, field potentials, and FMRI in human auditory cortex

Functional magnetic resonance imaging (fMRI) is an important tool for investigating human brain function, but the relationship between the hemodynamically based fMRI signals in the human brain and the underlying neuronal activity is unclear. We recorded single unit activity and local field potentials in auditory cortex of two neurosurgical patients and compared them with the fMRI signals of 11 healthy subjects during presentation of an identical movie segment. The predicted fMRI signals derived from single units and the measured fMRI signals from auditory cortex showed a highly significant correlation (r = 0.75, P < 10(-47)). Thus, fMRI signals can provide a reliable measure of the firing rate of human cortical neurons.