Single neurons are differently involved in stimulus-specific oscillations in cat visual cortex

Synchronised oscillatory population events (35-80 Hz; 60-300 ms) can be induced in the visual cortex of cats by specific visual stimulation. The oscillatory events are most prominent in local slow wave field potentials (LFP) and multiple unit spikes (MUA). We investigated how and when single cortical neurons are involved in such oscillatory population events. Simultaneous recordings of single cell spikes, LFP and MUA were made with up to seven microelectrodes. Three states of single cell participation in oscillations were distinguished in spike triggered averages of LFP or MUA from the same electrode: (1) Rhythmic states were characterised by the presence of rhythmicity in single cell spike patterns (35-80 Hz). These rhythms were correlated with LFP and MUA oscillations. (2) Lock-in states lacked rhythmic components in single cell spike patterns, while spikes were phase-coupled with LFP or MUA oscillations. (3) During non-participation states LFP or MUA oscillations were present, but single cell spike trains were neither rhythmic nor phase coupled to these oscillations. Stimulus manipulations (from "optimal" to "suboptimal" for the generation of oscillations) often led to systematic transitions between these states (from rhythmic to lock-in to non-participation). Single cell spike coupling was generally associated with negative peaks in LFP oscillations, irrespective of the cortical separation of single cell and population signals (0-6 mm). Our results suggest that oscillatory cortical population activities are not only supported by local and distant neurons with rhythmic spike patterns, but also by those with irregular patterns in which some spikes occur phase-locked to oscillatory events.     

Synchronization of visual responses between the cortex, lateral geniculate nucleus, and retina in the anesthetized cat

Synchronization of spatially distributed responses in the cortex is often associated with periodic activity. Recently, synchronous oscillatory patterning was described for visual responses in retinal ganglion cells that is reliably transmitted by the lateral geniculate nucleus (LGN), raising the question of whether oscillatory inputs contribute to synchronous oscillatory responses in the cortex. We have made simultaneous multi-unit recordings from visual areas 17 and 18 as well as the LGN and the retina to examine the interactions between subcortical and cortical synchronization mechanisms. Strong correlations of oscillatory responses were observed between retina, LGN, and cortex, indicating that cortical neurons can become synchronized by oscillatory activity relayed through the LGN. This feedforward synchronization occurred with oscillation frequencies in the range of 60-120 Hz and was most pronounced for responses to stationary flashed stimuli and more frequent for cells in area 18 than in area 17. In response to moving stimuli, by contrast, subcortical and cortical oscillations dissociated, proving the existence of independent subcortical and cortical mechanisms. Subcortical oscillations maintained their high frequencies but became transient. Cortical oscillations were now dominated by a cortical synchronizing mechanism operating in the 30-60 Hz frequency range. When the cortical mechanism dominated, LGN responses could become phase-locked to the cortical oscillations via corticothalamic feedback. In summary, synchronization of cortical responses can result from two independent but interacting mechanisms. First, a transient feedforward synchronization to high-frequency retinal oscillations, and second, an intracortical mechanism, which operates in a lower frequency range and induces more sustained synchronization.     

Stimulus-dependent neuronal oscillations and local synchronization in striate cortex of the alert cat

Neuronal responses to visual stimuli that are correlated on a millisecond time scale are well documented in several areas of the mammalian visual cortex. This coherent activity often takes the form of synchronous rhythmic discharges ranging in frequency from 20 to 70 Hz. We performed experiments to determine the incidence and properties of this rhythmic activity in the striate cortex of alert cats and to compare this activity to similar data collected in the striate cortex of anesthetized cats. The results demonstrate that optimal visual stimuli evoke robust, locally synchronous, 20-70 Hz oscillatory responses in the striate cortex of cats that are fully alert and performing a visual fixation task. The oscillatory activity is stimulus dependent, largely absent during periods of spontaneous activity, and shows a systematic increase in frequency with increasing stimulus velocity. Thus, the synchronous oscillatory activity observed in this and earlier studies cannot be explained as an artifact of anesthesia nor as a phenomenon that occurs independent of visual stimulation. Rather, it is a robust process that is present in the alert state and is dependent on the presence and specific properties of visual stimuli.     

The structure and precision of retinal spike trains

Assessing the reliability of neuronal spike trains is fundamental to an understanding of the neural code. We measured the reproducibility of retinal responses to repeated visual stimuli. In both tiger salamander and rabbit, the retinal ganglion cells responded to random flicker with discrete, brief periods of firing. For any given cell, these firing events covered only a small fraction of the total stimulus time, often less than 5%. Firing events were very reproducible from trial to trial: the timing jitter of individual spikes was as low as 1 msec, and the standard deviation in spike count was often less than 0.5 spikes. Comparing the precision of spike timing to that of the spike count showed that the timing of a firing event conveyed several times more visual information than its spike count. This sparseness and precision were general characteristics of ganglion cell responses, maintained over the broad ensemble of stimulus waveforms produced by random flicker, and over a range of contrasts. Thus, the responses of retinal ganglion cells are not properly described by a firing probability that varies continuously with the stimulus. Instead, these neurons elicit discrete firing events that may be the fundamental coding symbols in retinal spike trains.     

Relationship between afferent and central temporal patterns in the locust olfactory system

Odors evoke synchronized oscillations and slow temporal patterns in antennal lobe neurons and fast oscillations in the mushroom body local field potential (LFP) of the locust. What is the contribution of primary afferents in the generation of these dynamics? We addressed this question in two ways. First, we recorded odor-evoked afferent activity in both isolated antennae and intact preparations. Odor-evoked population activity in the antenna and the antennal nerve consisted of a slow potential deflection, similar for many odors. This deflection contained neither oscillatory nor odor-specific slow temporal patterns, whereas simultaneously recorded mushroom body LFPs exhibited clear 20-30 Hz oscillations. This suggests that the temporal patterning of antennal lobe and mushroom body neurons is generated downstream of the olfactory receptor axons. Second, we electrically stimulated arrays of primary afferents in vivo. A brief shock to the antennal nerve produced compound PSPs in antennal lobe projection neurons, with two peaks at an approximately 50 msec interval. Prolonged afferent stimulation with step, ramp, or slow sine-shaped voltage waveforms evoked sustained 20-30 Hz oscillations in projection neuron membrane potential and in the mushroom body LFP. Projection neuron and mushroom body oscillations were phase-locked and reliable across trials. Synchronization of projection neurons was seen directly in paired intracellular recordings. Pressure injection of picrotoxin into the antennal lobe eliminated the oscillations evoked by electrical stimulation. Different projection neurons could express different temporal patterns in response to the same electrical stimulus, as seen for odor-evoked responses. Conversely, individual projection neurons could express different temporal patterns of activity in response to step stimulation of different spatial arrays of olfactory afferents. These patterns were reliable and remained distinct across different stimulus intensities. We conclude that oscillatory synchronization of olfactory neurons originates in the antennal lobe and that slow temporal patterns in projection neurons can arise in the absence of temporal patterning of the afferent input.     

Effects of flicker adaptation and temporal gain control on the flicker ERG

The flicker electroretinogram (ERG) to stimuli varying in temporal frequency and modulation depth was recorded to investigate retinal gain control. With increasing modulation of a sinusoidal flickering stimulus, the flicker ERG shows an amplitude compression and a phase retardation (of the fundamental component) at 16 Hz, an amplitude expansion and a phase advance around 40-48 Hz, and an approximately linear response at 72 Hz. With sum-of-two-sinusoids stimuli, the second stimulus enhances the fundamental response to a 40 or 48 Hz test stimulus at low modulations, and reduces the variation in phase with modulation. This interaction depends primarily on the amplitude of the response to the second stimulus, but not its frequency. With temporally alternating stimuli, a similar but smaller interaction effect is measured. The results suggest that there is an active nonlinear gain control mechanism in the outer retina and this gain control works by adjusting the phase delay of the retinal response. The phase control mechanism is set by the amplitude of the outer retinal response integrated over time.     

Flicker is a primitive visual attribute in visual search.

At the earliest processing stages, visual stimuli are decomposed by a set of filters tuned to specific values of such attributes as colour, orientation, and motion. These filters have been characterised both neurophysiologically and behaviourally. The single exception is the attribute of flicker that has been characterised neurophysiologically but not behaviourally. Using a visual search paradigm, the authors provide the first behavioural demonstration that flicker is indeed a primitive attribute used by the visual system in stimulus encoding. Consistent with the temporal contrast-sensitivity function, sensitivity to flicker was highest at about 10 Hz and decreased as the flicker rate was either increased or decreased. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal patterns of the responses of auditory-nerve fibers to low-frequency tones

The temporal response patterns of auditory-nerve fibers to low-frequency tones were studied in anesthetized cats using period histograms. 'Peak-splitting' was observed mostly in fibers with lower characteristic frequencies (CF < 2 kHz) and with lower-frequency stimulation (< or = 500 Hz). The occurrence of peak-splitting, the number of peaks, and the time between the peaks were all dependent upon the stimulus frequency. The phases of responses, although complex functions of stimulus frequency, intensity, and the fiber's CF, clearly showed traveling-wave characteristics for all frequencies at or above 100 Hz. The amount of phase change with intensity was generally small for lower-frequency stimuli (< approximately 50 degrees), although larger phase changes (e.g., approximately 180 degrees) were occasionally seen with higher-frequency stimuli. At 50 and 100 Hz, the phase of neural responses in the basal region roughly corresponds to the maximum velocity of the basilar membrane towards scala tympani (as inferred from cochlear microphonic recordings).     

Antimicrobial action of propolis and some of its components: the effects on growth, membrane potential and motility of bacteria

Vertebrates are able to perceive the pitch of a series of harmonics, even when the fundamental frequency has been removed from the acoustic stimulus. Neural periodicity responses corresponding to the "missing fundamental" frequency of sonic stimuli have been observed in the auditory system of several animal species, including our own. This paper examines periodic cochlear neural responses of the gerbil. Periodicity responses to both sonic and ultrasonic stimuli originate within the cochlea of this animal. Acoustic stimuli, consisting of 2-12 successive harmonic frequencies, were used to generate an ensemble cochlear nerve periodicity response that was recorded from the round window of the cochlea. This response had a frequency equal to that of the missing fundamental, and not to those of the harmonic stimuli. Forward masking of the stimuli used to produce the periodicity response was used to generate sharp tuning curves, with tip frequencies corresponding to the harmonics and not to the periodicities. The sharpness of these functions increased as the frequencies of the harmonics increased, up to at least 38 kHz. This property could be related to reception of ultrasonic vocalizations utilized by many rodent species.     

Frequency discrimination of stylized synthetic vowels with a single formant

Just-noticeable differences (jnd's) in the center frequency of bandlimited harmonic complexes were measured for normal-hearing subjects. A triangular and a rounded spectral envelope were used. The center frequency ranged from 500 to 600 Hz in a region representing the first formant of vowels, and from 2000 to 2100 Hz in a second formant region. The slope of the spectral envelope was either 50 or 100 dB/oct for the first formant region and 100 or 200 dB/oct for the second formant region. For the fundamental frequency of the complexes 100 and 200 Hz were used. The jnd's were determined for various phase relations between the individual components of the complexes. For comparison we also determined jnd's for a Gaussian white noise that was filtered with the same spectral envelopes as the harmonic complexes. A three-interval, three-alternative forced-choice task was used. All measurements were performed with roving stimulus level. The jnd's found for center frequencies that were halfway between two harmonics were smaller than those found for center frequencies that coincided with a harmonic. The jnd's for the noise bands were mostly between those of the two aforementioned groups. Except for a small group of stimuli, the phase relations had little effect on the jnd's. The majority of the results for both the harmonic and the noise band stimuli can be described by a model using a spectral profile comparison. Most of the remaining data can be explained in the temporal domain from changes in the temporal envelope of the stimuli.     

Intensity and frequency dependence of laryngeal afferent inputs to respiratory hypoglossal motoneurons

Inspiratory hypoglossal motoneurons (IHMs) mediate contraction of the genioglossus muscle and contribute to the regulation of upper airway patency. Intracellular recordings were obtained from antidromically identified IHMs in anesthetized, vagotomized cats, and IHM responses to electrical activation of superior laryngeal nerve (SLN) afferent fibers at various frequencies and intensities were examined. SLN stimulus frequencies <2 Hz evoked an excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequence or only an IPSP in most IHMs that did not change in amplitude as the stimulus was maintained. During sustained stimulus frequencies of 5-10 Hz, there was a reduction in the amplitude of SLN-evoked IPSPs with time with variable changes in the EPSP. At stimulus frequencies >25 Hz, the amplitude of EPSPs and IPSPs was reduced over time. At a given stimulus frequency, increasing stimulus intensity enhanced the decay of the SLN-evoked postsynaptic potentials (PSPs). Frequency-dependent attenuation of SLN inputs to IHMs also occurred in newborn kittens. These results suggest that activation of SLN afferents evokes different PSP responses in IHMs depending on the stimulus frequency. At intermediate frequencies, inhibitory inputs are selectively filtered so that excitatory inputs predominate. At higher frequencies there was no discernible SLN-evoked PSP temporally locked to the SLN stimuli. Alterations in SLN-evoked PSPs could play a role in the coordination of genioglossal contraction during respiration, swallowing, and other complex motor acts where laryngeal afferents are activated.     

Frequency-specific receptive field plasticity in the medial geniculate body induced by Pavlovian fear conditioning is expressed in the anesthetized brain.

Fear conditioning modifies the processing of frequency information; receptive fields (RFs) in the auditory cortex and the medial geniculate body (MGB) are altered to favor processing the frequency of the conditioned stimulus/stimuli (CS) over the pretraining best frequency (BF) and other frequencies. This experiment was designed to determine whether brief conditioning in the waking state produces RF plasticity that is expressed under general anesthesia. Guinea pigs bearing electrodes in the MGB received 20 trials on tone-shock pairing in a single training session. RFs were determined with animals under ketamine anesthesia before conditioning and 1–3 hrs and 24 hrs after conditioning. Frequency-specific RF plasticity was evident for both postconditioning periods: The BF shifted toward or to the CS frequency, responses to the BF decreased, and responses to the CS increased. Broadly tuned cells developed greater RF plasticity than narrowly tuned neurons. Results demonstrate that the specific neuronal results of brief learning experiences can be expressed in the anesthetized brain. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Diversity in frequency response properties of saccular afferents of the toadfish, Opsanus tau

The frequency response of primary saccular afferents of toadfish (Opsanus tau) was studied in the time and frequency domains using the reverse correlation (revcor) method. Stimuli were noise bands with flat acceleration spectra delivered as whole-body motion. The recorded acceleration waveform was averaged over epochs preceding and following each spike. This average, termed the revcor, is an estimate of the response of an equivalent linear filter intervening between body motion and spike initiation. The spectrum of the revcor estimates the shape of the equivalent linear filter. Revcor responses were brief, damped oscillations indicative of relatively broadly tuned filters. Filter shapes were generally band-pass and differed in bandwidth, band edge slope, and characteristic frequency (74 Hz to 140 Hz). Filter shapes tend to be independent of stimulus level. Afferents can be placed into two groups with respect to characteristic frequency (74-88 Hz and 140 Hz). Some high-frequency afferents share a secondary peak at the characteristic frequency of low-frequency afferents, suggesting that an afferent may receive differently tuned peripheral inputs. For some afferents having similar filter shapes, revcor responses often differ only in polarity, probably reflecting inputs from hair cells oriented in opposite directions. The origin of frequency selectivity and its diversity among saccular afferents may arise from a combination of hair cell resonance and micromechanical processes. The resulting frequency analysis is the simplest yet observed among vertebrate animals. During courtship, male toadfish produce the 'boatwhistle' call, a periodic vocalization having several harmonics of a 130 Hz fundamental frequency. The saccule encodes the waveform of acoustic particle acceleration between < 50 and about 250 Hz. Thus, the fundamental frequency component of the boatwhistle is well encoded, but the successive higher harmonics are filtered out. The boatwhistle is thus encoded as a time-domain representation of its fundamental frequency or pulse repetition rate.     

Arousal effects on visual preferences in neonates.

Investigated the influence of arousal level on visual preferences by observing the looking preferences of 12 full-term neonates once in a more aroused condition (before feeding while unswaddled) and once in a less aroused condition (after feeding while swaddled). The stimuli were unpatterned light panels illuminated at temporal frequencies of 1, 2, 4, and 8 Hz. Significant linear relationships between amount of looking and stimulus frequency were found in both arousal conditions. However, the effects were in opposite directions in the 2 conditions. Ss looked more as temporal frequency increased when they were less aroused and looked less as temporal frequency increased when more aroused. Thus, even when awake and attending, infants' visual preferences vary systematically with changes in both internal and external amounts of stimulation such that there is an inverse relationship between level of arousal and preferred temporal frequency. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Optimization of preparation of mitochondria from 25-100 mg skeletal muscle

We measured the gain and phase of vertical vergence in response to disjunctive vertical oscillations of dichoptic textured displays. The texture elements were m-scaled to equate visibility over the area of the display and were aperiodic and varied in shape so as to avoid spurious binocular matches. The display subtended 65 degrees and oscillated through peak-to-peak amplitudes from 18 arc min to 4 degrees at frequencies from 0.05 to 2 Hz - larger ranges than used in previous investigations. The gain of vergence was near 1 when the stimulus oscillated at 18 arc min at a frequency of 0.1 Hz or less. As the amplitude of stimulus oscillation increased from 18 arc min to 4 degrees, vergence gain decreased at all frequencies, which is evidence of a nonlinearity. Gain declined with increasing stimulus frequency but was still about 0.5 at 2 Hz for an amplitude of 18 arc min. Phase lag increased from less than 10 degrees at a stimulus frequency of 0.05 Hz to between 100 degrees and 145 degrees at 2 Hz. Overall, the dynamics of vertical vergence resemble the dynamics of horizontal vergence and cyclovergence.     

Dynamics and bifurcations of two coupled neural oscillators with different connection types

In this paper we present an oscillatory neural network composed of two coupled neural oscillators of the Wilson-Cowan type. Each of the oscillators describes the dynamics of average activities of excitatory and inhibitory populations of neurons. The network serves as a model for several possible network architectures. We study how the type and the strength of the connections between the oscillators affect the dynamics of the neural network. We investigate, separately from each other, four possible connection types (excitatory-->excitatory, excitatory-->inhibitory, inhibitory-->excitatory, and inhibitory-->inhibitory) and compute the corresponding bifurcation diagrams. In case of weak connections (small strength), the connection of populations of different types lead to periodic in-phase oscillations, while the connection of populations of the same type lead to periodic anti-phase oscillations. For intermediate connection strengths, the networks can enter quasiperiodic or chaotic regimes, and can also exhibit multistability. More generally, our analysis highlights the great diversity of the response of neural networks to a change of the connection strength, for different connection architectures. In the discussion, we address in particular the problem of information coding in the brain using quasiperiodic and chaotic oscillations. In modeling low levels of information processing, we propose that feature binding should be sought as a temporally coherent phase-locking of neural activity. This phase-locking is provided by one or more interacting convergent zones and does not require a central ?top level? subcortical circuit (e.g., the septo-hippocampal system). We build a two layer model to show that although the application of a complex stimulus usually leads to different convergent zones with high frequency oscillations, it is nevertheless possible to synchronize these oscillations at a lower frequency level using envelope oscillations. This is interpreted as a feature binding of a complex stimulus.     

Development of arousal-modulated visual preferences in early infancy.

Looking preferences to visual temporal frequencies between 1 and 8 Hz were studied longitudinally at 3 ages (newborn, 1 month, 4 months; N?=?77) in 3 conditions: less aroused (after feeding), more aroused-internal (before feeding), and more aroused-external (after feeding with 8 Hz visual stimulation before each trial). Replicating and extending previous results, a strong interaction between arousal level and stimulus frequency was found at newborn and 1 month. Infants preferred faster stimuli when less aroused and slower stimuli when more aroused, with no differences between the 2 more aroused conditions even though produced by different operations. At 4 months, the interaction with arousal no longer existed; faster stimuli were preferred in all conditions. Thus, after the transition in visual behavior normally occurring at 2–3 months, arousal no longer played a major role, possibly as a result of emergent cortical sensory-specific attention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The surface epithelium of recurrent infected palatine tonsils is rich in gammadelta T cells

Thresholds for detecting alterations in the timbre and harmonicity of complex harmonic signals were measured in zebra finches, budgerigars, and humans. The stimuli used in this experiment were designed to have particular salience for zebra finches by modeling them after natural zebra finch calls. All 3 species showed similar abilities for detecting an amplitude decrement in a single component of a harmonic complex. However, zebra finches and budgerigars were extraordinarily sensitive to the mistunings of single harmonics and exhibited significantly lower thresholds compared with humans at 2 different fundamental frequencies, 570 Hz and 285 Hz. Randomizing relative phases of components in a harmonic stimulus resulted in a significant increase in threshold for detecting mistunings in zebra finches but not in humans. Decreasing the duration of mistuned harmonic stimuli resulted in higher thresholds for both birds and humans. The overall superiority of birds in discriminating inharmonicity suggests that birds and mammals may use different strategies in processing these complex harmonic sounds.     

A further report on a case of Floating-Harbor Syndrome in a mother and daughter

The effects of frequency differences between the lead and lag stimuli on auditory apparent motion (AAM--the perception of continuous changes in the location of a sound image over time) were examined in two experiments. In experiment 1, three standard frequencies (500, 1000, and 5000 Hz) and three SOAs (40, 60, and 100 ms) were tested. Both standard frequency and stimulus onset asynchrony (SOA) were constant throughout a session. Eleven comparison frequencies were tested within each session, with the range dependent on the standard frequency. At standard frequencies of 500 and 1000 Hz, AAM was heard when the frequencies of the lead and lag stimuli were within 100 Hz of each other. At 5000 Hz, the range of frequencies producing AAM increased with SOA. In experiment 2, two standards (500 and 5000 Hz) were tested with a wider range of SOAs (10-210 ms) varied within a session, and a narrower range of comparison frequencies. Here, comparison frequency was constant throughout a session. At 500 Hz, the SOAs producing AAM did not depend on comparison frequency. At 5000 Hz, the SOAs producing AAM increased with comparison frequency, consistent with Korte's third law of visual apparent motion.     

NMR spectroscopy and brain diseases. Clinical applications

We studied the relationship between auditory activity in the midbrain and selective phonotaxis in females of the treefrog, Pseudacris crucifer. Gravid females were tested in two-stimulus playback tests using synthetic advertisement calls of different frequencies (2600 versus 2875 Hz; 2800 versus 3500 Hz; 2600 versus 3500 Hz). Tests were conducted with and without a background of synthesized noise, which was filtered to resemble the spectrum of a chorus of spring peepers. There were no significant preferences for calls of any frequency in the absence of background noise. With background noise, females preferred calls of 3500 Hz to those of 2600 Hz. Multi-unit recordings of neural responses to synthetic sounds were made from the torus semicircularis of the same females following the tests of phonotaxis. We measured auditory threshold at 25 frequencies (1800-4200 Hz) as well as the magnitude of the neural response when stimulus amplitude was held constant and frequency was varied. This procedure yielded isointensity response contours, which we obtained at six amplitudes in the absence of noise and at the stimulus amplitude used during the phonotaxis tests with background noise. Individual differences in audiograms and isointensity responses were poorly correlated with behavioural data except for the test of 2600 Hz versus 3500 Hz calls in noise. The shape of the neural response contours changed with stimulus amplitude and in the presence of the simulated frog chorus. At 85 dB sound pressure level (SPL), the level at which females were tested, the contours of females were quite flat. The contours were more peaked at lower SPLs as well as during the broadcast of chorus noise and white noise at an equivalent spectrum level (45-46 dB/Hz). Peaks in the isointensity response plots of most females occurred at stimulus frequencies ranging from 3200 to 3400 Hz, frequencies close to the median best excitatory frequency (BEF) of 3357 Hz but higher than the mean of the mid-frequency of the male advertisement call (3011 Hz). Addition of background noise may cause a shift in the neural response-intensity level functions. Our results highlight the well-known nonlinearity of the auditory system and the danger inherent in focusing solely on threshold measures of auditory sensitivity when studying the proximate basis of female choice. The results also show an unexpected effect of the natural and noisy acoustic environment on behaviour and responses of the auditory system. Copyright 1998 The Association for the Study of Animal Behaviour.