Chronic multiunit recordings in behaving animals: advantages and limitations

Spectrotemporal analysis of evoked and induced electroencephalographic responses in primary auditory cortex (A1) of the awake monkey

Electroencephalography is increasingly being used to probe the functional organization of auditory cortex. Modulation of the electroencephalographic (EEG) signal by tones was examined in primary auditory cortex (A1) of awake monkeys. EEG data were measured at 4 laminar depths defined by current source density profiles evoked by best frequency (BF) tones. Midlaminar multiunit activity was used to define the tuning characteristics of A1 sites. Presentation of BF tones increased EEG power across the range of frequencies examined (4-290 Hz), with maximal effects evident within the first 100 ms after stimulus onset. The largest relative increases in EEG power generally occurred at very high gamma frequency bands (130-210 Hz). Increases in EEG power for frequencies less than 70 Hz primarily represented changes in phase-locked activity, whereas increases at higher frequencies primarily represented changes in non-phase-locked activity. Power increases in higher gamma bands were better correlated with the A1 tonotopic organization than power increases in lower frequency bands. Results were similar across the 4 laminar depths examined. These findings highlight the value of examining high-frequency EEG components in exploring the functional organization of auditory cortex and may enhance interpretation of related studies in humans.

Looming biases in monkey auditory cortex

Looming signals (signals that indicate the rapid approach of objects) are behaviorally relevant signals for all animals. Accordingly, studies in primates (including humans) reveal attentional biases for detecting and responding to looming versus receding signals in both the auditory and visual domains. We investigated the neural representation of these dynamic signals in the lateral belt auditory cortex of rhesus monkeys. By recording local field potential and multiunit spiking activity while the subjects were presented with auditory looming and receding signals, we show here that auditory cortical activity was biased in magnitude toward looming versus receding stimuli. This directional preference was not attributable to the absolute intensity of the sounds nor can it be attributed to simple adaptation, because white noise stimuli with identical amplitude envelopes did not elicit the same pattern of responses. This asymmetrical representation of looming versus receding sounds in the lateral belt auditory cortex suggests that it is an important node in the neural network correlate of looming perception.

Choice of cranial window type for in vivo imaging affects dendritic spine turnover in the cortex

Determining the degree of synapse formation and elimination is essential for understanding the structural basis of brain plasticity and pathology. We show that in vivo imaging of dendritic spine dynamics through an open-skull glass window, but not a thinned-skull window, is associated with high spine turnover and substantial glial activation during the first month after surgery. These findings help to explain existing discrepancies in the degree of dendritic spine plasticity observed in the mature cortex.

Detection of the subthalamic nucleus in microelectrographic recordings in Parkinson disease using the high-frequency (> 500 Hz) neuronal background

✓Accurate and fast localization of the subthalamic nucleus (STN) during intraoperative electrophysiological monitoring can improve the outcome of deep brain stimulation surgery. The authors show a simple method of detecting the STN that is based on an analysis of the high-frequency (> 500 Hz) background (HFB) activity of neurons. The HFB reflects multiunit spiking activity close to the recording electrode, and its characteristic profile, which is higher in the STN than in neighboring structures, and facilitates delineation of both the dorsal and ventral STN borders.

The maximum range and timing of excitatory contextual modulation in monkey primary visual cortex

Contextual modulations of receptive field properties by distal stimulus configurations have been shown for a variety of stimulus paradigms. A survey of excitatory contextual modulation data for V1 shows the maximum scale of interactions, measured in terms of distance in V1, to be between 10 mm and 30 mm. Different types of excitatory contextual modulation in V1 occur throughout the interval of 40-250 ms after stimulus delivery. This window provides opportunity for global propagation of visual contextual information to a subset of V1 neurons, via several routes within the visual system. We propose a number of experiments and analyses to confirm the results from this empirical survey.

Spectral resolution of monkey primary auditory cortex (A1) revealed with two-noise masking

An important function of the auditory nervous system is to analyze the frequency content of environmental sounds. The neural structures involved in determining psychophysical frequency resolution remain unclear. Using a two-noise masking paradigm, the present study investigates the spectral resolution of neural populations in primary auditory cortex (A1) of awake macaques and the degree to which it matches psychophysical frequency resolution. Neural ensemble responses (auditory evoked potentials, multiunit activity, and current source density) evoked by a pulsed 60-dB SPL pure-tone signal fixed at the best frequency (BF) of the recorded neural populations were examined as a function of the frequency separation (DeltaF) between the tone and two symmetrically flanking continuous 80-dB SPL, 50-Hz-wide bands of noise. DeltaFs ranged from 0 to 50% of the BF, encompassing the range typically examined in psychoacoustic experiments. Responses to the signal were minimal for DeltaF = 0% and progressively increased with DeltaF, reaching a maximum at DeltaF = 50%. Rounded exponential functions, used to model auditory filter shapes in psychoacoustic studies of frequency resolution, provided excellent fits to neural masking functions. Goodness-of-fit was greatest for response components in lamina 4 and lower lamina 3 and least for components recorded in more superficial cortical laminae. Physiological equivalent rectangular bandwidths (ERBs) increased with BF, measuring nearly 15% of the BF. These findings parallel results of psychoacoustic studies in both monkeys and humans, and thus indicate that a representation of perceptual frequency resolution is available at the level of A1.

Attention lights up new object representations before the old ones fade away

We investigated how attention shifts from one object to another by recording neuronal activity in the primary visual cortex. Monkeys performed a contour-grouping task in which they had to select a target curve and ignore a distractor curve. Some trials required a shift of attention, because the target and distractor curves were switched during the course of the trial. We monitored the dynamics of this attention shift in area V1, in which neuronal responses evoked by the target curve are stronger than those evoked by the distractor. The reallocation of attention was associated with a rapid and strong enhancement of responses to the newly attended curve, followed, after approximately 60 ms, by a weaker suppression of responses to the curve from which attention was removed. We conclude that attention can be rapidly allocated to a new object before it disengages from the previously attended one.

Juxtacellular labeling and chemical phenotyping of extracellularly recorded neurons in vivo

Extracellular recording of the action potential discharge of individual neurons has been an indispensable electrophysiological method for more than 50 yr. Although it requires relatively modest instrumentation, extracellular recording nevertheless provides critically important information concerning the patterning of intercellular communication in the nervous system. In 1996, Didier Pinault described "juxtacellular labeling" as "a novel and very effective single-cell labeling method" for revealing the morphology of extracellularly recorded neurons. Of particular interest for neuroscience is that juxtacellular labeling can be combined with immunocytochemistry and in situ hybridization histochemistry to reveal new and exciting information concerning the chemical phenotype of neurons whose electrophysiological properties have been characterized in vivo. By providing investigators with a means to "match" functional information from electrophysiological recordings with morphological and protein/gene expression data at the level of the single neuron, juxtacellular labeling has opened a new era in neuroscience research, one that holds the promise of an accelerated pace of discovery.

Neural responses in cat visual cortex reflect state changes in correlated activity

Cortical state is characterized by ongoing rhythmic neural activity. Changes in rhythmic activity and thus in cortical state are shown to occur spontaneously in the anesthetized cat. We were interested in whether these state changes have an affect on the cortical processing of sensory stimuli. This was investigated by recording spontaneous and stimulus-evoked local field potentials and multi-unit neuronal activity (MUA) from trans-cortical electrode arrays in the visual cortex of the anesthetized cat. Changes in cortical state were identified by calculating the cross-correlation strength and cross-coherency, between MUA channels at different layers and on separate electrode arrays. Spontaneous changes in rhythmic activity were associated with changes in the strength of stimulus-evoked multiple unit responses of cortical neurons. The highest multi-unit responses were found in periods when low-frequency rhythms of the electroencephalogram increase in magnitude and high-frequency rhythms decrease. Such changes in evoked responses were maximal at layer IV, the input layer of the visual cortex. Our findings suggest that stimulus response magnitude depends on rhythmic state and reflects changes in functional connectivity within the visual cortex.

Synchrony dynamics in monkey V1 predict success in visual detection

Behavioral measures such as expectancy and attention have been associated with the strength of synchronous neural activity. On this basis, it is hypothesized that synchronous activity affects our ability to detect and recognize visual objects. To investigate the role of synchronous activity in visual perception, we studied the magnitude and precision of correlated activity, before and after stimulus presentation within the visual cortex (V1), in relation to a monkey's performance in a figure-ground discrimination task. We show that during the period of stimulus presentation a transition in synchronized activity occurs that is characterized by a reduction of the correlation peak height and width. Before stimulus onset, broad peak correlations are observed that change towards thin peak correlations after stimulus onset, due to a specific decrease of low-frequency components. The magnitude of the transition in correlated activity is larger, i.e. a stronger desynchronization occurs, when the animal perceives the stimulus correctly than when the animal fails to detect the stimulus. These results therefore show that a transition in synchronous firing is important for the detection of sensory stimuli. We hypothesize that the transition in synchrony reflects a change from loose and global neuronal interactions towards a finer temporal and spatial scale of neuronal interactions, and that such a change in neuronal interactions is required for figure-ground discrimination.