Ariel Rokem | University of California, Berkeley (original) (raw)

Papers by Ariel Rokem

Nitime is a library for the analysis of time-series developed as part of the Nipy project, an eff... more Nitime is a library for the analysis of time-series developed as part of the Nipy project, an effort to build open-source libraries for neuroimaging research. While nitime is developed primarily with neuroimaging data in mind (espespecially functional Magnetic Resonance Imaging data), its design is generic enough that it should be useful to other fields with experimental time-series. The package starts from a purely functional set of algorithms for time-series analysis, including spectral transforms, event-related analysis and coherency. An objectoriented layer is separated into lightweight data container objects for the representation of time-series data and high-level analyzer objects that couple data storage and algorithms. Each analyzer is designed to deal with a particular family of analysis methods and exposes a high-level object oriented interface to the underlying numerical algorithms. We briefly describe functional neuroimaging and some of the unique considerations applicable to time-series analysis of data acquired using these techniques, and provide examples of using nitime to analyze both synthetic data and real-world neuroimaging time-series.

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Experiments were conducted on adult Locusta Migratoria. These grasshoppers communicate by means o... more Experiments were conducted on adult Locusta Migratoria. These grasshoppers communicate by means of species-specific songs which are broadband carrier SUMMARY Based on the characteristics of the ionic channels that compose the cellular membrane, some neurons have a tendency to alternate high frequency periods, with silent intervals. This is called burst firing. In order to characterize the type and amount of information transmitted through burst firing in sensory systems, the activity of grasshopper acoustic receptor neurons was recorded, for several stimulating sound waves. The analysis of these data reveals that the probability of generating bursts is strongly influenced by the statistical properties of the acoustic stimulus. Hence, the tendency to burst is not only determined by intrinsic neuronal properties but also by the way in which those properties interact with signals, amplitude modulated. The behavioural response of them is related to the shape of the amplitude modulation. The stimuli used in the experiments were low-pass filtered Gaussian noise with different cut-off frequencies and standard deviation. Spike trains in response to these stimuli were recorded intracellularly from the axons of auditory receptor neurons. Each

Journal of Vision, Jan 1, 2011

Abstract Perceptual learning is a persistent and specific improvement in the performance of a per... more Abstract Perceptual learning is a persistent and specific improvement in the performance of a perceptual discrimination. Previous research found that perceptual learning of a texture discrimination task is sleep-dependent (Karni et al., 1994) and that repeated, within-day ...

Journal of Vision, Jan 1, 2010

Journal of Vision, Jan 1, 2008

We used the motion aftereffect (MAE) to psychophysically characterize tuning of motion perception... more We used the motion aftereffect (MAE) to psychophysically characterize tuning of motion perception in the human visual system. Hiris and Blake (1992) measured the strength of the MAE for random dot kinematogram (RDK) adapter stimuli containing either one direction of motion or a ...

Journal of …, Jan 1, 2009

Voluntary visual spatial attention can be allocated in a goal-oriented manner to locations contai... more Voluntary visual spatial attention can be allocated in a goal-oriented manner to locations containing behaviorally relevant information. In contrast, involuntary attention is automatically captured by salient events. Allocation of attention is known to be modulated by release of the neurotransmitter acetylcholine (ACh) in cerebral cortex. We used an anti-predictive spatial cueing task to assess the effects of pharmacological enhancement of cholinergic transmission on behavioral measures of voluntary and involuntary attention in healthy human participants. Each trial began with the presentation of a cue in a peripheral location. In 80% of the trials, a target then appeared in a location opposite the cue. In the remaining 20% of trials, the target appeared in the cue location. For trials with short stimulus onset asynchrony (SOA) between cue and target, involuntary capture of attention resulted in shorter reaction times (RTs) to targets presented at the cue location. For long SOA trials, allocation of voluntary attention resulted in the opposite pattern: RTs were shorter when the target appeared in the expected (opposite) location. Each subject participated in two sessions: one in which the cholinesterase inhibitor donepezil was administered to increase synaptic ACh levels and one in which placebo was administered. Donepezil selectively improved performance (reduced RT) for long SOA trials in which targets appeared in the expected location. Thus, cholinergic enhancement augments the benefits of voluntary attention but does not affect involuntary attention, suggesting that they rely on different neurochemical mechanisms.

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In addition to donepezil!s beneficial effects on perceptual learning, there was an overall delete... more In addition to donepezil!s beneficial effects on perceptual learning, there was an overall deleterious effect of the drug on task performance. However, as shown in the Results section, this effect on overall task performance does not account for the effects of the drug on the magnitude of perceptual learning.

Journal of Vision, Jan 1, 2011

Abstract In studies of the behavioral effects of attention, performance is often rendered difficu... more Abstract In studies of the behavioral effects of attention, performance is often rendered difficult by degrading the stimulus in some way (brief stimulus presentations, addition of noise, reduction of contrast). The goal of this research was to determine which stimulus ...

Frontiers in Human …, Jan 1, 2011

Abstract Reduced gamma-aminobutyric acid (GABA) levels in cerebral cortex are thought to contribu... more Abstract Reduced gamma-aminobutyric acid (GABA) levels in cerebral cortex are thought to contribute to information processing deficits in patients with schizophrenia (SZ), and we have previously reported lower in vivo GABA levels in the visual cortex of patients with SZ. GABA-mediated inhibition plays a role in sharpening orientation tuning of visual cortical neurons. Therefore, we predicted that tuning for visual stimulus orientation would be wider in SZ. We measured orientation tuning with a psychophysical procedure in which subjects ...

Journal of Vision, Jan 1, 2007

Journal of …, Jan 1, 2010

Abstract The neurotransmitter dopamine has been implicated in cognitive control and working memor... more Abstract The neurotransmitter dopamine has been implicated in cognitive control and working memory. Specifically, the D2 dopamine receptor agonist bromocriptine has been demonstrated to affect task switching and resistance to distraction in visual working memory. Here, we systematically manipulated spatial attention in a cueing paradigm and assessed the effects of bromocriptine administration on behavioral performance. Subjects performed a visual discrimination task in which they reported the tilt of a target grating that appeared in ...

Journal of …, Jan 1, 2006

Frontiers in …, Jan 1, 2008

Various classes of neurons alternate between high-frequency discharges and silent intervals. This... more Various classes of neurons alternate between high-frequency discharges and silent intervals. This phenomenon is called burst firing. To analyze burst activity in an insect system, grasshopper auditory receptor neurons were recorded in vivo for several distinct stimulus types. The experimental data show that both burst probability and burst characteristics are strongly influenced by temporal modulations of the acoustic stimulus. The tendency to burst, hence, is not only determined by cell-intrinsic processes, but also by their interaction with the stimulus time course. We study this interaction quantitatively and observe that bursts containing a certain number of spikes occur shortly after stimulus deflections of specific intensity and duration. Our findings suggest a sparse neural code where information about the stimulus is represented by the number of spikes per burst, irrespective of the detailed interspike-interval structure within a burst. This compact representation cannot be interpreted as a firing-rate code. An information-theoretical analysis reveals that the number of spikes per burst reliably conveys information about the amplitude and duration of sound transients, whereas their time of occurrence is reflected by the burst onset time. The investigated neurons encode almost half of the total transmitted information in burst activity.

The Journal of …, Jan 1, 2010

Frontiers in …, Jan 1, 2009

At the single-neuron level, precisely timed spikes can either constitute firing-rate codes or spi... more At the single-neuron level, precisely timed spikes can either constitute firing-rate codes or spike-pattern codes that utilize the relative timing between consecutive spikes. There has been little experimental support for the hypothesis that such temporal patterns contribute substantially to information transmission. By using grasshopper auditory receptors as a model system, we show that correlations between spikes can be used to represent behaviorally relevant stimuli. The correlations reflect the inner structure of the spike train: a succession of burst-like patterns. We demonstrate that bursts with different spike counts encode different stimulus features, such that about 20% of the transmitted information corresponds to discriminating between different features, and the remaining 80% is used to allocate these features in time. In this spike-pattern code, the what and the when of the stimuli are encoded in the duration of each burst and the time of burst onset, respectively. Given the ubiquity of burst firing, we expect similar findings also for other neural systems.