Biomedical subjects

Alexander Gail

Publications and source records attributed to Alexander Gail.

4 recordsLinked to original sources

PubMed2006

Neural dynamics in monkey parietal reach region reflect context-specific sensorimotor transformations.

We investigated the neural dynamics of sensorimotor transformations in the parietal reach region (PRR) of monkeys. To dissociate sensory from motor goal representations, we used a memory-guided anti-reach task. The monkeys had to reach either to a visually instructed, memorized peripheral target position (pro-reach) or to a diametrically opposed position (anti) while keeping central ocular fixation. Pro- and anti-reaches were randomly interleaved and indicated by a color instruction from the beginning of each trial. We analyzed spatiotemporal single-cell tuning and performed time-resolved population decoding to quantify the dynamic representation of the spatial visual cue, the reach goal, and the currently valid task rule (pro/anti mapping). Sensory information regarding the visual cue position was represented weakly during a short period of cue visibility. PRR predominantly encoded the reach goal from the end of the cue period on. The representation of the reach goal in the memory task evolves later for the anti- compared with pro-reaches, consistent with a 40-50 ms difference in reaction time between the two task rules. The task rule could be decoded before the appearance of the spatial cue, which indicates that abstract rule information is present in PRR that is independent of spatial cue or motor goal representations. Our findings support the hypothesis that PRR immediately translates current sensory information into reach movement plans, rather than storing the memorized cue location in the instructed-delay task. This finding indicates that PRR represents integrated knowledge on spatial sensory information combined with abstract behavioral rules to encode the desired movement goal.

Action Potentials↗

PubMed2004

Perception-related modulations of local field potential power and coherence in primary visual cortex of awake monkey during binocular rivalry.

Cortical synchronization at gamma-frequencies (35-90 Hz) has been proposed to define the connectedness among the local parts of a perceived visual object. This hypothesis is still under debate. We tested it under conditions of binocular rivalry (BR), where a monkey perceived alternations among conflicting gratings presented singly to each eye at orthogonal orientations. We made multi-channel microelectrode recordings of multi-unit activity (MUA) and local field potentials (LFP) from striate cortex (V1) during BR while the monkey indicated his perception by pushing a lever. We analyzed spectral power and coherence of MUA and LFP over 4-90 Hz. As in previous work, coherence of gamma-signals in most pairs of recording locations strongly depended on grating orientation when stimuli were presented congruently in both eyes. With incongruent (rivalrous) stimulation LFP power was often consistently modulated in consonance with the perceptual state. This was not visible in MUA. These perception-related modulations of LFP occurred at low and medium frequencies (< 30 Hz), but not at gamma-frequencies. Perception-related modulations of LFP coherence were also restricted to the low-medium range. In conclusion, our results do not support the expectation that gamma-synchronization in V1 is related to the perceptual state during BR, but instead suggest a perception-related role of synchrony at low and medium frequencies.

Algorithms↗

PubMed2004

Neural mechanisms of visual associative processing.

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans--related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma-waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma-activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma-waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma-synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.

Animals↗

PubMed2003

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup.

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: approximately 2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: approximately 0.025 degrees visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Animals↗