Biomedical subjects

Stephan Quessy

Publications and source records attributed to Stephan Quessy.

5 recordsLinked to original sources

PubMed2005

Evaluating the operations underlying multisensory integration in the cat superior colliculus.

It is well established that superior colliculus (SC) multisensory neurons integrate cues from different senses; however, the mechanisms responsible for producing multisensory responses are poorly understood. Previous studies have shown that spatially congruent cues from different modalities (e.g., auditory and visual) yield enhanced responses and that the greatest relative enhancements occur for combinations of the least effective modality-specific stimuli. Although these phenomena are well documented, little is known about the mechanisms that underlie them, because no study has systematically examined the operation that multisensory neurons perform on their modality-specific inputs. The goal of this study was to evaluate the computations that multisensory neurons perform in combining the influences of stimuli from two modalities. The extracellular activities of single neurons in the SC of the cat were recorded in response to visual, auditory, and bimodal visual-auditory stimulation. Each neuron was tested across a range of stimulus intensities and multisensory responses evaluated against the null hypothesis of simple summation of unisensory influences. We found that the multisensory response could be superadditive, additive, or subadditive but that the computation was strongly dictated by the efficacies of the modality-specific stimulus components. Superadditivity was most common within a restricted range of near-threshold stimulus efficacies, whereas for the majority of stimuli, response magnitudes were consistent with the linear summation of modality-specific influences. In addition to providing a constraint for developing models of multisensory integration, the relationship between response mode and stimulus efficacy emphasizes the importance of considering stimulus parameters when inducing or interpreting multisensory phenomena.

Acoustic Stimulation↗

PubMed2004

Electrical stimulation of rhesus monkey nucleus reticularis gigantocellularis. I. Characteristics of evoked head movements.

The nucleus reticularis gigantocellularis (NRG) receives monosynaptic input from the superior colliculus (SC) and projects directly to neck motor neuron pools. Neurons in NRG are well situated to play a critical role in transforming SC signals into head movement commands. A previous study of movements evoked by NRG stimulation in the primate reported a variety of ipsilateral and contralateral head movements with horizontal, vertical and torsional components. In addition to head movements, it was reported that NRG stimulation could evoke movements of the pinnae, face, upper torso, and co-contraction of neck muscles. In this report, the role of the rhesus monkey NRG in head movement control was investigated using electrical stimulation of the rostral portion of the NRG. The goal was to characterize head movements evoked by NRG stimulation, describe the effects of altering stimulation parameters, and assess the relative movements of the eyes and head. Results indicate that electrical stimulation in the rostral portion of the NRG of the primate can consistently evoke ipsilateral head rotations in the horizontal plane. Head movement amplitude and peak velocity depend upon stimulation parameters (primarily frequency and duration of stimulation trains). During stimulation-induced head movements the eyes counter-rotate (presumably a result of the vestibulo-ocular reflex: VOR). At 46 stimulation sites from two subjects the average gain of this counter-rotation was -0.38 (+/-0.18). After the end of the stimulation train the head generally continued to move. During this epoch, after electrical stimulation ceased, VOR gain remained at this reduced level. In addition, VOR gain was similarly low when electrical stimulation was carried out during active fixation of a visual target. These data extend existing descriptions of head movements evoked by electrical stimulation of the NRG, and add to the understanding of the role of this structure in producing head movements.

Animals↗

PubMed2004

Electrical stimulation of rhesus monkey nucleus reticularis gigantocellularis. II. Effects on metrics and kinematics of ongoing gaze shifts to visual targets.

Saccade kinematics are altered by ongoing head movements. The hypothesis that a head movement command signal, proportional to head velocity, transiently reduces the gain of the saccadic burst generator (Freedman 2001, Biol Cybern 84:453-462) can account for this observation. Using electrical stimulation of the rhesus monkey nucleus reticularis gigantocellularis (NRG) to alter the head contribution to ongoing gaze shifts, two critical predictions of this gaze control hypothesis were tested. First, this hypothesis predicts that activation of the head command pathway will cause a transient reduction in the gain of the saccadic burst generator. This should alter saccade kinematics by initially reducing velocity without altering saccade amplitude. Second, because this hypothesis does not assume that gaze amplitude is controlled via feedback, the added head contribution (produced by NRG stimulation on the side ipsilateral to the direction of an ongoing gaze shift) should lead to hypermetric gaze shifts. At every stimulation site tested, saccade kinematics were systematically altered in a way that was consistent with transient reduction of the gain of the saccadic burst generator. In addition, gaze shifts produced during NRG stimulation were hypermetric compared with control movements. For example, when targets were briefly flashed 30 degrees from an initial fixation location, gaze shifts during NRG stimulation were on average 140% larger than control movements. These data are consistent with the predictions of the tested hypothesis, and may be problematic for gaze control models that rely on feedback control of gaze amplitude, as well as for models that do not posit an interaction between head commands and the saccade burst generator.

Animals↗

PubMed2003

Visual search is facilitated by scene and sequence familiarity in rhesus monkeys.

Whether searching for targets in a familiar scene leads to improved performance was tested in monkeys. We found that search performance improved for a familiar scene when target locations were always randomized. However, when target locations repeatedly followed a predictable sequence, performance improvement for a familiar scene was manifested only for targets presented in a familiar sequence, suggesting that scene memory might be masked by the learning of target sequences. These results suggest that information about a visual scene can facilitate the performance of visual search, and that this memory is coupled to the learned sequence of target locations.

Animals↗

PubMed2003

Activity in the supplementary motor area related to learning and performance during a sequential visuomotor task.

Monkeys were trained in a serial reaction time task to produce hand movements according to changing locations of visual targets. In most trials, targets followed the same sequence repeatedly, whereas in other trials targets were presented in random locations or switched unpredictably between two alternative sequences. Single-unit activity was recorded from the caudal supplementary motor area (SMA-proper). Based on the activity associated with random movement sequences, effects of hand position and movement direction were evaluated. Activity was influenced by the hand position in ~60% of the neurons, and the movement direction influenced the activity of 51% of the neurons. In addition, 37 and 71% of SMA neurons displayed nonstationarity in their activity across successive movements within a given trial and across trials, respectively. Such nonstationarity in the ongoing neural activity and the effects of performance-related variables were evaluated using a regression model and separated from learning-related activity changes. About a third of SMA neurons displayed gradual changes in neural activity related to experience with a movement sequence across trials. Furthermore, about a quarter of SMA neurons showed similar changes within individual trials. When the individual movements included in the frequently repeated movement sequences were introduced unexpectedly, learning-related changes in neural activity were reduced, indicating that many SMA neurons changed their activity in relation to the learning of particular movement sequences. These results suggest that the pattern of neural activity in the cortical network involved in the control of movement sequences can be modified continuously by experience.

Animals↗