Biomedical subjects

F Bremmer

Publications and source records attributed to F Bremmer.

25 records · Page 2Linked to original sources

PubMed1997

Visual responses of neurons from areas V1 and MT in a monkey with late onset strabismus: a case study.

One adult monkey (Macaca fascicularis) was investigated psychophysically and electrophysiologically after at least 5 years of late onset esotropic macrostrabismus (squint angle 52 deg). Behavioural tests revealed normal monocular visual and visuomotor functions. No indications of deep amblyopia or oculomotor asymmetry were found. The monkey used the left or right eye alternately at about equal frequencies. Single unit recordings from area VI disclosed a normal ocular dominance distribution. Most VI neurons from both hemispheres received binocular input. Thus, discordant visual information from corresponding retinal locations of the two eyes converged onto the cortical neurons. No evidence for anomalous retinal correspondence was found. Diplopia and confusion must therefore be avoided by suppression of vision through one eye to allow stable, unambiguous perception. Possible suppression was investigated by stimulating a neuron through the same eye when it was actively used for fixation in one set of trials, and when it was not used for fixation in another set of trials. Significant differences in these two stimulus conditions were found in 20/39 neurons from area VI and in 11/34 motion sensitive neurons recorded in the middle superior temporal area (MT). The normalized population activity in VI and MT was higher if cells were stimulated through the fixating eye. The data are discussed with respect to possible suppressive mechanisms helping to prevent double vision in strabismus and in binocular rivalry.

Action Potentials↗

PubMed1997

Eye position effects in monkey cortex. I. Visual and pursuit-related activity in extrastriate areas MT and MST.

We studied the effect of eye position on visual and pursuit-related activity in neurons in the superior temporal sulcus of the macaque monkey. Altogether, 109 neurons from the middle temporal area (area MT) and the medial superior temporal area (area MST) were tested for influence of eye position on their stimulus-driven response in a fixation paradigm. In this paradigm the monitored eye position signal was superimposed onto the stimulus control signal while the monkey fixated at different locations on a screen. This setup guaranteed that an optimized stimulus was moved across the receptive field at the same retinal location for all fixation locations. For 61% of the MT neurons and 82% of the MST neurons the stimulus-induced response was modulated by the position of the eyes in the orbit. Directional selectivity was not influenced by eye position. One hundred sixty-eight neurons exhibited direction-specific responses during smooth tracking eye movements and were tested in a pursuit paradigm. Here the monkey had to track a target that started to move in the preferred direction with constant speed from five different locations on the screen in random order. Pursuit-related activity was modulated by eye position in 78% of the MT neurons as well as in 80% of the MST neurons tested. Neuronal activity varied linearly as a function of both horizontal and vertical eye position for most of the neurons tested in both areas, i.e., two-dimensional regression planes could be approximated to the responses of most of the neurons. The directions of the gradients of these regression planes correlated neither with the preferred stimulus direction tested in the fixation paradigm nor with the preferred tracking direction in the pursuit paradigm. Eighty-six neurons were tested with both the fixation and the pursuit paradigms. The directions of the gradients of the regression planes fit to the responses in both paradigms tended to correlate with each other, i.e., for more than two thirds of the neurons the angular difference between both directions was less than +/-90 degrees. The modulatory effect of the position of the eyes in the orbit proved to balance out at the population level for neurons in areas MT and MST, tested with the fixation as well as the pursuit paradigm. Results are discussed in light of the hypothesis of an ongoing coordinate transformation of the incoming sensory signals into a nonretinocentric representation of the visual field.

Animals↗

PubMed1997

Eye position effects in monkey cortex. II. Pursuit- and fixation-related activity in posterior parietal areas LIP and 7A.

We studied the effect of eye position on pursuit-related discharges and activity during fixation in darkness for neurons of monkey visual cortical areas (lateral intraparietal area) LIP and 7A. In a first step, neurons were tested for direction-specific activity related to pursuit eye movements while the monkey tracked a moving target. In consecutive trials the pursuit target moved in random order in one of four directions on a translucent screen. For 39% of the neurons, located mostly in a dorsoposterior region of area LIP, as well as 42% of the neurons tested in area 7A, a direction-specific pursuit-related activity could be found. To test whether responsiveness of these neurons was modulated by eye position, we employed a pursuit paradigm. In this paradigm, the monkey had to track a target that started to move in the preferred direction with constant speed from five different locations on the screen in random order. For the majority of cells in both areas, pursuit-related activity was modulated by eye position. Most of the neurons tested also revealed an influence of eye position on their spontaneous activity during fixation in darkness (fixation paradigm). For the majority of cells (> 50%) recorded in both areas, two-dimensional regression planes could be approximated significantly (P < 0.05) or nearly significantly (P < 0.1) to the neuronal discharges observed on the fixation paradigm and pursuit paradigm. For 79% of the LIP neurons and 83% of the 7A neurons tested in both experimental paradigms, the directions of the gradients of the regression planes pointed into the same hemifield, suggesting a common neuronal mechanism mediating the eye position effect regardless of the behavioral task the monkey was performing. The observed effects very much resemble the effects of eye position on light-sensitive and saccade-related responses already described for areas LIP and 7A. Regarding also our results observed for the middle temporal and medial superior temporal areas, it is suggested that the observed modulatory effect of eye position on neuronal activity is a common phenomenon in the macaque visual cortical system subserving an internal representation of the external space in a nonretinocentric frame of reference.

Analysis of Variance↗

PubMed1996

Optic flow processing in monkey STS: a theoretical and experimental approach.

How does the brain process visual information about self-motion? In monkey cortex, the analysis of visual motion is performed by successive areas specialized in different aspects of motion processing. Whereas neurons in the middle temporal (MT) area are direction-selective for local motion, neurons in the medial superior temporal (MST) area respond to motion patterns. A neural network model attempts to link these properties to the psychophysics of human heading detection from optic flow. It proposes that populations of neurons represent specific directions of heading. We quantitatively compared single-unit recordings in area MST with single-neuron simulations in this model. Predictions were derived from simulations and subsequently tested in recorded neurons. Neuronal activities depended on the position of the singular point in the optic flow. Best responses to opposing motions occurred for opposite locations of the singular point in the visual field. Excitation by one type of motion is paired with inhibition by the opposite motion. Activity maxima often occur for peripheral singular points. The averaged recorded shape of the response modulations is sigmoidal, which is in agreement with model predictions. We also tested whether the activity of the neuronal population in MST can represent the directions of heading in our stimuli. A simple least-mean-square minimization could retrieve the direction of heading from the neuronal activities with a precision of 4.3 degrees. Our results show good agreement between the proposed model and the neuronal responses in area MST and further support the hypothesis that area MST is involved in visual navigation.

Animals↗

PubMed1996

Neuronal responses in the motion pathway of the macaque monkey to natural optic flow stimuli.

Neurones in higher visual motion areas in the superior temporal sulcus (STS) of the macaque monkey respond to abstract random dot optic flow stimuli. Higher motion areas may not only represent, but in a next computational stage also analyse the flow field to determine, for instance, the direction of heading for navigation purposes. Real world visual scenes differ in several aspects from these abstract optic flow stimuli. We tested the neuronal response to naturalistic optic flow stimuli which simulated egomotion in different virtual environments and contained different numbers of visual cues. Neuronal activity depended mainly on the position of the focus of expansion rather than on other visual cues. This finding supports the hypothesis that higher motion areas within the STS analyse optic flow in natural scenes and can thus signal the direction of heading.

Animals↗

PubMed1996

Monkey saccadic latency and pursuit velocity show a preference for upward directions of target motion.

Saccadic latency was studied as a function of the direction of sudden target displacements (steps) and of subsequent smooth target motion (ramps) in Macaca fascicularis. The monkey fixated a central spot that suddenly changed its position and then moved constantly at 10 deg s-1, thus eliciting initial saccades and subsequent pursuit eye movements (recorded by a magnetic search-coil technique). Latencies for initial saccades differed markedly in the vertical axis, being shorter in upward than downward directions for both step and ramp components of target motion. Saccadic latency was also related to the mean pursuit velocity, indicating that the oculomotor system accounts for the direction of step and ramp components of target motion in an integrative way.

Animals↗

PubMed1993

Optokinetic and pursuit system: a case report.

Several studies have demonstrated the importance of the pretectal Nucleus of the Optic Tract (NOT) and the Dorsal Terminal Nucleus of the accessory optic system (DTN) for the generation of horizontal optokinetic nystagmus (OKN). Although single unit data from trained rhesus monkey NOT/DTN cells are available it is still unclear if there is a link between the pursuit and the optokinetic system at this level of motion analysis. In order to address the question whether the NOT/DTN is important for the optokinetic as well as the pursuit system an electrolytic lesion was placed where NOT/DTN activity was recorded previously. The monkey was tested on optokinetic and pursuit paradigms. Immediately following the lesion the monkey performed a spontaneous nystagmus with slow phases directed away from the lesioned side. This spontaneous nystagmus persisted even during optokinetic stimulation in the opposite direction. During the first week postlesion the spontaneous nystagmus disappeared and the monkey regained the ability to perform optokinetic nystagmus toward the lesioned side. The gain of the mean slow phase eye velocity was, however, largely reduced for this stimulus direction. The onset of OKN following the onset of optokinetic stimulation was not affected by the lesion. During smooth pursuit the mean eye velocity was more reduced for pursuit towards the lesioned side. The resulting position error was compensated by an increase in the number of catch-up saccades. In addition to the confirmation of the well-known directional deficits of the optokinetic system caused by a lesion of the pretectum, a directional deficit in the pursuit system was demonstrated.

Animals↗