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K P Hoffmann

Publications and source records attributed to K P Hoffmann.

At least 19 recordsLinked to original sources
PubMed2001

Morphological changes in the neuronal substrate for the optokinetic reflex in albino ferrets.

Albino mammals show very characteristic deficits in their optokinetic system, and albino ferrets are even optokinetically blind. To investigate the neuronal causes for this defect we compared the morphology of retinal slip cells in the pretectal nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOT-DTN) in pigmented and albino ferrets (Mustela putorius furo) using retrograde tracing techniques. After tracer injections into the inferior olive, equal numbers of NOT-DTN neurons were retrogradely labelled in pigmented and albino animals. However, NOT-DTN cells in albino ferrets had fewer stem dendrites, and the cumulative dendritic length was reduced by 30% when compared with NOT-DTN neurons in pigmented animals. In addition, the prominent network formed by distal dendrites observed in the NOT-DTN of pigmented ferrets was largely diminished in albinos. Taken together with behavioural and physiological data, these findings indicate that the NOT-DTN as the main visuomotor interface in the optokinetic system is clearly defective in albino ferrets.

Albinism↗
PubMed2001

Directional effect of inactivation of the nucleus of the optic tract on optokinetic nystagmus in the cat.

The goal of the present investigation was to elucidate the role of the nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOT-DTN) for slow eye movements other than horizontal. Retinal slip neurons in the NOT-DTN in the awake behaving cat respond direction selectively to the ipsiversive component of horizontal and oblique image motion. They are, however, influenced neither by pure vertical stimulus movement nor by eye movements in the dark. Electrical stimulation of the NOT-DTN leads to pure horizontal optokinetic nystagmus with ipsiversive slow phases and does not influence vertical eye position. In addition, unilateral reversible inactivation of the NOT-DTN with muscimol elicits spontaneous contraversive horizontal nystagmus without vertical component. During oblique optokinetic stimulation, the ipsiversive OKN component is significantly decreased in all directions. After bilateral NOT-DTN inactivation, OKN can only be elicited in a narrow range of upward directions. These data indicate that the NOT-DTN is the only source to drive the horizontal component of OKN.

Animals↗
PubMed2001

A possible role of the superior colliculus in eye-hand coordination.

Reaching with the arm to a newly appearing target is usually preceded by a saccadic eye movement. Neurons in the superior colliculus (SC) constitute one important brain structure controlling saccades. Yet, the SC also contains reach neurons activated during arm movements, whose location extends also deeper into the underlying mesencephalic reticular formation. Reach neurons can be divided into two classes based on their different modulation with respect to gaze position. For the first class, the gaze-independent reach neurons, the activity does not depend on which location is currently fixated, but solely on the position and movement of the (usually contralateral) arm. There is a correlation of the activity of these neurons with the activity of shoulder muscles. The second class, the gaze-related reach neurons, are active for reaches into a specific area relative to the current point of gaze. This means the target has to project on a certain part of the retina, while it is not important which arm is used or by which trajectory the target will be reached. Many fixation neurons in the rostral pole of the SC discharge tonically during fixation and pause during saccades. For some fixation neurons, the activity can be increased during simultaneous arm movements, for others decreased. Two psychophysical experiments with healthy human subjects show possible behavioral correlates of an interaction between these reach neurons and visuomotor neurons within the SC.

Animals↗
PubMed2001

Cortical input to the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) in macaques: a retrograde tracing study.

Using retrograde tracing methods, we investigated the cortical projection to the nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system (NOT-DTN) in macaque monkeys. Tracer injections at electrophysiologically identified recording sites in the NOT-DTN resulted in retrogradely labelled neurons in layer V of various cortical areas. The strongest projection always arose from the middle temporal area (MT) and the adjoining cortex anterior to MT in the superior temporal sulcus. A less dense projection came from the middle superior temporal area (MST). In addition, retrogradely labelled cells were consistently found in areas V1 and V2 at moderate to high density. Furthermore, sparse to moderate labelling occurred in prestriate area V3. These findings were compared with the label resulting from control injections into the superior colliculus in two additional cases. Our results indicate that the cortical input to the NOT-DTN as the sensorimotor interface for the pathway subserving stabilizing eye movements during the optokinetic reflex and smooth pursuit mainly arises from the motion-sensitive areas MT and MST in the superior temporal sulcus, as well as from areas V1 and V2. Clearly the projection to the NOT-DTN does not arise from a single cortical area.

Animals↗
PubMed2000

Retinal ganglion cells projecting to the nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system in macaque monkeys.

Using classical neuroanatomical retrograde tracing methods we investigated the retinal ganglion cells projecting to the nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system (NOT-DTN) in macaque monkeys. Our main aim was to quantify the strength of the projection from the ipsilateral retina to the NOT-DTN. We therefore examined the number, distribution, and soma size of retinal ganglion cells involved in this projection. Electrophysiologically controlled small injections into the NOT-DTN revealed a clearly bilateral retinal projection originating mainly from the central retina but also involving peripheral retinal regions. Labelled cells were found nasally in the contralateral retina and temporally in the ipsilateral retina with some overlap in the fovea. The projection from the ipsilateral retina was 36-43% of that from the contralateral retina. On average, only 1-6% of the local population of ganglion cells projected to the NOT-DTN. Small soma size and large dendritic fields imply that in monkey rarely encountered, 'specialized' ganglion cells provide the direct retinal input to the accessory optic system (AOS). These results are discussed with respect to the symmetry of monocular horizontal optokinetic nystagmus (OKN) in primates.

Animals↗
PubMed2000

Influence of arm movements on saccades in humans.

When reaching for an object we usually look at it before we touch it with the hand. This often unconscious eye movement prior to the arm movement allows guiding of the final part of the hand trajectory by visual feedback. We examined the temporal and spatial coordination of this control system by psychophysical measurements of eye and arm movements of naive human subjects looking or looking and pointing as fast as possible to visual targets in physical and virtual-reality setups. The reaction times of saccades to a step-displaced target were reduced, and the number of corrective saccades decreased, when the subject had to produce a corresponding simultaneous hand movement to the same target. The saccadic reaction time was increased when saccade and hand movement went in opposite directions. In a double-step task the reaction time for the second saccade was longer than for the first. Co-use of the hand leads to an additional increase of saccadic reaction time. Taken together this study shows an improvement in initial saccades if they are accompanied by hand movements to the same target. This effect might ensure that the reach target is foveated early and accurately enough to support the visual feedback control of the hand near the target. Longer reaction times for the second saccade to double-step displaced targets might reflect a saccadic refractory time intensified by simultaneous arm movements. These results are discussed in the light of recent findings from our laboratory on saccade- and reach-related neurons in the superior colliculus of macaque monkeys.

Adult↗
PubMed2000

Neurons in the primate superior colliculus coding for arm movements in gaze-related coordinates.

In the intermediate and deep layers of the superior colliculus (SC), a well-established oculomotor structure, a substantial population of cells is involved in the control of arm movements. To examine the reference frame of these neurons, we recorded in two rhesus monkeys (Macaca mulatta) the discharges of 331 neurons in the SC and the underlying mesencephalic reticular formation (MRF) while monkeys reached to the same target location during different gaze orientations. For 65 reach-related cells with sufficient data and for simultaneously recorded electromyograms (EMGs) of 11 arm muscles, we calculated an ANOVA (factors: target position, gaze angle) and a gaze-dependency (GD) index. EMGs and the activity of many (60%) of the reach-related neurons were not influenced by the target representation on the retina or eye position. We refer to these as "gaze-independent" reach neurons. For 40%, however, the GD fell outside the range of the muscle modulation, and the ANOVA showed a significant influence of gaze. These "gaze-related" reach neurons discharge only when the monkey reaches for targets having specific coordinates in relation to the gaze axis, i.e., for targets in a gaze-related "reach movement field" (RMF). Neuronal activity was not modulated by the specific path of the arm movement, the muscle pattern that is necessary for its realization or the arm that was used for the reach. In each SC we found gaze-related neurons with RMFs both in the contralateral and in the ipsilateral hemifield. The topographical organization of the gaze-related reach neurons in the SC could not be matched with the well-known visual and oculomotor maps. Gaze-related neurons were more modulated in their strength of activity with different directions of arm movements than were gaze-independent reach neurons. Gaze-related reach neurons were recorded at a median depth of 2.03 mm below SC surface in the intermediate layers, where they overlap with saccade-related burst neurons (median depth: 1.55 mm). Most of the gaze-independent reach cells were found in a median depth of 4.01 mm below the SC surface in the deep layers and in the underlying MRF. The gaze-related reach neurons operating in a gaze-centered coordinate system could signal either the desired target position with respect to gaze direction or the motor error between gaze axis and reach target. The gaze-independent reach neurons, possibly operating in a shoulder- or arm-centered reference frame, might carry signals closer to motor output. Together these two types of reach neurons add evidence to our hypothesis that the SC is involved in the sensorimotor transformation for eye-hand coordination in primates.

Algorithms↗
PubMed2000

Responses to continuously changing optic flow in area MST.

We studied the temporal behavior and tuning properties of medial superior temporal (MST) neurons in response to constant flow-field stimulation and continuously changing flow-field stimulation (transitions), which were obtained by morphing one flow field into another. During transitions, the flow fields resembled the motion pattern seen by an observer during changing ego-motion. Our aim was to explore the behavior of MST cells in response to changes in the flow-field pattern and to establish whether the responses of MST cells are temporally independent or if they are affected by contextual information from preceding stimulation. We first tested whether the responses obtained during transitions were linear with respect to the two stimuli defining the transition. In over half of the transitions, the cell response was nonlinear: the response during the transition could not be predicted by the linear interpolation between the stimulus before and after the transition. Nonlinearities in the responses could arise from a dependence on temporal context or from nonlinearities in the tuning to flow-field patterns. To distinguish between these two hypotheses, we fit the responses during transitions and during continuous stimuli to the predictions of a temporally independent model (temporal-independence test) and we compared the responses during transitions to the responses elicited by inverse transitions (temporal-symmetry test). The effect of temporal context was significant in only 7.2% and 5.5% of cells in the temporal-independence test and in the temporal-symmetry test, respectively. Most of the nonlinearities in the cell responses could be accounted for by nonlinearities in the tuning to flow-field stimuli (i.e., the responses to a restricted set of flow fields did not predict the responses to other flow fields). Tuning nonlinearities indicate that a complete characterization of the tuning properties of MST neurons cannot be obtained by testing only a small number of flow fields. Because the cells' responses do not depend on temporal context, continuously changing stimulation can be used to characterize the receptive field properties of cells more efficiently than constant stimulation. Temporal independence in the responses to transitions indicates that MST cells do not code for second-order temporal properties of flow-field stimuli, i.e., for changes in the flow field through time that can be construed as paths through the environment. Information about ego-motion three-dimensional paths through the environment may either be processed at the population level in MST or in other cortical areas.

Action Potentials↗
PubMed1999

Linear vestibular self-motion signals in monkey medial superior temporal area.

The present study was aimed at investigating the sensitivity to linear vestibular stimulation of neurons in the medial superior temporal area (MST) of the macaque monkey. Two monkeys were moved on a parallel swing while single-unit activity was recorded. About one-half of the cells (28/51) responded in the dark either to forward motion (n = 10), or to backward motion (n = 11), or to both (n = 7). Twenty cells responding to vestibular stimulation in darkness were also tested for their responses to optic flow stimulation simulating forward and backward self-motion. Forty-five percent (9/20) of them preferred the same self-motion directions, that is, combined visual and vestibular signals in a synergistic manner. Thirty percent (6/20) of the cells were not responsive to visual stimulation alone. The remaining 25% (5/20) preferred directions that were antialigned. Our results provide strong evidence that neurons in the MST area are at least in part involved in the processing of self-motion.

Animals↗
PubMed1999

Ocular responses to radial optic flow and single accelerated targets in humans.

Self-movement in a structured environment induces retinal image motion called optic flow. Optic flow on one hand provides information about the direction of self-motion. On the other hand optic flow presents large field visual motion which will elicit eye movements for the purpose of image stabilization. We investigated oculomotor behavior in humans during the presentation of radial optic flow fields which simulated forward or backward self-motion. Different conditions and oculomotor tasks were compared. In one condition, subjects had to actively pursue single dots in a radial flow pattern. In a second condition, subjects had to pursue single dots over a dark background. These dots accelerated or decelerated similar to single dots in radial optic flow. In a third condition, subjects were asked to passively view the entire optic flow stimulus. Smooth pursuit eye movements with high gain were observed when dots were actively pursued. This was true for single dots moving over a homogeneous background and for single dots in the optic flow. Passive viewing of optic flow stimuli evoked eye movements that resembled an optokinetic nystagmus. Slow phase eye movements tracked the motion of elements in the optic flow. Gain was low for simulated forward self-motion (expanding optic flow) and high for simulated backward movement self-motion (contracting optic flow). Thus, voluntary pursuit and passive optokinetic responses yielded different gain for the tracking of elements of an expanding optic flow pattern. During passive viewing of the optic flow stimulus, gaze was usually at or near the focus of radial flow. Our results give insights into the oculomotor performances and needs for image stabilization during self-motion and in the role of gaze strategy for the detection of the direction of heading.

Eye Movements↗
PubMed1999

Development of the optokinetic system in macaque monkeys.

Optokinetic nystagmus in response to horizontal movement of a whole field random dot pattern was measured in infant macaque monkeys from the first week to about 5 months after birth using electrooculography. During monocular and binocular viewing conditions stimulus velocities were varied between 10 and 120 degrees/s. Monocular stimulation in the temporonasal direction yielded slow phase gain of the optokinetic system which was relatively constant for a given stimulus velocity over the whole period of observation. Gain during nasotemporal stimulation was also clearly present but significantly lower at early stages and increased during further development. This asymmetry of monocular horizontal optokinetic nystagmus (OKN) clearly depended on the stimulus velocity. At lower stimulus velocities (10-20 degrees/s) OKN was largely symmetrical at 2-5 weeks of age. At higher stimulus velocities (40 degrees/s) symmetry was reached at about 12 weeks of age or even much later (80-120 degrees/s).

Aging↗
PubMed1999

Decision-related activity in the macaque dorsal visual pathway.

Brain areas at higher levels of cortical organization are thought to be more involved in decision processes than are earlier, i.e. lower, sensory areas. Hence, neuronal activity correlated with decisions should vary with an area's position in the cortical hierarchy. To test this proposal, we investigated whether a change in neuronal activity during error trials depends in a systematic way on cortical hierarchical position. While macaque monkeys discriminated the direction of moving visual stimuli, the activity of direction-selective neurons was recorded in four extrastriate visual areas: V3A, the middle temporal area, the middle superior temporal area and the posterior part of the superior temporal polysensory area. Neuronal activity was significantly reduced in all areas when the monkeys made errors in judging the direction of stimuli moving in the preferred direction with low and intermediate luminance contrast. The amount of activity reduction was approximately 50% in all of the visual areas. Thus, the activity on error trials is reduced in early visual processing, independent of the hierarchy in the dorsal visual pathway. The activity reduction depended on stimulus contrast and the direction of the decision relative to the stimulus motion. It was profound and significant in all areas at low stimulus contrast. However, it was nonsignificant at high stimulus contrast. Our data suggest that activity reduction on error trials is due to lack of attention in association with stimulus expectation.

Animals↗
PubMed1999

[Animal models for the optokinetic system of the human].

BACKGROUND: We compare the neuronal and behavioural consequences of abnormal visual experience during early infancy for the optokinetic system of cat and monkey with the neuroopthalmological results in man. MATERIAL AND METHODS: Optokinetic eye movements were recorded with the search coil method and electrooculography. In addition, the response properties of single neurons in the visual cortex and pretectum of anesthetized and paralyzed cats and monkeys were determined in electrophysiological experiments. RESULTS: Our data show that monocular deprivation and strabismus lead to an increase of asymmetry of monocular optokinetic nystagmus and a decrease in gain of slow phase optokinetic eye movements during monocular stimulation as well as to a significant decrease of binocular convergence in the oculomotor system of both cat and monkey. Although these changes are basically similar in both species they are not identical. These differential effects may be explained by structural differences. CONCLUSIONS: Assuming a similar neuronal substrate for the optokinetic system in all primates the monkey represents a more suitable animal model for the human visual system than the cat.

Animals↗
PubMed1999

Correlation of primate superior colliculus and reticular formation discharge with proximal limb muscle activity.

We studied the discharge of neurons from both the superior colliculus (SC) and the underlying mesencephalic reticular formation (MRF) and its relation to the simultaneously recorded activity of 11 arm muscles. The 242 neurons tested with a center-out reach task yielded 2,586 pairs of neuron/muscle cross-correlations (normalized, such that perfect correlations are +/-1.0). Of these, 43% had peaks with magnitude as large as 0.15, a value that corresponds to the 5% level of significance, and 16% were as large as 0.25. The great majority of peaks in this latter group was positive. The median lag time within this group was 52 ms, indicating that the neuronal discharge tended to precede the correlated muscle activity. We found a small but significantly higher proportion of cells with these relatively strong correlations in the MRF than in the SC. For both areas, these occurred most frequently with muscles of the shoulder girdle and became less frequent for axial as well as for increasingly distal arm musculature. The results support a role for the SC and MRF in guiding the arm during reach movements via the control of proximal limb musculature.

Action Potentials↗
PubMed1998

Correlation of electrophysiology, morphology, and functions in corticotectal and corticopretectal projection neurons in rat visual cortex.

In most mammals the superior colliculus (SC) and the pretectal nucleus of the optic tract (NOT) receive direct input from the ipsilateral visual cortex via projection neurons from infragranular layer V. We examined whether these projection neurons belong to different populations and, if so, whether it is possible to correlate the electrophysiological features with the suggested function of these neurons. Projection cells were retrogradely labeled in vivo by rhodamine-coupled latex beads or fast blue injections into the SC or the NOT 2-5 days prior to the electrophysiological experiment. Intracellular recordings of prelabeled neurons were made from standard slice preparations and cells were filled with biocytin in order to reveal their morphology. Both cell populations consist of layer V pyramids with long apical dendrites that form terminal tufts in layer I. In electrophysiological terms, 12 of the corticotectal cells could be classified as intrinsically bursting (IB), while two neurons showed a doublet firing characteristic and one neuron was classified as regular-spiking (RS). Intracortical microstimulation of cortical layer II/III revealed that SC-projecting neurons responded optimally to stimulation sites up to a distance of 1000 microm from the recorded cell. The morphological features of the SC-projecting cells reveal an apical dendritic tuft in layer I with a lateral extension of 300 microm, a mean spine density of 65 spines per 40 microm on the apical dendrites located in layer II/III, and a bouton density of 13 boutons per 100 microm on the intracortical axons. Sixteen NOT-projecting neurons exhibited an IB and five cells an RS characteristic. Intracortical microstimulation of cortical layer II/III showed that NOT-projecting neurons responded optimally to stimulation sites up to a distance of 1500 microm. Their morphological features consist of an apical dendritic tuft with a lateral extension of 500 microm, a mean spine density of 25 spines per 40 microm on the apical dendrites located in layer II/III, and a bouton density of 6 boutons per 100 microm on the intracortical axons. When the passive membrane parameters, responses to intracortical microstimulation in layer V, the extension of the basal dendritic field, and spine densities in layers I or V were compared between SC- and NOT-projecting cells, no differences were revealed. Differences were only consistently found in the supragranular layers, either for morphological parameters or for intracortical microstimulation. The results suggest that NOT-projecting and SC-projecting neurons, although biophysically similar, could integrate and transmit different spatial aspects of cortical visual information to their target structures.

Action Potentials↗
PubMed1998

Eye position encoding in the macaque posterior parietal cortex.

In two previous studies, we had demonstrated the influence of eye position on neuronal discharges in the middle temporal area, medial superior temporal area, lateral intraparietal area and area 7A of the awake monkey (Bremmer et al., 1997a,b). Eye position effects also have been found in visual cortical areas V3A and V6 and even in the premotor cortex and the supplementary eye field. These effects are generally discussed in light of a coordinate transformation of visual signals into a non-retinocentric frame of reference. Neural network studies dealing with the eye position effect succeeded in constructing such non-retinocentric representations by using model neurones whose response characteristics resembled those of 'real' neurones. However, to our knowledge, response properties of real neurones never acted as input into these neural networks. In the present study, we thus investigated whether, theoretically, eye position could be estimated from the population discharge of the (previously) recorded neurones and, if so, we intended to develop an encoding algorithm for the position of the eyes in the orbit. The optimal linear estimator proved the capability of the ensemble activity for determining correctly eye position. We then developed the so-called subpopulation encoding of eye position. This algorithm is based on the partition of the ensemble of neurones into two pairs of subpopulations. Eye position is represented by the differences of activity levels within each pair of subpopulations. Considering this result, encoding of the location of an object relative to the head could easily be accomplished by combining eye position information with the intrinsic knowledge about the retinal location of a visual stimulus. Taken together, these results show that throughout the monkey's visual cortical system information is available which can be used in a fairly simple manner in order to generate a non-retinocentric representation of visual information.

Animals↗
PubMed1998

Optokinetic reflex in squirrel monkeys after long-term monocular deprivation.

Horizontal optokinetic nystagmus (OKN) as well as neuronal response properties in the nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOT-DTN) were investigated in three monocularly deprived squirrel monkeys. In two monkeys occlusion of one eye was performed at birth (early) and in the third after 7 weeks (late). In adulthood, in early deprived monkeys monocular horizontal OKN tested through the non-deprived eye was symmetrical and in no way different from normal, i.e. stimulation in the temporonasal and nasotemporal direction elicited equal and robust responses. OKN through the early occluded eye, however, was grossly abnormal with low gain and great variability in the consistency of nasotemporal and temporonasal slow phase eye movements. When in the late deprived monkey the non-deprived eye was occluded a strong spontaneous nystagmus developed despite the deprived eye viewing a stationary pattern. The slow phases were directed from nasal to temporal for the deprived eye. When tested through the non-deprived eye all neuronal responses of the NOT-DTN were normal. The deprived eye's influence on NOT-DTN neurons was extremely weak. No neuron with a moderate or even dominant input from the deprived eye was found after early deprivation. In the late deprived case the deficit was not as severe but still the non-deprived eye was clearly dominating the responses in all neurons tested. Velocity tuning of neurons tested through the non-deprived eye was normal and qualitatively corresponded well to slow phase eye velocity in response to equivalent retinal slip during OKN. Through the early deprived eye, however, velocity tuning was extremely poor. It was somewhat better through the late deprived eye. We suggest that the dramatic deterioration in the optokinetic reflex found after long-term monocular deprivation for the amblyopic eye is probably caused by the almost complete loss of retinal and cortical input driven by that eye to the NOT-DTN. These results are discussed in relation to our previous results in cats and reports in the literature for humans with occlusion amblyopia.

Animals↗