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Biomedical subjects

J M Groh

Publications and source records attributed to J M Groh.

13 recordsLinked to original sources
PubMed2001

Converting neural signals from place codes to rate codes.

The nervous system uses two basic types of formats for encoding information. The parameters of many sensory (and some premotor) signals are represented by the pattern of activity among an array of neurons each of which is optimally responsive to a different parameter value. This type of code is commonly referred to as a place code. Motor commands, in contrast, use rate coding: the desired force of a muscle is specified as a monotonic function of the aggregate rate of discharge across all of its motor neurons. Generating movements based on sensory information often requires converting signals from a place code to a rate code. In this paper I discuss three possible models for how the brain does this.

Animals↗
PubMed2001

Afferent delays and the mislocalization of perisaccadic stimuli.

Determining the precise moment a visual stimulus appears is difficult because visual response latencies vary. This temporal uncertainty could cause localization errors to brief visual targets presented before and during eye movements if the oculomotor system cannot determine the position of the eye at the time the stimulus appeared. We investigated the effect of varying neural processing time on localization accuracy for perisaccadic visual targets that differed in luminance. Although systematic errors in localization were observed, the effect of luminance was surprisingly small. We explore several hypotheses that may explain why processing delays are not more disruptive to localization performance.

Humans↗
PubMed2001

Eye position influences auditory responses in primate inferior colliculus.

We examined the frame of reference of auditory responses in the inferior colliculus in monkeys fixating visual stimuli at different locations. Eye position modulated the level of auditory responses in 33% of the neurons we encountered, but it did not appear to shift their spatial tuning. The effect of eye position on auditory responses was substantial-comparable in magnitude to that of sound location. The eye position signal appeared to interact with the auditory responses in at least a partly multiplicative fashion. We conclude that the representation of sound location in primate IC is distributed and that the frame of reference is intermediate between head- and eye-centered coordinates. The information contained in these neurons appears to be sufficient for later neural stages to calculate the positions of sounds with respect to the eyes.

Acoustic Stimulation↗
PubMed2000

Segregation of object and background motion in visual area MT: effects of microstimulation on eye movements.

To track a moving object, its motion must first be distinguished from that of the background. The center-surround properties of neurons in the middle temporal visual area (MT) may be important for signaling the relative motion between object and background. To test this, we microstimulated within MT and measured the effects on monkeys' eye movements to moving targets. We found that stimulation at "local motion" sites, where receptive fields possessed antagonistic surrounds, shifted pursuit in the preferred direction of the neurons, whereas stimulation at "wide-field motion" sites shifted pursuit in the opposite, or null, direction. We propose that activating wide-field sites simulated background motion, thus inducing a target motion signal in the opposite direction. Our results support the hypothesis that neuronal center-surround mechanisms contribute to the behavioral segregation of objects from the background.

Animals↗
PubMed1998

Neurophysiology: electrically evoking sensory experience.

Monkeys trained to distinguish touch stimuli that 'flutter' with different frequencies can similarly distinguish electrical stimulation of the somatosensory cortex according to its frequency; the implication is that the electrically-evoked patterns of cortical activity cause flutter sensations similar to those induced by touch.

Animals↗
PubMed1997

How is a sensory map read Out? Effects of microstimulation in visual area MT on saccades and smooth pursuit eye movements.

To generate behavioral responses based on sensory input, motor areas of the brain must interpret, or "read out," signals from sensory maps. Our experiments tested several algorithms for how the motor systems for smooth pursuit and saccadic eye movements might extract a usable signal of target velocity from the distributed representation of velocity in the middle temporal visual area (MT or V5). Using microstimulation, we attempted to manipulate the velocity information within MT while monkeys tracked a moving visual stimulus. We examined the effects of the microstimulation on smooth pursuit and on the compensation for target velocity shown by saccadic eye movements. Microstimulation could alter both the speed and direction of the motion estimates of both types of eye movements and could also cause monkeys to generate pursuit even when the visual target was actually stationary. The pattern of alterations suggests that microstimulation can introduce an additional velocity signal into MT and that the pursuit and saccadic systems usually compute a vector average of the visually evoked and microstimulation-induced velocity signals (pursuit, 55 of 122 experiments; saccades, 70 of 122). Microstimulation effects in a few experiments were consistent with vector summation of these two signals (pursuit, 6 of 122; saccades, 2 of 122). In the remainder of the experiments, microstimulation caused either an apparent impairment in motion processing (pursuit, 47 of 122; saccades, 41 of 122) or had no effect (pursuit, 14 of 122; saccades, 9 of 122). Within individual experiments, the effects on pursuit and saccades were usually similar, but the occasional striking exception suggests that the two eye movement systems may perform motion computations somewhat independently.

Animals↗
PubMed1996

Neurophysiology: neural fingerprints of visual attention.

Pronounced effects of attention have been demonstrated in a region of visual cortex previously thought to be devoid of such influences; identifying the features critical for eliciting these effects should teach us a great deal about the neural underpinnings of visual attention.

Animals↗
PubMed1996

Saccades to somatosensory targets. I. behavioral characteristics.

1. We compared the properties of saccades to somatosensory and visual targets. This comparison provides insight into the translation of sensory signals coding target location in different sensory coordinate frameworks into motor commands of a common format. Vibrotactile stimuli were delivered to the hands, which were fixed in position and concealed beneath a barrier. Saccades of different directions and amplitudes were elicited by the same somatosensory target from different initial eye positions. Both monkeys and humans served as subjects. 2. Somatosensory saccades were less accurate than visual saccades in both humans and monkeys. When the barrier concealing the hands was removed, somatosensory saccade accuracy improved. While the hands were concealed, the visual frame of reference provided by room illumination did not greatly affect saccade accuracy: accuracy was not degraded in complete darkness for two of three monkeys. 3. The endpoints of saccades to a single somatosensory target varied with initial eye position for the monkeys, but not for the human subjects. 4. We also found evidence of an effect of limb position on somatosensory saccades: when human subjects performed the task with crossed hands, the incidence of curved saccades increased. Saccades often began in the direction of the unstimulated hand and curved markedly toward the stimulated hand. When one subject was required to delay the saccade by 600-1,000 ms after target onset (the delayed saccade task), the saccades were straight. Somatosensory saccades were also straight when the hands were not crossed. 5. The reaction times of somatosensory saccades were longer than the reaction times of visual saccades, and they decreased as a function of saccade amplitude. The delayed saccade task reduced the differences between somatosensory and visual saccade reaction times. The reaction times of saccades to very dim visual targets increased into the range found for saccades to somatosensory targets. When the saccade target was the combination of the somatosensory and visual stimuli at the same location, the reaction time was slightly lower than for visual targets alone. 6. The peak velocities of somatosensory saccades were lower than those of visual saccades of the same amplitude. The velocities of saccades to combined somatosensory and visual targets were indistinguishable from those of saccades to visual targets alone. The differences between somatosensory and visual saccade velocity were maintained in the delayed trial type. These differences suggest that the main sequence or velocity-amplitude relationship characteristic of saccades depends on the modality of the target. 7. The implications of these modality-dependent differences in accuracy, reaction time, and saccade velocity are discussed with regard to models of the saccade generator and the coordinate transformation necessary for somatosensory saccades.

Afferent Pathways↗
PubMed1996

Saccades to somatosensory targets. II. motor convergence in primate superior colliculus.

1. We examined cells with saccade-related activity in the superior colliculus (SC) of monkeys performing saccades to both somatosensory and visual targets. Our goals were 1) to determine whether signals from these separate sensory systems have converged onto a common motor pathway by the level of the SC; 2) to determine the frame of reference of somatosensory saccade signals in the SC; and 3) to relate collicular motor activity to the behavioral characteristics of somatosensory saccades. 2. Somatosensory targets consisted of vibrotactile stimuli delivered to the hands, which were held in fixed spatial positions. Saccades of different directions and amplitudes were elicited from different initial eye positions. Of 86 cells with motor-related activity, 85 (99%) discharged for saccades to both visual and somatosensory targets. The remaining cell was active only for visual saccades. 3. Cells with saccade-related activity had movement fields representing the direction and amplitude of saccades to both visual and somatosensory targets. We found no cells that discharged for saccades to a particular somatosensory target regardless of the vector of the saccade. 4. Small modality-dependent differences in the spatial tuning of the movement fields were observed, but these variations formed no clear pattern. Given the large population of cells active in conjunction with each saccade, these small tuning differences may have no net effect. Because the visual and somatosensory movement fields of individual cells were similar to each other, the inaccuracy of somatosensory saccades is likely to be the result of inaccurate signals reaching the SC, rather than an error signal added downstream. 5. The peak discharge frequency of collicular motor cells was lower for somatosensory saccades than for visual saccades, although the number of spikes in the discharge was about the same. 6. The latency of the onset of the prelude of motor activity following the cue to initiate a saccade was about the same for somatosensory and visual trials, even though somatosensory saccades have longer reaction times than visual saccades. However, the peak of the motor activity was delayed on somatosensory trials such that the timing of the peak was the same with respect to the movement on somatosensory and visual trials. 7. We conclude that the same population of saccade-related neurons in the SC that represents saccades to visual targets also represents saccades to somatosensory targets. Somatosensory saccades are encoded by these cells as the change in eye position necessary to bring the target onto the fovea, rather than the location of the stimulus on the body surface. Modality-dependent differences in the frequency and timing of collicular motor activity may contribute to velocity and reaction time differences.

Afferent Pathways↗
PubMed1996

Saccades to somatosensory targets. III. eye-position-dependent somatosensory activity in primate superior colliculus.

1. We recorded from cells with sensory responses to somatosensory stimuli in the superior colliculus (SC) of awake monkeys. Our goal was to determine the frame of reference of collicular somatosensory signals by seeing whether the positions of the eyes influenced the responses of cells to a given tactile stimulus. Somatosensory targets consisted of vibrotactile stimuli delivered to the hands, which were held in fixed spatial positions. Monkeys performed a delayed saccade task from different initial fixation positions to the locations of these tactile stimuli or to visual stimuli at approximately the same location. 2. The responses of a majority of somatosensory cells (25 of 34 or 74%) were significantly affected by eye position. Nearly all somatosensory cells also responded to visual targets (28 of 30, 93%). Cells whose somatosensory responses depended on eye position responded to visual and somatosensory targets located at approximately the same direction in space with respect to the eyes. 3. The activity of these cells exhibited both sensory and motor qualities. The discharge was more closely linked in time to stimulus onset than to the movement. Sensory features of the stimulus were reflected in the responses: the discharge of a number of cells was phase-locked to the pulses of vibration. The sensory responses occurred even if the animal's next saccade was not directed into the response field of the cell. However, two thirds of the cells also exhibited a burst of motor activity in conjunction with the saccade to the somatosensory target. Sensory and motor activity were not always spatially coextensive. When different, the tuning of motor activity was broader. 4. Cells with somatosensory responses to vibratory stimulation of the hands were found in a wide region of the SC, spanning a 40 degrees range of movement amplitudes. 5. These data show that somatosensory signals in the SC are not purely somatotopic but are dependent on eye position. For stimuli at a fixed location, this eye position dependence allows somatosensory and visual signals to be in register and share a premotor circuitry for guiding saccadic eye movements. 6. The dependence of the somatosensory responses on eye position suggests that the somatosensory receptive fields may either shift on the body surface or they may be restricted to a limited region of the body surface but be gated by eye (and body) position. Future experiments varying body position and the location of the stimulus on the body surface are needed to determine which of these alternatives is correct. Cells with either type of receptive field could provide an unambiguous signal of the location of somatosensory saccade targets with respect to the eyes. The transformation of somatosensory signals from a body-centered frame of reference to a frame of reference that depends on the position of the stimulus with respect to the eyes is necessary for the correct activation of collicular neurons with motor activity, because this activity encodes saccades as desired changes in eye position.

Afferent Pathways↗
PubMed1992

Two models for transforming auditory signals from head-centered to eye-centered coordinates.

Two models for transforming auditory signals from head-centered to eye-centered coordinates are presented. The vector subtraction model subtracts a rate-coded eye position signal from a topographically weighted auditory target position signal to produce a rate-code of target location with respect to the eye. The rate-code is converted into a place-code through a graded synaptic weighting scheme and inhibition. The dendrite model performs a mapping of head-centered auditory space onto the dendrites of eye-centered units. Individual dendrites serve as logical comparators of target location and eye position. Both models produce a topographic map of auditory space in eye-centered coordinates like that found in the primate superior colliculus. Either type can be converted into a model for transforming visual signals from retinal to head-centered coordinates.

Auditory Perception↗