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M A Steinmetz

Publications and source records attributed to M A Steinmetz.

15 recordsLinked to original sources
PubMed2001

Neuronal responses in area 7a to multiple-stimulus displays: I. neurons encode the location of the salient stimulus.

The primate posterior parietal cortex (PPC) plays an important role in representing and recalling spatial relationships and in the ability to orient visual attention. This is evidenced by the parietal activation observed in brain imaging experiments performed during visuo- spatial tasks, and by the contralateral neglect syndrome that often accompanies parietal lesions. Individual neurons in monkey parietal cortex respond vigorously to the appearance of single, behaviorally relevant stimuli, but little is known about how they respond to more complex visual displays. The current experiments addressed this issue by recording activity from single neurons in area 7a of the PPC in monkeys performing a spatial version of a match-to-sample task. The task required them to locate salient stimuli in multiple-stimulus displays and release a lever after a subsequent stimulus appeared at the same location. Neurons responded preferentially to the appearance of salient stimuli inside their receptive fields. The presence of multiple stimuli did not affect appreciably the spatial tuning of responses in the majority of neurons or the population code for the location of the salient stimulus. Responses to salient stimuli could be distinguished from background stimuli approximately 100 ms after the onset of the cue. These results suggest that area 7a neurons represent the location of the stimulus attracting the animal's attention and can provide the spatial information required for directing attention to a salient stimulus in a complex scene.

Animals↗
PubMed2001

Neuronal responses in area 7a to multiple stimulus displays: II. responses are suppressed at the cued location.

Everyday visual scenes contain a variety of stimuli that vary in their significance. The companion paper demonstrates that neurons in the posterior parietal cortex (PPC) are capable of encoding the spatial locations of the salient stimulus in multiple stimulus scenes. The present experiment sought to address how neuronal responses to stimuli appearing in the receptive field are modulated after attention has been drawn to one of multiple stimuli in a visual scene. We recorded from area 7a of the PPC in monkeys trained to do a spatial version of a match-to-sample task. The results show that neuronal responses are greatly suppressed when stimuli appear at previously attended locations. No reduction in responsiveness is observed for locations where stimuli had previously appeared but did not draw attention. These results support the hypothesis that area 7a has a role in redirecting attention to stimuli appearing at novel, unattended locations.

Animals↗
PubMed1996

Neuronal activity in posterior parietal area 7a during the delay periods of a spatial memory task.

1. Neuronal activity was recorded from area 7a of monkeys performing a delayed match-to-sample task requiring release of a behavioral key when a visual stimulus appeared at a remembered spatial location. 2. Activity in the delay periods was significantly elevated in 28% of 405 neurons studied and could be classified as either sustained or anticipatory in nature. 3. Sustained activity was characterized by maintained or slowly decreasing discharge rates that were typically greater when the preceding stimulus was at a location that evoked a strong response. Sustained activity was terminated when a subsequent stimulus appeared at another location. 4. Anticipatory activity was characterized by accelerating discharge rates that were ordinarily greater after a stimulus at a location that evoked a weak response. Anticipatory activity was often associated with facilitated responses to the next stimulus. 5. These data demonstrate that a population of neurons in area 7a is active during the delay periods of a spatial memory task that does not require a behavioral response directed toward the location of the stimulus. This activity could represent a short-term memory trace for the spatial location of the preceding stimulus.

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PubMed1994

Covert attention suppresses neuronal responses in area 7a of the posterior parietal cortex.

1. The effect of covert attention was studied in area 7a of the posterior parietal cortex of rhesus monkeys performing a spatial match-to-sample task. The task required the animals to fixate a central target light, to detect and remember the location of a transient spatial cue, and to respond when one of a series of stimuli appeared at the cued location. Neuronal responses evoked by the visual stimuli were recorded during each behavioral trial. 2. Thirty-eight percent of the neurons isolated and studied in these experiments responded to visual stimuli. The responses of 55% of the neurons tested were suppressed, and 5% enhanced for stimuli presented at the attended location. Responses in the remaining neurons (40%) were unaffected by shifts in attention. 3. Activity in 57% of the suppressed neurons was reduced to rates not significantly different from spontaneous activity. 4. The extent of suppression for individual neurons was often restricted to the attended portion of the receptive field. 5. These data suggest a potential role for these neurons in the redirection of visual attention.

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PubMed1991

Adaptation of the Reitboeck method of multiple microelectrode recording to the neocortex of the waking monkey.

We adapted to the neocortex of waking monkeys a method for multiple microelectrode recording devised by Reitboeck. A sliding platform allows micropositioning of 7 electrodes independently, in 2 microns steps. Microelectrodes are quartz glass filaments (80 microns o.d.) with central metal cores (30 microns) of tungsten-platinum alloy. Filaments are drawn in a high temperature chamber, and ground to the desired form and tip size. The microdrive is held over the region to be explored, and the microelectrodes passed through 300 microns o.d. guide tubes fixed in implant thimbles of chosen size an and x - y arrangement of tubes, sealed by an O-ring into a small craniotomy opening. A microprocessor controlled recording system provides gain, noise and wave-shape filtering, impedance testing, and differential amplitude discrimination for each channel. Electrode movement is obtained via the microprocessor which displays and updates on the console terminal the electrode depth, impedance, and the channel assignment of each electrode. A second microprocessor based system is used to collect, buffer, and encode in real time all event data, which are transferred whenever convenient to a minicomputer that controls the experiment. Exploratory recordings were made in the posterior parietal, somatic sensory, and motor cortical areas. The system has now been used successfully in a number of investigations.

Action Potentials↗
PubMed1990

Frequency discrimination in the sense of flutter: psychophysical measurements correlated with postcentral events in behaving monkeys.

The capacities of humans and monkeys to discriminate between the frequencies of mechanical sinusoids delivered to the glabrous skin of the hand have been measured in psychophysical experiments. The 2 primates have similar capacities; they make discriminations with Weber fractions that change little over the frequency range from 20 to 200 Hz. The discriminatory capacities are similar whether stimuli are received passively or acquired actively. Combined experiments have been made in monkeys in which the electrical signs of the activity of quickly adapting (QA) and slowly adapting (SA) neurons of postcentral areas 3b and 1 were recorded, both in the working state as the animal made discriminations and in the irrelevant state in which the stimuli did not guide behavior. The neuronal responses were analyzed in terms of discharge rates, periodicities in the neuronal discharges, and harmonic contents. It was shown that discriminatory capacity depends upon the period lengths in the sets of periodically entrained activity evoked by stimuli readily discriminated, and not upon the small differences in rates of discharge evoked by those stimuli. The periodicities were shown by harmonic analysis to be sharply limited to stimulus frequencies. Low-frequency stimuli evoke periodicities at the second and third harmonics in some neurons, in addition to strongly periodic signals at the fundamental frequency of the stimuli. Their presence does not appear to interfere with frequency discrimination. Neuronal responses recorded in the stimulus-irrelevant state were not distinguishable from those recorded as monkeys made discriminations. The responses of SA neurons, recorded under similar conditions, resembled those of QA neurons in almost every feature, but reasons are given for concluding that the SA system plays no role in frequency discrimination in the sense of flutter.

Adult↗
PubMed1987

Common and differential effects of attentive fixation on the excitability of parietal and prestriate (V4) cortical visual neurons in the macaque monkey.

The excitability of cortical neurons of prestriate area V4 and area PG of the inferior parietal lobule were examined using the method of single-neuron analysis in awake macaque monkeys. Levels of excitability were measured as the intensity of response to optimal visual stimuli placed in the most responsive region of the cell's receptive field. Physically and retinotopically identical stimuli were delivered during eye movement pauses under 3 conditions: during a no-task state in which the animal was awake and alert, but not receiving or expecting rewards or working in any task; between trials of the task state, the intertrial interval, while the animal awaited the appearance of a fixation target; and during the foreperiod of the task state, as the animal attentively fixated a small target light, waiting to detect its dimming in order to receive liquid reward. Experiments were carried out in 6 hemispheres of 4 monkeys; both V4 and PG were examined through the same chamber placements in 2 hemispheres. A total of 478 neurons in V4 and PG were identified as visual; quantitative studies were done on 146 in V4 and 54 in PG. We found in these experiments a common effect, a 3-4-fold facilitation of the responses of both V4 and PG visual neurons during the task state as compared to in the no-task state, and a differential effect, in that V4 neurons showed a similar 3-4-fold facilitation of responses to stimuli presented during the intertrial interval, whereas PG neuronal responses during this interval were similar to those evoked in the no-task state. We describe the functional properties of V4 neurons studied in the waking state. The findings are discussed in relation to the positions of these 2 areas in the occipitoparietal and occipitotemporal transcortical visual systems and to their respective roles in visuospatial perception and pattern recognition. They are also discussed with regard to the candidate neural mechanisms through which the changes in cortical neuronal excitability might be mediated.

Action Potentials↗
PubMed1987

Functional properties of parietal visual neurons: mechanisms of directionality along a single axis.

The directional properties of parietal visual neurons (PVNs) were examined using the method of single-neuron analysis in waking monkeys. PVN properties were determined with passive visual stimuli as the animal executed a simple detection task. Parietal area PG was studied in 10 hemispheres of 6 male Macaca mulatta. Each class of parietal neurons was identified in PG: the fixation, projection, visual, and oculomotor neurons; 613 PVNs were identified, 323 were studied quantitatively, and 188 were studied with one or more of the protocols described. The receptive fields of PVNs are commonly large and bilateral, and at the limit some may fill the visual field; for many, the central zone of the visual field is spared when the fields are determined by stimuli that enter from the periphery and transit meridians. The receptive fields vary with the behavioral state, the angle of gaze, and the parameters of the stimuli used to determine them. PVNs are sensitive to stimulus movement but relatively insensitive to stimulus speed; many respond over a speed range of 5 degrees-500 degrees/sec. Stimulus-response relations may be incremental or decremental with increasing speed or show maxima or minima in the midrange of speed, but the response variation over the full range is rarely greater than 2:1. The directional preferences of PVNs with bilateral receptive fields are opponently organized; the preferred directions point either inward toward or outward away from the central line of gaze along the 4 meridians tested, which were equally spaced in the circular dimension of the visual field. The mechanism of the axis directionality of PVNs was studied using conditioning-test paradigms. They revealed a feed-forward inhibition preceding a stimulus, an effect that extends from the leading edge of the stimulus for 10 degrees-20 degrees in front of the moving stimulus and lasts for several hundred milliseconds. A double-Gaussian model of superimposed but unequal excitatory and inhibitory effects suffices to explain the present observations. It places demand upon the projection of functional properties from the contralateral hemisphere or from the ipsilateral prestriate areas that project upon PG over multistaged pathways and minimal demands upon intracortical processing mechanisms.

Action Potentials↗
PubMed1987

Functional properties of parietal visual neurons: radial organization of directionalities within the visual field.

Parietal visual neurons (PVNs) were studied in waking monkeys as they executed a simple fixation-detection task. Test visual stimuli of varied direction, speed, and extent were presented during the fixation period; these stimuli did not control behavior. Most PVNs subtend large, bilateral receptive fields and are exquisitely sensitive to stimulus motion and direction but insensitive to stimulus speed. The directional preferences of PVNs along meridians are opponently organized, with the preferred directions pointing either inward toward or outward away from the fixation point. Evidence presented in the preceding paper (Motter et al., 1987) indicates that opponent directionality along a single meridian is produced by a feed-forward inhibition of 20 degrees-30 degrees spatial extent. The observations fit a double-Gaussian model of superimposed but unequal excitatory and inhibitory receptive fields: When the former is larger, inward directionality results; when smaller, outward directionality results. We examine here the distribution of the meridional directional preferences in the visual field. Tests showed that opponent organization is not produced by differences in local directional properties in different parts of the receptive field. The distribution of response intensities from one meridian to another is adequately described by a sine wave function. These data indicate a best radial direction for each neuron with a broad distribution of response intensities over successive meridians. Thus, any single PVN, with rare exceptions, cannot signal radial stimulus direction precisely. We then determined how accurately the population response predicted radial stimulus direction by the application of a linear vector summation model. The resulting population vector varied from stimulus direction by an average of 9 degrees. Whether or not the perception of the direction of motion depends upon a population vector remains uncertain. PVNs are especially sensitive to object movement in the visual surround, particularly in the periphery of the visual field. This, combined with their large receptive fields and their wide but flat sensitivity to stimulus speed, makes them especially sensitive to optic flow. This is discussed in relation to the role of the parietal visual system in the visual guidance of projected movements of the arm and hand, in the guidance of locomotion, and in evoking the illusion of vection.

Action Potentials↗
PubMed1983

An improved method for water vapor detection.

We describe improvements in and details for the construction, calibration and use of a device using a thermal conductivity cell for the measurement of low-level rates of water evaporation (E) from a small surface area. E is measured from 0.0 to 1.0 mg . min-1 with a correlation coefficient of 0.999 between measured and independently verified rates and amounts of water evaporation. Data are available as a recordable analog d.c. voltage as well as in digital display for E and for the amount of water evaporated during an operator defined time period. The device we describe is noninvasive and it is designed to be constructed of conventional components. It is useful not only for measuring transcutaneous water diffusion in normal and diseased skin, but also it is adequately sensitive and rapidly responding to follow thermoregulatory and psychogenic sweating in small (nom. 1.0 cm2) skin areas. It can also be used to measure accurately and precisely the rates at which water is adsorbed by and removed from inanimate materials, as well as to determine how much water they store.

Animals↗
PubMed1980

Thermodynamic technique for the quantification of regional blood flow.

A method is described to quantify regional blood flow by thermal analysis. A weak temperature field is established in a tissue and for a thermal steady state, unidirectional heat flux and the associated temperature gradient are measured simultaneously across a small fixed segment of the tissue. This information is evaluated with probe calibrations for homogeneous isotropic fluids, with data from ancillary measurements in the nonperfused tissue and with values of specific heat and density of blood to express local blood flow in heat transfer [effective thermal conductivity (W. degrees C-1 . cm-1 x 10(-3) and/or in perfusion (ml . min-1 . cm-3)] terms. The technique measures local perfusion in small tissue volumes and is usable in acute or chronic experiments. Its accuracy is not a function of the absolute steady-state temperature of the tissue or of its metabolic heat production.

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PubMedDate not supplied

Neurophysiological evidence for a role of posterior parietal cortex in redirecting visual attention.

Lesions of the posterior parietal cortex in humans and monkeys result in a spatial neglect syndrome characterized by defects in visual-spatial perception, oculomotor function, and directing visual attention. Although symptoms of spatial neglect can result from lesions to other cortical and subcortical areas, patients with posterior parietal lesions are particularly impaired in their ability to disengage and reorient visual attention. Neurophysiological experiments in area 7a of behaving monkeys reveal a large class of neurons that respond to visual stimuli and are powerfully modulated by states of attention. These cells respond better to passive visual stimuli presented during states of attentive fixation than to identical stimuli presented in nonattentive states. The responses of the majority of these cells are also influenced by covert shifts of attention away from the point of fixation; they respond to stimuli presented anywhere within their receptive fields except the covertly attended location. The combined effect of facilitation during attentive fixation and lack of response at the attended location results in a sensitivity for visual stimuli that appear at one location while attention is directed to another. The special sensitivity for unattended stimuli in this group of neurons in area 7a suggests that they may play a role in reorienting attention, possibly by providing signals of the spatial locations of novel stimuli.

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