Comparison of recordings from microelectrode arrays and single electrodes in the visual cortex.
Explore the source record for details and available documents.
Action Potentials↗
Biomedical subjects
Matthew A Smith
Publications and source records attributed to Matthew A Smith.
Dynamics of suppression in macaque primary visual cortex.
The response of a neuron in primary visual cortex (V1) to an optimal stimulus in its classical receptive field (CRF) can be reduced by the presence of an orthogonal mask, a phenomenon known as cross-orientation suppression. The presence of a parallel stimulus outside the CRF can have a similar effect, in this case known as surround suppression. We used a novel stimulus to probe the time course of cross-orientation suppression and found that it is very fast, starting even before the response to optimal excitatory stimuli. However, it occurs with some delay after the offset response, considered to be a measure of the earliest excitatory signals that reach the CRF. We also examined the time course of response to a stimulus presented outside the CRF and found that cross-orientation suppression begins substantially earlier than surround suppression measured in the same cells. Together, these findings suggest that cross-orientation suppression is attributable to either direct feedforward signal paths to V1 neurons or a circuit involving fast local interneurons within V1. Feedback from higher cortical areas is implicated in surround suppression, but our results make this an implausible mechanism for cross-orientation suppression. We conclude that suppression from inside and outside the CRF occur through different mechanisms.
Action Potentials↗
Surround suppression in the early visual system.
Explore the source record for details and available documents.
Action Potentials↗
Stimulus dependence of neuronal correlation in primary visual cortex of the macaque.
Nearby cortical neurons often have correlated trial-to-trial response variability, and a significant fraction of their spikes occur synchronously. These two forms of correlation are both believed to arise from common synaptic input, but the origin of this input is unclear. We investigated the source of correlated responsivity by recording from pairs of single neurons in primary visual cortex of anesthetized macaque monkeys and comparing correlated variability and synchrony for spontaneous activity and activity evoked by stimuli of different orientations and contrasts. These two stimulus manipulations would be expected to have different effects on the cortical pool providing input to the recorded pair: changing stimulus orientation should recruit different populations of cells, whereas changing stimulus contrast affects primarily the relative strength of sensory drive and ongoing cortical activity. Consistent with this predicted difference, we found that correlation was affected by these stimulus manipulations in different ways. Synchrony was significantly stronger for orientations that drove both neurons well than for those that did not, but correlation on longer time scales was orientation independent. Reducing stimulus contrast resulted in a decrease in the temporal precision of synchronous firing and an enhancement of correlated response variability on longer time scales. Our results thus suggest that correlated responsivity arises from mechanisms operating at two distinct timescales: one that is orientation tuned and that determines the strength of temporally precise synchrony, and a second that is contrast sensitive, of low temporal frequency, and present in ongoing cortical activity.
Action Potentials↗
Dynamics of motion signaling by neurons in macaque area MT.
Most neurons in macaque area MT are selective for the direction of stimulus motion. By comparing direction selectivity for gratings and plaids, we classified MT neurons as pattern direction selective (PDS) or component direction selective (CDS). We compared the time course of responses in CDS and PDS neurons in opiate-anesthetized macaques, using a rapid pseudorandom sequence of gratings and plaids that moved in different directions. On average, responses began 6 ms earlier in CDS neurons than in PDS neurons. More importantly, the pattern-selective responses of PDS neurons did not reach their fully selective state until 50-75 ms after the responses of CDS neurons had stabilized. The population motion response of MT is therefore initially dominated by component motion signals, and does not completely represent pattern motion until substantially later. The circuits that compute pattern motion take more time to finish their work than those signaling component motion.
Action Potentials↗
The vomeronasal organ of greater bushbabies (Otolemur spp.): species, sex, and age differences.
The present study examined interspecies, intersexual, and age-related changes in size of the vomeronasal neuroepithelium (VNNE) of two species of greater bushbabies (genus Otolemur, Infraorder Lorisiformes, Suborder Strepsirrhini). Tissue blocks containing the vomeronasal organs of nine O. crassicaudatus (8 adults, 1 neonate) and ten O. garnettii (9 adults, 1 neonate) were studied by means of serial paraffin sectioning and computer-based reconstruction of VNNE volume. In addition, the immunoreactivity of the VNNE to two neuronal markers, neuron-specific beta tubulin (BT) and olfactory marker protein (OMP) was compared between species, sexes, and ages. Results indicated that a clear VNNE is present at birth in both species, and OMP immunoreactivity was verified in O. garnettii at birth. Male and female adults of both species showed OMP-immunoreactive and BT-immunoreactive neurons in the VNNE. Immunohistochemical findings indicated that all males and the youngest females had the thickest VNNE, especially at the marginal junctions with the receptor-free epithelium. Results of a 2-way Analysis of Variance (ANOVA, species x sex) revealed no significant differences in VNNE length or volume between species, but O. crassicaudatus had significantly (p < 0.05) greater palatal length. Significant (p < 0.05) differences also were found between sexes in VNNE volume, but no significant differences in palatal length or VNNE length. The distribution of VNNE volume against age indicated that the sex differences were more pronounced in O. crassicaudatus than O. garnettii. For both species and sexes, distribution of VNNE volume against age suggested an age-related reduction in volume. These findings demonstrate postnatal plasticity in VNNE size in Otolemur that is reminiscent of that found for olfactory structures in some rodents. Bushbabies or other strepsirrhine primates may offer an opportunity for further understanding of behavioral correlates of VNNE postnatal plasticity, which may represent primitive functional characteristics of the order Primates.
Aging↗
Cortical responses to visual motion in alert and anesthetized monkeys.
Explore the source record for details and available documents.
Action Potentials↗
Signals in macaque striate cortical neurons that support the perception of glass patterns.
Glass patterns are texture stimuli made by pairing randomly placed dots with partners at specific offsets. The strong percept of global form that arises from the sparse local orientation cues has made these patterns the subject of psychophysical investigations, yet neuronal responses to Glass patterns have not been studied. We measured the responses of neurons in macaque striate cortex (V1) to dynamic, translational Glass patterns as a function of dot separation and dot-pair orientation. Responses were selective, but were on average more than an order of magnitude weaker than responses to sinusoidal gratings. Response and selectivity were greatest when the dot-pair orientation matched that of the preferred grating and when dot separation was between one-quarter and one-half of the spatial period of the optimal grating; changing the dot-pair separation or inverting the contrast of one of the dots radically changed the orientation selectivity. We computed the expected responses for a receptive field model to translational Glass patterns and found that the complexity of our V1 tuning curves could be understood in terms of the responses of linear filters to pairs of dots. This modeling connects our understanding of V1 receptive fields as rectified, quasi-linear filters with results from psychophysical studies of Glass patterns. Our results provide a basis for studying how subsequent visual areas integrate weak, local signals into global form percepts.
Action Potentials↗
The timing of response onset and offset in macaque visual neurons.
We used fast, pseudorandom temporal sequences of preferred and antipreferred stimuli to drive neuronal firing rates rapidly between minimal and maximal across the visual system. Stimuli were tailored to the preferences of cells recorded in the lateral geniculate nucleus (magnocellular and parvocellular), primary visual cortex (simple and complex), and the extrastriate motion area MT. We found that cells took longer to turn on (to increase their firing rate) than to turn off (to reduce their rate). The latency difference (onset minus offset) varied from several to tens of milliseconds across cell type and stimulus class and was correlated with spontaneous or driven firing rates for most cell classes. The delay for response onset depended on the nature of the stimulus present before the preferred stimulus appeared, and may result from persistent inhibition caused by antipreferred stimuli or from suppression that followed the offset of the preferred stimulus. The onset delay showed three distinct types of dependence on the temporal sequence of stimuli across classes of cells, implying that suppression may accumulate or wear off with time. Onset latency is generally longer, can be more variable, and has marked stimulus dependence compared with offset latency. This suggests an important role for offset latency in assessing the speed of information transmission in the visual system and raises the possibility that signal offsets provide a timing reference for visual processing. We discuss the origin of the delay in onset latency compared with offset latency and consider how it may limit the utility of certain feedforward circuits.
Action Potentials↗