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R C Reid

Publications and source records attributed to R C Reid.

At least 37 records · Page 2Linked to original sources
PubMed1998

Synaptic integration in striate cortical simple cells.

Simple cells in the visual cortex respond to the precise position of oriented contours (Hubel and Wiesel, 1962). This sensitivity reflects the structure of the simple receptive field, which exhibits two sorts of antagonism between on and off inputs. First, simple receptive fields are divided into adjacent on and off subregions; second, within each subregion, stimuli of the reverse contrast evoke responses of the opposite sign: push-pull (Hubel and Wiesel, 1962; Palmer and Davis, 1981; Jones and Palmer, 1987; Ferster, 1988). We have made whole-cell patch recordings from cat area 17 during visual stimulation to examine the generation and integration of excitation (push) and suppression (pull) in the simple receptive field. The temporal structure of the push reflected the pattern of thalamic inputs, as judged by comparing the intracellular cortical responses to extracellular recordings made in the lateral geniculate nucleus. Two mechanisms have been advanced to account for the pull-withdrawal of thalamic drive and active, intracortical inhibition (Hubel and Wiesel, 1962; Heggelund, 1968; Ferster, 1988). Our results suggest that intracortical inhibition is the dominant, and perhaps sole, mechanism of suppression. The inhibitory influences operated within a wide dynamic range. When inhibition was strong, the membrane conductance could be doubled or tripled. Furthermore, if a stimulus confined to one subregion was enlarged so that it extended into the next, the sign of response often changed from depolarizing to hyperpolarizing. In other instances, the inhibition modulated neuronal output subtly, by elevating spike threshold or altering firing rate at a given membrane voltage.

Action Potentials↗
PubMed1998

Paired-spike interactions and synaptic efficacy of retinal inputs to the thalamus.

In many neural systems studied in vitro, the timing of afferent impulses affects the strength of postsynaptic potentials. The influence of afferent timing on postsynaptic firing in vivo has received less attention. Here we study the importance of afferent spike timing in vivo by recording simultaneously from ganglion cells in the retina and their targets in the lateral geniculate nucleus of the thalamus. When two spikes from a single ganglion-cell axon arrive within 30 milliseconds of each other, the second spike is much more likely than the first to produce a geniculate spike, an effect we call paired-spike enhancement. Furthermore, simultaneous recordings from a ganglion cell and two thalamic targets indicate that paired-spike enhancement increases the frequency of synchronous thalamic activity. We propose that information encoded in the high firing rate of an individual retinal ganglion cell becomes distributed among several geniculate neurons that fire synchronously. Because synchronous geniculate action potentials are highly effective in driving cortical neurons, it is likely that information encoded by this strategy is transmitted to the next level of processing.

Action Potentials↗
PubMed1998

Coding of visual information by precisely correlated spikes in the lateral geniculate nucleus.

Correlated firing among neurons is widespread in the nervous system. Precisely correlated spiking, occurring on a millisecond time scale, has recently been observed among neurons in the lateral geniculate nucleus with overlapping receptive fields. We have used an information-theoretic analysis to examine the role of these correlations in visual coding. Considerably more information can be extracted from two cells if temporal correlations between them are considered. The percentage increase in information depends on the degree of correlation; the average increase is approximately 20% for strongly correlated pairs. Thus, precise temporal correlation could be used as an additional information channel from thalamus to visual cortex.

Action Potentials↗
PubMed1997

Linkage of the gene for equine combined immunodeficiency disease to microsatellite markers HTG8 and HTG4; synteny and FISH mapping to ECA9.

Equine combined immunodeficiency disease (CID) is caused by homozygosity for an autosomal recessive gene. To identify linked markers for the disease, we studied a family segregating for the equine CID gene. A stallion and 19 of his CID-affected offspring were tested for marker segregation at 23 microsatellite DNA loci. His CID-affected offspring inherited only one of his two alleles at the HTG8 and HTG4 loci, namely HTG8-186 and HTG4-124, respectively. Lod scores for linkage to the CID gene using a theta of 0.01 were 5.34 for HTG8 and 2.37 for HTG4. The apparent genotypes also suggested linkage disequilibrium between the HTG8-186 allele and the gene for CID. The gene for the DNA protein kinase catalytic subunit (DNA-PK) was recently suggested as a candidate gene for equine CID. A defect of this gene causes a disease in mice that is similar to equine CID. Therefore, we investigated whether this gene might be associated with the microsatellite markers. Analysis of a somatic cell hybrid panel demonstrated synteny of DNA-PK with HTG4 and HTG8 (Kentucky Synteny Group 3). Fluorescence in situ hybridization (FISH) studies demonstrated that DNA-PK is located on horse chromosome ECA9p12. This work supports the hypothesis of DNA-PK as the probable cause of equine CID.

Alleles↗
PubMed1996

Precisely correlated firing in cells of the lateral geniculate nucleus.

Simple cells within layer IV of the cat primary visual cortex are selective for lines of a specific orientation. It has been proposed that their receptive-field properties are established by the pattern of connections that they receive from the lateral geniculate nucleus (LGN) of the thalamus. Thalamic inputs, however, represent only a small proportion of the synapses made onto simple cells, and others have argued that corticocortical connections are likely to be important in shaping simple-cell response properties. Here we describe a mechanism that might be involved in selectively strengthening the effect of thalamic inputs. We show that neighbouring geniculate neurons with overlapping receptive fields of the same type (on-centre or off-centre) often fire spikes that are synchronized to within 1 millisecond. Moreover, these neurons often project to a common cortical target neuron where synchronous spikes are more effective in evoking a postsynaptic response. We propose that precisely correlated firing within a group of geniculate neurons could serve to reinforce the thalamic input to cortical simple cells.

Action Potentials↗
PubMed1996

Efficient coding of natural scenes in the lateral geniculate nucleus: experimental test of a computational theory.

A recent computational theory suggests that visual processing in the retina and the lateral geniculate nucleus (LGN) serves to recode information into an efficient form (Atick and Redlich, 1990). Information theoretic analysis showed that the representation of visual information at the level of the photoreceptors is inefficient, primarily attributable to a high degree of spatial and temporal correlation in natural scenes. It was predicted, therefore, that the retina and the LGN should recode this signal into a decorrelated form or, equivalently, into a signal with a "white" spatial and temporal power spectrum. In the present study, we tested directly the prediction that visual processing at the level of the LGN temporarily whitens the natural visual input. We recorded the responses of individual neurons in the LGN of the cat to natural, time-varying images (movies) and, as a control, to white-noise stimuli. Although there is substantial temporal correlation in natural inputs (Dong and Atick, 1995b), we found that the power spectra of LGN responses were essentially white. Between 3 and 15 Hz, the power of the responses had an average variation of only +/-10.3%. Thus, the signals that the LGN relays to visual cortex are temporarily decorrelated. Furthermore, the responses of X-cells to natural inputs can be well predicted from their responses to white-noise inputs. We therefore conclude that whitening of natural inputs can be explained largely by the linear filtering properties (Enroth-Cugell and Robson, 1966). Our results suggest that the early visual pathway is well adapted for efficient coding of information in the natural visual environment, in agreement with the prediction of the computational theory.

Animals↗
PubMed1996

The processing and encoding of information in the visual cortex.

Our knowledge of the response properties of visual cortical neurons has increased steadily since the original studies of Hubel and Wiesel in the 1960s. By comparison, an understanding of the neural mechanisms responsible for these properties has proved more elusive. Models for the mechanisms involved in even the most basic responses, such as the orientation tuning of simple cells in the primary visual cortex of the cat, remain controversial. Recent studies, however, are providing further support for a simple model (first suggested by Hubel and Wiesel), in which it is hypothesized that the response properties of simple cells in layer 4 of the cortex are dominated by the convergence of highly specific thalamic inputs.

Animals↗
PubMed1995

Visually evoked calcium action potentials in cat striate cortex.

Early intracellular studies of cerebral cortical neurons indicated that synaptic input evokes dendritic action potentials that convey information towards the soma. Subsequent work in vitro established that neocortical neurons produce dendritic Ca2+ action potentials. To determine whether natural stimuli elicit Ca2+ spikes, we combined the techniques of whole-cell recording, pharmacology and quantitative receptive field mapping. Our findings show that visual stimulation routinely evoked Ca2+ spikes in distinct functional and anatomical classes of cells in different layers of the cat striate cortex. Hence regenerative Ca2+ potentials appear to play a role in both the initial and later stages of cortical sensory processing.

Action Potentials↗
PubMed1995

Specificity of monosynaptic connections from thalamus to visual cortex.

In cortical area 17 of the cat, simple receptive fields are arranged in elongated subregions that respond best to bright (on) or dark (off) oriented contours, whereas the receptive fields of their thalamic inputs have a concentric on and off organization. This dramatic transformation suggests that there are specific rules governing the connections made between thalamic and cortical neurons (see ref. 4). Here we report a study of these rules in which we recorded from thalamic (lateral geniculate nucleus; LGN) and cortical neurons simultaneously and related their receptive fields to their connectivity, as measured by cross-correlation analysis. The probability of finding a monosynaptic connection was high when a geniculate receptive field was superimposed anywhere over an elongated simple-cell subregion of the same signature (on or off). However, 'inappropriate' connections from geniculate cells of the opposite receptive field signature were extremely rare. Together, these findings imply that the outline of the elongated, simple receptive field, and thus of cortical orientation selectivity, is laid down at the level of the first synapse from the thalamic afferents.

Animals↗
PubMed1995

Evolution in action.

The alpha/beta barrel enzyme phosphotriesterase from soil bacteria appears to have evolved the ability to hydrolyze the insecticide paraoxon at the diffusion limit in only a few decades. A newly-identified open reading frame from Escherichia coli may offer a clue to its origins.

Amino Acid Sequence↗
PubMed1994

On temporal codes and the spatiotemporal response of neurons in the lateral geniculate nucleus.

1. The present work relates recent experimental studies of the temporal coding of visual stimuli (McClurkin, Optican, Richmond, and Gawne, Science 253: 675, 1991) to the measurements of the spatiotemporal receptive fields of neurons within the lateral geniculate of primate. 2. We analyze both new and previously described magnocellular and parvocellular single units. The spatiotemporal impulse response function of the unit, defined as the time-resolved average firing rate in response to a weak stimulus flashed at a given location and time, is characterized by the singular value decomposition. This analysis allows one to represent the impulse response by a small number, two to three, of spatial and temporal modes. Both magnocellular and parvocellular units are weakly nonseparable, with major and minor modes that account, respectively, for approximately 78 and 22% of the response. The major temporal mode for both types is essentially identical for the first 100 ms. At later times the response of magnocellular units changes sign and decays slowly, whereas the response of parvocellular units decays relatively rapidly. 3. The spatiotemporal impulse response function completely determines the response of a unit to an arbitrary stimulus when linear response theory is valid. Using the measured impulse response, combined with a rectifying neuronal input-output relation, we calculate the responses to a complete set of spatial luminance patterns constructed of "Walsh" functions. Our predicted temporal responses are in qualitative agreement with those reported for parvocellular units (McClurkin, Optican, Richmond, and Gawne, J. Neurophysiol. 66: 794, 1991). Under the additional assumptions of Poisson statistics for the probability of spiking and a plausible background firing rate, we predict the performance of a unit in the Walsh pattern discrimination task as quantified by mutual information. Our prediction is again consistent with the reported results. 4. Last, we consider the issue of temporal coding within linear response. For stimuli presented for fixed time intervals, the singular value decomposition provides a natural relation between the temporal modes of the neuronal response and the spatial pattern of the stimulus. Although it is tempting to interpret each temporal mode as an independent channel that encodes orthogonal features of the stimulus, successively higher order modes are increasingly unreliable and do not significantly increase the discrimination capabilities of the unit.

Animals↗
PubMed1994

Luteolytic effect of prostaglandin F2 alpha and two metabolites in ewes.

Twenty-three ewes were used in an experiment to determine the point in metabolism at which prostaglandin (PG) F2 alpha loses its luteolytic activity. Ewes exhibited at least one normal estrous cycle (15 to 19 d) and then were randomly assigned to one of four treatment groups: 1) control, 2) PGF2 alpha, 3) 15-keto-PGF2 alpha, or 4) 13,14-dihydro-15-keto-PGF2 alpha (PGFM). Each ewe received an i.m. injection of the designated treatment (5 mg in 1 mL of sterile saline) during the midluteal phase of the estrous cycle (d 8, 9, 10, or 11 after estrus). An identical second injection was given to each ewe 3 h after the first injection. Beginning at 9 h after injection, concentrations of progesterone were lower (P < .01) in the PGF2 alpha-treated ewes than in ewes in the other three groups. These differences were maintained throughout the duration of the 48-h sampling period in five of the six ewes that received PGF2 alpha. None of the ewes in the other three treatment groups exhibited any change in concentrations of progesterone. Ewes were classified as completing luteolysis if concentrations of progesterone decreased to less than 1 ng/mL within 48 h after the first injection. Five of the six ewes receiving PGF2 alpha completed luteolysis (P < .05), whereas none of the ewes in the other three groups completed luteolysis. The interestrous interval was reduced by approximately 5 d in the PGF2 alpha-treated group compared with the other three treatment groups (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Spatial structure of cone inputs to receptive fields in primate lateral geniculate nucleus.

Human colour vision depends on three classes of cone photoreceptors, those sensitive to short (S), medium (M) or long (L) wavelengths, and on how signals from these cones are combined by neurons in the retina and brain. Macaque monkey colour vision is similar to human, and the receptive fields of macaque visual neurons have been used as an animal model of human colour processing. P retinal ganglion cells and parvocellular neurons are colour-selective neurons in macaque retina and lateral geniculate nucleus. Interactions between cone signals feeding into these neurons are still unclear. On the basis of experimental results with chromatic adaptation, excitatory and inhibitory inputs from L and M cones onto P cells (and parvocellular neurons) were thought to be quite specific (Fig. 1a). But these experiments with spatially diffuse adaptation did not rule out the 'mixed-surround' hypothesis: that there might be one cone-specific mechanism, the receptive field centre, and a surround mechanism connected to all cone types indiscriminately (Fig. 1e). Recent work has tended to support the mixed-surround hypothesis. We report here the development of new stimuli to measure spatial maps of the linear L-, M- and S-cone inputs to test the hypothesis definitively. Our measurements contradict the mixed-surround hypothesis and imply cone specificity in both centre and surround.

Animals↗
PubMed1992

Broadband temporal stimuli decrease the integration time of neurons in cat striate cortex.

We have studied the responses of striate cortical neurons to stimuli whose contrast is modulated in time by either a single sinusoid or by the sum of eight sinusoids. The sum-of-sinusoids stimulus resembles white noise and has been used to study the linear and nonlinear dynamics of retinal ganglion cells (Victor et al., 1977). In cortical neurons, we have found different linear and second-order responses to single-sinusoid and sum-of-sinusoids inputs. Specifically, while the responsivity near the optimal temporal frequency is lower for the sum-of-sinusoids stimulus, the responsivity at higher temporal frequencies is relatively greater. Along with this change in the response amplitudes, there is a systematic change in the time course of responses. For complex cells, the integration time, the effective delay due to a combination of actual delays and low-pass filter stages, changes from a median of 85 ms with single sinusoids to 57 ms with a sum of sinusoids. For simple cells, the integration times for single sinusoids range from 44-100 ms, but cluster tightly around 40 ms for the sum-of-sinusoids stimulus. The change in time constant would argue that the increased sensitivity to high frequencies cannot be explained by a static threshold, but must be caused by a fundamental alteration in the response dynamics. These effects are not seen in the retina (Shapley & Victor, 1981) and are most likely cortical in origin.

Animals↗
PubMed1991

Directional selectivity and spatiotemporal structure of receptive fields of simple cells in cat striate cortex.

1. Simple cells in cat striate cortex were studied with a number of stimulation paradigms to explore the extent to which linear mechanisms determine direction selectivity. For each paradigm, our aim was to predict the selectivity for the direction of moving stimuli given only the responses to stationary stimuli. We have found that the prediction robustly determines the direction and magnitude of the preferred response but overestimates the nonpreferred response. 2. The main paradigm consisted of comparing the responses of simple cells to contrast reversal sinusoidal gratings with their responses to drifting gratings (of the same orientation, contrast, and spatial and temporal frequencies) in both directions of motion. Although it is known that simple cells display spatiotemporally inseparable responses to contrast reversal gratings, this spatiotemporal inseparability is demonstrated here to predict a certain amount of direction selectivity under the assumption that simple cells sum their inputs linearly. 3. The linear prediction of the directional index (DI), a quantitative measure of the degree of direction selectivity, was compared with the measured DI obtained from the responses to drifting gratings. The median value of the ratio of the two was 0.30, indicating that there is a significant nonlinear component to direction selectivity. 4. The absolute magnitudes of the responses to gratings moving in both directions of motion were compared with the linear predictions as well. Whereas the preferred direction response showed only a slight amount of facilitation compared with the linear prediction, there was a significant amount of nonlinear suppression in the nonpreferred direction. 5. Spatiotemporal inseparability was demonstrated also with stationary temporally modulated bars. The time course of response to these bars was different for different positions in the receptive field. The degree of spatiotemporal inseparability measured with sinusoidally modulated bars agreed quantitatively with that measured in experiments with stationary gratings. 6. A linear prediction of the responses to drifting luminance borders was compared with the actual responses. As with the grating experiments, the prediction was qualitatively accurate, giving the correct preferred direction but underestimating the magnitude of direction selectivity observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Brightness induction by local contrast and the spatial dependence of assimilation.

Two mechanisms of brightness perception (1) brightness induction by local contrast and (2) assimilation, were examined for a variety of visual stimuli. Local contrast is the primary determinant of brightness perception, making objects appear brighter on a background of lower luminance and darker on a background of greater luminance. Assimilation is the opposite effect, whereby objects on a brighter (but not necessarily more luminant) background appear brighter or on a dark background appear darker. We have compared the relative strength of the two effects using stimuli which permit them to be studied separately. Brightness induction by local contrast is quantitatively stronger in all situations. Further, the strength of assimilation is strongly dependent on spatial parameters in the visual scene. These results are shown to be true both for simple visual stimuli as well as for complicated Mondrian-like patterns. The Retinex theory of brightness perception predicts that the two effects are equal. Our results show a range of relative strengths (assimilation vs brightness induction due to contrast) from 0.59 to 0.63 at 5' down to 0.34 at 43'.

Humans↗