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Robert Shapley

Publications and source records attributed to Robert Shapley.

13 recordsLinked to original sources
PubMed2005

LFP power spectra in V1 cortex: the graded effect of stimulus contrast.

We recorded local field potentials (LFPs) and single-unit activity simultaneously in the macaque primary visual cortex (V1) and studied their responses to drifting sinusoidal gratings that were chosen to be "optimal" for the single units. Over all stimulus conditions, the LFP spectra have much greater power in the low-frequency band (< or = 10 Hz) than higher frequencies and can be described as "1/f." Analysis of the total power limited to the low, gamma (25-90 Hz), or broad (8-240 Hz) frequency bands of the LFP as a function of stimulus contrast indicates that the LFP power gradually increases with stimulus strength across a wide band in a manner roughly comparable to the increase in the simultaneously recorded spike activity. However, the low-frequency band power remains approximately constant across all stimulus contrasts. More specifically the gamma-band LFP power increases differentially more with respect to baseline than either higher or lower bands as stimulus contrast increases. At the highest stimulus contrasts, we report as others have previously, that the power spectrum of the LFP typically contains an obvious peak in the gamma-frequency band. The gamma-band peak emerges from the overall broadband enhancement in LFP power at stimulus contrasts where most single units' responses have begun to saturate. The temporal/spectral structures of the LFP located in the gamma band-which become most evident at the highest contrasts-provide additional constraints on potential mechanisms underlying the stimulus response properties of spiking neurons in V1.

Action Potentials↗
PubMed2004

Entrainment to video displays in primary visual cortex of macaque and humans.

Cathode ray tubes (CRTs) display images refreshed at high frequency, and the temporal waveform of each pixel is a luminance impulse only a few milliseconds long. Although humans are perceptually oblivious to this flicker, we show in V1 in macaque monkeys and in humans that extracellularly recorded action potentials (spikes) and visual-evoked potentials (VEPs) align with the video impulses, particularly when high-contrast stimuli are viewed. Of 91 single units analyzed in macaque with a 60 Hz video refresh, 29 cells (32%) significantly locked their firing to a uniform luminance display, but their number increased to 75 (82%) when high-contrast stimuli were shown. Of 92 cells exposed to a 100 Hz refresh, 21 (23%) significantly phase locked to high-contrast stimuli. Phase locking occurred in both input and output layers of V1 for simple and complex cells, regardless of preferred temporal frequency. VEPs recorded in humans showed significant phase locking to the video refresh in all seven observers. Like the monkey neurons, human VEPs more typically phase locked to stimuli containing spatial contrast than to spatially uniform stimuli. Phase locking decreased when the refresh rate was increased. Thus in humans and macaques phase locking to the high strobe frequency of a CRT is enhanced by a salient spatial pattern, although the perceptual impact is uncertain. We note that a billion people worldwide manage to watch TV without obvious distortion of their visual perception despite extraordinary phase locking of their V1s to a 50 or 60 Hz signal.

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PubMed2004

Correlation of local and global orientation and spatial frequency tuning in macaque V1.

Visual cortical neurones display a variety of visual properties. Among those that emerge in the primary visual cortex V1 are sharpening of selectivity for spatial frequency and for orientation. The selectivity for these stimulus attributes can be measured around the peak of the tuning function, usually as bandwidth. Other selectivity measures take into account the response across a broader range of stimulus values. An example of such a global measure is the circular variance of orientation tuning. Here we introduce a similar measure in the spatial frequency domain that takes into account the shape of the tuning curve at frequencies lower than the peak, called the low-spatial frequency variance. Our recent studies with dynamic stimuli suggest that the selectivity for spatial frequency and orientation is strongly correlated with the degree of suppression at low spatial frequencies and off-axis orientations. Here we extend the study of the global tuning to stimulus conditions that measure the response of cells to the presentation of drifting sinusoidal grating stimuli for periods of a few seconds. We find that under such steady-state stimulus conditions there is a strong correlation between the global selectivity measures, orientation circular variance and low spatial frequency variance. Consistent with previous studies, there is a weaker correlation between the local tuning measures, orientation and spatial frequency bandwidth. These results support the idea that there are multiple factors that contribute to tuning and that suppression observed in dynamic experiments is also likely to contribute to the global selectivity for steady-state stimuli.

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PubMed2004

Cone inputs in macaque primary visual cortex.

To understand the role of primary visual cortex (V1) in color vision, we measured directly the input from the 3 cone types in macaque V1 neurons. Cells were classified as luminance-preferring, color-luminance, or color-preferring from the ratio of the peak amplitudes of spatial frequency responses to red/green equiluminant and to black/white (luminance) grating patterns, respectively. In this study we used L-, M-, and S-cone-isolating gratings to measure spatial frequency response functions for each cone type separately. From peak responses to cone-isolating stimuli we estimated relative cone weights and whether cone inputs were the same or opposite sign. For most V1 cells the relative S-cone weight was <0.1. All color-preferring cells were cone opponent and their L/M cone weight ratio was clustered around a value of -1, which is roughly equal and opposite L and M cone signals. Almost all cells (88%) classified as luminance cells were cone nonopponent, with a broad distribution of cone weights. Most cells (73%) classified as color-luminance cells were cone opponent. This result supports our conclusion that V1 color-luminance cells are double-opponent. Such neurons are more sensitive to color boundaries than to areas of color and thereby could play an important role in color perception. The color-luminance population had a broad distribution of L/M cone weight ratios, implying a broad distribution of preferred colors for the double-opponent cells.

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PubMed2003

An egalitarian network model for the emergence of simple and complex cells in visual cortex.

We explain how simple and complex cells arise in a large-scale neuronal network model of the primary visual cortex of the macaque. Our model consists of approximately 4000 integrate-and-fire, conductance-based point neurons, representing the cells in a small, 1-mm(2) patch of an input layer of the primary visual cortex. In the model the local connections are isotropic and nonspecific, and convergent input from the lateral geniculate nucleus confers cortical cells with orientation and spatial phase preference. The balance between lateral connections and lateral geniculate nucleus drive determines whether individual neurons in this recurrent circuit are simple or complex. The model reproduces qualitatively the experimentally observed distributions of both extracellular and intracellular measures of simple and complex response.

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PubMed2003

Dynamics of orientation selectivity in the primary visual cortex and the importance of cortical inhibition.

To test theories of orientation selectivity in primary visual cortex (V1), we have done experiments to measure the dynamics of orientation tuning of single neurons in the V1 cortex of macaque monkeys. Based on our dynamics results, we propose that a V1 cell's orientation selectivity is generated mainly by both tuned enhancement and global suppression. Enhancement near the preferred orientation is probably caused by feed-forward input from LGN (plus amplification by cortical-cortical interaction). Global suppression could be supplied by cortical inhibition. Additionally, in about 1/3 of V1 neurons (usually the most sharply tuned) there is tuned suppression, centered near the cell's preferred orientation but broader than tuned enhancement. These mechanisms also can explain important features of steady-state selectivity in the V1 neuron population. Furthermore, similar neuronal mechanisms may be used generally throughout the cerebral cortex.

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PubMed2003

Dynamics of orientation tuning in macaque V1: the role of global and tuned suppression.

The temporal development of neural selectivity to physical attributes of a visual stimulus, such as its orientation and spatial frequency, can provide important clues about mechanisms of cortical tuning. We measured the dynamics of orientation tuning in macaque primary visual cortex (V1) and found several dynamical features in the data: changes in global enhancement and suppression, narrowing of orientation bandwidth, small but significant shifts in preferred orientation, and "Mexican-hat" tuning curves. The dynamics data were analyzed with a model that sums two fixed, tuned components (enhancement and suppression) and one global (untuned) component. The analysis suggests that there is early global enhancement followed by global and tuned suppression. Tuned suppression accounts for the dynamical reduction of orientation bandwidth and for the generation of Mexican-hat tuning profiles. Our findings imply that global and tuned suppression are important factors that determine the selectivity and dynamics of V1 responses to orientation.

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PubMed2002

Neural mechanisms for color perception in the primary visual cortex.

New neurophysiological results show the existence of multiple transformations of color signals in the primary visual cortex (V1) in macaque monkey. These different color mechanisms may contribute separately to the perception of color boundaries and colored regions. Many cells in V1 respond to color and to black-white (luminance) patterns. These neurons are spatially selective and could provide signals about boundaries between differently colored regions. Other V1 neurons that prefer color over luminance respond without much spatial selectivity to colored stimuli, and could be the neural basis for the response to local color modulation within a region. How these different types of color cells combine inputs from cone photoreceptors is what gives them their different spatial selectivities for color.

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PubMed2002

Contrast-dependent changes in spatial frequency tuning of macaque V1 neurons: effects of a changing receptive field size.

Previous studies on single neurons in primary visual cortex have reported that selectivity for orientation and spatial frequency tuning do not change with stimulus contrast. The prevailing hypothesis is that contrast scales the response magnitude but does not differentially affect particular stimuli. Models where responses are normalized over contrast to maintain constant tuning for parameters such as orientation and spatial frequency have been proposed to explain these results. However, our results indicate that a fundamental property of receptive field organization, spatial summation, is not contrast invariant. We examined the spatial frequency tuning of cells that show contrast-dependent changes in spatial summation and have found that spatial frequency selectivity also depends on stimulus contrast. These results indicate that contrast changes in the spatial frequency tuning curves result from spatial reorganization of the receptive field.

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PubMed2002

Receptive field structure of neurons in monkey primary visual cortex revealed by stimulation with natural image sequences.

Probing the visual system with the ensemble of signals that occur in the natural environment may reveal aspects of processing that are not evident in the neural responses to artificial stimulus sets, such as conventional bars and sinusoidal gratings. However, unsolved is the question of how to use complex natural stimulation, many aspects of which the experimenter cannot completely specify, to study neural processing. Here a method is presented to investigate the structure of a neuron's receptive field based on its response to movie clips and other stimulus ensembles. As a particular case, the technique provides an estimate of the conventional first-order receptive field of a neuron, similar to what can be obtained with other reverse-correlation schemes. This is demonstrated experimentally and with computer simulations. Our analysis also revealed that the receptive fields of both simple and complex cells had regions where image boundaries, independent of their contrast sign, would enhance or suppress the cell's response. In some cases, these signals were tuned for the orientation of the boundary. This demonstrates for the first time that it might be feasible to investigate the receptive field structure of visual neurons from their responses to natural image sequences.

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PubMedDate not supplied

Large-scale modeling of the primary visual cortex: influence of cortical architecture upon neuronal response.

A large-scale computational model of a local patch of input layer 4 [Formula: see text] of the primary visual cortex (V1) of the macaque monkey, together with a coarse-grained reduction of the model, are used to understand potential effects of cortical architecture upon neuronal performance. Both the large-scale point neuron model and its asymptotic reduction are described. The work focuses upon orientation preference and selectivity, and upon the spatial distribution of neuronal responses across the cortical layer. Emphasis is given to the role of cortical architecture (the geometry of synaptic connectivity, of the ordered and disordered structure of input feature maps, and of their interplay) as mechanisms underlying cortical responses within the model. Specifically: (i) Distinct characteristics of model neuronal responses (firing rates and orientation selectivity) as they depend upon the neuron's location within the cortical layer relative to the pinwheel centers of the map of orientation preference; (ii) A time independent (DC) elevation in cortico-cortical conductances within the model, in contrast to a "push-pull" antagonism between excitation and inhibition; (iii) The use of asymptotic analysis to unveil mechanisms which underly these performances of the model; (iv) A discussion of emerging experimental data. The work illustrates that large-scale scientific computation--coupled together with analytical reduction, mathematical analysis, and experimental data, can provide significant understanding and intuition about the possible mechanisms of cortical response. It also illustrates that the idealization which is a necessary part of theoretical modeling can outline in sharp relief the consequences of differing alternative interpretations and mechanisms--with final arbiter being a body of experimental evidence whose measurements address the consequences of these analyses.

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PubMedDate not supplied

States of high conductance in a large-scale model of the visual cortex.

This paper reports on the consequences of large, activity dependent, synaptic conductances for neurons in a large-scale neuronal network model of the input layer 4Calpha of the Macaque primary visual cortex (Area V1). This high conductance state accounts for experimental observations about orientation selectivity, dynamics, and response magnitude (D. McLaughlin et al. (2000) Proc. Natl. Acad. Sci. USA 97: 8087-8092), and the linear dependence of Simple cells on visual stimuli (J. Wielaard et al. (2001) J. Neuroscience 21: 5203-5211). The source of large conductances in the model can be traced to inhibitory corticocortical synapses, and the model's predictions of large conductance changes are consistent with recent intracellular measurements (L. Borg-Graham et al. (1998) Nature 393: 369-373; J. Hirsch et al. (1998) J. Neuroscience 15: 9517-9528; J.S. Anderson et al. (2000) J. Neurophysiol. 84: 909-926). During visual stimulation, these conductances are large enough that their associated time-scales become the shortest in the model cortex, even below that of synaptic interactions. One consequence of this activity driven separation of time-scales is that a neuron responds very quickly to temporal changes in its synaptic drive, with its intracellular membrane potential tracking closely an effective reversal potential composed of the instantaneous synaptic inputs. From the effective potential and large synaptic conductance, the spiking activity of a cell can be expressed in an interesting and simplified manner, with the result suggesting how accurate and smoothly graded responses are achieved in the model network. Further, since neurons in this high-conductance state respond quickly, they are also good candidates as coincidence detectors and burst transmitters.

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