Showing papers on "Receptive field published in 1999"
TL;DR: Results suggest that rather than being exclusively feedforward phenomena, nonclassical surround effects in the visual cortex may also result from cortico-cortical feedback as a consequence of the visual system using an efficient hierarchical strategy for encoding natural images.
Abstract: We describe a model of visual processing in which feedback connections from a higher- to a lower- order visual cortical area carry predictions of lower-level neural activities, whereas the feedforward connections carry the residual errors between the predictions and the actual lower-level activities. When exposed to natural images, a hierarchical network of model neurons implementing such a model developed simple-cell-like receptive fields. A subset of neurons responsible for carrying the residual errors showed endstopping and other extra-classical receptive-field effects. These results suggest that rather than being exclusively feedforward phenomena, nonclassical surround effects in the visual cortex may also result from cortico-cortical feedback as a consequence of the visual system using an efficient hierarchical strategy for encoding natural images.
4,149 citations
TL;DR: Non-spatial, feature-based attentional modulation of visual motion processing is demonstrated, and it is shown that attention increases the gain of direction-selective neurons in visual cortical area MT without narrowing the direction-tuning curves.
Abstract: Changes in neural responses based on spatial attention have been demonstrated in many areas of visual cortex, indicating that the neural correlate of attention is an enhanced response to stimuli at an attended location and reduced responses to stimuli elsewhere. Here we demonstrate non-spatial, feature-based attentional modulation of visual motion processing, and show that attention increases the gain of direction-selective neurons in visual cortical area MT without narrowing the direction-tuning curves. These findings place important constraints on the neural mechanisms of attention and we propose to unify the effects of spatial location, direction of motion and other features of the attended stimuli in a 'feature similarity gain model' of attention.
1,435 citations
TL;DR: A quantitative neural model of attention is proposed to account for these findings, which are consistent with the idea that attention biases competitive interactions among neurons, causing them to respond primarily to the attended stimulus.
Abstract: stimulus presented alone inside the receptive field or paired with a second receptive field stimulus, while the monkey attended to a location outside the receptive field. Adding the second stimulus typically caused the neuron’s response to move toward the response that was elicited by the second stimulus alone. Then, we directed the monkey’s attention to one element of the pair. This drove the neuron’s response toward the response elicited when the attended stimulus appeared alone. These findings are consistent with the idea that attention biases competitive interactions among neurons, causing them to respond primarily to the attended stimulus. A quantitative neural model of attention is proposed to account for these results.
1,253 citations
TL;DR: It is found that attention did not cause systematic changes in the undriven activity of the neurons, and selectivity, as measured by the width of its orientation-tuning curve, was not systematically altered by attention.
Abstract: We examined how attention affected the orientation tuning of 262 isolated neurons in extrastriate area V4 and 135 neurons in area V1 of two rhesus monkeys. The animals were trained to perform a delayed match-to-sample task in which oriented stimuli were presented in the receptive field of the neuron being recorded. On some trials the animals were instructed to pay attention to those stimuli, and on other trials they were instructed to pay attention to other stimuli outside the receptive field. In this way, orientation-tuning curves could be constructed from neuronal responses collected in two behavioral states: one in which those stimuli were attended by the animal and one in which those stimuli were ignored by the animal. We fit Gaussians to the neuronal responses to twelve different orientations for each behavioral state. Although attention enhanced the responses of V4 neurons (median 26% increase) and V1 neurons (median 8% increase), selectivity, as measured by the width of its orientation-tuning curve, was not systematically altered by attention. The effects of attention were consistent with a multiplicative scaling of the driven response to all orientations. We also found that attention did not cause systematic changes in the undriven activity of the neurons.
1,215 citations
TL;DR: It is found that the extent of spatial summation in macaque V1 neurons depended on contrast, and was on average 2.3-fold greater at low contrast.
Abstract: Stimulation outside the receptive field of a primary visual cortical (V1) neuron reveals intracortical neural interactions. However, previous investigators implicitly or explicitly considered the extent of cortical spatial summation and, therefore, the size of the classical receptive field to be fixed and independent of stimulus characteristics or of surrounding context. On the contrary, we found that the extent of spatial summation in macaque V1 neurons depended on contrast, and was on average 2.3-fold greater at low contrast. This adaptive increase in spatial summation at low contrast was seen in cells throughout V1 and was independent of surround inhibition.
626 citations
TL;DR: These findings suggest that subthreshold responses in the unresponsive region surrounding the classical discharge field result from the integration of visual activation waves spread by slowly conducting horizontal axons within primary visual cortex.
Abstract: The receptive field of a visual neuron is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. At the cortical level, a challenging issue concerns the roles of feedforward, local recurrent, intracortical, and cortico-cortical feedback connectivity in receptive field properties. Intracellular recordings in cat area 17 showed that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing responses to stimuli flashed at increasing distances from the center of the receptive field decreased in strength, whereas their onset latency increased. These findings suggest that subthreshold responses in the unresponsive region surrounding the classical discharge field result from the integration of visual activation waves spread by slowly conducting horizontal axons within primary visual cortex.
505 citations
TL;DR: It is proposed that the attentional modulation of contextual effects represents a gating of long range horizontal connections within area V1 by feedback connections to V1 and that this gating provides a mechanism for shaping responses under attention to stimulus configuration.
458 citations
TL;DR: The authors showed that complex cells in striate cortex of macaque showed a rapid pattern-specific adaptation, which reduced correlations among the responses of populations of cells, thereby increasing the information transmitted by each action potential.
Abstract: Complex cells in striate cortex of macaque showed a rapid pattern-specific adaptation. Adaptation made cells more sensitive to orientation change near the adapting orientation. It reduced correlations among the responses of populations of cells, thereby increasing the information transmitted by each action potential. These changes were brought about by brief exposures to stationary patterns, on the time scale of a single fixation. Thus, if successive fixations expose neurons' receptive fields to images with similar but not identical structure, adaptation will remove correlations and improve discriminability.
457 citations
TL;DR: The results suggest that in L4 of the barrel cortex synaptic transmission between spiny neurones is largely restricted to a single barrel and the connections are very reliable, probably due to a high release probability, and have a high efficacy because of the compact structure of the dendrites and axons of spiny neurons.
Abstract: Dual whole-cell recordings were made from pairs of synaptically coupled excitatory neurones in the ‘barrel field’ in layer (L) 4 in slices of young (postnatal day 12–15) rat somatosensory cortex. The majority of interconnected excitatory neurones were spiny stellate cells with an asymmetrical dendritic arborisation largely confined to a single barrel. The remainder were star pyramidal cells with a prominent apical dendrite terminating in L2/3 without forming a tuft.
Excitatory synaptic connections were examined between 131 pairs of spiny L4 neurones. Single presynaptic action potentials evoked unitary EPSPs with a peak amplitude of 1·59 ± 1·51 mV (mean ± s.d.), a latency of 0·92 ± 0·35 ms, a rise time of 1·53 ± 0·46 ms and a decay time constant of 17·8 ± 6·3 ms.
At 34–36 °C, the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0·37 ± 0·16 and the percentage of failures to evoke an EPSP was 5·3 ± 7·8%. The c.v. and failure rate decreased with increasing amplitude of the unitary EPSP.
Postsynaptic glutamate receptors in spiny L4 neurones were of the AMPA and NMDA type. At −60 mV in the presence of 1 mM Mg2+, NMDA receptors contributed 39·3 ± 12·5% to the EPSP integral. In Mg2+-free solution, the NMDA receptor/AMPA receptor ratio of the EPSC was 0·86 ± 0·64.
The number of putative synaptic contacts established by the projection neurone with the target neurone varied between two and five with a mean of 3·4 ± 1·0 (n = 11). Synaptic contacts were exclusively found in the barrel in which the cell pair was located and were preferentially located on secondary to quarternary dendritic branches. Their mean geometric distance from the soma was 68·8 ± 37·4 μm (range, 33·4-168·0 μm). The number of synaptic contacts and mean EPSP amplitude showed no significant correlation.
The results suggest that in L4 of the barrel cortex synaptic transmission between spiny neurones is largely restricted to a single barrel. The connections are very reliable, probably due to a high release probability, and have a high efficacy because of the compact structure of the dendrites and axons of spiny neurones. Intrabarrel connections thus function to amplify and distribute the afferent thalamic activity in the vertical directions of a cortical column.
In rodents, whisker-related patterns of neuronal architecture are present at the level of the brainstem trigeminal nuclei, the ventrobasal thalamus and the somatosensory cortex. In 1970, Woolsey & van der Loos described neuronal clusters in layer (L) 4 of the mouse somatosensory cortex that mirror the topography of the whisker pad on the animal's muzzle. These cell clusters were named ‘barrels’ and it is now established that each cortical barrel represents a principal whisker on the contralateral side of the face in a one-to-one relationship (Woolsey & van der Loos, 1970; Welker, 1976; Wallace, 1987; Agmon & Connors, 1991; for a review see Jones & Diamond, 1995). The columnar organisation and receptive field properties of these neurones appear to be comparatively simple. This should facilitate elucidation of cortical signal flow at the cellular level.
Afferent nerve fibres from the thalamic ‘relay nuclei’ terminate in L4 of the sensory cortices (Hubel & Wiesel, 1962; Killackey, 1973; Jensen & Killackey, 1987; Lu & Lin, 1993; for reviews see Ebner & Armstrong-James, 1990; Jones & Diamond, 1995). The primary targets of thalamic afferents are excitatory spiny neurones, the majority being spiny stellate cells while a smaller fraction appears to be made up of star pyramidal cells (Lund, 1984; Ahmed et al. 1994; Hirsch, 1995; Stratford et al. 1996). The first step of intracortical signal processing takes place between synaptically coupled spiny neurones in L4; from there, information is relayed to supragranular laminae (Armstrong-James et al. 1992). The main aim of the present study was to investigate the physiological and morphological factors responsible for fast excitatory synaptic transmission within L4 of the barrel cortex. We used dual whole-cell voltage recordings and morphological reconstruction of biocytin-filled spiny neurones to determine the functional properties of EPSPs and the number and dendritic location of synaptic contacts. The results suggest that synapses of spiny L4 neurones are functionally different from other neocortical synapses (Thomson & West, 1993; Thomson et al. 1996; Markram et al. 1997; for a review see Thomson & Deuchars, 1997). Connected neurones were confined to a single barrel and synaptic contacts were established exclusively within the barrel. In comparison to other cortical connections, unitary EPSPs had a low variability and failure rate and, on occasion, were sufficiently large to evoke action potentials (APs) in the target neurone. These properties render synaptically connected spiny stellate and star pyramidal neurones of the barrel cortex well suited to amplify and subsequently distribute incoming thalamic signals within the cortical column.
Preliminary data from this study have been presented in abstract form (Feldmeyer & Egger, 1998).
435 citations
TL;DR: Recording intracellularly from pairs of neurons less than 500 microm distant from one another in V1 of anesthetized cats revealed significant correlations between the cells in each pair, suggesting local, rather than global, synchrony.
432 citations
TL;DR: Findings show that attention modulates motion processing from stages early in the dorsal visual pathway by selectively enhancing the representation of attended stimuli and simultaneously reducing the influence of unattended stimuli.
Abstract: The visual system is continually inundated with information received by the eyes. Only a fraction of this information appears to reach visual awareness. This process of selection is one of the functions ascribed to visual attention. Although many studies have investigated the role of attention in shaping neuronal representations in cortical areas, few have focused on attentional modulation of neuronal signals related to visual motion. We recorded from 89 direction-selective neurons in middle temporal (MT) and medial superior temporal (MST) visual cortical areas of two macaque monkeys using identical sensory stimulation under various attentional conditions. Neural responses in both areas were greatly influenced by attention. When attention was directed to a stimulus inside the receptive field of a neuron, responses in MT and MST were enhanced an average of 20 and 40% compared with a condition in which attention was directed outside the receptive field. Even stronger average enhancements (70% in MT and 100% in MST) were observed when attention was switched from a stimulus moving in the nonpreferred direction inside the receptive field to another stimulus in the receptive field that was moving in the preferred direction. These findings show that attention modulates motion processing from stages early in the dorsal visual pathway by selectively enhancing the representation of attended stimuli and simultaneously reducing the influence of unattended stimuli.
TL;DR: The main finding is that FS units possess spatially tuned sensory, motor, and delay activity similar to those found in RS units, which establishes that a large part of inhibition in prefrontal cortex is spatially oriented rather than being untuned and simply regulating the threshold response of pyramidal cell output.
Abstract: Studies on the cellular mechanisms of working memory demonstrated that neurons in dorsolateral prefrontal cortex (dPFC) exhibit directionally tuned activity during an oculomotor delayed response. To determine the particular contributions of pyramidal cells and interneurons to spatial tuning in dPFC, we examined both individually and in pairs the tuning properties of regular-spiking (RS) and fast-spiking (FS) units that represent putative pyramidal cells and interneurons, respectively. Our main finding is that FS units possess spatially tuned sensory, motor, and delay activity (i. e., "memory fields") similar to those found in RS units. Furthermore, when recorded simultaneously at the same site, the majority of neighboring neurons, whether FS or RS, displayed isodirectional tuning, i.e., they shared very similar tuning angles for the sensory and delay phases of the task. As the trial entered the response phase of the task, many FS units shifted their direction of tuning and became cross-directional to adjacent RS units by the end of the trial. These results establish that a large part of inhibition in prefrontal cortex is spatially oriented rather than being untuned and simply regulating the threshold response of pyramidal cell output. Moreover, the isodirectional tuning between adjacent neurons supports a functional microcolumnar organization in dPFC for spatial memory fields similar to that found in other areas of cortex for sensory receptive fields.
TL;DR: The results show that RF dimensions are regulated in a dynamic manner that depends both on local stimulus characteristics, such as contrast, and on global relationships between a stimulus and its surroundings.
Abstract: One of the fundamental tasks of the visual cortex is to integrate input from different parts of the retina, parsing an image into contours and surfaces, and then assembling these features into coherent representations of objects. To examine the role of the primary visual cortex in the integration of visual information, we measured the response properties of neurons under different stimulus conditions. Surprisingly, we found that even the most conventional measures of receptive field (RF) size were not fixed, but could vary depending on stimulus contrast and foreground–background relationships. On average, the length of the excitatory RF was 4-fold greater for a low-contrast stimulus than for a stimulus at high contrast. Embedding a high-contrast stimulus in a textured background tended to suppress neuronal responses and produced an enlargement in RF size similar to that observed by decreasing the contrast of an isolated stimulus. The results show that RF dimensions are regulated in a dynamic manner that depends both on local stimulus characteristics, such as contrast, and on global relationships between a stimulus and its surroundings.
TL;DR: It is shown that nearby neurons seldom have very similar receptive fields, when these fields are characterized in space and time, and that spatial phase is the single parameter that accounts for most of the difference between receptive fields of nearby neurons.
Abstract: It is well established that multiple stimulus dimensions (e.g., orientation and spatial frequency) are mapped onto the surface of striate cortex. However, the detailed organization of neurons within a local region of striate cortex remains unclear. Within a vertical column, do all neurons have the same response selectivities? And if not, how do they most commonly differ and why? To address these questions, we recorded from nearby pairs of simple cells and made detailed spatiotemporal maps of their receptive fields. From these maps, we extracted and analyzed a variety of response metrics. Our results provide new insights into the local organization of striate cortex. First, we show that nearby neurons seldom have very similar receptive fields, when these fields are characterized in space and time. Thus, there may be less redundancy within a column than previously thought. Moreover, we show that correlated discharge increases with receptive field similarity; thus, the local dissimilarity between neurons may allow for noise reduction by response pooling. Second, we show that several response variables are clustered within striate cortex, including some that have not received much attention such as response latency and temporal frequency. We also demonstrate that other parameters are not clustered, including the spatial phase (or symmetry) of the receptive field. Third, we show that spatial phase is the single parameter that accounts for most of the difference between receptive fields of nearby neurons. We consider the implications of this local diversity of spatial phase for population coding and construction of higher-order receptive fields.
TL;DR: The effects of contextual modulation in area V1 of anesthetized macaque monkeys are studied, suggesting that physiological mechanisms contributing to the saliency ("popout") of textural stimuli operate, at least to some degree, even under anesthesia.
Abstract: We studied the effects of contextual modulation in area V1 of anesthetized macaque monkeys. In 146 cells, responses to a single line over the center of the receptive field were compared with those to full texture patterns in which the center line was surrounded by similar lines at either the same orientation ( uniform texture )o r the orthogonal orientation (orientation contrast). On average, the responses to single lines were reduced by 42% when texture was presented in the surround. Uniform textures often produced stronger suppression (7% more, on average) so that lines with orientation contrast on average evoked larger responses than lines in uniform texture fields. This difference is correlated with perceptual differences between such stimuli, suggesting that physiological mechanisms contributing to the saliency (“popout”) of textural stimuli operate, at least to some degree, even under anesthesia. Significant response modulation by the texture surround was seen in 112 cells (77%). Fifty-three cells (36%) responded differently to the two texture patterns; response preferences for orientation contrast (35 cells; 24%) were seen more often than preferences for uniform textures (18 cells; 12%). The remaining 59 cells (40%) were similarly suppressed by both texture surrounds. Detailed analysis of texture modulation revealed two major components of surround effects: (1) fast nonspecific (“general”) suppression that occurred at about the same latency as excitatory responses and was found in all layers of striate cortex; and (2) differential response modulation that began about 60‐70 ms after stimulus onset (about 15‐20 ms after the onset of the excitatory response) and was less homogeneously distributed over cortical layers.
TL;DR: Neurons in the brain of macaque monkeys that represent the auditory space surrounding the head, within roughly 30 cm, have spatial receptive fields that extend a limited distance outward from the head.
Abstract: Humans can accurately perceive the location of a sound source—not only the direction, but also the distance1,2,3,4,5,6,7,8,9. Sounds near the head, within ducking or reaching distance, have a special saliency. However, little is known about this perception of auditory distance. The direction to a sound source can be determined by interaural differences, and the mechanisms of direction perception have been studied intensively1; but except for studies on echolocation in the bat10, little is known about how neurons encode information on auditory distance. Here we describe neurons in the brain of macaque monkeys (Macaca fascicularis) that represent the auditory space surrounding the head, within roughly 30 cm. These neurons, which are located in the ventral premotor cortex, have spatial receptive fields that extend a limited distance outward from the head.
TL;DR: It is concluded that the PrV input mediates the near "one-to-one" correspondence between a neuronal response in V PM and a single mystacial whisker and alterations in VPM responses produced by changing the depth of anesthesia are due to its selective influence on the properties mediated by SpVi inputs at the level of the thalamus.
Abstract: Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia. The dominant frequency of electrocorticographic (ECoG) recordings was used to determine the de...
TL;DR: It is concluded that neurons in rat V1 have complex and diverse visual properties, necessary for precise visual form perception with low spatial resolution.
Abstract: The rat is used widely to study various aspects of vision including developmental events and numerous pathologies, but surprisingly little is known about the functional properties of single neurons...
TL;DR: By fitting the data to a linear filter model, the effect of contrast on the temporal-frequency response is subsumed into a change in a single parameter in the model, and the dynamic effect of Contrast on the M cells' response is demonstrated.
Abstract: The retinal ganglion cells (RGCs) of the primate form at least two classes--M and P--that differ fundamentally in their functional properties M cells have temporal-frequency response characteristics distinct from P cells (Benardete et al, 1992; Lee et al, 1994) In this paper, we elaborate on the temporal-frequency responses of M cells and focus in detail on the contrast gain control (Shapley & Victor, 1979a,b) Earlier data showed that the temporal-frequency response of M cells is altered by the level of stimulus contrast (Benardete et al, 1992) Higher contrast shifts the peak of the frequency-response curve to higher temporal frequency and produces a phase advance In this paper, by fitting the data to a linear filter model, the effect of contrast on the temporal-frequency response is subsumed into a change in a single parameter in the model Furthermore, the model fits are used to predict the response of M cells to steps of contrast, and these predictions demonstrate the dynamic effect of contrast on the M cells' response We also present new data concerning the spatial organization of the contrast gain control in the primate and show that the signal that controls the contrast gain must come from a broadly distributed network of small subunits in the surround of the M-cell receptive field
TL;DR: The results suggest that although geniculate neurons often receive input from several retinal afferents, these multiple afferentials represent a select subset of the retinal ganglion cells with overlapping receptive-field centers.
Abstract: Retinal ganglion cells and their target neurons in the principal layers of the lateral geniculate nucleus (LGN) of the thalamus have very similar, center-surround receptive fields. Although some geniculate neurons are dominated by a single retinal afferent, others receive both strong and weak inputs from several retinal afferents. In the present study, experiments were performed in the cat that examined the specificity and strength of monosynaptic connections between retinal ganglion cells and their target neurons. The responses of 205 pairs of retinal ganglion cells and geniculate neurons with overlapping receptive-field centers or surrounds were studied. Receptive fields were mapped quantitatively using a white-noise stimulus; connectivity was assessed by cross-correlating the retinal and geniculate spike trains. Of the 205 pairs, 12 were determined to have monosynaptic connections. Both the likelihood that cells were connected and the strength of connections increased with increasing similarity between retinal and geniculate receptive fields. Connections were never found between cells with <50% spatial overlap between their centers. The results suggest that although geniculate neurons often receive input from several retinal afferents, these multiple afferents represent a select subset of the retinal ganglion cells with overlapping receptive-field centers.
TL;DR: The brain location, extent and functional organization of the cortical visual area V6A was investigated in macaque monkeys by using single cell recording techniques in awake, behaving animals in the view that this area could be involved in the control of reaching out and grasping objects.
Abstract: The brain location, extent and functional organization of the cortical visual area V6A was investigated in macaque monkeys by using single cell recording techniques in awake, behaving animals. Six hemispheres of four animals were studied. Area V6A occupies a horseshoe-like region of cortex in the caudalmost part of the superior parietal lobule. It extends from the medial surface of the brain, through the anterior bank of the parieto-occipital sulcus, up to the most lateral part of the fundus of the same sulcus. Area V6A borders on areas V6 ventrally, PEc dorsally, PGm medially and MIP laterally. Of 1348 neurons recorded in V6A, 61% were visual and 39% non-visual in nature. The visual neurons were particularly sensitive to orientation and direction of movement of visual stimuli. The inferior contralateral quadrant was the most represented one. Visual receptive fields were also found in the inferior ipsilateral quadrant and in the upper visual field. Receptive fields were on average smaller in the lower visual field than in the upper one. Both central and peripheral parts of the visual field were represented. Large parts of the visual field were represented in small regions of area V6A, and the same regions of the visual field were re-represented many times in different parts of this area, without any apparent topographical order. The only reliable sign of retinotopic organization was the predominance of central representation dorsally and far periphery ventrally. The functional organization of area V6A is discussed in the view that this area could be involved in the control of reaching out and grasping objects.
TL;DR: The spatial organization of surrounds of single-cell receptive fields in the primary visual cortex of anesthetized, paralyzed cats is examined to demonstrate that the surrounds are more specific than previously realized, and this specialization has implications for the processing of visual information in thePrimary visual cortex.
Abstract: Areas beyond the classical receptive field (CRF) can modulate responses of the majority of cells in the primary visual cortex of the cat (). Although general characteristics of this phenomenon have been reported previously, little is known about the detailed spatial organization of the surrounds. Previous work suggests that the surrounds may be uniform regions that encircle the CRF or may be limited to the "ends" of the CRF. We have examined the spatial organization of surrounds of single-cell receptive fields in the primary visual cortex of anesthetized, paralyzed cats. The CRF was stimulated with an optimal drifting grating, whereas the surround was probed with a second small grating patch placed at discrete locations around the CRF. For most cells that exhibit suppression, the surrounds are spatially asymmetric, such that the suppression originates from a localized region. We find a variety of suppressive zone locations, but there is a slight bias for suppression to occur at the end zones of the CRF. The spatial pattern of suppression is independent of the parameters of the suppressive stimulus used, although the effect is clearest with iso-oriented surround stimuli. A subset of cells exhibit axially symmetric or uniform surround fields. These results demonstrate that the surrounds are more specific than previously realized, and this specialization has implications for the processing of visual information in the primary visual cortex. One possibility is that these localized surrounds may provide a substrate for figure-ground segmentation of visual scenes.
TL;DR: The result suggests that the thalamic plasticity that is seen immediately after a peripheral deafferentation is dependent upon both descending corticofugal projections and ascending trigeminothalamic projections.
Abstract: Multiple neuron ensemble recordings were obtained simultaneously from both the primary somatosensory (SI) cortex and the ventroposterior medial thalamus (VPM) before and during the combined administration of reversible inactivation of the SI cortex and a reversible subcutaneous block of peripheral trigeminal nerve fibers. This procedure was performed to quantify the contribution of descending corticofugal projections on (i) the normal organization of thalamic somatosensory receptive fields and (ii) the thalamic somatosensory plastic reorganization that immediately follows a peripheral deafferentation. Reversible inactivation of SI cortex resulted in immediate changes in receptive field properties throughout the VPM. Cortical inactivation also significantly reduced but did not completely eliminate the occurrence of VPM receptive field reorganization resulting from the reversible peripheral deafferentation. This result suggests that the thalamic plasticity that is seen immediately after a peripheral deafferentation is dependent upon both descending corticofugal projections and ascending trigeminothalamic projections.
TL;DR: The link between morphology and physiology for some of the cell types of the macaque monkey retina is reviewed with emphasis on understanding the neural mechanism for spectral opponency in the light response of ganglion cells.
TL;DR: The spatial extent over which one stimulus would divide the response to another and found effective normalization from stimuli quite far removed from the RF center is found, which supports models under which normalization occurs both in MT and in earlier stages.
Abstract: Receptive fields (RFs) of cells in the middle temporal area (MT or V5) of monkeys will often encompass multiple objects under normal image viewing. We therefore have studied how multiple moving stimuli interact when presented within and near the RF of single MT cells. We used moving Gabor function stimuli, <1 degrees in spatial extent and approximately 100 msec in duration, presented on a grid of possible locations over the RF of the cell. Responses to these stimuli were typically robust, and their small spatial and temporal extent allowed detailed mapping of RFs and of interactions between stimuli. The responses to pairs of such stimuli were compared against the responses to the same stimuli presented singly. The responses were substantially less than the sum of the responses to the component stimuli and were well described by a power-law summation model with divisive inhibition. Such divisive inhibition is a key component of recently proposed "normalization" models of cortical physiology and is presumed to arise from lateral interconnections within a region. One open question is whether the normalization occurs only once in primary visual cortex or multiple times in different cortical areas. We addressed this question by exploring the spatial extent over which one stimulus would divide the response to another and found effective normalization from stimuli quite far removed from the RF center. This supports models under which normalization occurs both in MT and in earlier stages.
TL;DR: The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used.
TL;DR: This work has confirmed that nonlinear responses persisted in OFF ganglion cells when ON bipolar cell responses were blocked by l-AP-4, and suggested that the single class of bipolar cells (either ON or OFF) that drives the center/surround response also drives the nonlinear response.
Abstract: A retinal ganglion cell commonly expresses two spatially overlapping receptive field mechanisms. One is the familiar “center/ surround,” which sums excitation and inhibition across a region somewhat broader than the ganglion cell’s dendritic field. This mechanism responds to a drifting grating by modulating firing at the drift frequency (linear response). Less familiar is the “nonlinear” mechanism, which sums the rectified output of many small subunits that extend for millimeters beyond the dendritic field. This mechanism responds to a contrastreversing grating by modulating firing at twice the reversal frequency (nonlinear response). We investigated this nonlinear mechanism by presenting visual stimuli to the intact guinea pig retina in vitro while recording intracellularly from large brisk and sluggish ganglion cells. A contrast-reversing grating modulated the membrane potential (in addition to the firing rate) at twice the reversal frequency. This response was initially hyperpolarizing for some cells (either ON or OFF center) and initially depolarizing for others. Experiments in which responses to bars were summed in-phase or out-of-phase suggested that the single class of bipolar cells (either ON or OFF) that drives the center/surround response also drives the nonlinear response. Consistent with this, nonlinear responses persisted in OFF ganglion cells when ON bipolar cell responses were blocked by L-AP-4. Nonlinear responses evoked from millimeters beyond the ganglion cell were eliminated by tetrodotoxin. Thus, to relay the response from distant regions of the receptive field requires a spiking interneuron. Nonlinear responses from different regions of the receptive field added linearly.
TL;DR: The results suggest that spatial attention alone is not sufficient to induce strong attentional effects in MT even when two competing motion stimuli appear within the RF of the recorded neuron.
Abstract: This study examines the influence of spatial attention on the responses of neurons in the middle temporal visual area (MT or V5) of extrastriate cortex. Two monkeys were trained to perform a direction-discrimination task. On each trial, two apertures of random-dot stimuli appeared simultaneously at two spatially separated locations; the monkeys were required to discriminate the direction of stimulus motion at one location while ignoring the stimulus at the other location. After extensive training, we recorded the responses of MT neurons in two configurations: 1) Both apertures placed "within" the neuron's receptive field (RF) and 2) one aperture covering the RF while the other was presented at a "remote" location. For each unit we compared the responses to identical stimulus displays when the monkey was instructed to attend to one or the other aperture. The responses of MT neurons were 8.7% stronger, on average, when the monkey attended to the spatial location that contained motion in the "preferred" direction. Attentional effects were equal, on average, in the within RF and remote configurations. The attentional modulations began approximately 300 ms after stimulus onset, gradually increased throughout the trial, and peaked near stimulus offset. An analysis of the neuronal responses on error trials suggests that the monkeys failed to attend to the appropriate spatial location on these trials. The relatively weak attentional effects that we observed contrast strikingly with recent results of Treue and Maunsell, who demonstrated very strong attentional modulations (median effect >80%) in MT in a task that shares many features with ours. Our results suggest that spatial attention alone is not sufficient to induce strong attentional effects in MT even when two competing motion stimuli appear within the RF of the recorded neuron. The difference between our results and those of Treue and Maunsell suggests that the magnitude of the attentional effects in MT may depend critically on how attention is directed to a particular stimulus and on the precise demands of the task.
TL;DR: It is suggested that brightness information is explicitly represented in the responses of neurons in striate cortex as part of a neural representation of object surfaces.
Abstract: Brightness changes can be induced in a static gray field by modulating the luminance of surrounding areas. We used this induction phenomenon to investigate the neural representation of perceived brightness. Extracellular recordings were made in striate cortex, the lateral geniculate nucleus (LGN), and the optic tract of anesthetized cats using stimuli that produced brightness induction. While a cell’s receptive field (RF) was covered by uniform gray illumination, the luminance of rectangular flanking regions was modulated sinusoidally in time, inducing brightness changes in the RF. We looked for a correspondence between the modulation of a cell’s response and stimulus conditions that did or did not produce perceptual changes in brightness. We found that the responses of retinal ganglion cell axons in the optic tract were never correlated with brightness. On the other hand, many neurons in striate cortex and a small fraction in the LGN responded in a phase-locked manner at the temporal frequency of the flank modulation, even though the flanks were 3–7° beyond the edges of the RF. Only in striate cortex were cells found that had responses correlated with brightness under all stimulus conditions. These findings suggest that brightness information is explicitly represented in the responses of neurons in striate cortex as part of a neural representation of object surfaces.
TL;DR: A nonhomogeneous topographic distribution of inhibitory properties is consistent with the hypothesis that cat primary auditory cortex (A1) is composed of at least two functionally distinct subdivisions that may be part of different auditory cortical processing streams.
Abstract: Based on properties of excitatory frequency (spectral) receptive fields (esRFs), previous studies have indicated that cat primary auditory cortex (A1) is composed of functionally distinct dorsal an...