Showing papers on "Receptive field published in 2009"

High-frequency, long-range coupling between prefrontal and visual cortex during attention.

Abstract: Electrical recordings in humans and monkeys show attentional enhancement of evoked responses and gamma synchrony in ventral stream cortical areas. Does this synchrony result from intrinsic activity in visual cortex or from inputs from other structures? Using paired recordings in the frontal eye field (FEF) and area V4, we found that attention to a stimulus in their joint receptive field leads to enhanced oscillatory coupling between the two areas, particularly at gamma frequencies. This coupling appeared to be initiated by FEF and was time-shifted by about 8 to 13 milliseconds across a range of frequencies. Considering the expected conduction and synaptic delays between the areas, this time-shifted coupling at gamma frequencies may optimize the postsynaptic impact of spikes from one area upon the other, improving cross-area communication with attention.

Stimulus contrast modulates functional connectivity in visual cortex

Abstract: By simultaneously recording spikes and local field potentials (LFPs) in cat and monkey visual cortex, the authors demonstrate that the magnitude and spread of LFP waves from the originating spike are reduced with increasing stimulus contrast. This suggests that visual cortex functional connectivity is not fixed, but is instead modulated by stimulus contrast.

Visual field maps, population receptive field sizes, and visual field coverage in the human MT+ complex.

Abstract: Human neuroimaging experiments typically localize motion-selective cortex (MT+) by contrasting responses to stationary and moving stimuli. It has long been suspected that MT+, located on the lateral surface at the temporal–occipital (TO) boundary, contains several distinct visual field maps, although only one coarse map has been measured. Using a novel functional MRI model–based method we identified two maps—TO-1 and TO-2—and measured population receptive field (pRF) sizes within these maps. The angular representation of the first map, TO-1, has a lower vertical meridian on its posterior side at the boundary with the lateral–occipital cortex (i.e., the LO-2 portion). The angular representation continues through horizontal to the upper vertical meridian at the boundary with the second map, TO-2. The TO-2 angle map reverses from upper to lower visual field at increasingly anterior positions. The TO maps share a parallel eccentricity map in which center-to-periphery is represented in the ventral-to-dorsal direction; both maps have an expanded foveal representation. There is a progressive increase in the pRF size from V1/2/3 to LO-1/2 and TO-1/2, with the largest pRF sizes in TO-2. Further, within each map the pRF size increases as a function of eccentricity. The visual field coverage of both maps extends into the ipsilateral visual field, with larger sensitivity to peripheral ipsilateral stimuli in TO-2 than that in TO-1. The TO maps provide a functional segmentation of human motion-sensitive cortex that enables a more complete characterization of processing in human motion-selective cortex.

A normalization model of attentional modulation of single unit responses.

Abstract: Although many studies have shown that attention to a stimulus can enhance the responses of individual cortical sensory neurons, little is known about how attention accomplishes this change in response. Here, we propose that attention-based changes in neuronal responses depend on the same response normalization mechanism that adjusts sensory responses whenever multiple stimuli are present. We have implemented a model of attention that assumes that attention works only through this normalization mechanism, and show that it can replicate key effects of attention. The model successfully explains how attention changes the gain of responses to individual stimuli and also why modulation by attention is more robust and not a simple gain change when multiple stimuli are present inside a neuron's receptive field. Additionally, the model accounts well for physiological data that measure separately attentional modulation and sensory normalization of the responses of individual neurons in area MT in visual cortex. The proposal that attention works through a normalization mechanism sheds new light a broad range of observations on how attention alters the representation of sensory information in cerebral cortex.

Receptive field properties of ON- and OFF-ganglion cells in the mouse retina.

Abstract: There are two subclasses of alpha cell in the mammalian retina, which are morphologically identical in plain view but have opposite responses to a luminance change: one is ON center and the other is OFF center. Recent studies have shown that the neural circuitries, which underlie light responses in such ON- and OFF-ganglion cell pairs, are not mirror symmetric with respect to the ON and OFF pathways (Pang et al., 2003; Zaghloul et al., 2003; Murphy & Rieke, 2006). This study examines alpha-cell homologues in the mouse retina and elucidates the synaptic mechanisms that generate their light responses. Morphological analysis of recorded cells revealed three subclasses that were essentially identical in plan view but had distinct vertical stratification levels. We refer to these cells as the sustained ON (ON-S), sustained OFF (OFF-S), and transient OFF (OFF-T) cells (Murphy & Rieke, 2006; Margolis & Detwiler, 2007). Both ON-S and OFF-S cells were largely driven through the ON pathway via changes in excitatory and inhibitory inputs, respectively. Light responses of OFF-T cells were driven by transient changes in excitatory and inhibitory inputs. Light responses of OFF-S cells were also measured in connexin 36 knockout mice in order to dissect glycinergic input arising from AII amacrine cells. At photopic/mesopic intensities, peak glycinergic input to OFF-S cells in the connexin 36 knockout mouse was reduced by ~85% compared to OFF-S cells in the wild-type retina. This is consistent with the idea that AII cells receive their input from ON-cone bipolar cells through gap junctions and in turn provide glycinergic inhibition to OFF-S cells.

Influence of context and behavior on stimulus reconstruction from neural activity in primary auditory cortex.

Abstract: Population responses of cortical neurons encode considerable details about sensory stimuli, and the encoded information is likely to change with stimulus context and behavioral conditions. The details of encoding are difficult to discern across large sets of single neuron data because of the complexity of naturally occurring stimulus features and cortical receptive fields. To overcome this problem, we used the method of stimulus reconstruction to study how complex sounds are encoded in primary auditory cortex (AI). This method uses a linear spectro-temporal model to map neural population responses to an estimate of the stimulus spectrogram, thereby enabling a direct comparison between the original stimulus and its reconstruction. By assessing the fidelity of such reconstructions from responses to modulated noise stimuli, we estimated the range over which AI neurons can faithfully encode spectro-temporal features. For stimuli containing statistical regularities (typical of those found in complex natural sounds), we found that knowledge of these regularities substantially improves reconstruction accuracy over reconstructions that do not take advantage of this prior knowledge. Finally, contrasting stimulus reconstructions under different behavioral states showed a novel view of the rapid changes in spectro-temporal response properties induced by attentional and motivational state.

Rapid synaptic depression explains nonlinear modulation of spectro-temporal tuning in primary auditory cortex by natural stimuli.

Abstract: In this study, we explored ways to account more accurately for responses of neurons in primary auditory cortex (A1) to natural sounds. The auditory cortex has evolved to extract behaviorally relevant information from complex natural sounds, but most of our understanding of its function is derived from experiments using simple synthetic stimuli. Previous neurophysiological studies have found that existing models, such as the linear spectro-temporal receptive field (STRF), fail to capture the entire functional relationship between natural stimuli and neural responses. To study this problem, we compared STRFs for A1 neurons estimated using a natural stimulus, continuous speech, with STRFs estimated using synthetic ripple noise. For about one-third of the neurons, we found significant differences between STRFs, usually in the temporal dynamics of inhibition and/or overall gain. This shift in tuning resulted primarily from differences in the coarse temporal structure of the speech and noise stimuli. Using simulations, we found that the stimulus dependence of spectro-temporal tuning can be explained by a model in which synaptic inputs to A1 neurons are susceptible to rapid nonlinear depression. This dynamic reshaping of spectro-temporal tuning suggests that synaptic depression may enable efficient encoding of natural auditory stimuli.

Color perception in the intermediate periphery of the visual field.

Abstract: Color perception changes across the visual field. It is best in the fovea and declines in the periphery. Sensitivity to red-green color variations declines more steeply toward the periphery than sensitivity to luminance or blue-yellow colors. It is thought that this decline is due to the increasing size of receptive fields of parvocellular retinal ganglion cells and the unselective or random contribution of L- and M-cones to the receptive field surround. In earlier psychophysical studies it has been found that L - M cone opponency becomes absent above 30 deg. However, physiological experiments in macaque monkeys have shown that midget ganglion cells exist in the intermediate zone of the peripheral retina (20-50 deg) that are strongly cone opponent. Here we explore this contradiction between physiological and psychophysical research, using stimuli of variable size at eccentricities of up to 50 deg. We found that chromatic detection gets worse with increasing eccentricity but is still possible even at large eccentricities. Our results show that chromatic detection at these eccentricities is mediated by cone-opponent mechanisms.

Visual receptive field structure of cortical inhibitory neurons revealed by two-photon imaging guided recording.

Abstract: Synaptic inhibition plays an important role in shaping receptive field (RF) properties in the visual cortex. However, the underlying mechanisms remain not well understood, partly because of difficulties in systematically studying functional properties of cortical inhibitory neurons in vivo. Here, we established two-photon imaging guided cell-attached recordings from genetically labeled inhibitory neurons and nearby "shadowed" excitatory neurons in the primary visual cortex of adult mice. Our results revealed that in layer 2/3, the majority of excitatory neurons exhibited both On and Off spike subfields, with their spatial arrangement varying from being completely segregated to overlapped. In contrast, most layer 4 excitatory neurons exhibited only one discernable subfield. Interestingly, no RF structure with significantly segregated On and Off subfields was observed for layer 2/3 inhibitory neurons of either the fast-spike or regular-spike type. They predominantly possessed overlapped On and Off subfields with a significantly larger size than the excitatory neurons and exhibited much weaker orientation tuning. These results from the mouse visual cortex suggest that different from the push-pull model proposed for simple cells, layer 2/3 simple-type neurons with segregated spike On and Off subfields likely receive spatially overlapped inhibitory On and Off inputs. We propose that the phase-insensitive inhibition can enhance the spatial distinctiveness of On and Off subfields through a gain control mechanism.

High-Frequency, Long-Range Coupling Between Prefrontal and Visual Cortex During Attention

Abstract: Electrical recordings in humans and monkeys show attentional enhancement of evoked responses and gamma synchrony in ventral stream cortical areas. Does this synchrony result from intrinsic activity in visual cortex or from inputs from other structures? Using paired recordings in the frontal eye field (FEF) and area V4, we found that attention to a stimulus in their joint receptive field leads to enhanced oscillatory coupling between the two areas, particularly at gamma frequencies. This coupling appeared to be initiated by FEF and was time-shifted by about 8 to 13 milliseconds across a range of frequencies. Considering the expected conduction and synaptic delays between the areas, this time-shifted coupling at gamma frequencies may optimize the postsynaptic impact of spikes from one area upon the other, improving cross-area communication with attention.

Comparison of Spatial Summation Properties of Neurons in Macaque V1 and V2

Abstract: In visual cortex, responses to stimulation of the receptive field (RF) are modulated by simultaneous stimulation of the RF surround. The mechanisms for surround modulation remain unidentified. We previously proposed that in the primary visual cortex (V1), near surround modulation is mediated by geniculocortical and horizontal connections and far surround modulation by interareal feedback connections. To understand spatial integration in the secondary visual cortex (V2) and its underlying circuitry, we have characterized spatial summation in different V2 layers and stripe compartments and compared it to that in V1. We used grating stimuli in circular and annular apertures of different sizes to estimate the extent and sensitivity of RF and surround components in V1 and V2. V2 RFs and surrounds were twice as large as those in V1. As in V1, V2 RFs doubled in size when measured at low contrast. In both V1 and V2, surrounds were about fivefold the size of the RF and the far surround could exceed 12.5° in radius, averaging 5.5° in V1 and 9.2° in V2. The strength of surround suppression was similar in both areas. Thus although differing in spatial scale, the interactions among RF components are similar in V1 and V2, suggesting similar underlying mechanisms. As in V1, the extent of V2 horizontal connections matches that of the RF center, but is much smaller than the largest far surrounds, which likely derive from interareal feedback. In V2, we found no laminar or stripe differences in size and magnitude of surround suppression, suggesting conservation across stripes of the basic circuit for surround modulation.

Receptive fields and functional architecture in the retina

Abstract: Functional architecture of the striate cortex is known mostly at the tissue level – how neurons of different function distribute across its depth and surface on a scale of millimetres. But explanations for its design – why it is just so – need to be addressed at the synaptic level, a much finer scale where the basic description is still lacking. Functional architecture of the retina is known from the scale of millimetres down to nanometres, so we have sought explanations for various aspects of its design. Here we review several aspects of the retina's functional architecture and find that all seem governed by a single principle: represent the most information for the least cost in space and energy. Specifically: (i) why are OFF ganglion cells more numerous than ON cells? Because natural scenes contain more negative than positive contrasts, and the retina matches its neural resources to represent them equally well; (ii) why do ganglion cells of a given type overlap their dendrites to achieve 3-fold coverage? Because this maximizes total information represented by the array – balancing signal-to-noise improvement against increased redundancy; (iii) why do ganglion cells form multiple arrays? Because this allows most information to be sent at lower rates, decreasing the space and energy costs for sending a given amount of information. This broad principle, operating at higher levels, probably contributes to the brain's immense computational efficiency.

Noise Correlations in Cortical Area MT and Their Potential Impact on Trial-by-Trial Variation in the Direction and Speed of Smooth-Pursuit Eye Movements

Abstract: Smooth-pursuit eye movements are variable, even when the same tracking target motion is repeated many times. We asked whether variation in pursuit could arise from noise in the response of visual motion neurons in the middle temporal visual area (MT). In physiological experiments, we evaluated the mean, variance, and trial-by-trial correlation in the spike counts of pairs of simultaneously recorded MT neurons. The correlations between responses of pairs of MT neurons are highly significant and are stronger when the two neurons in a pair have similar preferred speeds, directions, or receptive field locations. Spike count correlation persists when the same exact stimulus form is repeatedly presented. Spike count correlations increase as the analysis window increases because of correlations in the responses of individual neurons across time. Spike count correlations are highest at speeds below the preferred speeds of the neuron pair and increase as the contrast of a square-wave grating is decreased. In computational analyses, we evaluated whether the correlations and variation across the population response in MT could drive the observed behavioral variation in pursuit direction and speed. We created model population responses that mimicked the mean and variance of MT neural responses as well as the observed structure and amplitude of noise correlations between pairs of neurons. A vector-averaging decoding computation revealed that the observed variation in pursuit could arise from the MT population response, without postulating other sources of motor variation.

Receptive fields in primate retina are coordinated to sample visual space more uniformly.

Abstract: In the visual system, large ensembles of neurons collectively sample visual space with receptive fields (RFs). A puzzling problem is how neural ensembles provide a uniform, high-resolution visual representation in spite of irregularities in the RFs of individual cells. This problem was approached by simultaneously mapping the RFs of hundreds of primate retinal ganglion cells. As observed in previous studies, RFs exhibited irregular shapes that deviated from standard Gaussian models. Surprisingly, these irregularities were coordinated at a fine spatial scale: RFs interlocked with their neighbors, filling in gaps and avoiding large variations in overlap. RF shapes were coordinated with high spatial precision: the observed uniformity was degraded by angular perturbations as small as 15°, and the observed populations sampled visual space with more than 50% of the theoretical ideal uniformity. These results show that the primate retina encodes light with an exquisitely coordinated array of RF shapes, illustrating a higher degree of functional precision in the neural circuitry than previously appreciated.

Attention Reshapes Center-Surround Receptive Field Structure in Macaque Cortical Area MT

Abstract: Directing spatial attention to a location inside the classical receptive field (cRF) of a neuron in macaque medial temporal area (MT) shifts the center of the cRF toward the attended location Here we investigate the influence of spatial attention on the profile of the inhibitory surround present in many MT neurons Two monkeys attended to the fixation point or to 1 of 2 random dot patterns (RDPs) placed inside or next to the cRF, whereas a third RDP (the probe) was briefly presented in quick succession across the cRF and surround The probe presentation responses were used to compute a map of the excitatory receptive field and its inhibitory surround Attention systematically reshapes the receptive field profile, independently shifting both center and surround toward the attended location Furthermore, cRF size is changed as a function of relative distance to the attentional focus: attention inside the cRF shrinks it, whereas directing attention next to the cRF expands it In addition, we find systematic changes in surround inhibition and cRF amplitude This nonmultiplicative push-pull modulation of the receptive field's center-surround structure optimizes processing at and near the attentional focus to strengthen the representation of the attended stimulus while reducing influences from distractors

Parallel ON and OFF cone bipolar inputs establish spatially coextensive receptive field structure of blue-yellow ganglion cells in primate retina.

Abstract: In the primate retina the small bistratified, "blue-yellow" color-opponent ganglion cell receives parallel ON-depolarizing and OFF-hyperpolarizing inputs from short (S)-wavelength sensitive and combined long (L)- and middle (M)-wavelength sensitive cone photoreceptors, respectively. However, the synaptic pathways that create S versus LM cone-opponent receptive field structure remain controversial. Here, we show in the macaque monkey retina in vitro that at photopic light levels, when an identified rod input is excluded, the small bistratified cell displays a spatially coextensive receptive field in which the S-ON-input is in spatial, temporal, and chromatic balance with the LM-OFF-input. ON pathway block with l-AP-4, the mGluR6 receptor agonist, abolished the S-ON response but spared the LM-OFF response. The isolated LM component showed a center-surround receptive field structure consistent with an input from OFF-center, ON-surround "diffuse" cone bipolar cells. Increasing retinal buffering capacity with HEPES attenuated the LM-ON surround component, consistent with a non-GABAergic outer retina feedback mechanism for the bipolar surround. The GABAa/c receptor antagonist picrotoxin and the glycine receptor antagonist strychnine did not affect chromatic balance or the basic coextensive receptive field structure, suggesting that the LM-OFF field is not generated by an inner retinal inhibitory pathway. We conclude that the opponent S-ON and LM-OFF responses originate from the excitatory receptive field centers of S-ON and LM-OFF cone bipolar cells, and that the LM-OFF- and ON-surrounds of these parallel bipolar inputs largely cancel, explaining the small, spatially coextensive but spectrally antagonistic receptive field structure of the blue-ON ganglion cell.

Transformation of Polarized Light Information in the Central Complex of the Locust

Abstract: Many insects perceive the E-vector orientation of polarized skylight and use it for compass navigation. In locusts, polarized light is detected by photoreceptors of the dorsal rim area of the eye. Polarized light signals from both eyes are integrated in the central complex (CC), a group of neuropils in the center of the brain. Thirteen types of CC neuron are sensitive to dorsally presented, polarized light (POL-neurons). These neurons interconnect the subdivisions of the CC, particularly the protocerebral bridge (PB), the upper and lower divisions of the central body (CBU, CBL), and the adjacent lateral accessory lobes (LALs). All POL-neurons show polarization-opponency, i.e., receive excitatory and inhibitory input at orthogonal E-vector orientations. To provide physiological evidence for the direction of information flow through the polarization vision network in the CC, we analyzed the functional properties of the different cell types through intracellular recordings. Tangential neurons of the CBL showed highest signal-to-noise ratio, received either ipsilateral polarized-light input only or, together with CL1 columnar neurons, had eccentric receptive fields. Bilateral polarized-light inputs with zenith-centered receptive fields were found in tangential neurons of the PB and in columnar neurons projecting to the LALs. Together with other physiological parameters, these data suggest a flow of information from the CBL (input) to the PB and from here to the LALs (output). This scheme is supported by anatomical data and suggests transformation of purely sensory E-vector coding at the CC input stage to position-invariant coding of 360 degrees -compass directions at the output stage.

Hierarchical computation in the canonical auditory cortical circuit.

Abstract: Sensory cortical anatomy has identified a canonical microcircuit underlying computations between and within layers. This feed-forward circuit processes information serially from granular to supragranular and to infragranular layers. How this substrate correlates with an auditory cortical processing hierarchy is unclear. We recorded simultaneously from all layers in cat primary auditory cortex (AI) and estimated spectrotemporal receptive fields (STRFs) and associated nonlinearities. Spike-triggered averaged STRFs revealed that temporal precision, spectrotemporal separability, and feature selectivity varied with layer according to a hierarchical processing model. STRFs from maximally informative dimension (MID) analysis confirmed hierarchical processing. Of two cooperative MIDs identified for each neuron, the first comprised the majority of stimulus information in granular layers. Second MID contributions and nonlinear cooperativity increased in supragranular and infragranular layers. The AI microcircuit provides a valid template for three independent hierarchical computation principles. Increases in processing complexity, STRF cooperativity, and nonlinearity correlate with the synaptic distance from granular layers.

Visual avoidance in Xenopus tadpoles is correlated with the maturation of visual responses in the optic tectum.

Abstract: The optic tectum is central for transforming incoming visual input into orienting behavior. Yet it is not well understood how this behavior is organized early in development and how it relates to the response properties of the developing visual system. We designed a novel behavioral assay to study the development of visually guided behavior in Xenopus laevis tadpoles. We found that, during early development, visual avoidance—an innate, tectally mediated behavior—is tuned to a specific stimulus size and is sensitive to changes in contrast. Using in vivo recordings we found that developmental changes in the spatial tuning of visual avoidance are mirrored by changes in tectal receptive field sharpness and the temporal properties of subthreshold visual responses, whereas contrast sensitivity is affected by the gain of the visual response. We also show that long- and short-term perturbations of visual response properties predictably alter behavioral output. We conclude that our assay for visual avoidance is a useful functional measure of the developmental state of the tectal circuitry. We use this assay to show that the developing visual system is tuned to facilitate behavioral output and that the system can be modulated by neural activity, allowing it to adapt to environmental changes it encounters during development.

Motor-Related Signals in the Intraparietal Cortex Encode Locations in a Hybrid, rather than Eye-Centered Reference Frame

Abstract: The reference frame used by intraparietal cortex neurons to encode locations is controversial. Many previous studies have suggested eye-centered coding, whereas we have reported that visual and auditory signals employ a hybrid reference frame (i.e., a combination of head- and eye-centered information) (Mullette-Gillman et al. 2005). One possible explanation for this discrepancy is that sensory-related activity, which we studied previously, is hybrid, whereas motor-related activity might be eye centered. Here, we examined the reference frame of visual and auditory saccaderelated activity in the lateral and medial banks of the intraparietal sulcus (areas lateral intraparietal area [LIP] and medial intraparietal area [MIP]) of 2 rhesus monkeys. We recorded from 275 single neurons as monkeys performed visual and auditory saccades from different initial eye positions. We found that both visual and auditory signals reflected a hybrid of head- and eye-centered coordinates during both target and perisaccadic task periods rather than shifting to an eye-centered format as the saccade approached. This account differs from numerous previous recording studies. We suggest that the geometry of the receptive field sampling in prior studies was biased in favor of an eye-centered reference frame. Consequently, the overall hybrid nature of the reference frame was overlooked because the non--eye-centered response patterns were not fully characterized.

Functional Groups in the Avian Auditory System

Abstract: Auditory perception depends on the coding and organization of the information-bearing acoustic features of sounds by auditory neurons. We report here that auditory neurons can be classified into functional groups, each of which plays a specific role in extracting distinct complex sound features. We recorded the electrophysiological responses of single auditory neurons in the songbird midbrain and forebrain to conspecific song, measured their tuning by calculating spectrotemporal receptive fields (STRFs), and classified them using multiple cluster analysis methods. Based on STRF shape, cells clustered into functional groups that divided the space of acoustical features into regions that represent cues for the fundamental acoustic percepts of pitch, timbre, and rhythm. Four major groups were found in the midbrain, and five major groups were found in the forebrain. Comparing STRFs in midbrain and forebrain neurons suggested that both inheritance and emergence of tuning properties occur as information ascends the auditory processing stream.

Receptive field organization across multiple electrosensory maps. I. Columnar organization and estimation of receptive field size.

Abstract: The electric fish Apteronotus leptorhynchus emits a high-frequency electric organ discharge (EOD) sensed by specialized electroreceptors (P-units). Amplitude modulations (AMs) of the EOD are caused by objects such as prey as well as by social interactions with conspecifics. The firing rate of P-units is modulated by the AMs due to both objects and communication signals. P-units trifurcate as they enter the medulla; they terminate topographically with three maps of the electrosensory lateral line lobe (ELL): the centromedial (CMS), centrolateral (CLS), and lateral (LS) segments. Within each map P-units terminate onto the basal dendrites of pyramidal cells. Anterograde filling of P-units and retrograde filling of the basal bushes of pyramidal cells were used to estimate their respective spreads and spacing in the three maps. These estimates were used to compute the receptive field structure of the pyramidal cells: receptive fields were small in CMS and very large in LS with intermediate values in CLS. There are several classes of pyramidal cells defined by morphological and functional criteria; these cells are organized into columns such that each column contains one member of each class and all cells within a column receive the same P-unit input.

Neurons in the lateral intraparietal area create a priority map by the combination of disparate signals

Abstract: Primates search for objects in the visual field with eye movements. We recorded the activity of neurons in the lateral intraparietal area (LIP) in animals performing a visual search task in which they were free to move their eyes, and reported the results of the search with a hand movement. We distinguished three independent signals: (1) a visual signal describing the abrupt onset of a visual stimulus in the receptive field; (2) a saccadic signal predicting the monkey’s saccadic reaction time independently of the nature of the stimulus; (3) a cognitive signal distinguishing between the search target and a distractor independently of the direction of the impending saccade. The cognitive signal became significant on average 27 ms after the saccadic signal but before the saccade was made. The three signals summed in a manner discernable at the level of the single neuron.