How the brain creates imagery: projection to primary visual cortex.
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A disorder of colour perception associated with abnormal colour after-images: a defect of the primary visual cortex.
A 64 year old woman with posterior cortical atrophy secondary to probable Alzheimer's disease is described. Her presenting symptom was of seeing objects as abnormally coloured after prior exposure to a coloured stimulus. Formal testing disclosed that the patient experienced colour after-images of abnormal latency, duration, and amplitude.The demonstration of prolonged colour after-images in a patient with a cortical disease process provides strong evidence that the generation of colour after-images is mediated at least in part by the visual cortex. A mechanism for the generation of colour after-images is proposed in which abnormal prolongation of the images results from excessive rebound inhibition of previously excited wavelength selective neurons in V1. This may occur as a consequence of the relative sparing of inhibitory interneurons in V1 in the context of the degeneration of excitatory neurons that occurs in Alzheimer's disease.
Afterimage↗
role of primary visual cortex in canine postural control.
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Animals↗
Temporal encoding of two-dimensional patterns by single units in primate primary visual cortex. I. Stimulus-response relations.
1. Previously we developed a new approach for investigating visual system neuronal activity in which single neurons are considered to be communication channels transmitting stimulus-dependent codes in their responses. Application of this approach to the stimulus-response relations of inferior temporal (IT) neurons showed that these carry stimulus-dependent information in the temporal modulation as well as in the strength of their responses. IT cortex is a late station in the visual processing stream. Presumably the neuronal properties arise from the properties of the inputs. However, the discovery that IT neuronal spike trains transmit information in stimulus-dependent temporally modulated codes could not be assumed to be true for those earlier stations, so the techniques used in the earlier study were applied to single-striate cortical neurons in the studies reported here. 2. Single-striate cortical neurons were recorded from three awake, fixating rhesus monkeys. The neurons were stimulated by two sets of patterns. The first set was made up of 128 black-and-white patterns based on a complete, orthogonal set of two-dimensional Walsh-Hadamard functions. These stimuli appear as combinations of black-and-white rectangles and squares, and they fully span the range of all possible black-and-white pictures that can be constructed in an 8 x 8 grid. Except for the stimulus that appeared as an all-white or all-black square, each stimulus had equal areas of white and black. The second stimulus set was made up of single bars constructed in the same 8 x 8 grid as the Walsh stimuli. These were presented both as black against a gray background and white against a gray background. The stimuli were centered on the receptive field, and each member of the stimulus set was presented once before any stimulus appeared again. 3. The responses of 21 striate cortical neurons were recorded and analyzed. Two were identified as simple cells and the other 19 as complex cells according to the criteria originally used by Hubel and Wiesel. The stimulus set elicited a wide variety of response strengths and patterns from each neuron. The responses from both the bars and the Walsh set could be used to differentiate and classify simple and complex cells. 4. The responses of both simple and complex cells showed striking stimulus-related strength and temporal modulation. For all of the complex cells there were instances where the responses to a stimulus and its contrast-reversed mate were substantially different in response strength or pattern, or both.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Temporal encoding of two-dimensional patterns by single units in primate primary visual cortex. II. Information transmission.
1. Previously, we studied how picture information was processed by neurons in inferior temporal cortex. We found that responses varying in both response strength and temporal waveform carried information about briefly flashed stationary black-and-white patterns. Now, we have applied that same paradigm to the study of striate cortical neurons. 2. In this approach the responses to a set of basic black and white pictures were quantified through use of a set of basic waveforms, the principal components (extracted from all the responses of each neuron). We found that the first principal component, which corresponds to the response strength, and others, which correspond to different basic temporal activity patterns, were significantly related to the stimuli, i.e., the stimulus drove both the response strength and its temporal pattern. 3. Our previous study had shown that, when information theory was used to quantify the stimulus-response relation, inferior temporal neurons convey over twice as much information in a response code that includes temporal modulation as in a response code that includes only the response strength. This study shows that striate cortical neurons also carry twice as much information in a temporal code as in a response strength code. Thus single visual neurons at both ends of a cortical processing chain for visual pattern use a multidimensional temporal code to carry stimulus-related information. 4. These results support our multiplex-filter hypothesis, which states that single visual system neurons can be regarded as several simultaneously active parallel channels, each of which conveys independent information about the stimulus.
Analysis of Variance↗
Visual resolution and sensitivity of single cells in the primary visual cortex (V1) of a nocturnal primate (bush baby): correlations with cortical layers and cytochrome oxidase patterns.
1. This study describes the response properties of V1 cortical cells in a nocturnal primate and examines the receptive field organization of these cells in relationship to anatomically defined layers and cytochrome oxidase (CO) rich blobs and CO poor interblob compartments. Visual resolution and contrast sensitivity are consistent with other physiological and behavioral measures in this species. Comparisons are made with response properties of the same zones in macaque monkey, as well as of area 17 of a distantly related species (cat) that also occupies a nocturnal niche. 2. The responses of single cells to drifting sinusoidal gratings were recorded in V1 (striate cortex) of anesthetized, paralyzed bush babies (Galago crassicaudatus). Cells tended to be grouped with respect to ocular dominance, orientation preference, and direction selectivity. There was a high proportion of monocularly driven cells as in macaque monkey. Only 6% of the cells were nonoriented. These were poorly tuned complex cells and bore no resemblance to nonoriented lateral geniculate nucleus (LGN)-like cells reported in layer IV of macaque monkeys. Unidirectional cells were most frequently encountered in cortical layers that receive input from the magnocellular layers of the LGN. 3. Cells were classified as simple (31%) or complex (69%) according to standard criteria. Simple cells were significantly more narrowly tuned than complex cells for both orientation and spatial frequency. Complex cells had significantly higher average optimal spatial frequencies and spatial frequency cutoffs than simple cells. Contrast sensitivity of simple and complex cells averaged 38 and 34, respectively. Spatial resolution and sensitivity of these cells matches behavioral measures in bush baby. The spatial and temporal resolution of bush baby cells are similar to those of cats, which is likely related to the nocturnal niche of both species. 4. Cells in supragranular (I-III) and infragranular (V, VI) layers differed significantly in their response characteristics. The cells in the supragranular layers had significantly higher contrast sensitivity than did the cells in the infragranular layers. Cells in the supragranular layers likewise had higher temporal frequency cutoffs, significantly lower optimal spatial frequencies, lower spatial frequency cutoffs, and tighter orientation tuning than did cells in the infragranular layers. 5. Properties of cells in individual layers and CO blob and interblob compartments also showed differentiation. Layer III had the narrowest orientation and spatial frequency tuning with the tightest tuning in layer IIIC (IVB).(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Length and width tuning of neurons in the cat's primary visual cortex.
1. The classically defined receptive field of a visual neuron is the area of visual space over which the cell responds to visual stimuli. It is well established, however, that the discharge produced by an optimal stimulus can be modulated by the presence of additional stimuli that by themselves do not produce any response. This study examines inhibitory influences that originate from areas located outside of the classical (i.e., excitatory) receptive field. Previous work has shown that for some cells the response to a properly oriented bar of light becomes attenuated when the bar extends beyond the receptive field, a phenomenon known as end-inhibition (or length tuning). Analogously, it has been shown that increasing the number of cycles of a drifting grating stimulus may also inhibit the firing of some cells, an effect known as side-inhibition (or width tuning). Very little information is available, however, about the relationship between end- and side-inhibition. We have examined the spatial organization and tuning characteristics of these inhibitory effects by recording extracellularly from single neurons in the cat's striate cortex (Area 17). 2. For each cortical neuron, length and width tuning curves were obtained with the use of rectangular patches of drifting sinusoidal gratings that have variable length and width. Results from 82 cells show that the strengths of end- and side-inhibition tend to be correlated. Most cells that exhibit clear end-inhibition also show a similar degree of side-inhibition. For these cells, the excitatory receptive field is surrounded on all sides by inhibitory zones. Some cells exhibit only end- or side-inhibition, but not both. Data for 28 binocular cells show that length and width tuning curves for the dominant and nondominant eyes tend to be closely matched. 3. We also measured tuning characteristics of end- and side-inhibition. To obtain these data, the excitatory receptive field was stimulated with a grating patch having optimal orientation, spatial frequency, and size, whereas the end- or side-inhibitory regions were stimulated with patches of gratings that had a variable parameter (such as orientation). Results show that end- and side-inhibition tend to be strongest at the orientation and spatial frequency that yield maximal excitation. However, orientation and spatial frequency tuning curves for inhibition are considerably broader than those for excitation, suggesting that inhibition is mediated by a pool of neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Development of neuronal selectivity in primary visual cortex of cat.
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Afferent Pathways↗
Neuronal origin of the transcorneal electrically evoked response in area 17 (primary visual cortex) of cats.
We investigated how the neuronal activity of the cells in area 17 of cats evoked by transcorneal electrical stimuli is associated with the transcorneal electrically evoked response (EER). Neuronal firings were associated with the negative components of the EER, N1 (9 ms latency) and N2 (20 ms latency). Excitation of a longer latency was suppressed and periodic firings were rarely found. These findings indicate that the early negative components of the EER consist of the synaptic activities over the discharge thresholds of cortical cells, and also suggest that inhibition in the visual cortex may affect the EER.
Animals↗
Neural field model of receptive field restructuring in primary visual cortex.
Receptive fields (RF) in the visual cortex can change their size depending on the state of the individual. This reflects a changing visual resolution according to different demands on information processing during drowsiness. So far, however, the possible mechanisms that underlie these size changes have not been tested rigorously. Only qualitatively has it been suggested that state-dependent lateral geniculate nucleus (LGN) firing patterns (burst versus tonic firing) are mainly responsible for the observed cortical receptive field restructuring. Here, we employ a neural field approach to describe the changes of cortical RF properties analytically. Expressions to describe the spatiotemporal receptive fields are given for pure feedforward networks. The model predicts that visual latencies increase nonlinearly with the distance of the stimulus location from the RF center. RF restructuring effects are faithfully reproduced. Despite the changing RF sizes, the model demonstrates that the width of the spatial membrane potential profile (as measured by the variance sigma of a gaussian) remains constant in cortex. In contrast, it is shown for recurrent networks that both the RF width and the width of the membrane potential profile generically depend on time and can even increase if lateral cortical excitatory connections extend further than fibers from LGN to cortex. In order to differentiate between a feedforward and a recurrent mechanism causing the experimental RF changes, we fitted the data to the analytically derived point-spread functions. Results of the fits provide estimates for model parameters consistent with the literature data and support the hypothesis that the observed RF sharpening is indeed mainly driven by input from LGN, not by recurrent intracortical connections.
Electroencephalography↗
Topographic receptive fields and patterned lateral interaction in a self-organizing model of the primary visual cortex.
This article presents a self-organizing neural network model for the simultaneous and cooperative development of topographic receptive fields and lateral interactions in cortical maps. Both afferent and lateral connections adapt by the same Hebbian mechanism in a purely local and unsupervised learning process. Afferent input weights of each neuron self-organize into hill-shaped profiles, receptive fields organize topographically across the network, and unique lateral interaction profiles develop for each neuron. The model demonstrates how patterned lateral connections developed based on correlated activity and explains why lateral connection patterns closely follow receptive field properties such as ocular dominance.
Brain Mapping↗
Modulations of primary visual cortex activity representing attentive and conscious scene perception.
In the visual cortex, information is transferred from one area to the next by means of feedforward connections. These connections shape the receptive field properties of neurons in subsequent visual areas. Horizontal and feedback connections modulate this neuronal activity, resulting in the phenomenon of contextual modulation. In area V1, where receptive field properties reflect only low level processing, contextual modulation can be observed that represents fully evaluated perceptual saliency of the features within the receptive field. Here, we discuss to what extent these modulations are related to high level visual processes like perceptual organization, attention and visual awareness. Contextual modulation appears to reflects a process very distinct from receptive field based processing. This process seems to integrate information from distant areas in visual cortex to neurophysiologically 'highlight' those neurons that represent image elements or features of objects that stand out perceptually. Moreover, similar modulations are observed in relation to whether objects are attended to or not. Finally, these modulations are only present when subjects are aware of the visual input.
Animals↗
The topography and variability of the primary visual cortex in man.
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Blindness↗
Development and organization of ocular dominance bands in primary visual cortex of the sable ferret.
Thalamocortical afferents in the visual cortex of the adult sable ferret are segregated into eye-specific ocular dominance bands. The development of ocular dominance bands was studied by transneuronal labeling of the visual cortices of ferret kits between the ages of postnatal day 28 (P28) and P81 after intravitreous injections of either tritiated proline or wheat germ agglutinin-horseradish peroxidase. Laminar specificity was evident in the youngest animals studied and was similar to that in the adult by P50. In P28 and P30 ferret kits, no modulation reminiscent of ocular dominance bands was detectable in the pattern of labeling along layer IV. By P37 a slight fluctuation in the density of labeling in layer IV was evident in serial reconstructions. By P50, the amplitude of modulation had increased considerably but the pattern of ocular dominance bands did not yet appear mature. The pattern and degree of modulation of the ocular dominance bands resembled that in adult animals by P63. Flat mounts of cortex and serial reconstructions of layer IV revealed an unusual arrangement of inputs serving the two eyes in the region rostral to the periodic ocular dominance bands. In this region, inputs serving the contralateral eye were commonly fused along a mediolateral axis, rostral to which were large and sometimes fused patches of ipsilateral input.
Aging↗
Functional micro-organization of primary visual cortex: receptive field analysis of nearby neurons.
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.
Animals↗
[What image characteristics are selected by neurons in the cat primary visual cortex?].
About 40% of neurons (114/289) studied in the cat area 17 gave a larger (by 3.06 +/- 0.32 times on average) response to a flashed cross, corner or y-like figures centered in the RF than to an optimal single bar. Most such neurons (72%) were found to be highly selective both to shape (angle between the lines) and to orientation of these figures. In the studied neuronal selection we have also found all possible types of invariance of sensitivity to orientation and/or shape of these figures. Separated and combined stimulation of RF center and surrounding area revealed in 44 units summation, antagonism or absence of interaction of these zones by the selectivity index (cross/bar response ratio). Cross-sensitivity was investigated in 85 V1 neurons before, during and after local blockade of GABAA ergic inhibition by microiontophoretical application of bicuculline. Inhibition either emerged or increased cross-sensitivity (32% of cells), or depressed it (36% of cases), while in some neurons it does not influenced the function. Possible mechanisms of the described effects are discussed as well as their functional implication for second-order feature extraction in the visual cortex: selective or invariant sensitivity of neurons to the shape and orientation of the line-crossings.
Animals↗
Membrane potential and firing rate in cat primary visual cortex.
We have investigated the relationship between membrane potential and firing rate in cat visual cortex and found that the spike threshold contributes substantially to the sharpness of orientation tuning. The half-width at half-height of the tuning of the spike responses was 23 +/- 8 degrees, compared with 38 +/- 15 degrees for the membrane potential responses. Direction selectivity was also greater in spike responses (direction index, 0.61 +/- 0.35) than in membrane potential responses (0.28 +/- 0.21). Threshold also increased the distinction between simple and complex cells, which is commonly based on the linearity of the spike responses to drifting sinusoidal gratings. In many simple cells, such stimuli evoked substantial elevations in the mean potential, which are nonlinear. Being subthreshold, these elevations would be hard to detect in the firing rate responses. Moreover, just as simple cells displayed various degrees of nonlinearity, complex cells displayed various degrees of linearity. We fitted the firing rates with a classic rectification model in which firing rate is zero at potentials below a threshold and grows linearly with the potential above threshold. When the model was applied to a low-pass-filtered version of the membrane potential (with spikes removed), the estimated values of threshold (-54.4 +/- 1.4 mV) and linear gain (7.2 +/- 0.6 spikes. sec(-1). mV(-1)) were similar across the population. The predicted firing rates matched the observed firing rates well and accounted for the sharpening of orientation tuning of the spike responses relative to that of the membrane potential. As it was for stimulus orientation, threshold was also independent of stimulus contrast. The rectification model accounted for the dependence of spike responses on contrast and, because of a stimulus-induced tonic hyperpolarization, for the response adaptation induced by prolonged stimulation. Because gain and threshold are unaffected by visual stimulation and by adaptation, we suggest that they are constant under all conditions.
Action Potentials↗
Contextual modulation in primary visual cortex of macaques.
Recent studies have suggested that V1 neurons extract figures from their backgrounds, in that they respond better to interior features of figures than to equivalent features of background stimuli. This is reportedly true even when the figure boundaries are distant from the borders of the classical receptive field (RF). To test the role of V1 neurons in figure-ground segregation, we recorded their responses to texture figures on texture backgrounds, centered on the RF. The texture elements of the figures remained identical across trials, and figure boundaries were defined by orientation differences between the elements in the background texture relative to elements in the figure. For nearly all neurons (98/102), responses to a large texture figure did not differ from the responses to a uniform-texture background. Although many neurons gave enhanced responses to texture boundaries, this occurred only when the boundaries were within or close to the RF borders. Similar effects were found in V2. For neurons in V1, the limited spatial extent of the contextual modulation was not increased either at low stimulus contrast or when the animal was rewarded for detecting an orientation-defined figure. Thus, V1 neurons appear to signal texture boundaries rather than figures per se. Unexpectedly, many V1 neurons gave significant long-latency responses to texture stimuli located entirely outside the classical RF, up to 5 degrees from the RF border in some cases. However, these responses did not depend on the stimulus forming a figure that contained the RF. Although V1 neurons are influenced by stimuli outside the classical RF, they do not appear to segregate figures from ground.
Animals↗